Determining the Chiller Size

Chiller size is specified by the quantity of heat they can remove per unit of time, which depends on the heat characteristics of the application. Calculating the size required may seem daunting, but it really is quite simple as long as the formula is known and a few other facts. However, the chiller manufacturer will be willing to assist and even perform all of the calculations to accurately size the chiller for your specific process.

There is a an easy to follow formula for determining the size of the chiller you require. However, there are a few factors you must know before you begin:

• The incoming water temperature
• The water temperature required
• The flow rate

General sizing formula:

1. Calculate Temperature Differential (ΔT°F) ΔT°F = Incoming Water Temperature (°F) – Required Chill Water Temperature
2. Calculate BTU/hr. BTU/hr. = Gallons per hr x 8.33 x ΔT°F
3. Calculate tons of cooling capacity Tons = BTU/hr. ÷ 12,000
4. Oversize the chiller by 20% Ideal Size in Tons = Tons x 1.2
5. You have the ideal size for your needs

For example, what size chiller is required to cool 3GPM from 72°F to 62°F?

1. ΔT°F = 72°F – 62°F = 10°F
2. BTU/hr. =3 gpm x 60 x 8.33 x 10°F =14,994 BTU/hr.
3. Ton Capacity = 14,994 BTU/hr. ÷ 12,000 = 1.2495 Tons
4. Oversize the chiller = 1.2495 x 1.2 =1.4994 Tons
5. A 1.5 Ton (5256 Watts) Chiller is required

Chiller capacity is specified in most often tons but can be watts as well. The size specified often is a measure of heat capacity in BTU’s. A refrigeration ton is approximately equivalent to 12,000 BTU/h. Benchtop chillers which are frequently used in labs are available in capacities of one-quarter ton (876 Watts) to one-third ton (1051Watts) which is  3,000 and 4,000 BTUs.