Trane SYS-APM001-EN manual Cooling-tower options with low flow, Smaller tower, System design, 15.6

System Design Options

performance of this coil when it is selected with a 44°F [6.7°C] entering fluid temperature and a 10°F [5.6°C] fluid temperature rise (ΔT). To provide the required 525 MBh [154 kW] of cooling capacity, the coil requires 105 gpm [6.6 L/s] of water.

The right-hand column shows the performance of same coil, but in this case it is selected with 40°F [4.4°C] entering fluid and a 15.6°F [8.7°C] ΔT. To provide the equivalent capacity, the coil requires only 67.2 gpm [4.2 L/s] of water.

Table 9. Impact of supply temperature and flow rate on cooling coil selection

	“Conventional”	“Low flow”
	system design	system design
Coil face area, ft2 [m2]	29.01 [2.69]	29.01 [2.69]
Face velocity, fpm [m/s]	448 [2.3]	448 [2.3]
Coil rows	6 rows	6 rows
Fin spacing, fins/ft [fins/m]	85 [279]	85 [279]
Total cooling capacity, MBh [kW]	525 [154]	525 [154]
Entering fluid temperature, °F [°C]	44 [6.7]	40 [4.4]
Leaving fluid temperature, °F [°C]	54 [12.2]	55.6 [13.1]
Fluid ΔT, °F [°C]	10 [5.6]	15.6 [8.7]
Fluid flow rate, gpm [L/s]	105 [6.6]	67.2 [4.2]
Fluid pressure drop, ft H2O [kPa]	14.0 [41.8]	6.3 [18.8]

By lowering the entering fluid temperature, this coil can deliver the same cooling capacity with 36% less flow, at less than half of the fluid pressure drop, with no impact on the airside system.

Cooling-tower options with low flow

Smaller tower

Like coils, cooling towers are heat exchangers—although often misunderstood heat exchangers. The tower exchanges heat between the entering (warmest) water temperature and the ambient wet-bulb temperature. Therefore, in a new system or when a cooling tower is replaced, a low-flow system design allows a smaller, more efficient cooling tower to be selected. How is this possible?

Keep in mind that a cooling tower is not limited to a specific tonnage. A cooling tower is a heat exchanger that exchanges heat between the entering water temperature and the ambient wet bulb. By varying the flow or the temperature, the tower capacity can be changed—often increased.

Since the amount of heat to be rejected, Q, is approximately the same in standard-rating-condition and low-flow systems, we can estimate the heat exchange area necessary to reject the heat:

34 Chiller System Design and Control SYS-APM001-EN