Chiller Capacity and Chilled Water Flow Calculator (Preliminary)

How is chiller capacity decided?

A chiller removes the heat collected from a building through chilled water and rejects it outdoors. Its capacity is chosen for the largest cooling load that occurs at one time. Because the spaces peak at different hours, that load is smaller than the sum of the peaks. The ratio is the diversity factor:

Q = Q_p × e × (1 + s)

Capacity is quoted in kW or tons of refrigeration (TR): 1 TR = 3.517 kW = 12 000 BTU/h.

Chilled water flow

Heat carried by water is Q = ρ × c_p × V × Δt, so the flow is V = Q / (ρ × c_p × Δt). In the common 7/12 °C regime the temperature difference is 5 K, which takes about 0.172 m³/h per kW. A larger temperature difference means less flow, smaller pipes and less pump power. In US units the same balance is the familiar 2.4 gpm per ton at 10 °F.

Example

With peak loads adding up to 500 kW, 90 percent diversity and a 10 percent margin, the required capacity is 500 × 0.9 × 1.1 = 495 kW, or 141 TR. At 5 K the chilled water flow is 85 m³/h. At a COP of 3 the compressor draws 165 kW and the condenser rejects 660 kW.

Condenser side

A chiller rejects the heat it removes plus the compressor work: Q_cond = Q × (1 + 1 / COP). In water cooled plant the cooling tower and condenser water pump are sized for that heat. To find condenser water flow, enter this value in place of capacity in the second tab.

Glycol systems

Adding glycol for freeze protection lowers specific heat. More flow is needed to carry the same heat and pressure drop rises. Enter density and specific heat for the mixture ratio and temperature from manufacturer data.

Limits

This is a preliminary estimate. Load calculation, part load efficiency, ambient conditions, redundancy and the number of machines call for engineering work.