Page 213 - Process Equipment and Plant Design Principles and Practices by Subhabrata Ray Gargi Das

7.2 Cooling tower 211

There are two possible causes for an increase in range:
- The inlet (to the cooling tower) water temperature is increased (and the cold-water temperature at
the exit remains the same). In this case, it is economical to invest in removing the additional heat
picked up.
- The exit water temperature is decreased (and the hot water temperature at the inlet remains the
same). In this case, the tower size would have to be considerably increased because the approach
is also reduced (discussed later), and this is not always economical.
3
Capacity - Heat rejected (Q kcal/hr), and water circulation rate (C m /hr) indicates the capacity of
cooling towers. Q in kcal/hr is a product of mass flow rate of water, specific heat (C p ¼ 1000 kcal/

mt. C for water) and temperature difference (R)in C.

3 3

Q ¼ C m=hr C p R ¼ C m=hr C p ðT h T c Þ (7.2)
Heat load imposed on a cooling tower is determined by the process being served. Typical numbers
used to estimate cooling load for some common equipment/devices are presented in Table 7.1.

Table 7.1 Typical cooling capacity requirement for different equipment.
Equipment Cooling load requirement
Air compressor
- Single-stage 130 kcal/kW$hr
- Single-stage with aftercooler 860 kcal/kW$hr
- Two-stage with intercooler 518 kcal/kW$hr
- Two-stage with intercooler and aftercooler 860 kcal/kW$hr
Refrigeration, compression 65 kcal/min/ton refrigeration load
Steam turbine condenser 555 kcal/kg of steam
Diesel engine, four stroke 880 kcal/kW$hr
2
Natural gas engine, four stroke 1525 kcal/kW$hr (for 18 kg/cm compression)

Wet-bulb temperature e The amount of cooling that can be obtained from a cooling tower de-
pends on the relative humidity or the wet-bulb temperature of the air. Irrespective of the size of cooling
tower, range or heat load, it is not possible to cool water below the wet-bulb temperature of air with
evaporative cooling. Thus, the wet-bulb temperature of air entering the cooling tower determines the
minimum operating temperature for the process or system that uses cooling water. In practice T c is

always a few degrees above T amb;WBT , typically a minimum of 4 C. However, it is theoretically
possible to cool water to temperatures below the wet bulb temperature of the ambient air since air on
entering the cooling tower gets heated and can hold more moisture.