Answer to Question #172734 in Mechanical Engineering for randy

Question

The evaporative condenser of an ammonia refrigeration plant has a water flow rate of 230 kg/s and enters the natural draft cooling tower at 40 C. The water is cooled to 30 C by air entering at 36 C db and 38% RH. The air leaves the tower as saturated at 45 C db. Calculate:

(a) Amount of makeup water in kg/hr.

(b) The cooling tower efficiency (%).

Answer 1

a) Water make-up (M ) = Total water losses = Drift Losses ( D) + Evaporation Losses (E ) + Blow down Losses (B)

D = 0.1 to 0.3 percent of Circulating water (C ) for an induced draft cooling tower

C = Circulating water in m³/hr

 $\lambda$ = Latent heat of vaporization of water = 540 kcal/kg or 2260 kJ / kg

T_i – T₀ = water temperature difference from tower top to tower bottom in °C ( cooling tower inlet hot water and outlet cold water temperature difference)

C_p = specific heat of water = 1 kcal/kg / °C (or) 4.184 kJ / kg / °C

C = 230 kg/s = (230 kg / 998 kg/m³) / (1/3600 h) = 830 m³/h

E = (830 m³/h x (40 - 30°C) x 4.184 kJ / kg / °C ) / 2260 kJ / kg = 15.4 m³/h

M = 830 x 0.1 + 15.4 = 98.4 m³/h or 98 203 kg/h

b) Cooling tower efficiency can be expressed as

μ = (t_i - t_o) 100 / (t_i - t_wb)               

where

μ = cooling tower efficiency (%) - the common range is between 70 - 75%

t_i = inlet temperature of water to the tower (^oC)

t_o = outlet temperature of water from the tower (^oC)

t_wb = wet bulb temperature of air (^oC, ^oF)

μ = (40 - 30) / (40 - 26) * 100 = 71.4%