As run energy efficiency of Cooling Towers

D.Pawan Kumar

Thermal performance of an evaporative tower is primarily dependent upon the entering air wet bulb temperature (WBT) and relative humidity (RH).

The entering WBT is an independent variable that dictates cooling tower selection.

It is both the theoretical limit to the leaving cold water temperature and the only air parameter involved in cooling tower selection.

The difference between the WBT and the tower leaving or cold water temperature (CWT) is called the approach temperature or the approach.

Approach temperatures generally fall between 5 to 20 F.

The difference between the tower leaving or hot water temperature (HWT) and the CWT is called the range

Cooling tower effectiveness is the ratio of range (achieved value)and range plus approach (achievable value theoretically).Highest effectiveness value is desirable.

Simultaneous achievement of maximum Range, Capacity and Effectiveness with lowest input energy needs are desirable attributes of a cooling tower in operation.

We get higher cooling water temperature than design

Higher Cooling water temperature means higher back pressure for a steam turbine, higher heat rate, higher cost of generation.

Higher cooling water temperature means poor efficiency and performance of diesel generating sets, air compressors,AC plants,furnaces and process equipment.

Is it giving the capacity, range,effectiveness, as per design?

Cooling tower may not be subjected to the design conditions in as run conditions.

To assess the as run performance, also refer to cooling tower performance curves.

They are drawn for a flow variation of 80-120 %

They are drawn for a WBT variation of +/- 5 deg.C with respect to design value

They are drawn for a cooling range variation of +/- 20%

The factors the tower can be assessed for, include:• Heat load(capacity),• Cooling water flow,• Fan Power,• Cooling Range, approach, and effectiveness.

Cooling Towers normally do have spare cells by design:

• Examine if operation of spare cell would beneficial

• Careful analysis of various operational costs and

performance data would help optimize

Return water distribution system to ensure:• uniform hot water flow to all the cells• Clean nozzles• Splash bars in original location• All fills in place and in clean condition• Air inlet is clear of obstructions.• Fans are operating at design condition• Operating cells only get water inflow

Water losses include evaporation, drift (water entrained in discharge vapor), and blow down (water released to discard solids). 

Drift losses are estimated to be between 0.1 and 0.2% of water supply.

Evaporation Loss = 0.00085 x water flow rate x (T1-T2)  

                               Blow down Loss = Evaporation Loss/(COC-1)                                              where cycles of concentration (COC) is the ratio solids in the circulating water to the solids in the make-up water

Total Losses = Drift Losses + Evaporation Losses + Blow down Losses     

Cooling Water Treatment: With increasing costs of water, increase Cycles of

Concentration (COC) would help to reduce make up water requirements significantly.

Drift Loss in the Cooling Towers: drift loss minimization to as low as 0.003 – 0.001% by

change in drift eliminator shapes & opting for more efficient designs.

Cooling Tower Fans: Adoption of fans (lightweight, efficient designs)with better

aerodynamic profile with optimum, twist, taper for achieving fan total efficiency as high as 85–92 %desirable.

Cooling Tower loads: Segregation of high temperature (like furnace cooling) and

temperature sensitive loads is desirable for better overall efficiency.

Condition of Nozzles and Fills Upkeep of Nozzles and fills is crucial for efficiency and

capacity management

Optimizing fan operations desirable through on-off controls or multi speed fan motors, automatically adjustable pitch fans, variable speed fan drives.

Adoption of varying combinations in L/G ratio for optimum cooling tower performance during summer, monsoon and winter times desirable, to achieve highest cooling tower effectiveness alongside capacity simultaneously.

Adoption of on-line conductivity monitoring with optimized blow-down and makeup water management desirable.

THANK YOU