REG261 - Air-Conditioning

AIR-CONDITIONING

Compiled by

Mohd. Rodzi IsmailSchool of Housing Building & Planning

INTRODUCTION

Definitions

The cooling of air (simple definition) The process of treating air in an internal environment to

establish and maintain required standards of temperature, humidity, cleanliness and motion. Temperature (air temperature) is controlled by cooling

(the removal of heat) the air Humidity, the water vapour content of the air, is controlled

by adding or removing water vapour from the air (humidification or dehumidification)

Cleanliness or air quality is controlled by either filtration or by ventilation. Often both are used in an installation

Motion refers to air velocity and to where the air is distributed. It is controlled by appropriate air distributing equipment

Why do we need air-conditioning? Comfort Work performance and production Process Health Conservation of equipment and material Symbol of status

Air-conditioning & comfort Comfort temperature range from 23 oC to 27 oC (25 oC 2) Effective humidity form 40% to 60% Air velocity less than 0.2 ms-1 Sufficient clean air Noise from equipment and ductwork should be avoided

HEAT & REFRIGERATION

What is ................ ?

Heat A form of energy. Every object

contains heat energy in both quantity and intensity.

Refrigeration The process of removing heat from

one substance and transferring it to another substance.

Heat intensity is measured by its temperature, commonly in either F or C

The quantity of heat is not the same as intensity of heat

Quantity and Intensity of Heat

Dessert Candle

High in quantity of

heat

High in intensity of

heat

43.3C (110F)15.6C (60F)

same quantity of heat

contains more heat per unit of mass

(the heat energy is more concentrated)

Measuring Heat Quantity

60F 61F

15C 16C

1 Btu

1 kcal

1 lbwater

1 kgwater

(kilocalorie)

Btu - the quantity of heat energy required to raise the temperature of 1 lb of water by 1F.

(British Thermal Unit)

kcal - the amount of heat energy required to raise the temperature of 1 kg of water by 1C

Principles of Heat Transfer

Heat energy cannot be destroyed Heat always flows from a higher temperature substance

to a lower temperature substance Heat can be transferred from one substance to another

Heat can only be transferred to another substance - the principle of conservation of energy.

Heat energy cannot be destroyed

As heat is transferred from the beverage to the ice, the temperature of the beverage is lowered. The heat transferred/removed is not destroyed but instead is absorbed by the ice, changing the ice from a solid to a liquid.

Heat flows from hot to cold

Heat always flows from a higher temperature substance to a lower temperature substance

Three basic methods of heat transfer Conduction is the process of transferring heat through a

solid

RADIATIONRADIATION

hotwater

hotwater CONDUCTIONCONDUCTION

CONVECTIONCONVECTION

cool aircool air

warm airwarm air

Heat can be transferred from one substance to another

Convection is the process of transferring heat as the result of the movement of air caused by temperature (density) differences

Radiation is the process of transferring heat by means of electromagnetic waves, emitted due to the temperature difference between two objects

The quantity of heat that flows from one substance to another within a given period of time

Commonly expressed in terms of Btu/hrthe quantity of heat, in Btus, that flows from one substance to another over a period of 1 hour

In the SI metric system of units, it is expressed in terms of kilowatts (kW), which are equivalent to kJ/sec - the quantity of heat, in kJ, that flows from one substance to another over a period of 1 second

A larger and more convenient measure of the rate of heat flow in English system of units called a ton of refrigeration (TR) 1TR is defined as the transfer of heat at the rate of 12,000 Btu/hr (3.517 kW).

Heat Flow Rate

REFRIGERANTS

Icemelts at 32oF [0oC]

Dry ice evaporates at -109.4F [-78.6C]

Refrigerant-22 (R-22) boils at -41.4F [-40.8C]

Coolant

Each of these three substances (pure ice, dry ice, and R-22) absorbs heat and changes phase at its own fixed temperature

Effect of Transferring Heat

60F 61F

15C 16C

1 Btu

1 kcal

1 lbwater

1 kgwater

By adding or subtracting heat energy, the temperature is raised or lowered


60F 212F

15C 100C

+ 152 Btu =

+ 85 kcal =

1 lbwater

1 kgwater

The amount of energy added will raise the water temperature to its boiling point


212F

100C

+ 970.3 Btu =

+ 244.5 kcal =

1 lbsteam

1 kgsteam

1 lbwater

1 kgwater

212F

100CAt its boiling point, with the amount of energy added, water will completely transform into steam at the same temperature


212F

100C

1 lbwater

1 kgwater

1 lbsteam

1 kgsteam

- 970.3 Btu =

- 244.5 kcal =

212F

100C

Condensation

Latent Heat

212F

100C

970.3 Btu

244.5 kcal

1 lbsteam

1 kgsteam

1 lbwater

1 kgwater

212F

100CLatent heat - the energy involved in changing the phase of a substance (e.g. from a liquid to a vapor)The temperature during the phase change remain the same

Sensible Heat

60F 61F

15C 16C

1 Btu

1 kcal

1 lbwater

1 kgwater

Sensible heat - heat energy that, when added to or removed from a substance, results in a measurable change in temperature.

AA BB

Specific Heat140F[60C]

200F[93.3C]

The specific heat of a substance is defined as the quantity of heat, in Btus, required to raise the

temperature of 1 lb of that substance 1F the quantity of heat, in kJs, required to raise the

temperature of 1 kg of that substance 1C

Higher capacity for absorbing

heat

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Modern Refrigerants Refrigerants are substances that are used to absorb

and transport heat for the purpose of cooling. When selecting a refrigerant to use for a given

application, in addition to these heat transfer properties the manufacturer considers;

EfficiencyOperating pressuresCompatibility with materialsStabilityToxicity

FlammabilityCostAvailabilitySafety, andEnvironmental impact.

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Historically, space cooling systems have relied on chlorofluorocarbon (CFC) refrigerants (R-11, R-12) and hydrochlorofluorocarbon (HCFC) refrigerant (R-22)

Refrigerant-22 has been the most widely used refrigerant in residential, commercial, and industrial applications since the 1940s

The most common refrigerants used in mechanical refrigeration systems today are Refrigerant-123 (or R-123), R-134a, and R-22

Ammonia (R-717) and, under certain operating pressures, even water (R-718) and carbon dioxide (R-744) can be used as refrigerants

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Example of Types of Refrigerants and Their Use

Refrigerant Name Equipment Use

HCFC-22 (R-22) residential air conditioning (central and window)commercial rooftop air conditioning, residential and commercial heat pumpslarge air conditioning equipment (chillers)commercial refrigeration equipment for supermarkets, food storage, beverage coolers, etc.

HCFC-123 (R-123) large air conditioning equipment (chillers)

HCFC Blends (R-401 A&B, 402A&B, 405A, 406A, 408A, 409A, 411A&B, 414A&B and 416A

commercial refrigeration equipment (e.g. supermarkets, food processing, storage & distribution)beverage & large commercial coolersice machinesice drinks

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These refrigerants will be phased out due to environmental concern (ozone layer depletion) & to comply with the world CFC reduction standards (Montreal Protocol)

The most widely accepted replacement option for HCFCs is the use of hydrofluorocarbons (HFCs)

Ammonia is also a replacement option in the large commercial air conditioning and refrigeration sectors

These refrigerants do not deplete the ozone layer and can replace both CFC and HCFC uses

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Phase-out schedule for HCFCs based on the terms of the Montreal Protocol

Jan. 1, 1996 baseline annual allowable amount of HCFCs based on Montreal Protocol

Jan. 1, 2004 annual allowable amount of HCFCs reduced by 35%


Jan. 1, 2010 no new R-22 equipment manufactured or imported


Jan. 1, 2020 annual allowable amount of HCFCs reduced by 99.5% except HCFC-123, which can be imported or manufactured until 2030 to service large air conditioning units (chillers) under the remaining .5% allowance. No new HCFC equipment to be manufactured or imported

Jan. 1, 2030 HCFCs no longer permitted to be imported or manufactured

The HCFC Phase-out Schedule

REFRIGERATION CYCLE

Basic Refrigeration System

compressor

condenser

evaporator

expansion device

dischargeline

suctionline

liquidline

A

B

C

D

Cooling equipment moves heat from cool indoor spaces to warmer outdoor locations. It moves heat by causing a refrigerant to evaporate and condense. Refrigerants capture a lot of heat when they evaporate, and the captured heat is released when refrigerant vapour condenses.

4 cooling cycles components

A compressor circulates refrigerantthrough the loop and an expansion valve maintains low pressure on the suction side of the compressor and high pressure on the discharge side.

AIR-CONDITIONING SYSTEMS & COMPONENTS

Air-Conditioning System Common Components Condenser

Part of the system that pressurizes refrigerant to cool it by changing it from a vapor to a liquid

CompressorMotorised equipment that circulates coolant through the system.

EvaporatorA system of coils that, when filled with cold refrigerant, cools the air around it

Expansion valve/DeviceControlling pressure

Air handlerShort for air-handling unit (AHU), the blower equipment designed for circulating cooled air through a central air-conditioning system.

Vapor-Compression Refrigeration

expansiondevice

condenser

compressor

evaporator

Large Air-Conditioning Equipment

SCHEMATIC COOLING COMPONENTS

PHYSICAL COOLING COMPONENTS

AHUChiller

Cooling tower

Types of Air-Conditioning Systems

AIR-CONDITIONING SYSTEM

UNITARY SYSTEM

PLANT SYSTEM

LOCAL CONTROL SYSTEM

PACKAGE UNIT

SPLIT UNIT

WINDOW UNIT

CENTRAL HANDLING PLANT SYSTEM

CHILLED WATER PLANT SYSTEM

SELF-CONTAINED PACKAGED UNIT

AIR COOLED PACKAGE UNIT

WATER COOLED PACKAGE UNIT

SPLIT SYSTEM WITHOUT OUTSIDE AIR

SPLIT SYSTEM WITH OUTSIDE AIR

INDUCTION UNIT SYSTEM

FAN COIL UNIT SYSTEM

VARIABLE AIR VOLUME UNIT SYSTEM

Classification based on layout arrangement, equipment and components

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Classification based on air handling and distribution: Low velocity system High velocity system Constant volume system Variable air volume system

Classification based on air cooled method prior to distribution into space: Direct expansion system All air system Water to air system All water system

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Classification based on services to the cooled area: Single zone system Multi zone system Terminal Reheat system Dual-duct system

Room Air-ConditionerRoom air conditioner are a sensible cooling solutionfor situations where you only want to cool one or two rooms. Like central air conditioners, room air conditioners extract heat and moisture from the room air, cool it, and return the air to the room. A blower pulls warm room air through a filter.

The main difference between a room and a central air conditioner is that a room air conditioner is a single, self-contained unit with evaporator or cooling coils, a condenser, and refrigerant-filled tubing all in one box.

Split Air-ConditionerEvaporator

(indoor)

Condenser(outdoor)

Insulated refrigerant piping & wiring

TThermostat

42Cassette type

Split unit chiller

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Package Air Conditioners

They are bigger versions of the room air conditioners and available in nominal capacities of 3, 5, 7, 10 and 15 tons.

Like the room air conditioners, the package unit also houses:i. Air filteringii. Cooling-humidifyingiii. Air handling components

The package air conditioners are usually factory assembled and condensers can be air-cooled or water-cooled type.

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Water-cooled package units

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Indoor self-contained units (Capacity: 10 105 TR)

Single package units

Variable Refrigerant Volume (VRV)

Central Air-ConditionerIn a typical air conditioning system, a refrigerant circulates through a loop of copper tubing that runs between an outdoor coil (the condenser) and an indoor coil (the evaporator).

Refrigerant travels between the two coils, absorbing heat from the room and releasing it outside. In the process, the refrigerant cools the evaporator coils. A blower sends the chilled air into the room. The cooling effect causes the warm air to release its moisture, which drops into a drain pan and is carried away.

Centralised air-conditioning system

Air handling unit

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AC SYSTEM SELECTION CRITERIA

Air-conditioning design and selection criteria can be classified as follows: Comfort criteria

These criteria include noise considerations, accuracy of control space conditions, amount of fresh air and air filtration, and tolerance of the effects of failure in the AC system.

Space considerations AC systems occupy substantial space and AC components

may require special support from structure. Space considerations include space required to house the

equipment and the distribution ductwork and pipework. AHU room - 3% of total floor area cooled by that unit Plant room - 5% of total cooled floor area

Another major considerations is the accessibility of the equipment for maintenance purposes.

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First cost An important consideration in the design and selection of AC

systems. There is a tendency for owners to prefer low first cost. Careful consideration should be given to the expandability of

the system as well as to implications on the operating costs. Operating cost

One of the most important considerations in the design selection of air-conditioning systems.

The energy cost should be calculated and competing systems should be compared in terms of their life cycle cost which takes into account capital, energy and maintenance costs over the whole life of the systems.

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Flexibility & maintenance The system should be flexible enough to meet changes in the

use of building. The reliability, maintainability and the cost of maintenance

contracts as well as the cost of replacement of major components should be considered in deciding between alternative systems.

Others Fire protection & smoke control, interior & exterior appearance and

environmental effects

The final choice lies on the owner of the building/financier of the projects. This choice should be based on the recommendations of the consultants who should provide justifications for the recommended design solution against the requirements of the owner/financier.

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LOCATION OF PLANT ROOM

Top floorAdvantage Disadvantage

Main spaces at ground level are fully utilised for other purposes (no waste of spaces)

Load on building frame increased

Piping distance of chilled water from cooling tower to plant room could be reduced if located on the same floor

Noise from vibration of equipment

Good ventilation at the top Accessibility challenge for maintenance personnel

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Middle floorAdvantage Disadvantage

No waste of main spaces Increase load on building frame

Piping distance of chilled water from cooling tower to plant room could be reduced if located on the same floor

Noise from vibration of equipment

Good ventilation at this level

Zoning for air distribution could easily be done

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Ground floorAdvantage Disadvantage

Easy accessibility for maintenance personnel

Waste of valuable space on ground floor

Reduce load on building structure

Longer piping installation to connect the plant room to cooling tower at roof top

Good ventilation at this level

Ventilation problem occurs if the cooling tower is located on the ground floor since it emits heat

Zoning for air distribution could easily be done

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UndergroundAdvantage Disadvantage

No waste of valuable space

Longer piping installation to connect the plant room to cooling tower at roof top

Reduce load on building structure

The equipment would be at risk if flood occurs

Easy accessibility for maintenance personnel

Reduce space for parking

Reduce noise and vibration

Ventilation is very critical

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It could also be explained as follows.

Away from building Suitable to avoid noise to the

user

Alleviate in getting water supply, ventilation and maintenance work

Plant room

SECTION

PLAN

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Scattered and in between spaces For provisions of air-

conditioning requirements for multiple uses and various cooling load between spaces

Also acts as boundaries between buildings (but it is expensive to have many plant rooms as compared to just one huge plant room with similar capacity)

PLAN

PLAN

PLAN

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AIR HANDLING UNIT (AHU) ROOM

Location of AHU room Should be near to facility zone or noise zone (to assist in

maintenance work) Must consider fresh air intake Not too close to toilet to avoid contaminated air (minimum

around 6 m) Not too close to parking area (especially closed parking area) Should be in one line vertically As close as possible to the cooling area

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LOCATION OF COLING TOWER

Roof top Advantage DisadvantageAlleviate in getting optimum ventilation

Could be a problem for maintenance work

Valuable spaces can be fully utilised

Increase of load on buildings main frames

No eyesore view Vibration to building structure

Heat can be directly discharged outside

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Podium Advantage DisadvantageAlleviate in getting optimum ventilation

Eyesore view from the tower building

Valuable spaces can be fully utilised

Condensation of discharged heat will happen and move upward increase in building cooling load

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Ground level

Advantage Disadvantage

Alleviate in maintenance work

Waste of valuable spaces at this level

Load on buildings main frames could be reduced

Increase of heat to the surrounding

Location of cooling tower could be far from the building

Eyesore and vibration

COOLING LOAD

Space Heat Gain Components

The space cooling load is the rate at which heat must be removed from a space in order to maintain the desired conditions in the space, generally a dry-bulb temperature and relative humidity

Cooling load from external heat gainsi. Solar heat gain through the fenestration areas of the buildingii. Conduction heat gains through the fenestration areas, walls and roofiii. Conduction heat gains through internal partitions, ceilings and floorsiv. Heat gains through infiltration and ventilation air

Cooling load from internal heat gainsi. Peopleii. Lightingiii. Electrical equipment and appliances

roof

lights

equipment

floor

exteriorwall

glass solar

glassconduction

infiltrationpeople

partitionwall

Cooling Load Components

sensibleload

latentload

conduction through roof, walls, windows,and skylights

solar radiation through windows, skylightsconduction through ceiling, interior

partition walls, and floorpeoplelightsequipment/appliancesinfiltrationventilationsystem heat gains

spaceload

coilloadcooling load components

Cooling Load Estimation

Rule of thumb Cooling capacity for an enclosed area

= Area (ft2) x Cooling load factor (Btu/hr ft2)

Capacity of A/C:

1 HP (horsepower) = 9,000 Btu/hr1 TR (Ton refigerant) = 12,000 Btu/hr1 kW= 3,412 Btu/hr1 kcal= 3.968 Btu/hr

Detail estimation based on ASHRAEs recommendation

PSYCHROMETRICS

Psychrometry

Psychrometry the science of studying the thermodynamic properties of

moist air and the use of these properties to analyze conditions and processes involving moist air

The air condition can be determined by using a Psychrometric Chart.

Use of psychrometric chart determination of comfort zone prediction of condensation problems calculation of HVAC capacity/design

Common properties used in the psychrometric chart includesi. dry-bulb temperature (Tdb) ii. wet-bulb temperature (Twb)iii. relative humidity (RH) iv. humidity ratio (w) (moisture content/absolute humidity)v. specific volume (v)vi. dew point temperature (Tdp)vii. enthalpy (h)

With two known properties it is possible to characterise the air in the intersection of the property lines, the state-point. With the intersection point located on the chart or diagram, other air properties can be read directly.

DefinitionsDry Bulb Temperature (Tdb):

The air temperature measured with a standard thermometer , oC

Dew Point Temperature (Tdp): The temperature at which water vapor in the air will condense

from a gas to liquid if cooled at constant pressure and humidity ratio (w), oC

Examples: Condensation on cold window, cloud formationWet Bulb Temperature (Twb):

The temperature at which water vapour is evaporated into air bringing it to saturation conditions at the same temperature, oC

Temperature measured by the wetted thermometer in a psychrometer

Indirect measure of how saturated the air is

Humidity Ratio (w): The amount (mass) of water in the air, kg/water/kg/air

Relative Humidity (RH or ): A measure of the actual amount of water vapor in the air

compared to the saturated conditions, %

w

ws

w

ws

Pactual vapor pressure of water in the air

vapor pressure of water in the air @ saturation P

xmole fraction of water in the air

mole fraction of water in the air @ saturation x

Enthalpy (h): The total energy content of the water vapor mixture,

kJkgdry air

Specific Volume (): Inverse of the air density, m3

kgdry air

Related Physical Laws

Ideal Gas Law: A law that describes the relationships between measurable

properties of an ideal gas

pV = mRT

where

p = absolute pressure (N/m2)V = volume (m3)m = mass (kg)R = individual gas constant* (J/kg.oK)T = absolute temperature (oK)

* The Individual Gas Constant depends on the particular gas and is related to the molecular weight of the gas. The value is independent of temperature.

Daltons Law of Partial Pressures: Total pressure is sum of the partial pressures of the components

Ptotal = Pdry air + Pwater vapour

Psycrometric Chart

ASHRAE

Structure

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PSYCHROMETRIC CHART

Psychrometric Chart Applications

Obtaining conditions based on two known conditions, e.g. Given

Temperatures of 15Cdb and 10Cwb

Obtained from the chart The RH or percentage of

saturation is 52% The moisture content is 5.4

g/kg of air

Exercise

Given: T = 27C, RH = 40%Find: h _________

w _________ _________Twb _________Tdp _________

Psychrometric Process

1. Sensible heating and cooling Heat addition or removal

without moisture change

HEATING

COOLING

2. Evaporative cooling Adiabatic process

(constant h) Move along constant h

or Twb line toward saturation

Air loses sensible heat to gain latent heat from water added COOLING 1

2

3. Heating and humidification (of the air) Typical of ventilation air

that circulates through a building space

Air picks up both sensible and latent heat from entry to exit

HEATING + HUMDIFICATION

1

2

4. Adiabatic mixing Two separate conditions

of air mixing together to form a third final condition without exchange of energy to the environment

Addition of 1 and 2 in proportion to mass flow of each. Final condition will lie in straight line between the two on psychrometric chart.

x

Using the enthalpy/humidity protractor Protractor in upper left hand side of psychrometric chart is

useful in getting final conditions of complicated mixing problems

Example: Define supply conditions necessary for air conditioning air into a space

Outside: Enthalpy h

= Humidity Ratio W

S

T

QSensible Heat =

Total Heat Q

PSYCHROMETRICS & SYSTEM DESIGN

Air Mixing Recirculating ductwork containing air at 21Cdb and

15Cwb, mixing with fresh air at 36Cdb and 25Cwb before processing in an air-handling unit. If the ratio of mixed air is 3:1, i.e. 75% recirculated to 25% fresh. Find the state of mixed air:

Tdb Twb RH Moisture content

Plant Sizing

An air-conditioning is used to supply air at 27Cdb and 20Cwb to 20Cdb and 14Cwb, in a factory of 1500 m3volume requiring 5 air changes per hour.

Find: Chiller rating Reheater rating

Q = Volume x air changes

3600= 1500 x 5 = 2.1 m3/s

3600

Convert m3/s to kg/s by establishing specific volume plant commencing conditions:

At 27Cdb and 20Cwb = 0.87 m3/kg (chiller)At 10Cdb and 10Cwb = 0.81 m3/kg (reheater)

Therefore

2.1 m3/s = 2.4 kg/s (chiller)0.87 m3/kg

and2.1 m3/s = 2.6 kg/s (reheater)

0.81 m3/kg

Enthalpy values for chilling and reheating:

Chilling 57 45 kJ/kg = 12 kJ/kg

Reheating 39 29.5 kJ/kg = 9.5 kJ/kg

The chiller rating is

2.4 kg/s x 12 kJ/kg = 28.8 kW

The reheater rating is

2.6 kg/s x 9.5 kJ/kg = 24.7 kW

REFERENCES

Greeno, R.(1997). Building Services, Technology and Design. Essex: Longman.

Hall, F. & Greeno, R. (2005). Building Services Handbook. Oxford: Elsevier.

Trane Air-Conditioning Company. Mohamed Rashid Embi & Sulaiman Shariff (1996).

Pengudaraan dan Sistem Penyamanan Udara. Kuala Lumpur: Dewan Bahasa dan Pustaka.

Quiz

What is cooling load?

List down the components of heat gains.

Documents

REG261 - Air-Conditioning