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This section describes how the performance of cooling powers can be assessed.3 The performance of cooling towers is evaluated to assess present levels of approach and range against their design values, identify areas of energy wastage and to suggest improvements.
During the performance evaluation, portable monitoring instruments are used to measure the following parameters:
These measured parameters and then used to determine the cooling tower performance in several ways. (Note: CT = cooling tower; CW = cooling water). These are:
a) Range (see Figure 7). This is the difference between the cooling tower water inlet and outlet temperature. A high CT Range means that the cooling tower has been able to reduce the water temperature effectively, and is thus performing well.
The formula is:
CT Range (°C) = [CW inlet temp (°C) – CW outlet temp (°C)]
b) Approach This is the difference between the cooling tower outlet cold-water temperature and ambient wet bulb temperature. The lower the approach the better the cooling tower performance. Although, both range and approach should be monitored, the `Approach’ is a better indicator of cooling tower performance.
CT Approach (°C) = [CW outlet temp (°C) – Wet bulb temp (°C)]
c) Effectiveness. is the ratio between the range and the ideal range (in percentage), i.e. difference between cooling water inlet temperature and ambient wet bulb temperature, or in other words it is = Range / (Range + Approach). The higher this ratio, the higher the cooling tower effectiveness.
CT Effectiveness (%) = 100 x (CW temp – CW out temp) / (CW in temp – WB temp)
d) EFFICIENCY OF COOLING TOWER
N = (T1 – T2) / (T1 – WBT)
e) Cooling capacity. This is the heat rejected in kCal/hr or TR, given as product of mass flow rate of water, specific heat and temperature difference.
f) Evaporation loss. This is the water quantity evaporated for cooling duty. Theoretically the evaporation quantity works out to 1.8 m3 for every 1,000,000 kCal heat rejected. The following formula can be used (Perry):
Evaporation loss (m3/hr) = 0.00085 x 1.8 x circulation rate (m3/hr) x (T1-T2)
T1 - T2 = temperature difference between inlet and outlet water
f) Cycles of concentration (C.O.C). This is the ratio of dissolved solids in circulating water to the dissolved solids in make up water.
g) Blow down losses depend upon cycles of concentration and the evaporation losses and is given by formula:
Blow down = Evaporation loss / (C.O.C. – 1)
h) Liquid/Gas (L/G) ratio. The L/G ratio of a cooling tower is the ratio between the water and the air mass flow rates. Cooling towers have certain design values, but seasonal variations require adjustment and tuning of water and air flow rates to get the best cooling tower effectiveness. Adjustments can be made by water box loading changes or blade angle adjustments. Thermodynamic rules also dictate that the heat removed from the water must be equal to the heat absorbed by the surrounding air. Therefore the following formulae can be used:
L(T1 – T2) = G(h2 – h1)
L/G = (h2 – h1) / (T1 – T2)
Where: L/G = liquid to gas mass flow ratio (kg/kg) T1 = hot water temperature (0C) T2 = cold-water temperature (0C)
h2 = enthalpy of air-water vapor mixture at exhaust wet-bulb temperature (same units as above)
h1 = enthalpy of air-water vapor mixture at inlet wet-bulb temperature (same units as above)
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