Wednesday, May 24, 2017

Chiller Refrigeration Tons

The process of cooling is called refrigeration. Refrigeration or cooling capacity can be measured in 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)
where
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:
Cooling load = 500 (1 US gal/min) (10 oF) / 12000
    = 0.42 ton

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