Cooling Tower Approach, Most important Term!!

In cooling Tower it is important to understand importance of approach in cooling tower design / selection. Two terms are very important in the design of cooling tower :-

  1. Range : Difference between cooling tower inlet and outlet temperature
  2. Approach: Difference between cooling tower outlet temp and wet bulb temperature.

For establishing performance  of cooling tower, following parameter is measured :-

  1. Cooling tower heat rejection Kcal/hr or TR is measured
  2. Cooling Tower effectiveness: Range / (Range +Approch)
  3. L/G ratio, L/G ratio is between water and air mass flow rates

Thermodynamically, Heat removed from water must be equal to heat absorbed in surrounding atmosphere.

L (T1-T2) =G (h2=h1)

Or

L/G =(h2-h1)/(T1-T2)

Where,

L = mass of Water, G = Mass of Air

T1 = Inlet water Temp, T2 = Outlet water Temp

H1=Enthalpy of inlet air, h2 = Enthalpy of outlet air

L/G ratio is compared with the original design condition.

Impact of Approach in cooling tower :-

Cooling tower rejection (TR) and Range is not sufficient to understand cooling tower performance. Range depends upon the process where it is being used.

Let us take one example:-

Flow Vs. approach for a given tower (Tower is 21.65 m × 36.9 M; Three 7.32 M Fans; Three 25 kW Motors; 16.7°C Range with 26.7°C Wet Bulb)

Flow Approch Cold water Hot water M Kcal/hr TR
M3/hr Deg C Deg C Deg C
3632 2.78 29.4 46.1 60.7 20070
4086 3.33 30.0 46.7 68.3 22592
4563 3.89 30.5 47.2 76.2 25214
5039 4.45 31.1 47.8 84.2 27844
5516 5 31.6 48.3 92.2 30480
6061 5.56 32.2 48.9 101.3 33491
7151 6.67 33.3 50.0 119.5 39512
8736 8.33 35.0 51.7 145.6 48158
11590 11.1 37.8 54.5 193.0 63814
13620 13.9 40.6 57.2 226.9 75036
16276 16.7 43.3 60.0 271.3 89723

Source: BEE guide book.

Data shows if the approach of cooling tower increases there is increase in the TR capacity (Rejection capacity) of cooling tower subject to hydraulic load bearing capacity. It shows if cooling tower is selected at lower approach can give higher heat rejection at higher approach. Also the cost of cooling tower will increase if the approach of the cooling tower is selected at lower. Based on the process requirement and life cycle cost approach of cooling tower needs to be decided.

Suppose if we require cooling water for condensation of water vapour at atmospheric pressure, we may select cooling tower at higher approach to reduce capital cost. While if we need cooling tower for refrigeration application, in this case if approach is low there will be power saving in refrigeration system. In case of refrigeration system we should consider lower approach cooling tower.

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