Cooling towers are a very important part of utility system of many process plants. Basic function of cooling tower is remove heat from the system to atmosphere. In the cooling tower make up water is used as a refill water lost to evaporation. Hot water from chiller/condenser is sent to the cooling tower. After reducing the temperature of water in cooling tower and is sent back to the chiller/condenser for further process.
Dominant Parameter to calculate Cooling Tower Efficiency with Energy Conservation opportunity:-
Dominant parameter to determining the efficiency of cooling towers are:
1. Cooling Tower Range
Cooling tower range is the difference between the cooling tower water inlet/Hot inlet water temperature and cooling tower outlet/colt water outlet temperature.(See above Figure)
Range = cooling tower inlet water temp. – cooling tower outlet Water Temp.
ENCON Opportunity:-
Range °C = Heat Load in kcals/hour / Water Circulation Rate in LPH
Inversely with the range: The size of the cooling tower varies inversely with the temperature range.
2. Cooling Tower Approach
Cooling tower approach is the difference between the cooling tower outlet cold water temperature and ambient wet bulb temperature.(See above Figure) Cooling Tower approach is the better indicator for the performance.
Approach = Cold Water Temperature – Ambient Wet Bulb Temperature
ENCON Opportunity:-
A cooling tower is said to be performing well when approach is close to zero.
3. Make up Water
Cooling tower water mass balance gives an indication about make up water requirement in system. Due to Drift loss, Evaporation loss and blowdown, in the Cooling Tower Makeup water is required. To calculate the makeup water requirement use the below standard formula:-
Make up water Requirement in m3/hr(M)=Evaporation Loss in m3/hr(E) + Drift Loss in m3/hr (D) + Blow Down in m3/hr (B)
M = E + D + B
ENCON Opportunity:-
make sure Lower the make up water requirement lead to decrease the evaporation loss, drift loss and blowdown.
4. Cycle of Concentration (CoC)
Cycles of concentration (C.O.C) is the ratio of dissolved solids in circulating water to
the dissolved solids in make up water.
ENCON Opportunity:-
Consider COC improvement measures for water savings.
5. Evaporation loss
Evaporation loss is the water quantity evaporated during the heat rejection process. To calculate the evaporation loss use the below standard formula.
Evaporation Loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)
T1-T2= Temp. difference between inlet and outlet water.
ENCON Opportunity:-
Lower the evaporation loss reduce the make up water requirement.
6. Blow down loss
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)
ENCON Opportunity:-
Periodically done the blow down process increase the efficiency and life of cooling tower
7. Liquid to Gas ratio
Liquid/Gas (L/G) ratio, of a cooling tower is the ratio between the water and the air mass flow rates.
L/G= h2– h1/T1-T2
ENCON Opportunity:-
Monitor L/G ratio, CW flow rates w.r.t. design as well as seasonal variations. It would help to increase water load during summer and times when approach is high and increase air flow during monsoon times and when approach is narrow.
8. Cooling Tower Efficiency/Effectiveness
The calculation of cooling tower efficiency involves the Range and approach of the cooling Tower. Cooling tower efficiency is limited by the ambient wet bulb temperature. In the ideal case, the cold water temperature will be equal to the wet-bulb temperature. This is practically not possible to achieve. This requires very large tower and results in huge evaporation and drift loss resulting in a practically not viable solution. In practice, the cooling tower efficiency will be in between 70 to 75%.
Cooling Tower Efficiency =
(Hot Water Temperature – Cold water Temperature) x 100/
(Hot Water Temperature – Wet bulb temperature)
Or
In Simply term
Cooling Tower Efficiency = Range/ (Range + Approach) x 100
In summer the ambient air wet bulb temperature raises when compared to winter thus limiting the cooling tower efficiency.
General Energy Conservation Measures Tips for Cooling Towers:-
(Source:-Energy efficiency in electrical utilities Guide book by BEE)
Control cooling tower fans based on leaving water temperatures.
Control to the optimum water temperature as determined from cooling tower and chiller performance data.
Use two-speed or variable-speed drives for cooling tower fan control if the fans are few. Stage the cooling tower fans with on-off control if there are many.
Turn off unnecessary cooling tower fans when loads are reduced.
Cover hot water basins (to minimize algae growth that contributes to fouling).
Balance flow to cooling tower hot water basins.
Periodically clean plugged cooling tower water distribution nozzles.
Install new nozzles to obtain a more-uniform water pattern.
Replace splash bars with self-extinguishing PVC cellular-film fill.
An old counter flow cooling towers, replace old spray-type nozzles with new square-spray ABS practically-non-clogging nozzles.
Replace slat-type drift eliminators with high-efficiency, low-pressure-drop, self-extinguishing, PVC cellular units.
If possible, follow manufacturer’s recommended clearances around cooling towers and relocate or modify structures, signs, fences, dumpsters, etc. that interfere with air intake or exhaust.
Optimize cooling tower fan blade angle on a seasonal and / or load basis.
Correct excessive and / or uneven fan blade tip clearance and poor fan balance.
Use a velocity pressure recovery fan ring.
Divert clean air-conditioned building exhaust to the cooling tower during hot weather.
Re-line leaking cooling tower cold water basins.
Check water overflow pipes for proper operating level.
Optimise chemical use.
Consider side stream water treatment.
Restrict flows through large loads to design values.
Shut off loads that are not in service.
Take blow down water from the return water header.
Optimise blow down water from the return water header.
Automate blow down to minimise it.
Send blow down to other uses (Remembers, the blow down does not have to be removed at the cooling tower. It can be removed anywhere in the piping system.)
Implement a cooling tower winterisation plan to minimise ice build-up.
Install interlocks to prevent fan operation when there is no water flow.
Establish a cooling tower efficiency-maintenance program. Start with an energy audit and follow-up, then make a cooling tower efficiency-maintenance program a part of your continuous energy management program.
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