12.0 COOLING TOWER

Cooling Water System Introduction

Cooling tower: TypesNatural draftLarge concrete chimneys generally used for water flow rates above 45,000 m3/hrutility power stations Mechanical draft Lrge fans to force or suck air through circulated water. The water falls downward over fill surfaces, which help increase the contact time between the water and the air maximising heat transfer between the two. Cooling rates of Mechanical draft towers depend upon their fan diameter and speed of operation

Components of Cooling Tower Frame and casing Fill Cold water basin Drift eliminators Air inlet Louvers Nozzles Fans

Tower Materials Wooden components included the frame, casing, louvers, fill, and often the cold water basin Galvanized steel, various grades of stainless steel, glass fiber, and concrete enhance corrosion resistance, reduce maintenance, and promote reliability and long service life Plastics are widely used for fill, including PVC, polypropylene, and other polymers. Plastics also find wide use as nozzle materials Aluminum, glass fiber, and hot-dipped galvanized steel are commonly used fan materials. Centrifugal fans are often fabricated from galvanized steel. Propeller fans are fabricated from galvanized, aluminum, or molded glass fiber reinforced plastic

Fill Media EffectsHeat exchange between air and water is influenced by surface area of heat exchange, time of heat exchange (interaction) and turbulence in water effecting thoroughness of intermixing. Fill media in a cooling tower is responsible to achieve all of above.

Cooling Tower Performance

Cooling Tower Performance Range is the difference between the cooling tower water inlet and outlet temperatureApproach is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. Although, both range and approach should be monitored, the `Approach is a better indicator of cooling tower performance. Cooling tower effectiveness (in percentage) is the ratio of range, to the ideal range, i.e., difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach).

Cooling Tower Performance Cooling capacity is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.Evaporation loss is the water quantity evaporated for cooling duty and, theoretically, for every 10,00,000 kCal heat rejected, evaporation quantity works out to 1.8 m3. An empirical relation used often is:

Circulation Rate (m3/hr) * Temp. Difference in oC

m3/hr, Evaporation Loss = -----------------------------------------------------------

675

Cooling Tower Performance

Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.

Blow down losses depend upon cycles of concentration and the evaporation losses and is given by relation:

Blow Down=Evaporation Loss / (C.O.C. 1)

Cooling Tower Performance Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates. Against design values, seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness through measures like water box loading changes, blade angle adjustments.

Factors Affecting Cooling Tower Performance

Capacity Heat dissipation (in kCal/hour) and circulated flow rate (m3/hr) are not sufficient to understand cooling tower performance. For example, a cooling tower sized to cool 4540 m3/hr through a 13.9oC range might be larger than a cooling tower to cool 4540 m3/hr through 19.5oC range.

RangeRange is determined not by the cooling tower, but by the process it is serving Range oC = Heat Load in kcals/hour Water Circulation Rate in LPH Thus, Range is a function of the heat load and the flow circulated through the system Cooling towers are usually specified to cool a certain flow rate from one temperature to another temperature at a certain wet bulb temperature. For example, the cooling tower might be specified to cool 4540 m3/hr from 48.9oC to 32.2oC at 26.7oC wet bulb temperature.

Cold Water Temperature 32.2oC Wet Bulb Temperature (26.7oC) = Approach (5.5oC)As a generalization, the closer the approach to the wet bulb, the more expensive the cooling tower due to increased size. Usually a 2.8oC approach to the design wet bulb is the coldest water temperature that cooling tower manufacturers will guarantee. If flow rate, range, approach and wet bulb had to be ranked in the order of their importance in sizing a tower, approach would be first with flow rate closely following the range and wet bulb would be of lesser importance.

Heat Load The heat load imposed on a cooling tower is determined by the process being served In most cases, a low operating temperature is desirable to increase process efficiency or to improve the quality or quantity of the product. In some applications (e.g. internal combustion engines), however, high operating temperatures are desirable The size and cost of the cooling tower is proportional to the heat load

Wet Bulb Temperature minimum cold water temperature to which water can be cooled by the evaporative method Thus, the wet bulb temperature of the air entering the cooling tower determines operating temperature levels throughout the plant, process, or system. Theoretically, a cooling tower will cool water to the entering wet bulb temperature, when operating without a heat load. However, a thermal potential is required to reject heat, so it is not possible to cool water to the entering air wet bulb temperature, when a heat load is applied The temperature selected is generally close to the average maximum wet bulb for the summer months whether it is specified as ambient or inlet

Effect of approach on the size and cost of a cooling tower

Approach and Flow

Table 7.2 Flow vs. Approach for a Given Tower

(Tower is 21.65 m ( 36.9 M; Three 7.32 M Fans; Three 25 kW Motors; 16.7oC Range with 26.7oC Wet Bulb)

Flow m3/hr

Approach oC

Cold Water oC

Hot Water oC

Million kCal/hr

3632

2.78

29.40

46.11

60.691

4086

3.33

29.95

46.67

68.318

4563

3.89

30.51

47.22

76.25

5039

4.45

31.07

47.78

84.05

5516

5.00

31.62

48.33

92.17

6060.9

5.56

32.18

48.89

101.28

7150.5

6.67

33.29

50.00

119.48

8736

8.33

35.00

51.67

145.63

11590

11.1

37.80

54.45

191.64

13620

13.9

40.56

57.22

226.91

16276

16.7

43.33

60.00

271.32

Tower Size vs Approach

Range, Flow and Heat Load Range is a direct function of the quantity of water circulated and the heat load. Increasing the range as a result of added heat load does require an increase in the tower size. If the cold water temperature is not changed and the range is increased with higher hot water temperature, the driving force between the wet bulb temperature of the air entering the tower and the hot water temperature is increased, the higher level heat is economical to dissipate.If the hot water temperature is left constant and the range is increased by specifying a lower cold water temperature, the tower size would have to be increased considerably. Not only would the range be increased, but the lower cold water temperature would lower the approach. The resulting change in both range and approach would require a much larger cooling tower.

Approach & Wet Bulb TemperatureA 4540 m3/hr cooling tower selected for a 16.67oC range and a 4.45oC approach to 21.11oC wet bulb would be larger than a 4540 m3/hr tower selected for a 16.67oC range and a 4.45oC approach to a 26.67oC wet bulb. Air at the higher wet bulb temperature is capable of picking up more heat. Assume that the wet bulb temperature of the air is increased by approximately 11.1oC. 21.10C WB, 18.86 kCals/kg32.20C WB, 24.7 kCals/kg26.670C WB, 24.17 kCals/kg37.80C WB, 39.67 kCals/kgHeat Picked Up =12.1kCals/kgHeat Picked Up =15.5 kCals/kg

Efficient System Operation Cooling Water Treatment Drift Loss in the Cooling Towersdrift loss requirement to as low as 0.003 0.001%Cooling Tower FansFlow Control Strategies

Lighting

Basic Terms in Lighting System and Features

Lamps:Lamp is equipment, which produces light. Incandescent lamps: Incandescent lamps produce light by means of a filament heated to incandescence by the flow of electric current through it. The principle parts of an incandescent lamp, also known as GLS (General Lighting Service) lamp include the filament, the bulb, the fill gas and the cap. Reflector lamps: Reflector lamps are basically incandescent, provided with a high quality internal mirror, which follows exactly the parabolic shape of the lamp. The reflector is resistant to corrosion, thus making the lamp maintenance free and output efficient. Gas discharge lamps: The light from a gas discharge lamp is produced by the excitation of gas contained in either a tubular or elliptical outer bulb.

The most commonly used discharge lamps are as follows:

Fluorescent tube lamps (FTL) Compact Fluorescent Lamps (CFL) Mercury Vapour Lamps (MVL) Sodium Vapour Lamps (HPSV/LPSV) Metal Halide Lamps

Luminaire Luminaire is a device that distributes, filters or transforms the light emitted from one or more lamps. The luminaire includes, all the parts necessary for fixing and protecting the lamps, except the lamps themselves. principles used in optical luminaire are reflection, absorption, transmission and refraction. Control Gear The gears used in the lighting equipment are as follows: Ballast: A current limiting device, to counter negative resistance characteristics of any discharge lamps. In case of fluorescent lamps, it aids the initial voltage build-up, required for starting. Ignitors: These are used for starting high intensity Metal Halide and Sodium vapour lamps.

Illuminance This is the quotient of the illuminous flux incident on an element of the surface at a point of surface containing the point, by the area of that element. The illuminance provided by an installation affects both the performance of the tasks and the appearance of the space. Lux (lx): This is the illuminance produced by a luminous flux of one lux, uniformly distributed over a surface area of one square metre. One lux is equal to one lumen per square meter.Luminous Efficacy (lm/W)This is the ratio of luminous flux emitted by a lamp to the power consumed by the lamp. It is a reflection of efficiency of energy conversion from electricity to light form.Colour Rendering Index (RI)Is a measure of the degree to which the colours of surfaces illuminated by a given light source confirm to those of the same surfaces under a reference illuminant; suitable allowance having been made for the state of Chromatic adaptation.

Lighting System Approach

Lighting Quality Illumination level. Uniformity Absence of glare.Colour rendering index (CRI).

Luminous Performance Characteristics of Commonly Used Luminaries

Type of Lamp

Lum / Watt

Color Rendering Index

Typical Application

Life (Hours)

Range

Avg.

Incandescent

8-18

14

Excellent

Homes, restaurants, general lighting, emergency lighting

1000

Fluorescent Lamps

46-60

50

Good w.r.t. coating

Offices, shops, hospitals, homes

5000

Compact fluorescent lamps (CFL)

40-70

60

Very good

Hotels, shops, homes, offices

8000-10000

High pressure mercury (HPMV)

44-57

50

Fair

General lighting in factories, garages, car parking, flood lighting

5000

Halogen lamps

18-24

20

Excellent

Display, flood lighting, stadium exhibition grounds, construction areas

2000-4000

High pressure sodium (HPSV) SON

67-121

90

Fair

General lighting in factories, ware houses, street lighting

6000-12000

Low pressure sodium (LPSV) SOX

101-175

150

Poor

Roadways, tunnels, canals, street lighting

6000-12000

Recommended Illuminance Levels for Various Tasks / Activities / Locations 20305075100150200300500750100015002000, Lux

Lighting ControlsOn/off flip switchesTimer control & auto timed switch offPresence detectionLuminary grouping / Group SwitchingDay light linking, blinders, corrugated roof sheetsDimmers , Lighting voltage controllers Photo sensors

METHODOLOGY OF LIGHTING SYSTEM ENERGY EFFICIENCY STUDY Step-1 : Inventorise the Lighting System elements, & transformers in the facility as per following typical format.

S. No.

Plant Location

Lighting Device & Ballast Type

Rating in Watts Lamp & Ballast

Population Numbers

Use / Shifts as I / II / III shifts / Day

Energy savings in lighting SystemMake maximum use of natural light (North roof/translucent sheets/more windows and openings)Switch off when not requiredModify lighting layout to meet the needSelect light colours for interiorsProvide timer switches / PV controlsProvide lighting Transformer to operate at reduced voltageInstall energy efficient lamps, luminaries and controlsMetal halide in place of Mercury and SVL lampsCFT in place of incandescent lampsClean North roof glass, translucent sheet and luminaries regularlySeparate lighting TransformerTo isolate from power feeder, To avoid voltage fluctuation problemInstall Servo stabilizer if separate transformer is not feasible.

ENERGY PERFORMANCE ASSESSMENT OF LIGHTING SYSTEMS

Purpose of the Performance Test

The purpose of performance test is to calculate the installed efficacy in terms of lux/watt/m (existing or design) for general lighting installation. The calculated value can be compared with the norms for specific types of interior installations for assessing improvement options.The installed load efficacy of an existing (or design) lighting installation can be assessed by carrying out a survey.

Performance Terms and Definitions

Lumen is a unit of light flow or luminous flux. The lumen rating of a lamp is a measure of the total light output of the lamp. The most common measurement of light output (or luminous flux) is the lumen. Light sources are labeled with an output rating in lumens.Lux is the metric unit of measure for illuminance of a surface. One lux is equal to one lumen per square meter. Circuit Watts is the total power drawn by lamps and ballasts in a lighting circuit under assessment.

Installed Load Efficacy is the average maintained illuminance provided on a horizontal working plane per circuit watt with general lighting of an interior. Unit: lux per watt per square metre (lux/W/m)Lamp Circuit Efficacy is the amount of light (lumens) emitted by a lamp for each watt of power consumed by the lamp circuit, i.e. including control gear losses. This is a more meaningful measure for those lamps that require control gear. Unit: lumens per circuit watt (lm/W)Installed Power Density. The installed power density per 100 lux is the power needed per square metre of floor area to achieve 100 lux of average maintained illuminance on a horizontal working plane with general lighting of an interior. Unit: watts per square metre per 100 lux (W/m/100

Installed load efficacy (lux/W/m) Installed Load Efficacy Ratio (ILER) = Actual Lux/W/m or Target W/m/100lux Target Lux/W/m Actual W/m/100luxAverage maintained illuminance is the average of lux levels measured at various points in a defined area.Color Rendering Index (CRI) is a measure of the effect of light on the perceived color of objects. To determine the CRI of a lamp, the color appearances of a set of standard color chips are measured with special equipment under a reference light source with the same correlated color temperature as the lamp being evaluated. If the lamp renders the color of the chips identical to the reference light source, its CRI is 100. If the color rendering differs from the reference light source, the CRI is less than 100. A low CRI indicates that some colors may appear unnatural when illuminated by the lamp.

Installed power density (W/m/100 lux) = 100

Preparation (before Measurements)

Before starting the measurements, the following care should be taken:All lamps should be operating and no luminaires should be dirty or stained.There should be no significant obstructions to the flow of light throughout the interior, especially at the measuring points.Accuracies of readings should be ensured by- Using accurate illuminance meters for measurements- Sufficient number and arrangement of measurement points within the interior- Proper positioning of illuminance meterEnsuring that no obstructions /reflections from surfaces affect measurement

To Determine the Minimum Number and Positions of Measurement PointsCalculate the Room Index: RI = L x W Hm(L + W)Where L = length of interior; W = width of interior; Hm = the mounting height, which is the height of the lighting fittings above the horizontal working plane. The working plane is usually assumed to be 0.75m above the floor in offices and at 0.85m above floor level in manufacturing areas.It does not matter whether these dimensions are in metres, yards or feet as long as the same unit is used throughout. Ascertain the minimum number of measurement points from Table

For example, the dimensions of an interior are:Length = 9m, Width = 5m, Height of luminaires above working plane (Hm) = 2m Calculate RI = 9 x 5 = 1.93 2(9 + 5)From Table 10.1 the minimum number of measurement points is 16As it is not possible to approximate a square array of 16 points within such a rectangle it is necessary to increase the number of points to say 18, i.e. 6 x 3. These should be spaced as shown below:

Therefore in this example the spacing between points along rows along the length of the interior = 9 6 = 1.5m and the distance of the 'end' points from the wall = 1.5 2 = 0.75m.Similarly the distance between points across the width of the interior = 5 3 = 1.67m with half this value, 0.83m, between the 'end' points and the walls.If the grid of the measurement points coincides with that of the lighting fittings, large errors are possible and the number of measurement points should be increased to avoid such an occurrence.

Calculation of the Installed Load Efficacy and Installed Load Efficacy Ratio of a General Lighting Installation in an Interior

STEP 1

Measure the floor area of the interior:

Area = -------------------- m

STEP 2

Calculate the Room Index

RI = --------------------

STEP 3

Determine the total circuit watts of the installation by a power meter if a separate feeder for lighting is available. If the actual value is not known a reasonable approximation can be obtained by totaling up the lamp wattages including the ballasts:

Total circuit watts = -------------

STEP 4

Calculate Watts per square metre, Value of step 3 value of step 1

W/m = ------------

STEP 5

Ascertain the average maintained illuminance by using lux meter, Eav. Maintained

Eav.maint. = ------------

STEP 6

Divide 5 by 4 to calculate lux per watt per square

Metre

Lux/W/m = ------------

STEP 7

Obtain target Lux/W/m lux for type of the type of interior/application and RI (2):

Target Lux/W/m =

STEP 8

Calculate Installed Load Efficacy Ratio ( 6 7 ).

ILER =

45

Example of ILER Calculation

STEP 1

Measure the floor area of the interior:

Area = 45 m

STEP 2

Calculate the Room Index

RI = 1.93

STEP 3

Determine the total circuit watts of the installation by a power meter if a separate feeder for lighting is available. If the actual value is not known a reasonable approximation can be obtained by totaling up the lamp wattages including the ballasts:

Total circuit watts = 990 W

STEP 4

Calculate Watts per square metre, 3 1 :

W/m = 22

STEP 5

Ascertain the average maintained illuminance,

Eav. Maintained (average lux levels measured at 18 points)

Eav.maint. = 700

STEP 6

Divide 5 by 4 to calculate the actual lux per watt per square Metre

Lux/W/m = 31.8

STEP 7

Obtain target Lux/W/m lux for type of the type of interior/application and RI (2):(Refer Table 10.2)

Target Lux/W/m = 46

STEP 8

Calculate Installed Load Efficacy Ratio ( 6 7 ).

ILER = 0.7

Referring to table 3, ILER of 0.7 means that there is scope for review of the lighting system.

Annual energy wastage = (1 ILER) x watts x no. of operating hours

= (1 0.7) x 990 x 8 hrs/day x 300 days

= 712 kWh/annum

45

Table 10.4 IES Illuminance Categories and Values - For Generic Indoor Activities

ACTIVITY

CATEGORY

LUX

FOOTCANDLES

Public spaces with darksurroundings

A

20-30-50

2-3-5

Simple orientation for shorttemporary visits

B

50-75-100

5-7.5-10

Working spaces where visualtasks are only occasionally performed

C

100-150-200

10-15-20

Performance of visual tasks ofhigh contrast or large size

D

200-300-500

20-30-50

Performance of visual tasks ofmedium contrast or small size

E

500-750-1000

50-75-100

Performance of visual tasks oflow contrast or very small size

F

1000-1500-2000

100-150-200

Performance of visual tasks of low contrast or very small size over a prolonged period

G

2000-3000-5000

200-300-500

Performance of very prolongedand exacting visual tasks

H

5000-7500-10000

500-750-1000

Performance of very specialvisual tasks of extremely lowcontrast

I

10000-15000-20000

1000-1500-2000

A-C for illuminances over a large area (i.e. lobby space)

D-F for localized tasks

G-I for extremely difficult visual tasks

Areas for Improvement

Look for natural lighting opportunities through windows and other openingsIn the case of industrial lighting, explore the scope for introducing translucent sheetsAssess scope for more energy efficient lamps and luminaries Assess the scope for rearrangement of lighting fixtures