A chiller uses a refrigeration cycle to cool water. This chilled water then cools a larger area, such as a factory floor. Cooling equipment in this matter increases its efficiency by providing a steady thermal environment. You can derive a chiller's capacity from the temperature drop it creates and the volumetric flow of the water within it. This formula produces the chiller's capacity in British Thermal Units (BTUs). That scale directly corresponds with the more common unit of "tons" that cooling systems often use.

Subtract the temperature of water as its leaves the chiller from the temperature of the water as it enters it. If water enters the chiller at 64 degrees Fahrenheit and leaves at 51 degrees Fahrenheit: 64  51 = 13 degrees.

Multiply the result by your flow rate, which is measured in gallons per minute. If, for instance, the chiller must move 300 gallons per minute: 13 x 300 = 3,900 gallons per minute.

Multiply your answer by 500. This converts the volumetric flow rate to a mass flow rate, which is measured in pounds of water per hour. With this example: 3,900 x 500 = 1,950,000. This answer is the chiller's capacity measured in BTUs per hour.

Divide your answer by 12,000: 1,950,000 / 12,000 = 162.5. This is the chiller's capacity in tons.
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References
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Resources
 "HVAC Equations, Data, and Rules of Thumb"; Arthur A. Bell; 2007
 American Hermetics: Selection Guide