Just like refrigerator a chiller uses a refrigeration cycle to cool water or dehumidifies air. This chilled water then used to cool a larger space, such as a factory floor or for process uses. Cooling equipment in this matter increases its efficiency by providing a steady thermal environment. Before we begin to derive a chiller's capacity we must know three variables, they are:

1. The incoming water temperature

2. The chilled water (out going water) temperature required

3. The flow rate

This formula produces the chiller's capacity in British Thermal Units (BTU’s). That scale directly corresponds with the more common unit of "tons" that cooling systems often use.

For our example, we will calculate what size chiller is required to cool 60 GPM (gallons per minute) from 64 °F to 52 °F? Use the following five steps and general sizing formula:

**1. Calculate Temperature Differential (ΔT°F)**

Subtract the temperature of water as its leaves the chiller from the temperature of the water as it enters it.

ΔT°F = Incoming Water Temperature (°F) - Required Chilled Water Temperature.

· Example: ΔT°F = 64°F - 52°F = 12°F

2.

**Calculate BTU/hr.**(**British Thermal Unit/hr****)**
The cooling capacity of air conditioners is measured in BTUs, or British thermal units. The more Btu of capacity, the larger the room the air conditioner can cool. Scientifically, ‘one Btu is the amount of energy required to change the temperature of 1 pound of water by 1 degree Fahrenheit’. In terms of air-conditioner capacity, the rule of thumb is that it takes around 25 Btu to cool 1 square foot of room floor area.

To calculate BTU Multiply the temperature difference by the flow rate (that what we need), which is measured in gallons per minute. If, for instance,

BTU/hr. = Gallons per hr x 8.33 x ΔT°F

· Example: 60 gpm x 60 x 8.33 x 12°F = 3,59,856 BTU/hr.

(Or)

· 40 gpm x 500 x 12°F = 3,59,856 BTU/hr. (since 60 x 8.333 = 500)

**3. Calculate tons of cooling capacity**

Tons = BTU/hr. ÷ 12,000

· Example: Ton capacity = 239,904 BTU/hr. ÷ 12,000 = 29.998 tons

1 ton = 12000 BTU

**4**.

**Oversize the chiller by 20%**

Ideal Size in Tons = Tons x 1.2

· Example: 29.998 x 1.2 = 35.9976

**5. You have the ideal size for your needs**

· Example: a 35.9976 (or 35-Ton) chiller is required

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