What are the factors affecting Cooling Tower Performance?

 

 

Circulating cooling water passes through condenser to cool the exhaust steam of the turbine. This water is cooled in cooling tower and again re-circulated by pumping system. This can be natural draft or induced draft or forced draft cooling tower depending on availability of land , water and infrastructures etc. But while selecting the cooling tower following factors are to be considered:

1.       CW Flow

2.       Wet bulb temperature

3.       Hot water temperature

4.       Cold water temperature

5.       Air flow rate 

6.       Wind velocity and direction

7.       Make up water quality

8.       COC- cycle of concentration

9.       Pumping head or frictional losses in CT

10.   Relative Humidity

 

In case of Induced draft CT, two different types of wet cooling tower used in power plant.

1.       Cross Flow

2.       Counter Flow

 Cross Flow Cooling Tower                                            Counter Flow Cooling Tower

Counter flow is more efficient than cross flow type of cooling tower. But cross flow CT is required lesser maintenance. Sometimes it is found that films are got choked in counter flow cooling tower and distribution nozzle are chocked in cross flow CT.

In the process of cooling of the water, water is evaporated in cooling tower. The concentration of the total dissolved solids (TDS) in cooling water is increased and water level is also decreased in CT basin. Make up water is required to maintain the basin level of the CT. If the concentration of the dissolved solids is increased beyond to a certain level, it is deposited on the surface of the condenser tubes which has a detrimental effect on the performance of the condenser and reduces heat rate and plant capacity. So water quality monitoring and maintaining are very important factors for running of the cooling tower.  There are three types of water loss in cooling tower:

·         Evaporation loss(EL)

·         Drift loss(DL)

·         Blow down loss (BDL)

So makeup water requirement = EL +DL+BDL

 

Effectiveness of Cooling Tower = 

Range = (Hot water Temp) -  (Cold water temp)

Approach =( Hot water temp) – (Wet bulb temp )

COC= cycle of concentration= TDS of CW/ TDS makeup water or

                                                                Hardness of CW/ hardness of makeup water or

                                                                Conductivity of CW / Conductivity of makeup water

Blow down, BDL = 

 EL=Evaporation Loss

Evaporation Loss, EL= 0.00085xRx1.8xC, m3/h

                                R=Range

                                C= Circulating water flow, m3/hr

Make-Up water= COC x BDL

 

Cooling Tower Water Parameter (Typical)  :

Parameters	Control Limit for CW	Unit	Makeup Water
PH	>7.2  < 7.8		6-8
TSS	<20	ppm
Conductivity micro-S/cm	<6000
Ca Hardness as CaCO3	<1100	ppm
Chloride as Cl-	<1000	ppm
Sulphate as SO42-	<200	ppm
Phosphate as PO42-	<6	ppm
P.ALKALINITY	NIL
M-Alkalinity as CaCO3	<150	ppm	<50
Iron as Fe	<3	ppm	<0.3
Silica as Si2-	<150	ppm	<30
Residual Chlorine	0.1 to 0.2	ppm	nil

 

Best Practices:

1.       Chemical treatment of cooling water to control corrosion, fouling, scaling and increase COC,

2.       Chlorine dosing in cooling tower to reduce microbiological growth

3.       Automatic Blow down control with the conductivity controller

4.       Fan Speed control with VFD for energy saving

5.       Fan Blade Pitch control for energy saving

6.       Corrosion rate control and monitoring

7.       Automatic basin level control

8.       Monitoring CW flow/air flow ratio, COC, Blowdown water flow, make up water flow

9.        Pre-treatment of makeup water (to remove chemical contaminants and heavy metals, soften, adjust pH, reduce silica, TDS and TSS)

10.    Sidestream treatment (filtration and softening)of recirculating water(to soften, reduce silica, and reduce TSS)

11.    Post treatment of cooling tower blow down water – (to reduce volume)

12.   Maintain daily log sheet and maintenance record