The Cooling Tower is important utility equipment which plays a vital role for operation & energy consumption perspective by providing cold water (above ambient Wet Bulb Temperature Air) by utilizing atmospheric air to cool the incoming hot water. The application of Cooling Tower is very widespread ranging from large thermal power plants for barometric condenser, water cooled chillers for condenser heat exchange application to a process industry or even simple machine tool cooling application. Design, shape, capacity etc vary as per requirement & application. Hence hyperbolic, induced draft cross flow/counter flow, forced draft or natural draft cooling towers came into picture. In all the cases, it does the same operation of lowering the temperature of incoming hot water by exchanging the heat from atmospheric air. In this process, air and water mixture releases latent heat of vaporization, which has a cooling effect on water by turning a certain amount of liquid into its gaseous state, releasing the Latent Heat of Vaporization. In simple terms, the part of falling/circulating water removes heat from bulk of circulating water, by the way of evaporation with the help of ambient/atmospheric air. This gives rise to loss of water & hence the outlet air has higher Relative humidity (%RH).
Since Cooling Tower plays significant part in energy consumption and its performance is majorly driven by ambient WBT, the performance evaluation needs careful attention by effective analysis considering all the factors that are responsible in Cooling Tower Performance. Often we evaluate Cooling Tower performance based on its effectiveness (%) which is nothing but the simple arithmetic division of Range & Approach as follows:
CT Effectiveness (%) = Range (°C or °F) / [Range +Approach] (°C or °F)
Where, Range = (Inlet Water Temp. – Sump Water Temp.)
Approach = [Sump Water Temp. – Ambient Air Wet Bulb Temp. (WBT)]
But this is not the very correct way to get a judgment of cooling tower performance as shown in the following table to elaborate the same:
Now compare the ‘Range’ & ‘Approach’ values and corresponding % effectiveness.
Case 1: Base Case of 28 deg C WBT with 5 deg C Range & 4 deg C Approach has 55.5% effectiveness.
Case 2: Same ambient WBT of 28 deg C as Base case with same 4 deg C Approach but higher range of 7 deg. C. This gives higher effectiveness of 63.6%.
Note: Case-2 shows, higher the range higher becomes the effectiveness.
Case-3: The Range is further higher at 8 deg. C with lower ambient WBT of 26 deg. C with 6 deg. C approach. This gives little lower effectiveness of 57.14%.
Note: Case-3 shows, higher the range does not necessarily bring into higher effectiveness but higher the approach results in lower effectiveness as compared with Case-2 but on contrary the effectiveness is higher as compared with Case-1 (Base case).
Similarly, many combinations can become available for analysis but do not give accurate analysis just from the value of effectiveness. Following is another example where with same approach of 4 deg C & different range, the effectiveness values vary.
Following is an example where with different approach but same range of 5 deg C, the effectiveness values vary.
Hence, the Cooling Tower Performance is not to be decided based on the value of % effectiveness. But the same is also to be evaluated from L/G ratio (Liquid to Gas ratio) & then the same is to be compared with designed conditions for particular cooling tower. Liquid in this case means water & gas means ambient air.
Since the heat transfer takes between water and air, the approach practically could not be zero & hence the cooling tower effectiveness can’t be 100%.
Though the cooling tower performance wisely considered as ‘CT Effectiveness’ will be higher with higher ‘Range’ & Lower ‘Approach’ which means as low as possible the sump water temperature (CT O/L water temp) better the performance of Cooling Tower but the same gets largely affected by following five factors:
- Ambient WBT (Deg. C or Deg. F)
2. Hot Water Temperature (CT Inlet Water Temperature) (Deg. C or Deg. F)
3. Cold Water Temperature (CT outlet/Sump Water Temperature) (Deg. C or Deg. F)
4. Air Flow Rate [L-Liquid) (m3/sec, m3/hr, CFM etc.)
5. Water Flow Rate [G-Gas) (m3/sec, m3/hr, GPM, Liter/hr etc.)
The practical issues arising at site for measurement of circulating water flow rate and air flow rate have limitations especially for big size cooling towers & parabolic cooling towers, where the proper arrangement for measurement is not available. In these cases, proper calibrated instrumentation plays crucial role which needs to be cross checked with pumping & blower power consumptions as well as material & heat balance for a particular application.
With the help of these measured parameters, the capacity of Cooling Tower can easily be calculated from water side heat and material balance as per following formula:
TR = (m x Cp x Delta T)/3024
…..TR: Ton of Refrigeration
…..m: mass flow rate of circulating water (Kg/hr)
…..Cp: Specific heat of circulating water (Kcal/Kg.°C)
…..Delta T: Temperature difference (I/L & O/L water) (°C)
It is important to maintain the required L/G ratio as per designed conditions. If L/G ratio is less than the Rated this means either circulating water flow rate is lower or the air flow rate is higher. Other condition of higher L/G ratio means either circulating water flow rate is higher or the air flow rate is lower. The corresponding adjustments can be done either in liquid or gas flow rates, as per convenience by adjusting the VFD frequency to get the corresponding saving in energy consumption.
The cooling tower analysis in similar manner can also be done with the help of Merkel Equation as a KaV/L analysis but the same becomes little difficult for calculations & hence not very popularly been used where the air film is represented by the Water Operating Line on the Saturation Curve on Psychrometric Chart with corresponding enthalpies & temperatures.
The main air condition is represented by the Air Operating Line and the slope of which is the Water/Air [L/G] ratio. In this analysis, the value of KaV/L remains constant & not directly dependent on ambient WBT, range, approach by formula but depends mainly on L/G ratio and attracts different perspective view to analyze the cooling tower performance. This shows the importance of L/G ratio in analyzing the cooling tower performance.
The seasonal adjustment in CT fan air flow rate are suggested to do as per the variation in ambient WBT, to get better performance & corresponding energy savings. Hence, application of VFD on CT fan becomes useful application which mainly to be operated during winter season of low ambient WBT, otherwise allowing CT fan to run on full speed to maintain sump water temperature (CT O/L water temp.) as low as possible is preferable. In HVAC application for a case of water cooled chillers especially for high ambient WBT & RH conditions (non-winter conditions), since the capacity of CT fan (KW) is so smaller than corresponding compressor motor capacity (KW power consumption), reducing CT fan speed by VFD will save negligible energy than corresponding increase in chiller compressor energy consumption, due to marginal increase in CT O/L water temperature.
Abbreviations
WBT : Wet Bulb Temperature CT : Cooling Tower
RH : Relative Humidity VFD : Variable Frequency Drive
TR : Tonne of Refrigeration I/L : Inlet
O/L : Outlet
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