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Calculating Cooling Loads

Calculating chiller and cooling tower refrigeration loads - in tons.

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Cooling towers tons pairs the water-cooled chiller tons and the water-cooled condenser tons. 

Chiller Refrigeration Tons

The process of cooling is called refrigeration. For commercial and industrial refrigeration systems most of the world uses the kilowatt (kW) as the basic unit of refrigeration. Some commercial and industrial refrigeration systems are rated in Tons of Refrigeration (TR, tons).

A water-chiller refrigeration ton is defined as:

1 Refrigeration Ton (RT) = 1 TONScond = 12000 Btu/h = 200 Btu/min = 3025.9 k Calories/h = 12661 kJ/h = 3.517 kW

1 kW = 0.2843 Refrigeration Ton (RT)

A ton is the amount of heat removed by an air conditioning system that would melt 1 ton (2000 lbs.) of ice in 24 hours. The heat required to melt 1 lb of ice at 32 oF to water is 144 Btu.

1 Ton Refrigeration = (2000 lb) (144 Btu/lb) / (24 hr)

    = 12000 Btu/hr

Converting between Btu/h and Tons of Refrigeration

Btu/hRefrigeration TonkW
6000 1/2 1.76
12000 1 3.52
18000 1 1/2 5.28
24000 2 7.03
30000 2 1/2 8.79
36000 3 10.6
42000 3 1/2 12.3
48000 4 14.1
54000 4 1/2 15.8
60000 5 17.6

Cooling Tower Tons

A cooling tower ton is defined as:

1 cooling tower ton = 1 TONSevap = 1 TONScond x 1.25 = 15000 Btu/h = 3782 k Calories/h = 15826 kJ/h = 4.396 kW

The equivalent ton on the cooling tower side actually rejects about 15000 Btu/h due to the heat-equivalent of the energy needed to drive the chiller's compressor. This equivalent ton is defined as the heat rejection in cooling 3 U.S. gallons/minute (1500 pound/hour) of water 10°F, which amounts to 15000 Btu/hour, or a chiller coefficient-of-performance (COP) of 4.0 - a COP equivalent to an energy efficiency ratio (EER) of 13.65.

Heat Load and Water Flow

A water systems heat load in Btu/h can be simplified to:

h = cp ρ q dt

    = (1 Btu/lbm oF) (8.33 lbm/US gal) q (60 min/h) dt

    = 500 q dt                                                                  (1)


h = heat load (Btu/h)

cp = specific heat, 1 (Btu/lbm oF) for water

ρ = 8.33 (lbm/US gal) for water

q = water volume flow rate (US gal/min)

dt = temperature difference (oF)

Example - Water Chiller Cooling

Water flows with 1 gal/min and 10oF temperature difference. The ton of cooling load can be calculated as:

h = 500 (1 US gal/min) (10 oF)

    = 5000 Btu/h

    = (5000 Btu/h) / (12000 Btu/ton)

    = 0.42 ton

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