6.0 Cooling Tower and its performance assessment

on



COOLING TOWER

TYPE

A. Natural draft cooling tower (They are generally used for water flow rates above 45,000 m3/hr.)

1. Cross flow tower (Fill located outside tower)

2. Counter flow tower (Fill located inside tower) 

B. Mechanical draft cooling tower

1. Counter flows induced draft.

2. Cross flows induced draft.

3. Counter flows forced draft.



Cooling Tower Performance

1

Range

It is the difference between the cooling tower water inlet and outlet temperature. R= T1-T2

2

Approach

It is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature. A=T2-Twb

3

Cooling tower effectiveness

It is the ratio of range, to the ideal range,or in other words it is = Range / (Range + Approach). E=R/(R+A)

4

Cooling capacity

It is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.  
Cooling Capacity(KCal/hr or TR)=Mass flow rate of water * Sp. Heat * Temp. difference

The process of cooling is called refrigeration. Refrigeration or cooling capacity can be measured in tons. A ton is the amount of heat removed by an air conditioning system that would melt 1 ton (2000 lbs.) of ice in 24 hours. The heat required to melt 1 lb of ice at 32 oF to water is 144 Btu. 
1 Ton Refrigeration = (2000 lb) (144 Btu/lb) / (24 hr) = 12000 Btu/hr
A water-chiller refrigeration ton is defined as:
1 Refrigeration Ton (RT) = 1 TONScond = 12000 Btu/h = 200 Btu/min = 3025.9 k Calories/h = 12661 kJ/h = 3.517 kW
1 kW = 0.2843 Refrigeration Ton (RT)

5

Evaporation loss

It 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.   
Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)

It is approx. 1% of circulation rate.

6

C.O.C

Cycles of concentration is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.

7

Blow down losses

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)

8

Drift Loss

0.002 to 0.01% of recirculation water flow

9

Make up Water

Make up water = E + BD

10

Liquid/Gas (L/G) ratio

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. Thermodynamics also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air:
 L/G = (h2 – h1)/(T1– T2)

L/G = liquid to gas mass flow ratio (kg/kg),

h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature,                     

h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature

11

Hold Time Index

The time required to reduce the chemical or makeup water added to a system to 50% of its original concentration.
HTI= ( 0.693 * Holding Capacity) / Blow down Rate

12

Time per Cycle

The time it takes all the water in a system to make one trip around the recirculation loop.
Time per Cycle = Holding Capacity / Recirculation Rate

13

Windage  loss

Spray pond – 1 to 5 %

Cooling pond – 1 to 3 %

Cooling tower mechanical draft – 1 to 3 %

14

Langelier saturation index

LSI = pHact – PhD

If + => scaling tendency

(-) or 0 => no scaling

15

Ryznar stability Index

RSI = 2pHs – pHact

If >6 – Scaling tendency

<6 – No scaling

16

Puckorius modify Stability Index

PSI = 2pHs – pHe

If >6 – Scaling tendency

<6 – No scaling

pHs = pH of saturation for CaCO3

pHe = Equilibrium pH based on total alkalinity


Factors Affecting Cooling Tower Performance

1. Capacity

A cooling tower sized to cool 4540 m3/hr through a 13.9°C range might be larger than a cooling tower to cool 4540 m3/hr through 19.5°C range.

 

2, Range

Range °C = Heat Load in kcals/hour / Water Circulation Rate in LPH. The closer the approach to the wet bulb, the more expensive the cooling tower due to increased size.

 

3. Heat Load

Heat rejection requirements of various types of power equipment. 
* Air compressor - Single-stage                                                          - 129 kCal/kW/hr
                               - Single-stage with after cooler                            - 862 kCal/kW/hr
                               - Two-stage with intercooler                                - 518 kCal/kW/hr
                               - Two-stage with intercooler and after cooler   - 862 kCal/kW/hr
* Refrigeration, Compression                                                             - 63 kCal/min/TR
* Refrigeration, Absorption                                                                 - 127 kCal/min/TR
* Steam Turbine Condenser                                                                - 555 kCal/kg of steam
* Diesel Engine, Four-Cycle, Supercharged                                       - 880 kCal/kW/hr
* Natural Gas Engine, Four-cycle (18 kg/cm2 compression)          - 1523 kCal/kW/hr

 

4. Fill Media Effects

Splash fill media generates the required heat exchange area by splashing action of water over fill media and hence breaking into smaller water droplets. Thus, surface of heat exchange is the surface area of the water droplets, which is in contact with air.

In a Film fill, water forms a thin film on either side of fill sheets. Thus area of heat exchange is the surface area of the fill sheets, which is in contact with air.

 

Choosing a Cooling Tower

It is well known that counter flow heat exchange is more effective as compared to cross flow or parallel flow heat exchange.

Factor that effect cooling tower size

Cooling tower size is affected by heat load, range, approach and WBT. When three of these four quantities and held constant, tower size varies in following manner.

·                        Directly with the Heat load

·                        Inversely with the Range

·                        Inversely with the Approach

·                        Inversely with the entering WBT.

 

Improving Energy Efficiency of Cooling Towers

1

Clearances around cooling towers needs to be adequate to ensure uninterrupted air intake or exhaust.

2

Use right type of nozzles that do not clog & spray in a more uniform water pattern. Square spray nozzles are clog free as compared to spray type nozzles.

3

Increase contact surface and contact time between air and water may be with the use of PVC Film Type fills by replacing splash bars.

4

Clean distribution nozzles regularly.

5

Optimize the blow down flow rate, taking into account the cycles of concentration (COC)limit.

6

Keep the cooling water temperature to a minimum level by

(a) segregating high heat loads like furnaces, air compressors, DG sets and

(b) isolating cooling towers from sensitive applications like A/C plants, condensers of captive power plant etc.

7

Monitor approach, effectiveness and cooling capacity to continuously optimize the cooling tower performance Seasonal variations be taken into consideration.

8

Monitor liquid to gas ratio and cooling water flow rates and amend these depending on the design values and seasonal variations. For example: increase water loads during summer and times when approach is high and increase air flow during monsoon times and when approach is low.

9

Increase COC improvement for water savings. The use of water treatment chemicals, pretreatment such as softening & pH adjustment , and other techniques can affect the acceptable range of cycles of concentration.

10

Check cooling water pumps regularly to maximize their efficiency.

 

Example:- Performance Assessment of Cooling Towers

OBSERVATION

Parameters

 

Unit

Rated

Actual

Dry bulb temperature 

Tdb

oC

 

40.8

Wet bulb temperature

Twb

oC

27.5

29.3

Inlet temperature of water to the tower

Ti

oC

43

44

Outlet temperature of water from the tower

To

oC

33

37.6

Number of CT Cells on line with water flow

 

 

48

45

Total Measured Cooling Water Flow

C

M3/Hr

 

70426.76

Measured CT Fan Air Flow

 

M3/Hr

 

989544

Dissolved solid in the recirculation water

 

ppm

 

2700

Dissolved solid in the make up water

 

ppm

 

1000

Make up water flow

 

M3/Day

 

1150

Make up water flow

 

M3/Hr

 

47.92

Holding capacity or System volume

 

M3

 

2000

Constant

 

 

 

 

Specific Heat of Water

Cp

kCal / kg / °C

 

1

Latent heat of vaporization of water

Hv

kCal / kg

 

539.76

Density of Water per m3/Hr

 

kg/m3

 

1000

Density of Air per m3/Hr

 

kg/m3

 

1.08

 

Cooling Tower Calculation

ANALYSIS

Parameters

 

Unit

Rated

Actual

CT Water Flow/Cell

 

m3/hr

1875

1565

 

kg/hr

 

1565039

CT Fan Air Flow

 

m3/hr (Avg.)

997200

989544

 

kg/hr (Avg.)

 

1068708

L/G Ratio of C.T

 

Kg/Kg

 

1.46

Range

R or ΔT = (Ti - To)

 

 

6.4

Approch

(To - Twb)

 

 

8.3

Cooling Tower  Efficiency (μ ) =

 (Ti - To) x 100

%

 

43.54

(Ti - Twb)

Cooling Capacity =

C x Cp x R

kCal/hr

 

450731264

 

1 ton (refrigeration) = 3023.9491 kcal/h

TR

 

149053.81

Concentration Ratio or Cycles of Concentration

TDS of circulating water      TDS of makeup water

COC

 

2.70

 

 

 

 

 

Evaporation Loss

E = 0.00085 x 1.8 x R x C

M3/Hr

 

689.62

 

E = R x C x Cp / Hv

M3/Hr

 

835.06

 

 

 

 

 

Blowdown Loss

B = Evaporation loss / (COC - 1)

M3/Hr

 

405.66

 

 

 

 

 

Drift Loss

 

M3/Hr

 

4.23

For Natural Draft Cooling Tower 

D = 0.3 to 1.0 * C /100

 

 

 

For Induced Draft Cooling Tower 

D = 0.1 to 0.3 * C /100

 

 

 

For Cooling Tower with Drift Eliminator 

D = 0.01* C /100

 

 

 

 

 

 

 

 

Makeup Rate

M = E + B + D

M3/Hr

 

1099.50

 

 

 

 

 

 

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