Ventilation and Airconditioning-Electrical Rooms

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Handbook of Industrial Air Technology

Applications

VENTILATION AND AIR-CONDITIONING OF ELECTRICAL

EQUIPMENT ROOMS

Second, revised edition

Kim HagstrmJorma RailioEsko Thti

February 2003


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PREFACE

This document is the revised version of the first pilot Application booklet for the Handbook ofIndustrial Air Technology.

This text is based on INVENT project "Design Criteria for Ventilation in Electrical EquipmentRooms", done in Finland in 1991-94, reported in Finnish as INVENT Reports 36 to 39. It has

been re-structured to follow the basic structure of the Design Methodology for industrialventilation, also in order to test the methodology in practice.

After being published as a draft manuscript in 1996 (INVENT Report 52), the text has beenreviewed by Mr Martti Lagus (Nokia Telecommunications, Finland) and by Mr Peter Kiff

(British Telecom). In addition, the described design methodology has been validated by severalFinnish companies who actively apply the results of the INVENT project, either as end users ofequipment rooms, or as suppliers of air-conditioning systems and equipment.

The text has been revised after review by Mr Jan Gustavsson (Camfil, Sweden), Dr PaoloTronville (Politecnico di Torino, Italy) and Dr David Shao (Ericsson Radio Access, Sweden). TheEnglish language of the revision has been checked and corrected by Mr Eric Curd (U.K.)

Helsinki, December 2002


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VENTILATION AND AIR-CONDITIONING OF ELECTRICAL EQUIPMENTROOMS

CONTENTSPREFACE.......................................................................................................................... 2 CONTENTS....................................................................................................................... 3 SUMMARY....................................................................................................................... 4 1. INTRODUCTION......................................................................................................... 5

1.1. Classification of environmental parameters for electrical equipment.............. 5 1.2 Typical environments of electrical equipment located in ventilated indoorfacilities .......................................................................................................................... 5 1.3 Application scope .................................................. .................................................. 6

2 DESIGN CRITERIA ..................................................................................................... 8 2.1 Given data................................................................................................................ 8 2.2 Process description.................................................................................................. 9 2.3 Building layout and structures .................................................... ........................ 11 2.4 Target level assessment......................................................................................... 20 2.5 Source description.................................... ............................................................. 36 2.6 Load calculations ............................................... ................................................... 39

3 SYSTEM PERFORMANCE....................................................................................... 41 3.1 Selection of system ................................................................................................ 41 3.2 Selection of equipment.......................................................................................... 53 3.3 Implementation design ......................................................................................... 62

4 COMMISSIONING..................................................................................................... 66 4.1 The construction schedule.................................................................................... 66 4.2 Checks............................................................................................... ..................... 67 4.3 Spare parts............................................................................................................. 67 4.4 Documentation ................................................... ................................................... 68

APPENDIX 1 - BASIS FOR DESIGN FOR VENTILATION IN ELECTRICALEQUIPMENT ROOMS, CLIMATIC CONDITIONS ................................................ 76 APPENDIX 2 - THE MEASURING PRECONDITIONS OF GASES...................... 77 APPENDIX 3 - REFERENCES................................................ ..................................... 79


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SUMMARYThere has been a lack of "common language" for the design, construction and operation ofair-conditioning of industrial electric-, electrotechnical- and control rooms. Requirementsfor the equipment and its environmental conditions are presented in different standards andguidelines in such a way that many interpretations are possible. For example, a giventemperature limit can have been regarded by the end user as an absolute maximum, whilethe equipment supplier could allow it to be exceeded (within another range) for a short

period.

As a result of this, in some rooms the environment is too severe, resulting in operational errors and

equipment failures, which really can be worth more than the whole equipment some rooms are conditioned unnecessarily well in relation to the actual

environmental requirements, resulting in high investment costs, due to oversized (orunnecessarily double) air-conditioning equipment, or in high operation costs.

An INVENT project was done in 1991-94 to tackle this problem area.

The participants in the project represented different supplier and user groups such as: HVACequipment manufacturers and suppliers, HVAC consulting engineers, electrical equipmentmanufacturers, process automation- and -control system manufacturers, and end-users fromdifferent trades of process-industry: pulp and paper, chemical industry, metal industry. Aservice product was also developed, in order to analyze the state of existing equipmentrooms. This work was done in 1995, and the results can be utilized in commercial basisnow, and several project references already exist. The knowledge gained in these actions has

been the basis of this application.

Modern electrical equipment containing electronics is very sensitive to their environmentalconditions: temperature and humidity, chemically and mechanically active substances etc.

In highly automated process industry a failure in process control equipment may causelosses in production that are many times worth of the equipment itself. Just to mention oneexample: a paper machine breakdown can cost up to 30.000 or USD/hour.

Minor improvements can be also done with low costs. Very often the tightness of the roomcan be improved so that the ventilation rate for maintaining over-pressure can be adjusted toa much lower level. A typical pay-back time for sealing the room properly is 0,5-0,7 yearsand investment less than 1000 or USD/room respectively.

The benefits from proper design, construction and use can be summarized as follows: better operating conditions prolonged lifetime for electrical equipment improved reliability of the systems efficient use of the systems improved knowledge of the condition of the systems improved skills of he maintenance personnel


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1. INTRODUCTION

1.1. Classification of environmental parameters for electrical equipmentThe basis of the environmental classification for electrical equipment is covered in EN60721-3-0 standard. International recommendations for environmental conditions inelectrical equipment rooms are covered by the European standard EN 60721-3-3.

To standardise design practice the above should be used in ventilation systems design.Environmental factors covered in EN 60721-3-3 have been divided in the following groups.

Climatic conditions (K) Climatic special conditions (Z) Biological conditions (B) Chemically active substances (C) Mechanically active substances (S) Mechanical conditions (M)

The environmental conditions in EN 60721-3-3 for electrical equipment room are coveredin various classes.Electrical equipment manufacturers define the special requirements of each deviceaccording to EN 60721-3-3 in the following manner:

A definition such as the following 3K1/3Z1/3Z4/3B1/3C1/3S1/3M1:

The code-3K1 Defines climatic conditions.In class 3K1, the target temperature value is 20 - 25 o C in the range 18 -27o C.The relative humidity target value is 30-60 % in the range 20-75 % andthe absolute humidity ranges from 3,5 - 15 g.kg 1

-3Z1 Z-class describes the special climatic conditions, including thesurroundings thermal radiation.

-3Z4 shows the permitted air velocity if different from the K-class value. In thiscase it is 5 m.s -1. With special condition classes, it considers how a devicereacts to water.

-3B1 describes biological conditions. Organisms or animals are not accepted inthe 3B1 class.

-3C1 A demand that covers chemically active substances. This defines the permissible concentrations of corrosive gases in the space.

-3S1 Mechanically active substances. Defines the permissible concentration of particle contaminants e.g. dust, sand etc in the space.

1.2 Typical environments of electrical equipment located in ventilated indoor facilitiesTable 1.1.1 provides a general idea of the "environmental tolerance" of equipment indifferent spaces. A nomenclature of electrical equipment rooms has not yet been


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determined; Rooms of similar names in different industrial plants can contain very diverseequipment. Therefore, the environmental requirements for each room have to be checkedindividually during the various project stages.

1.3 Application scopeThe environmental classification for electrical equipment and the design basis forventilation represented here, consider all indoor spaces where electrical equipment islocated.Chapters 3 (System Performance) and 2.3 (Building Layout and Structures) emphasizeventilation of electrical equipment rooms (computer-, tele-, automation and current supplyrooms).


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Table 1.1.1 Operating Environments of Electrical and Electronic Equipment, VentilatedIndoor Spaces

TYPICAL ENVIRONMENT THE ENVIRONMENTCLASS OF THEEQUIPMENT ROOMaccording to the standardEN 60721-3-3. See 1.1 forexplanation ofclassification

A Control- and automation roomsa1) workspaces separated from the processa2) automation roomsa3) cross-connection rooms (measuring equipment,monitors and connections)Computer rooms

3K2 / 3Z2 / 3Z4 /3B1 / 3C1 / 3S1 /

(As above, except 3K1)

B Electrical equipment rooms-rooms that are separated from outdoor spaces and the

processb1) tele cross-connection and device rooms:b2) electrical equipment rooms of the production:

(motor use, MCC)b3) electrical exchange rooms:(main distribution centres,sub-main distribution centers)b4) transformer rooms (internal)

3K3 / 3Z2 / 3Z4 /3B1 / 3C1 / 3S1 /

C Assembling halls in the electronics industry-circuit card production, assembling of micro electroniccomponents testing and adjustments, assembling of finemechanisms and fibre optics

3K2 / 3B1 / 3C1 /3S1 / 3M1 /

D Production spaces in the metal industry-engineering workshops, light metal industry, bulkassembling: (motors, robots, control devices)-cable spaces

3K4 / 3Z2 / 3Z4 / 3Z7 /3B2 / 3C2 / 3S2 /

E Production spaces in the process industry-spaces where contaminants and corrosive gases are

typical:(instrumentation with protective cover, motors,control device)

3K5 / 3Z2 / 3Z4 / 3Z7 /3B2 / 3C2+C3 when

necessary/3S2 /

F An open, dirty industrial space-dusty spaces in direct contact with outdoor air, foundries,ore mills, waste treatment plants:(equipment with

protective cover, suitable for outdoor use)-outdoor transformers

3K6 / 3Z2 / 3Z4 / 3Z7 3B2 /3C2 / 3S2 /

G Movable containers -control cabinets and electricalrooms that are movable during use (for example military

purposes)

3K2 / 3Z2 / 3B1 / 3C1 3S1 /


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2 DESIGN CRITERIA

2.1 Given data

2.1.1 Meteorological dataConsideration of external temperature, humidity, wind forces and solar radiation arenecessary in order to determine the cooling, heating and moisture loads.The design criteria shown in 2.5 have been compiled to meet the requirements of thestandard EN 60721-3-3. If the maximum temperature 99% value is used i.e. (thetemperature for 99% of the time below the design value). However, if a customer wishes touse higher design temperature for operational reliability the design parameters have to beagreed with the customer.

2.1.2 Air contaminants.

2.1.2.1 Chemically active substances (corrosive gases).In certain areas of the process industry (such as the pulp, chemical and petrochemicalindustries) and in cities, the concentrations of corrosive gases may be higher than the

permissible concentrations for electrical equipment. In a corrosive environment, electricalequipment is easily damaged shortening the operating life.

It is difficult to obtain information on the exact environmental air purity since:

Outdoor air purity is a complex matter: the concentrations measured in the same place canvary within 1:100. This variation depends on wind direction air pressure and on processmalfunctionsIn recent years air purity is constantly improving due to various environmental protectionacts. For these reasons, details that could easily be used in defining the outside air on adesign basis cannot be developed.

The following approaches can be used to define the contaminant concentration of the intakeair

1. Accurate concentration measurements.This however is not often practical, as:The measurements have to be carried out over a long time period (minimum 6months) if exact initial values are required.To obtain accurate information measurements have to be obtained close to the areaof concern. Attention has to be paid to the fact that a new building will createchanges in airflows, which can influence standard design requirements.

2. Estimating the concentration beforehand. This can be done by the copper-stripmethod.


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If the measuring period is long enough, and the concentration is measured at the airintake, provided the copper-strip is in a warm location. The contaminant ratio can

be 1:4 within a few meters of the air inlet.

3. General measurements of the air conditions.For example in Finland, these results can be obtained from the following sources:

- The Environment Department of the company.- Measurements carried out by the county, city or town authorities.- Meteorological Institute data

4. By rough estimations. For example tables provided by the equipment manufacturerscan be used.

5. Experience. This is probably the most important way knowledge is gained; itdepends on experience, regarding filters life and equipment corrosion. This isaccomplished by comparing practical experiences with similar plants in theorganization or in other factories.

While dimensioning filters the possible sudden pressure changes caused bymalfunctions in the process have to be considered as these changes can cause thegas concentrations to momentarily rise up to 10-100 times the averageconcentrations.

Measuring preconditions are described in Appendix 1.

2.1.2.2 Mechanically active substances (sand, dust)The outdoor air data, relating to dust will not normally provide a base for selecting particlefilters. In 3.2.3 consideration is given on how to select a filter for electrical equipmentrooms.

The surrounding dust concentration can be defined by similar principles as gases.

2.2 Process description

2.2.1 IntroductionRegardless of the electrical equipment installed, there are usually no other emission sourcesin electrical equipment rooms. In control- and automation rooms however, employees mayoccupy the space for long periods of time.

Internal loads in electrical equipment rooms are: the heat developed by the equipment the emissions from humans (control cabinets and automation rooms)


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In battery rooms hydrogen is liberated into the air, and care has to be takenregarding explosion hazards.

The role of the ventilation in electrical equipment rooms is to: - remove the heat developed by electrical equipment keep the room clean of contaminants from the surroundings.

2.2.2 Typical electrical equipment roomA typical electrical equipment room has equipment cabinets positioned in several rows.Many different-sized cables are fed to and from these cabinets either from below or above.In the process industry the cable space is usually separated in its own compartment. Due tofire safety reasons the cable space is confined by a raised floor forming its own fire cell(compartment). Figures 2.2.1 and 2.2.2 show two alternative typical cross-sections ofelectrical equipment rooms.

Figure 2.2.1. The recommended minimum dimensions of aisles for an equipment room


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Figure 2.2.2 . A cable space

2.3 Building layout and structures

2.3.1 Location of electrical equipment rooms

2.3.1.1 IntroductionThe room shall be over-pressurised to reduce air infiltration from surrounding areas. Theair tightness of electrical equipment rooms has to be sound, and the degree of over-

pressurization has to be sufficient to neutralize the influence of wind forces, temperaturedifferences and surrounding process spaces, which may be operating under negative

pressure.

2.3.1.2 Effect of wind forcesIf an electrical equipment room has external wall it can be greatly influenced by windforces. These may, over-pressure on the wind-exposed wall resulting in polluted outdoorair entering the room. This effect has to be considered in the design process.The electrical equipment room if possible should be positioned, so that wind forces do notinfluence it. In spaces which rely on high air flows for cooling, the structural tightness andexhaust air openings sizes should be dimensioned so that the internal overpressure is keptto a reasonable level, e.g. not more than 20 Pa.

Figure 2.3.1 shows graphically the wind pressure on the outer wall of a building with thewind velocity in the 0, 5 and 10 m.s -1 range against the wall.


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The example results have been calculated by using the formula: -

p=K*0.5 * *v2 (1)

whereK= a pressure coefficient depending on building shape and the wind direction= air density = 1,2,kg.m -3

v= the wind velocity [m.s -1]

For air of standard density the above equation becomes p = K 0.6 v 2

Figure 2.3.1 . The wind pressure on the outer wall of a building with wind velocities of 0, 5and 10 m/s on the wall.

2.3.1.3 Vertical position in buildingA temperature difference between indoor and outdoor air causes a pressure difference onthe outer wall. An internal neutral plane is formed at some height above the building floor.The actual position of the neutral plane changes due to the influence of wind forces andopenings (crackage) in the structure. Above the neutral plane the inner parts of the buildingare over-pressurized with respect to the outdoor air and below this point it has a negative

pressure when outdoor air is colder than the indoor air. This causes problems to theresulting pressure ratios, especially when the room is located on the outer wall of a highquality process space.

Figure 2.3.2 shows graphically the pressure difference on the outer wall created bytemperature difference, as a function of the distance from the neutral plane with differenttemperatures, for a room temperature of 20 o C.


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Figure 2.3.2 . Pressure difference created by the temperature difference between the indoorand outdoor air, on the outer wall of the building with different outdoor air temperatures.

The interaction of the wind and temperature differences on the pressure difference of theouter wall is shown in figure 2.3.3.Problems of resulting pressure ratios will be created when the electrical equipment roomsare connected to both indoor and outdoor areas.If the room outlet is at a high position, the airflow will be outward from the space and if theoutlet is at low level the flow will be reversed.

Figure 2.3.3. The interaction of wind and temperature on the outer wall of a building when = -20 C.

2.3.1.4 Surrounding roomsThe external heat loads on electrical equipment rooms vary considerably. The external heatloads are due to open cable spaces, and adjacent process areas. The positioning of electricalequipment rooms close to hot process should be avoided.

The pressure ratios in surrounding rooms will influence the design pressure conditions.A typical example is that a strongly negative-pressurized cable space will reduce the over-

pressure in an electrical equipment room. Cable spaces should be designed to have even- pressures. If located beside, under or over an electrical equipment room. Process spacesmay be held at positive and negative pressure.


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2.3.2 Tightness of the structure

2.3.2.1 IntroductionThe air tightness of the structure is a critical factor when considering the influence thecontaminant loads have on electrical equipment. It also influences the design of theventilation system, as well as investment and operating costs. The design of the ventilationsupply is related to the structural air tightness. The required over-pressure necessary toavoid infiltration will not be achieved if air leakage through the room structure is greaterthan that calculated.The tighter the structure, the less outdoor air flow is needed to provide the desired roomover-pressure, the operating costs will also be reduced. The worst leakage areas are serviceholes and crackage in the structure, and it is essential that these be kept to a minimum.

The structure of an electrical equipment room is usually brick, concrete or sheet metal-mineral wool-sheet metal elements. Untreated tile and concrete surfaces and porous, and airflows through them. The structural leakage paths can be reduced by the use of special

paints.

If the electrical equipment room is built of sheet metal-mineral wool-sheet metal elements,the joints between the elements and connections to other structures have to be sealed toreduce external and internal air transfer. Partition walls also require sealing.

2.3.2.2 Estimation of the room tightnessThe general formula (Olander 1982) that describes the leakage air flow through walls, isfollowing:

QVL= C * (dp) 0,65 [m3s-1m-2, Pa 0,65 ] (2)

Factor C varies according to the tightness of the wall, typical values being

A tight wallAn average wallA leaky wall

0,00030,00050,0007

With the above formula (2) the air tightness of electrical equipment rooms can bedetermined.The required air volume flow for the room pressurization depends not only on the roomvolume but also on the room shape and size. Therefore the correct design criteria must be

based on room wall area. In normal cases the floor and ceiling can be considered as beingairtight due to the coatings on them. Measurement of the make-up airflow according to newdesign criteria can be made using the diagram shown in Figure 2.3.4.In properly sealed rooms the required make-up airflow, to maintain a 20 Pa overpressure inthe room, is 2,1 l.s 1, per m 2 of wall.


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Figure 2.3.4. Dimensioning diagram for pressurization airflow in electrical equipmentrooms

After the electrical equipment room has been completed, it should be tested to ensure thatthe required over-pressure in the room is achieved with the designed outdoor airflow. Innormal cases it is aimed to reach the over-pressure of 20 Pa with an outdoor airflow of 2,1l/(s m 2) of wall.For checking the actual over-pressure, a pressure difference meter should be installedoutside the door. The measurements recorded should logged and used for all future servicechecks.

If the required over-pressure is not reached, or if it reduces during use, leakages may be thecause, and extra sealing is required.

Fan problems also cause a pressure drop and fan airflow should be checked for designconditions. If the over-pressure is greater than the required design energy costs willincrease.

2.3.2.3 Effect of openings on the room tightnessDOORS: The number of doors must be kept to a minimum. Doors not in everyday use, likehauling and trap doors, should be securely sealed. It is recommended to use only one doorin the room and carefully seal other exits.

Doors in everyday use should have air locks to reduce uncontrollable airflow. Doors should

be self-closing, essential fire doors must be fitted with tight seals.

WINDOWS: These should be avoided in electrical equipment rooms. It is difficult to sealwindow frames. In addition solar radiation through the windows increases the external heatgain to the room.

STRUCTURAL PENETRATIONS FOR CABLES AND PIPES: The holes for cables and pipes in the walls of the room should be sealed carefully with an incombustible airtightmaterial. Gypsum can be used for sealing, but it can break down with movement.


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Leaks through penetrations must not unduly impair the air tightness of the wall. Thisconcerns the normal use, as not all of the so-called expanding sealants are suitable forstopping penetrations.

When old building stock is being rebuilt, it is essential that an asbestos study be carried out before work commences. Asbestos was in the past frequently used in structural firebreaks.

ELEMENT JOINTS: All the element joints have to be carefully sealed airtight before painting the walls. The expansion joints shall not be placed on the roof of an electricalequipment room. If an expansion joint has to be located in the room, it has to be carefullytightened Figures 2.3.5 give examples of expansion joint construction.

2.3.3 Effect of the structural mass on the heat dynamics of the roomOver a short time period, say, less than 10 minutes, the structural mass will not have amajor influence on the decrement and the resulting thermal transmission.

Obviously for a longer time period a heavy structure will balance out the room temperaturechanges.

In the approach used, thermally lightweight structure rooms are formed when the meansurface areas are covered with timber panels or other similar materials. Structures made oflight concrete are graded as medium structures. Spaces classed as heavy structures have atleast a half of their surfaces of uncovered concrete.

Figures 2.3.6 - 2.3.8 show calculated warming curves with the help of two time-constantsmodel for a 1000 m 3 type room during the period of time of eight hours. Figures indicatethe effect of different parameters (heat load, initial temperature and weight of structure) tothe warming of the room. Temperature of the environment was held at 25

C.


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Figure 2.3.5. Tightening the expansion joint, examples.


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Figure 2.3.6. Warming curves of a middle-heavy construction room with 4 different heat

loads, when the initial temperature is 25 C.

With heat loads less than 100 W.m 2 the room temperature will not rise above 40 Cregardless of the nature of the structure when the outdoor temperature is +25 C.With heat loads of 200 W.m 2 the temperature of the room will rapidly rise over +40 Cwhen the initial temperature is 35 C.With an initial temperature of 30 oC warming up to 40 C will take 3-8 hours regardless ofthe nature of the structure.In rooms of heavy structure, with heat loads over 300 W.m -2 the temperature will riseabove 40 C regardless of the initial temperature.

The temperature of an electrical equipment room cannot be controlled by structural meansalone. As in the case of air conditioning plant failure with high heat gains, it is impossibleto maintain the design temperature. In order to control the temperature the space must be

provided with back up air-conditioning.

Figure 2.3.7. Warming curves of a light construction room with 4 different heat loads, withinitial temperature 25 C.


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Figure 2.3.8 . Warming curves of a medium heavy weight construction room with fourdifferent heat loads, with initial temperature at 30C.

2.3.4 Materials

2.3.4.1 Construction materials Untreated tile and concrete surfaces liberate dust, causing electrical equipment problems.For this reason the walls of electrical equipment rooms have to be covered with a dust-

binding coat of paint. Precoated sheet metal-mineral wool-sheet metal elements are alsoused

Ceilings should not have mineral wool panels or other dust producing materials included inthem. Alternatives for mineral wool are bag wool with a fabric top. Closed suspendedceilings, however, should be avoided in electrical equipment rooms.

2.3.4.2. Material emissionsMaterials that liberate gases due to aging, which are harmful to equipment, should not beused in electrical equipment rooms. After applying a surface coating, adequate time should

be allowed for drying before the electrical equipment is installed.

A typical drying-time for epoxy paints is 7 days and for acrylate latex 2-4 days, for walltemperatures of +20 C.

2.3.5 Insulation

2.3.5.1 Moisture insulation and gas tightnessThe diffusion of moisture and gases through the walls should be avoided. Holes and cracksin the structure have to be filled with an airtight material. Surfaces to be sealed by speciallyselected paints such as Epoxy and acrylate latex paints.Alloprene and vinyl paints are not recommended, since they emit chlorine and hydrochloricacid during a fire.


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Vapour barriers should be on the high moisture content side, usually the process side. Theelectrical equipment room surfaces should be painted to bind any dust. The cavity in theouter walls should be painted from inside.

2.3.5.2 Thermal insulationThe heat load from the surrounding spaces into electrical equipment rooms might be highin excess of 100 W per-m 2 floors. Normally the main portion of the surrounding loadscomes through the roof and/or the floor from the cable spaces. In the case of a wallseparating a hot process area, thermal insulating is not normally economical unless otheradvantages are achieved such as improved reliability of temperature control in the case ofventilation plant failure.

2.3.6 Fire protectionRequirements concerning fire protection are covered in National Building Regulations andin the requirements of insurance companies.

During a structural or cable insulation fire toxic gases are emitted to a room (a typicalexample is PVC -> HCl). Care has to be taken in the design of exits from these areas. Thewater used for fire fighting these toxic gases forms an aggressive liquid that destroysequipment and primary structures. Typical conditions during a fire are covered in IEC60721-2-8.

2.4 Target level assessment

2.4.1 The effect of environmental parameters on electrical equipment

2.4.1.1 High temperatureEffects of temperature on electrical equipment must consider: -

The air temperature The equipment temperature.

The equipment temperature depends on its electrical loading and its ability to liberate thisheat to the surroundings.The convection to the surrounding air has a major influence on the rate of heat transfer.When the equipment has a cover, the thermal conductivity of the equipment and the path tothe cover is an essential factor to consider. The actual temperatures at which a fault occurs,varies with different equipment. For example semiconductor silicon components cantolerate a range of 125 to 175 C while germanium components can only tolerate 70 to100 C. The memory in a hard disk is damaged with a temperature in the 70 C range.

The failure frequency of plant depends on the surrounding conditions, the load and the ageof the equipment. A typical failure frequency curve for electronic equipment is shown infigure 2.4.1. The service life of equipment can be divided into three stages.


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The early operating period lasting for 0,5 to 2 years, when the failure frequency is2-10-fold compared to the actual operating period.

The actual operating period that in the normal conditions lasts for 10...20 years afterwhich the equipment is usually replaced with more efficient equipment.

The ageing period when the frequency of failure increases rapidly.

Figure 2.4.1. A typical graph for the fail frequency of an electronic equipment (Z (t)=failfrequency)

The temperature rise has a great influence on the failure frequency of electronic equipment.It is said that when the temperature rises by 10 C the frequency of failure doubles. It isassumed that the failure frequency of electronic components follows Arrhenius law:

z=z 0*e -E/(k*T) (3)

where z= failure frequencyz0=failure frequency in the normal conditionsk=Bolzmann's constantE=the activation energy of the fault mechanismT=the component temperature K

The formula shows that temperature increase influences exponentially the failure frequency

of components. If the electrical stress rate is high, the temperature effect increases thefailure rate. Figure 2.4.2 indicates how temperature, affects the failure frequency ofcomponents. The figure indicates the mean time between component failures as a functionof the temperature. The mean time between failures (MTBF) is the inverse value of thefailure frequency. Except for MTBF, the temperature will also influence also componentefficiency.


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Figure 2.4.2 . The predicted mean time between failures of equipment with differentcomponent choices (A and B) as a function of the temperature (m (h)= the mean time

between failures)

Excessive high temperatures ages electrical equipment reducing considerably their working

period. It has been claimed that a temperature rise of 14 C reduces the lifetime ofelectronic components by 50% Cable insulating materials age with temperature increase,the lifetime of rubber insulating materials at different operating temperatures is given intable 2.4.1:

Table 2.4.1 The rubber insulation service life atdifferent operating temperaturesTemperature Insulation service lifeC years25 3032 15

39 7,546 3,7

International research work has been carried out covering the environmental factors andtheir effects on electrical equipment. This work is published in the IEC (InternationalElectronic Commission) standards, many of which have been adopted as EuropeanStandards. In standard EN 60068-1 the principal effects of environmental factors andtypical faults caused by them are covered. The effects of high temperatures are shown intable 2.4.2:


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Table 2.4.2. The main effects of a high temperature and typical faults in electricalequipmentMain effects Typical faults Thermal agingOxidationCrackingChemical reactionsGrowth of mechanical stressSofteningMeltingSublimationReduction of viscosityEvaporatingThermal expansion

Insulation faultsStructural faultsImpairment of greasing propertiesIncrease in the wearing of moving parts

High temperatures also damages equipment indirectly by accelerating chemical reactions,resulting in corrosion by the air contaminants. For example when the temperature risesfrom 20 C to 30 C the reaction rate doubles. Evaporating of solvents and insulatingmaterials resulting in an acceleration of gaseous contaminants, which further increasecontact surfaces fouling.

2.4.1.2 Low temperatureLow temperatures alone do not increase the failure frequency, provided the temperaturestays above 0 C. See figure 2 where, the meantime between failures stay almost as constant

between 0 and 20 C. Instability of equipment can cause a change in the parameter values,such temperature reduction, humidity, and air movement. If the intake air temperature isnear to the room air dew point, moisture will condense on the surface of electricalequipment, with serious results.

When the temperature falls below 0 C, the rate of failures increases rapidly. When thetemperature drops to -40 C the failures of different components are about 10-foldcompared to normal conditions. When the temperature is below 0 C, moisture condensingon surfaces will freeze in narrow spaces causing joint failure.

The main effect of low temperatures and faults caused by them according to EN 60068-1

are shown in table 2.4.3:

Table 2.4.3 The main effects of low temperatures and typical faults in electrical equipmentMain effects Typical faults Increase in viscosityIce formationEmbrittlementShrinkingImpairment of mech. strength

Insulation faultsImpairment of greasing propertiesSealing faultsCrackingFailureStructural faultsIncrease in wearing of moving parts


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2.4.1.3 Rate of change in temperatureDue to different thermal expansion coefficients, component can develop serious stresses ifthe component temperature varies from its design value. If the temperature changesconstantly, the component will experience fatigue" and in time will fail. For examplememory protection batteries will be damaged if the temperature increases too often abovethe permitted value. A single temperature increase above the permitted value will affect thestorage capacity.

In the EN 60068-1 standard the main effects of the temperature changes are thermal shocksand local temperature differences are covered. Typical faults caused by these aremechanical and insulation faults resulting in cracking and electrical leakage.

Temperature changes influence the relative humidity of the surrounding air causing causemoisture to condense locally on components.

2.4.1.4 High relative humidityChanges in humidity influence the resistance of electrical insulating materials. Thesechanges result in static electricity, particle formation and corrosion between differentmaterials. It will also cause corrosion by:

1. Directly, by chemical reaction on metals and plastics.2. 1. Corrosive compounds forming with other gases in the air, e.g. sulphuric acid,

H2SO 4 with sulphur dioxide SO 2 and nitric acid HNO 2 with nitrogen oxides. NOx3. Electrochemically as an electrolyte on two different metals causing corrosion. For

example: - a copper plate coated with gold, if moisture condenses on it electrolysismay result causing hairline cracks.

The main problems and typical faults of high relative humidity is given in EN 60068-1 arelisted in table 2.4.4.

Some insulating materials adsorb moisture at high relative humidity. This results in theelectrical conductivity of insulating material increasing with electrical leakage causingequipment damage. Dust particles in the air, below a 1 m (micrometer), can adsorbmoisture and gaseous contaminants. This may accelerate equipment corrosion. Corrosiondue to gaseous contaminants grows exponentially with an increase in relative humidity

If the air relative humidity increases above 80%,the silver used in the circuit cards maydevelop a "migration phenomenon causing short circuits. The gold used to fasten chips totheir beds also suffers from humidity and the chips may loosen and fail.


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Table 2.4.4. The main effects of a high relative humidity and typical faults in electricalequipmentMain effects Typical faults Absorption of humidity and condensation on the surface of an articleSwellingImpairment of mechanicalStrengthChemical reactions such as: - corrosion and electrolysisIncreasing of the conductivity of the insulation

Mechanical faultsBreakingInsulation faults

When the relative humidity increases to 80 %, a water film forms on the equipmentsurfaces.

High humidity is not an actual a problem in an automation/electrical equipment room with

well-designed ventilation systems, as air movement at the correct temperature removes themoisture from the space. However if the intake air temperature is low, local moisture willcondense. A failure in room over-pressurization and a poor moisture barrier will cause anincrease in the local relative humidity.

2.4.1.5 Low relative humidityThe main effects due to low relative humidity and the typical faults caused by themaccording to EN 60068-1 are given in table 2.4.5:

Table 2.4.5 . The main effects of low relative humidity and typical faults in electrical

equipment.Main effects Typical faults DryingEmbrittlementShrinkingImpairment of mechanical strengthIncrease in wearing of contact surfacesDeveloping of static charge

Mechanical faults of non-metallic partsCrackingElectrical faults

The worst threat to electrical equipment from the above-causes is that low relative humidityincreases the incidence of static charges. A static charge is when similar charges build up ina substance and do not immediately become neutralized with opposite charges. A typicalelectrostatic phenomenon is to have high potential differences, but the appearance of smallquantities of electricity. Normally an electrical charge leaks slowly along the surface of amaterial or through it, without causing any problems. If the potential of a charged area

becomes high, a powerful discharge occurs which may cause damages to equipment wipingout memory, cause electrical shocks to employees and create fire hazards.

The aim of protection is to keep the leakage rate greater than its rate of generation. Bymaintaining the relative humidity in the 60-70 % range will eliminate static electricity

problems. As the temperature inside electrical motors is greater than that of the surroundingair, the relative humidity in the surrounding should be 65% or more. The problems of static


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electricity are minimised when the relative humidity of the surrounding air is greater than55%.

Relative humidity influences occupants safety. For example cotton clothing is safe due toits good electric conductance. This is true when the relative humidity is high, but below40% relative humidity, cotton is a good insulating material. The value of clothing electricconductance is important to neutralize the electrical charges between man and electricalequipment. Wrist and foot straps will prevent electrostatic discharge from the people toequipment; this is often a much cheaper solution than humidifying the room.

If the relative humidity drops below 40% static charges will attract dust particles and causeundesirable dust forming. The formed dust causes wear of contact surfaces, breaks andcorrosion depending on the dust properties. The increase in contact faults with a decrease inrelative humidity is covered in figure 2.4.3

Figure 2.4.3 . The effects of relative humidity on the functioning of tele-exchanges.

2.4.1.6 Rate of change of the relative humidityRapid changes in relative humidity may cause local condensation resulting in corrosion.The corrosion rate caused by gases increases considerably when the rate of change of the

relative humidity is greater than 6% in an hour.

2.4.1.7 Chemically active substancesThe effect of chemically active corrosive gases on electrical equipment according to EN60068-1 is given in table 2.4.6.


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Table 2.4.6 The main effects of chemically active gases and typical faults in electricalequipment.Main effects Typical faults Chemical reactionsCorrosionElectrolysisSurface decayIncrease in conductanceIncrease in contact resistance

Increased wearingMechanical faultsElectrical faults

2.4.1.8 Mechanically active substancesThe effect of mechanically active substances, such as particulate matter, on electricalequipment is given in table 2.4.7.

Table 2.4.7 . The main effects of mechanically active gases and typical faults in electricalequipment.Main effects Typical faults Friction, wearingCloggingGetting stuckFrictional electricityIncrease in thermalinsulation

Mechanical faultsIncreased wearingElectrical faultsOver heating

2.4.2 Basis for design of electrical equipment rooms

2.4.2.1 IntroductionThe equipment supplier defines the environmental demands for each room, and theequipment heat loads. Initial information is given by the electrical designer. Table 1.1.1 can

be used to initially define the room environmental information during the preliminarydesign. Other heat loads are calculated individually for each case.

Condition information/ checklist:Temperature

Environment class + the requirements of air conditioning Target value Accuracy (range of variation) Steadiness (rate of change) Maximum and minimum values in the case of disturbance

Humidity As temperature

Chemical contaminants Environment class Permitted concentration for each gas


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Particle contaminants Environment class

Over-pressure/outdoor airflow Electrical equipment rooms are designed in over-pressure against environment. In

normal case 20 Pa over-pressure is enough. It is reached in a tight room with make-up airflow of approx. 2,1 l.s 1 per m2 wall see figure 2.3.4.

The room design criteria will define the choice of the air conditioning system required. Inaddition to system selection the following have to be considered: - available space (foot

print and height) reliability in use, and possible room future extension.

2.4.2.2 PressurizationAs the requirements for electrical equipment room conditions are usually stricter than thosein the surrounding areas, they require pressurising. Normally 20 Pa excess is adequate forrooms that border on outdoor air, see 2.3 and figure 2.3.4 for details. When the conditionsare extreme such as exposed sites high buildings and spaces having high negative

pressures, the over-pressure and the air flow rates required, have to be calculatedseparately.

2.4.2.3 Introduction to the environmental parameter conceptWhen the operating conditions of electronic equipment are defined, all the condition factorshave to be determined for several operating parameters. The parameters can be dividedinto: -Base value i.e. normal conditions Range of variation above and below the base value Minimum and maximum values required when the equipment is in operation Minimum and maximum values required when the equipment is not operating The real limit values necessary to avoid equipment damage.

The relationship between the parameters is shown in figure 2.4.4:


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Figure 2.4.4. Operating parameters and reliability of electronic components.

The design value relates to the constant operating conditions (temperature, humidity, particulate matter etc.) required in the area where the equipment is in use. It is necessarythese parameters be met to ensure operation reliably over its lifetime. When perfect

functioning of equipment is required the conditions defined by the base values must beworked to.

The variation range design values define the choice of surroundings in which theequipment reliability remains constant. The reliability over this range is obviouslyinfluenced by the rate of change of conditions.

The design value and its variation range define the design conditions to be selected.Equipment in normal operating situations provides the initial values for the air conditioningdesigner.

Maximum and minimum conditions, when the equipment is operating, define the extremeenvironmental values surrounding the equipment in a special case. These cover the event offailure of the air conditioning plant. When conditions reach the extreme values, there is arisk of equipment failure the maximum and the equipment manufacturer provides minimumconditions for equipment, these are based on test performance under given conditions.When designing electrical equipment room ventilation systems, it should be considered thatthe air conditioning system must operate after a malfunction, before extreme environmentalconditions are reached and the manufacturing process fails.

Environmental tests for electrical equipment have been standardised. The key methods ofthe testing are given in EN 60068-1. The environmental standards and the tests related to


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them are used for defining the greatest short-term environmental stresses encountered for a product. However, these classes do not provide information regarding long-term stressesthat influence the equipment for its lifetime. Even remaining within the testedenvironmental class does not guarantee a perfect function of the product in these conditionsLong-term stresses can slowly influence the product quality and result finally in failure.

The classification considers that different environmental parameters (temperature, humidityof air etc.) are symmetrically divided. The extreme values of classes have been chosen sothat average equipment can tolerate that the conditions do not exceed the extreme valuefor no more than 1% of the operating time.

Maximum and minimum values for non-operating periods are used for defining theequipment storing conditions. During installation and shutdown periods the airconditioning must operate to ensure the operating conditions are achieved as soon as

possible. When conditions reach the equipment critical limit immediate failure may result.

2.4.2.4 Climatic conditions (temperature and humidity)As the basis for designing air conditioning systems, a conditions curve of 75% is used. In

practice the humidity can be varied over a wider range of this curve because air humidity ina space changes with the seasons, hence the risk of extreme humidity does not continueover the whole of the time.

Some classes are more flexible regarding the requirements of air conditioning. Hencecertain precautions can be permitted so that the temperature approaches the outdoorextreme design conditions. The extreme conditions of electrical equipment correspond tothe 99% values on the conditions curve.

Air-conditioning design has to ensure that the room conditions achieve average values inthe middle of the permitted area. Hence care has to be taken with selection of the airterminal devices and their actual positioning.

Design requirements are compiled by considering the electrical equipment in the room.Extra requirements for each room, such as workplaces, have also to be considered, see2.4.3.

Rooms with high heat loads must be prepared for a sudden temperature change due tomalfunction or failure of the air conditioning. This is achieved by reducing the roomtemperature level with backup equipment or by natural ventilation.

The conditions that correspond to individual rooms requirements should be maintainedduring shutdowns, as moisture condensing on electrical equipment may cause damage on

plant start up.

When the temperature rate of change is calculated during the design, the initial assumptionis perfect mixing. The rate of change has to remain in the given range during a period of 5


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minutes. The calculations have considered the structural damping effect on temperaturechange. This is achieved by the use of two time-constants models.

During operation, the room conditions have to stay within given limits of the electricalequipment. This means the volume around the equipment measured from the floor level upto two meters, and at least to the 50 cm from the equipment surface. The measuring shouldnot be carried out near an air conditioning unit (the minimum distance is 1 m) or in itsairflow. Hence positioning of control sensing devices must be taken into account.

The design basis given is intended for forced ventilation design. Natural ventilation can beapplied by applying the design values of electrical equipment (extreme values). Table 2.4.8and climatographs (see Appendix 1) the air conditioning design conditions for differentclimatic conditions have to be considered.

2.4.2.5 Special climatic conditionsMost equipment permits air velocities greater than those mentioned above. This is dealtwith separately in special climatic conditions categories. The permitted air velocities in thedifferent classes are given in table 2.4.9.

Table 2.4.9 . Special climatic conditions; the permitted air velocity in the differentcondition classes

Condition class Permitted air velocity m.s -1

3Z43Z53Z6

5 m.s -1 10 m.s -1 30 m.s 1

2.4.2.6 Chemically active substances (corrosive gases)The permitted concentrations of chemically active gases in electrical equipmentenvironments are defined in the classes C of the EN 60721-3-3, standard as are chemicallyactive substances. In table 2.4.10 the maximum values of different gases for airconditioning design are given. Since the concentrations in the strictest classifications are

extremely small, comparison at the size range level with classes (G1-G3, GX) of standardISA-71.04-85 are used. The copper-strip method in the above standard can be used toestimate the rooms surrounding classification.

The following are the two most critical classes, since concentrations of the other classes areso high, that these other classes are irrelevant for electrical equipment rooms.


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3C1: The average air concentration in areas with no emissions of harmful

gases. Near industrial plant emissions and in some city areas, chemicalfiltering is necessary to achieve this class. This corresponds to class ISAG1-G2, even though the concentrations are a little higher than in the ISA.

3C2: Corresponds to class ISA G3-GX. Causes corrosion of unprotectedelectrical equipment.

In laboratory tests for chemical filters it is shown that the filters in the tests could notremove nitrogen oxides from the air. Therefore the customer has to specify in hisequipment inquiry a wider class of filter that will deal with NOx if the concentrations ofnitrogen acids rise over 0,1 mg. (nitrogen oxides alone can cause corrosion of metalsurfaces, but together with other gases corrosion is accelerated)

The ozone concentration range in the standard, is critical as the concentration of class 3C2is exceeded e.g. in Finland on background levels. In practice the permitted ozoneconcentration should be classified to class 3C3 with the present outdoor concentrations.Ozone is not considered to be a corrosive substance, however it oxidizes plastics, rubberand textiles, and accelerates the corrosion caused by other gases.


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Table 2.4.10 . The permitted maximum concentrations of chemically active substances indifferent conditions classes, according to EN 60721-3-3.

Environmental factor Unit 3C1 3C2 3C3 3C4

salts mg/m 3

cm 3/m3

no 1) saltmist

saltmist

saltmist

sulphur dioxide mg/m 3

cm 3/m3

0,10,037

0,30,11

5,01,85

134,8

hydrogen sulphide mg/m 3

cm 3/m3

0,010,0071

0,10,071

3,02,1

149,9

chlorine mg/m 3

cm 3/m3

0,10,034

0,10,034

0,30,1

0,60,2

hydrogen chloride mg/m 3

cm 3/m3

0,10,066

0,10,066

1,00,66

1,00,66

hydrogen fluoride mg/m 3

cm 3/m3

0,0030,0036

0,010,012

0,10,12

0,10,12

ammonia mg/m3

cm 3/m30,30,42 1,01,4 1014 3549

ozone mg/m 3

cm 3/m3

0,010,005

0,050,025

0,10,05

0,20,1

nitrogen-oxides 3) mg/m 3

cm 3/m3

0,10,052

0,50,26

3,01,56

105,2

1) Sea salt mist can occur in weather protected spaces on the shore and in the spaces that are in costal areas.

2) Values are both calculated in cm 3/m3 And mg/m 3 values at 20 C temperature. The values of the table are rounded.

3) Is given as equivalent of nitrogen dioxide

2.4.2.7 Mechanically active substances (sand, dust)The permitted concentrations of particulate matter in the operation environment ofelectrical equipment are presented in table 2.4.11. The classification does not consider the

position for measuring the dust or its the origin.


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Table 2.4.11 . Maximum concentrations of mechanically active substances for differentclasses.

CONDITION CLASS SAND

mg.m -3

DUST(suspended

particulate)mg.m -3

DUSTSEDIMENTATION

mg.m -3 per day

3S1 no 0,01 103S2 30 0,2 353S3 300 0,4 350

2.4.2.8 Biological conditions

The classification concerns the protection of electrical equipment against mildew, fungi,organisms and small animals. Usually electrical equipment rooms are class 3B1 and theserooms have been protected against these factors. This is not normally considered instandard air conditioning design

2.4.2.9 Mechanical conditionsThis classification describes the influence of mechanical stress, including vibration andimpact directed to the electrical equipment. This factor is normally not considered in

basic air conditioning design.

2.4.2.10 Special requirements of different equipmentThe storage capacity of batteries decreases with a temperature decrease

2.4.3 Human occupancy.Work places should be located separately from the actual electrical equipment rooms. Ifthey are permanent (for more than occasional work) they must be taken into accountwhile designing the air conditioning. See 3.1.4 Ventilation.


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Table 2.4.12 . Requirements for work places

LIMIT VALUES TARGETVALUES

TEMPERATURE C-Sedentary work-Light, moving work

20-2818-25

21-25 1) 19-23 1)

RELATIVEHUMIDITY %

15-70 30-50 2)

AIR VELOCITY(m.s -1 at 20 C)-Sedentary work-Light, moving work

0,150,25

0,150,25

NOISE dB(A) 55 3) 55 3) 1) There should not be any extreme thermal radiation, hot or cold, which may cause damage to theequipment2) To reach the target value, means that the air humidity has to be controlled3) National regulations or standards may define a lower limit value.

The air purity has to achieve values necessary for office spaces. For electrical equipmentair purity of class 3C1 is usually adequate for plant and occupants

2.5 Source description

2.5.1 IntroductionWaste heat generated in electrical equipment is transferred almost entirely by airconvection to the room through the equipment cabinets. The surface temperature of thecabinets is normally not much higher than the surrounding temperature, so radiant heattransfer can be ignored.

2.5.2 Estimation of heat loadsThe maximum heat loads in electrical equipment rooms have to be calculated withaccuracy. The final design should be based on the heat loads given by the electricalequipment supplier. When changes are made to existing rooms, it is essential todetermine the heat loads both by measurement and calculation. Care has to be taken ifrule of thumb methods are used for this purpose

2.5.2.1 Rough estimation of heat loads in electrical equipment roomsThe power losses of low-voltage equipment and its associated cables in the cable spaceequal to the loading losses of the supplying transformer. The transformer power loss isobtained from the manufacturers technical specifications. A 1.6MVA transformerdissipates 14,6 kW at full load.


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The power losses at low-voltage (< 500 V) in equipment rooms are about 0,3.to 0,5 % ofthe electrical power supplied. The dissipation power losses in low-voltage centres can beestimated on floor area of the distribution centre. A useful estimation is 800 W. m 2 offloor area of a distribution centre. The heat load generated by other equipment in theroom, such as AC-inverters, lights, fans etc has to be determined separately and added tothe total load.

2.5.2.2 Heat loads of different equipmentThe following clause, gives power losses provided by electrical equipmentmanufacturers. These can be used if no other information available. The values areapproximate further checks are required with the chosen equipment supplier.

Motor control equipment (source: ABB Strmberg, Finland, 1993)

Motor control equipment having the following dissipation powers:

The supply voltage of the device Power losses (% of the nominal power)

380 V 2.0 %

500 V 1.5 %

When direct current is used, the power losses do not vary much. With alternating currentthe power losses depends on the rate of utilization.

Automation equipment (source: Valmet Automation, Finland, 1993)The power losses of automation cabinets is


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Table 2.5.3. Power losses of single and three phase UPS-equipment,range 5-150 kVA.Size [kVA] Number of phases Maximum heat losses[kW]

5 1 1,2

10 1 2,2

12 1 2,5

15 1 3,1

20 1 3,6

25 1 4,4

30 1 5,2

40 1 6,7

50 1 8,1

60 1 9,7

75 1 11,3

10 3 2,7

15 3 3,8

20 3 4,5

30 3 5

50 3 7

75 3 10100 3 12

150 3 18

ADP-equipmentThe power losses of ADP-equipment depend on the equipment supplier. Hence the

precise information must be obtained from the suppliers.

TELE-equipment

The information regarding power losses of tele-exchange equipment has to be determinedfrom the equipment supplier. The percentage of power losses has reduced by half, withthe new generation of equipment on the market. The room loads however have notreduced, as the equipment sizes become smaller in the same ratio. For coming thirdgeneration telecommunication system, heat dissipation will increase dramatically.

2.5.2.3 Total heat loads of electrical equipment room in Pulp and Paper millsTable 2.5.4 shows total heat load values (internal and external loads) measured in theelectrical equipment rooms of pulp and paper plants. The differences between differentrooms are considerable The loads in similar electrical equipment rooms for the same


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kind of plant can differ from each other depending on the location of the electricalequipment and the external load. When AC- is used the power losses depends on themotor utilization rate. The power losses for DC-use and for automatic equipment arenormally constant regardless of the utilization rate.

Table 2.5.4. Total heat loads measured in electrical equipment rooms for pulp and paper plants.

Heat load [W.m -2 ] Average load [W.m -2 ]]

Control rooms 60-200 160

Automation rooms 100-350 260

Electrical equipment rooms 100-2000 250-300

Cable spaces 10-130 50 *

* The designed heat load of cable spaces is usually 20-25% of the electrical equipmentload.

2.5.3 Battery rooms.Batteries produce hydrogen during charging. To ensure that fire and explosion does notoccur, adequate ventilation by flameproof fans is required the ventilation requirementsmust be based on electrical safety regulations.

2.5.4 Sources of the contaminant loads.In addition to the filtered intake, particle and gaseous loads enter by leakage from thesurrounding spaces and from employees. If smoking is forbidden in the room and theinstructions given in this text are followed in the ventilation design, the human loads can

be ignored.

2.6 Load calculations

2.6.1 Heat loads

The electrical equipment supplier provides the heat loads generated by his equipment.During the design the initial heat loads are provided by the electrical designer. Clause 2.4gives methods by which an approximate estimate of the maximum heat load can beobtained.

In addition to the maximum loads the average heat loads in the room have to bedetermined. They can differ considerably from the maximum values due to the equipmentuse.In addition to the heat load from electrical equipment, attention has to be paid to otherheat loads: such as: - uncooled intake air, other heat generating devices such as fans,occupants working in the space, surrounding spaces, outdoor conditions, lighting, solar


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radiation etc. External loads can be high both on heating and cooling and they must beallowed for, these loads vary according to the seasons. Clause 2.3.5.2 gives an exampleregarding calculating the surrounding heat loads.

If the mean heat loads differ considerably from the maximum loads, these have to betaken into account when designing the air-cooling control. See 3.3.2.2 for cooling control.

2.6.2 Contaminant loadsClause 2.1 gives the means of estimating the surrounding contaminant loads.

2.6.3 Pressure conditions.In clauses 2.3.1 and 2.3.2 consideration is given to wind effects, and temperature on the

pressure ratios in the electrical equipment room.


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3 SYSTEM PERFORMANCE

3.1 Selection of system

3.1.1 Air conditioning systems for electrical equipment roomsThis clause describes air conditioning systems suitable for serving transformer, cable,electrical equipment, automation, cross-connection and tele spaces and control rooms.The air conditioning solutions shown cover all electrical equipment rooms. These areconsidered from the industry point of view. There are five basic solutions, see figure3.1.1:

1. Natural ventilation2. Forced extract ventilation3. Over-pressure ventilation4. Cooling with circulated air5. Separate cooling placed to the room.

The systems and their most important properties are covered in table 3.1.1.

Factors influencing system selection are: the position, layout and structure of the room,environmental conditions, room heat load reliability of equipment use, spacerequirements, fire areas and initial costs. If necessary, the possible requirement of futureextension has to be considered. A summary on the conditioning classification fordifferent systems is shown in table 3.1.2.


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Table 3.1.2 . Condition classes that are reached with different air conditioning systems(number of system refers to clause 3.1.1)

CONDITION

CLASS

AIR CONDITIONING SYSTEM

1 2 3 4 5

3K1 - - - + + *)

3K2 - - - + +

3K3 +**)

+**)

+**)

+ +3K4 + **) +**) +**) + +

3K5 + **) +**) +**) + +

3S1 - - + + +

3S2-4 + ***) +***) + + +

3C1 ****) - - + + +

3C2 *****) + + + + +* Attention shall be paid to cooling control** Depending on the maximum temperature and heat loads*** Depending on the location**** Requires a chemical filtering in areas with highly polluted outdoor air***** Not close to process emissions without a chemical filtering


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Figure 3.1.1. Basic ventilation and air-conditioning systems for electrical equipmentrooms.

1.1. Natural ventilation1.2. Forced extract ventilation1.3. Over-pressure ventilation1.4. Cooling with circulated air1.5. Separate cooling unit within the room.


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3.1.2 ReliabilityThe basis for the design of different rooms and the reliability level should be discussedthoroughly with the customer as early as possible. If the reliability of use is taken intoaccount during the design stage, the study can include the whole factory. Spare parts areessentially to ensure operational reliability. Their amount can be reduced by careful

planning and at the same time improve the reliability of use.

The required reliability of use of air conditioning system depends on the importance ofthe air-conditioned room when related to the whole process. Equipment rooms can beconsidered to have three different stages of the reliability

1) The lowest requirements for the reliability of use are for rooms in which thedevice can be stopped for a while without influencing the main process. Thefunction of the air conditioning system in this type of room does not require a

backup system. The system consists of reliable components, and rapid servicing isnecessary to ensure the minimum of shut down time.

2) Rooms in which a temporary temperature increase cannot be allowed, requireattention in order to determine the reliability of use. Equipment in this kind ofroom cannot be stopped without disturbing the main process. However theequipment may tolerate a small temperature rise with associated malfunctionswithout causing major problems the air conditioning design must ensure the roomtemperature will not exceed the maximum operating temperature of the electricalequipment even during a malfunction.

This requirement can be met if the cooling capacity is divided into several unitsindependent of each other. In case of the failure of one or more units at the sametime will not cause the room temperature to rise above the maximum operatingtemperature. Any electrical work on the air-conditioning equipment can then becarried out without isolating all the equipment. In addition rapid service isnecessary. If damaged equipment cannot be replaced quickly enough by theequipment supplier, the user has to ensure that adequate spare parts are availablefor his own maintenance staff to get the plant on line again without delay.

The requirement for spare parts also depends on the extent of the heat loads. Forexample if the heat load is under 100 W.m -2, the room temperature will not riseabove +40 C when the outdoor temperature is +25 C.

3. Rooms in which equipment failure causes a shutdown should always beequipped with a double cooling system. Typical rooms are computer- andautomation rooms. In these malfunctions increase even with a small temperaturerise. Hence a rapid repair service is essential.

The reliability of use and the effect of damaging of different component can beobserved by analysis of the reliability of operation. An analysis of this type will

pin point the equipment weak spots.


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3.1.3 Extension reserveElectrical equipment rooms tend to fill up in time with extra electrical equipment, so it isimportant in air conditioning design to be prepared for system extension this is achievedin principle in two different ways.

1) Cooling equipment designed to deal with the "full" space. Then reliable andeconomical use of the cooling system must be secured on partial loads.

2) The space is cooled by the use of modular units, provision being made for extraspace that will allow more units to be added when necessary

3.1.4 Air distribution

3.1.4.1 Objectives of air distributionSupply air entering electrical equipment rooms should be mixed effectively with theroom air-cooling the whole space evenly. The room conditions should be determined inthe manner shown in 2.4.2.3 the air distribution has to be able to deal with the room

pressure ratios to stop uncontrollable air leaks. When floor or ceiling flow (laminarceiling) is used it is critical to pay attention to pressure ratios.

Air velocity is not a critical factor in electrical equipment room for temporary occupancythe permitted air velocities in different classes are given in 2.4.2.5 (see table 2.4.9).Usually the electrical equipment manufacturers permit higher air velocities in theequipment specifications (climatic special conditions: classes 3Z4-6). A critical factorconcerning the air distribution is when the space is permanently occupied

3.1.4.2 Air distribution systems used in electrical equipment roomsThe most common methods of air distribution are mixing air distribution and floordischarge. The mixing air distribution is achieved by using ceiling-diffusers, grilles ordirect discharge In rooms where people are working, laminar ceiling, active displacementand radial whirl diffusers for air distribution are used.

Floor dischargeIn floor discharge systems the intake air is discharged into the space through a raisedfloor. This space acts a plenum. Intake air flows to the air-conditioned room through floorgrilles or through the equipment cabinets. The raised floor works also serves as a cablespace and an air conditioning duct. This principle of air distribution is shown in figure3.1.2


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Figure 3.1.2 . Floor discharge

Floor discharge is the most common method of distributing air in medium-sized and largecomputer centres. Other air distribution solutions are not recommended for computerrooms except the floor discharge, together with the exhaust air removal from the ceiling.Comfort criteria can be met with this method up to a cooling load of 150 W.m -2

There are some problems with floor discharge systems, these being fire safety due to thefloor void. It is recommended that the cable space and the electrical space are differentfire areas. In such cases the raised floor has to be fire proof and the intake air grilles haveto be equipped with fire dampers. Additionally the space under a raised floor is difficultto keep clean. Therefore a cable space is often separated from other spaces, regulationsmay stipulate that the minimum height is two meters and has it own air conditioningsystem.

Mixing air distribution.When air distribution is by ceiling diffusers, excellent air mixing with good dilutionoccurs. The cooling capacity and the air flow, required in electrical equipment rooms arenormally high resulting in problems in achieving comfort conditions.

Laminar ceiling.In a laminar ceiling, air is discharged from the ceiling through a perforated plate into theroom. Cool air flows with low velocity through the holes and is mixed with room air. Theair flowing downwards warms up to the design temperature before it reaches the criticalareas in the space.

A laminar ceiling is an ideal air distribution system for control- and automation rooms.Air can be introduced into the room without draught up to a cooling capacity of 170-200W.m -2 A perforated ceiling requires accurate design to avoid uncontrollable flow. The

perforated area of the whole ceiling area should not be more than 50%. A laminar ceilingsystem can be used in electrical equipment rooms with the air being introduced above theaisles and the exhaust extracted above the equipment cabinets see figure 3.1.3.


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Figure 3.1.3. A laminar ceiling in an electrical equipment floor

Active displacementActive displacement operates by means of nozzle ducts. Manufacturers state that acooling capacity of 240 W.m -2 and airflow of 40 l.s -1 can be achieve without draught Thenozzle ducts system operates by means of small air jets that induce a large volume ofsecondary air ensuring good mixing Air distribution patterns can be adjusted by changingthe number of nozzles and the blowing sector. With nozzles evenly distributed around aduct, the conditions are almost equal to a perforated ceiling. The use of a nozzle duct inan electrical equipment room is shown in figure 3.1.4.

The nozzle duct it is a factory-made product, and provides good environmental conditionsFull use of the equipment manufacturer data should be used to achieve the desired result.

Figure 3.1.4 . Active displacement

Closed air circulationIf people are working in an electrical equipment room, a closed air circulation is the onlyway to provide comfortable working conditions for cooling capacity (> 400 W.m -2). The

principle of the closed air circulation is shown in figure 3.1.5.


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Figure 3.1.5. Closed air circulation

Designing and operating a closed air circulation is more complicated than other systems.It requires the electrical equipment supplier to designing airflows for each cabinet. Inaddition the cabinets have assemblies for the air conditioning. A dual ductwork systemrequires more space making the equipment cable- laying difficult. The closed aircirculation system is more sensitive to cooling equipment malfunctions, as the aircirculation capacity is less than the case of when the whole equipment room is ventilated.

Half-open systemsOne solution is the combination of a closed air circulation and ceiling discharge. A

portion of the air is discharged directly into the equipment cabinets and the remainder into the room space. The exhaust is placed above the equipment cabinets as in opensystems. This provides the best characteristics of both systems. The equipment cabinetsreceive controlled clean air, and the equipment space provides a buffer against thesurrounding. During a malfunction all of the room air capacity can be used. Another

problem in this case is how to introduce the correct air quantity into each cabinet. The principle of this arrangement is shown in figure 3.1.6.

Figure 3.1.6. A half-open system; blow into the equipment cabinets.

In rooms with high heat loads (500 W.m -2) the temperature difference between intake andexhaust air can be increased, and the air flow reduced. The warm exhaust air is induced


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directly from the equipment cabinets. In this case information regarding thermal powerand airflows for each cabinet has to be obtained from the electrical equipment supplier.The principle of the system is shown in figure 3.1.7.

Figure 3.1.7. A half-open system; suction from the equipment cabinets

3.1.4.3 Workplaces.Workplaces are normally in separate control rooms near to electrical equipment rooms.The conditions in these rooms correspond to office spaces. In addition to the abovemethods of air distribution in control cabinets a chilled ceiling may be used, this reducesthe airflow rate. Care has to be taken to ensure that condensation in or on the ceiling doesnot take place. This is achieved by ensuring the supply air is of the correct moisture

content (See 3.1.1, system 5).

In practice electrical equipment rooms, especially for automation spaces, have work places, where people may stay for long time. The draught and noise prevention in roomswith heat loads (>200 W.m -2) requires special attention. Workplaces should be separatedfrom the other parts of the room by a movable wall, or by air conditioning solutions.Using partly or totally closed air circulation in the cabinets can reduce the heat load andairflow required in the room. See 2.4.3 for conditions of workplaces.

3.1.5 Air conditioning costs

3.1.5.1 Building costsPurchasing air conditioning equipment is carried out by tender; the actual pricedepending on market forces. Material quality and component also influences the price.Components used for industrial applications are usually more expensive than standardcomfort-units. The cost is related to the required environmental conditions, the largestsingle cost is that of chemical filtering. The reliability of use of equipment has to beconsidered carefully, due to its effect on the initial costs.


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The air conditioning costs can be estimated approximately see. Table 3.1.3, which showsthe costs per square meter in the type room (200m 2) that can be used in calculations, fordifferent system solutions with two different values of cooling capacity.

3.1.5.2 Operating costs.The most important item in air conditioning operating costs is that of replacing thechemical filter medium.

Due to the high cooling load, the use of electricity by air conditioning and coolingdevices is large. Thus the economical performance and the choice of operating conditionsis critical, consider absorption refrigeration


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Table 3.1.3 . Purchase costs of air conditioning plant in a type room /m 2 (price level of1991-1992). Prices include the installation costs of the device, design costs excluded.

REQUIREMENTSOF THE ROOM

HEATLOAD

W/m 2

1 2 3 4 5

3K1/3S1/3C1450200

XX

XX

XX

450-570350-480

XX

3K1/3S1/ NO CHEMICALFILTERING

450200

XX

XX

XX

370280

XX

3K2/3S1/3C1 450200 XX XX XX 380-570300-480 300-570250-530


450200

XX

XX

XX

300-370220-280

220-280170-230

3K3-4/3S1/3C1

450200

XX

XX

XX

380-500300-480

300-570250-530

3K3-4/3S1/NO CHEMICAL

FILTERING

450200

XX

XX

430220

300-370220-280

220-280170-230


450200

XX

XX

230120

300-370220-280

220-280170-230


450200

XX

10050

230120

220-300130-220

800-22080-170

3K5/NOFILTERING

450200

XX

10050

170100

220130

150100

3K6/NOFILTERING

450200

X20

8030

15080

220130

150100

150%-EXTRACOOLING COST

450200

XX

100- -

130100

130100

X=required conditions are not reached with the system


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3.2 Selection of equipment

3.2.1 Introduction

EMC-compatibility:The European EMC-directive gives requirements for equipment in industrialenvironments. The EMC-directive allows determination of the permitted disturbanceradiation of electrical equipment to its environment, and disturbances along a flex.Standard EN 50081-1 gives general disturbance emissions experienced in light industry.Higher disturbance emissions are permitted in heavy industry according to standard EN50081-2.

It always possible that old equipment can achieve the essential requirements of the EMC-directive, covered in the above standards. This is the case with equipment having tyristorcontrol or similar, which cause disturbances that is not allowed in the standard, unlessthese disturbances have been covered in the design and documentation.

Equipment has to tolerate disturbances according to standard EN 50082-2 or EN 50082-1depending on the place of use.

3.2.2 Selection of chemical filter.

3.2.2.1 Basic data for filter selection.The following covers the basic data required in the selection of a chemical filter. Thecustomer should use this as the requirements and guarantee values in tendering. Inaddition to the basic data, the names of filter manufacturers should be given.

The minimum following basic data should be provided: The filtered air flow [m 3.s-1] The lifetime target of the filter medium [a] The average concentration of the filtered gases in the air [ppb] The maximum concentration of the filtered gases in design [ppm] The concentration of gases [ppb] after the filtering or the required filtering

efficiency

The lifetime of filter medium is normally assumed to be at least one year. Often theaverage concentrations of filtered gases are not based on measuring information and haveto be estimated. The actual replacement intervals can be considerably different from thetarget. The filter life for circulated air may be less than its estimated life due to pollutionleakage in to the filtered space and/or ductwork.

The capacity of a chemical filter has to be designed with the given maximumconcentration. The concentrations of filtered gases may not exceed the planned valueswhen the concentrations upstream the filter is below the maximum. Due to processdisturbances the maximum concentrations can be high. The efficiency of the outdoor


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filter has to be over 99% of the maximum critical gas concentration. In filtering circulatedair the collection efficiency is not normally a critical factor.

3.2.2.2 Design of a chemical filter.The time for air to pass through the filter varies usually between 0,5-2,0 secondsdepending on the outdoor air purity, selected lifetime of the filter and filter type. A delayunder 0,5 seconds should not be allowed for filtering outdoor air Usually the delay incirculated air filters is about 0,1-0,2 seconds.

The air velocity through the filter medium is designed to be less than 0,5 m.s -1.Increasing the velocity decreases the filtering efficiency and increases the pressure loss inthe filter. The pressure loss of a chemical filter varies between 250-2500 Pa depending onthe filter type and airflow; this has to be considered in the fan selection. Pressure lossdoes not usually change during use, as is the case with particle filters.

The filter frame and body have to be leak tight and by-pass leakages should not exceed1% of the nominal airflow in the outdoor air filters. Attention should be paid to corrosion

problems of the material. Acid-proof material is normally used in the casing of theoutdoor air filters.

When the filter is selected, its space requirement and pressure loss should be designingother pa

Documents

Ventilation and Airconditioning-Electrical Rooms