Trane SYS-APM001-EN manual System Design Options, ΔT1 = 94.2 - 78 = 16.2F or 34.6 - 25.6 = 9.0C

System Design Options

Q= U x A1 x ΔT1, where A = area,

U = coefficient of heat transfer, and ΔT = temperature difference

so, for a roughly equivalent heat rejection,

U x A1 x ΔT1= U x A2 x ΔT2

and for a constant coefficient of heat transfer,

A1 x ΔT1 = A2 x ΔT2

Using standard rating conditions, the temperature difference between tower entering temperature and ambient wet bulb, ΔT1 is

ΔT1 = 94.2 – 78 = 16.2°F or [34.6 – 25.6 = 9.0°C]

while at typical low-flow conditions, ΔT2 is

ΔT2 = 99.1 – 78 = 21.1°F or [37.3 – 25.6 = 11.7°C]

Therefore:

A1 × 16.2 = A2 × 21.1 or A2 = 0.77 A1

So, the tower would theoretically need only 77% of the heat exchange area to achieve the same heat rejection capacity, simply by reducing the flow rate from 3.0 gpm/ton [0.054 L/s/kW] to 2.0 gpm/ton [0.036 L/s/kW].

The heat exchange capacity can be altered by changing the surface area or airflow, or some combination of the two. A cooling-tower manufacturer’s selection program can give the exact size and power requirements. In the example previously summarized on pages 30-32,both the cooling-tower size and airflow (hence, required fan power) were reduced.

Same tower, smaller approach

Another option is to use the same cooling tower at a lower flow rate. In a new system, this is a design decision, but in an existing system, it is often a constraint that the tower cannot be changed. Given the same heat-rejection load, the low-flow system allows the cooling tower to return colder water; that is, the tower's approach to the ambient wet-bulb temperature decreases. In the previous example of 450 tons [1580 kW], the same cooling tower would have resulted in a leaving tower-water temperature of 83.5°F [28.6°C] instead of the 85°F [29.4°C] with the smaller cooling tower. It is important to realize that the entering temperature for the tower would be approximately 97.6°F [36.4°C]. Therefore, the effect of reduced flow rate on chiller energy consumption is partially offset by the lower leaving tower-water temperature. The system would use less pump energy at the lower flow conditions.

SYS-APM001-EN Chiller System Design and Control 35