What Should We Know About Subcooling?
By Robert Hyde


"Both the subcooling method and the booster pump method were effective in eliminating flash gas, however, only the booster pump could return the expansion valve to it's normal capacity."

-- Department of Mechanical Engineering, Kings College, London, England

The study at Kings College incorporated one of the best laboratory tests on the effects of operating a refrigeration system with reduced head pressures. True energy conservation is achieved by increasing the efficiency of the refrigeration or Air Conditioning system. The most direct method is to reduce the condensing temperature and pressure that the compressor must pump against.

Table 10 represents the KW per ton of cooling at different condensing temperatures. It is representative of an ideal refrigeration cycle. Notice that the percentage reduction in KW per ton from 120 degrees F to 100 degrees F equals slightly more than 1.5% per degree.

Table 10
Effect of Temperature on Theoretical Horsepower per Ton

Refrigerant 12 Theoretical Horsepower per Ton of Refr.
           Condensing Temperature

Evap. Temp F 60 F 80 F 100 F 120 F
-20 1.03 1.38 1.80 2.32
0 0.71 1.01 1.37 1.80
20 0.44 0.70 1.00 1.36
40 0.20 0.43 0.68 0.99

Refrigerant 22 Theoretical Horsepower per Ton of Refr.
           Condensing Temperature

Evap. Temp F 60 F 80 F 100 F 120 F
-20 1.02 1.35 1.75 2.22
0 0.71 1.00 1.34 1.75
20 0.44 0.69 0.99 1.34
40 0.20 0.43 0.68 0.98

(This information is also shown in graphical format.)

Normal minimum condensing temperature (head pressures) for air conditioning is 105 degrees F and for refrigeration system 86 degrees F. When considering the energy cost of running the compressors without addressing the generation of "flash gas" before entering the expansion device, these condensing temperatures have been found to be the most optimum condensing pressures. Floating below these condensing temperatures, for AC or Refrigeration, requires some method to reduce the presence of the generated "flash gas" before the expansion valve. The expansion valve capacity must also be maintained.

The use of subcooling is probably the best known method of suppressing the generation of "flash gas." Subcooling can be done by reducing the temperature of the liquid below the boiling point at its existing pressure. Subcooling can also be accomplished by increasing the pressure of the liquid refrigerant above the flash point without increasing the temperature of the liquid.

To maintain the rated capacity of the TXV (thermostatic expansion valve) it is necessary to eliminate the presence of vapor refrigerant displacing the liquid refrigerant at the TXV orifice. It is also necessary to consider both the effects of reduced temperature of the liquid and the pressure drop across the orifice of the valve. The reduction of liquid temperature can be accomplished by conventional subcooling methods. Only the use of a refrigerant pump allows for reduced condensing temperatures and addresses the pressure boost needed at the TXV to increase the mass flow through the TXV, thus maintaining full capacity of the TXV.


Effects of Pump Pressure on TXV capacity

The amount of liquid refrigerant flow (mass flow) into the evaporator will determine the capacity of the evaporator and the pounds of vapor the compressor pumps, We see from Table 10 that if the condensing temperature is reduced, the capacity of the compressor will increase. The expansion valve must also be able to feed enough refrigerant to sustain this increased capacity. The most significant loss of TXV capacity will occur from the formation of "flash gas" ahead of the TXV, displacing the solid liquid refrigerant at the valve orifice. However along with the presence of vapor other factors will effect the valve capacity. Both the temperature of the liquid refrigerant and the pressure drop across the TXV will effect the capacity of the valve.

To a first approximation the refrigerant mass flow rate through an orifice with a pressure difference of Pc- Pe is obtained from Bernoulli's equation, i.e.,

Mr=CdA(2 (Pc-Pe) / v1) 0.5

Where:
Mr=Mass flow rate of refrigerant
Cd=discharge coefficient
A=Area of the orifice
V1=specific volume of the refrigerant.

The expansion valve published ratings from manufacturers are almost identical to the engineering equations for both corrections due to liquid temperature and to the mass flow through the valve due to the difference in pressure drop across the orifice By looking at these charts we see clearly the capacity increase due to the pressure increase.

                  Pressure Drop Across Valve
                   40 F

Nomimal Capacity (Tons) 75 100 125 150 175 200
1/5 0.17 0.20 0.22 0.24 0.26 0.28
1/4 0.22 0.25 0.28 0.31 0.33 0.35
1/3 0.30 0.35 0.39 0.43 0.46 0.49
1/2 0.39 0.45 0.50 0.55 0.60 0.64
3/4 0.65 0.75 0.84 0.92 0.99 1.06
1 0.87 1.00 1.12 1.22 1.32 1.41

Refrigerant Liquid Temperature Correction Factors

Refrigerant Liquid Temperature F 0 10 20 30 40 50 60 70 80 90 100 110
Correction Factors 1.55 1.51 1.45 1.40 1.34 1.29 1.23 1.17 1.12 1.06 1.00 0.94

EXAMPLE:
Typical cooling system-
One ton R-22 valve (Nominal Capacity = 0.87 tons)
Evaporator temperature = 40 degrees F
80 degree condensing temperature = 143.6 psi
Pressure at the outlet of the valve = 68.51 psi
Pressure drop across the valve = 75 psi
Valve Capacity=(Nominal Valve Capacity) X (Refrigerant Temperature Correction Factor)
Valve capacity = 0.87 tons X 1.12 = 0.97 tons

The same cooling system with a Hy-Save Liquid Pressure Amplifier-
One ton R-22 valve (Nominal Capacity = 0.87 tons)
Evaporator temperature = 40 degrees F
80 degree condensing temperature = 143.6 psi
Pressure at the outlet of the valve = 68.51 psi
Pressure drop across the valve =75 psi + 25 psi from LPA = 100 psi
Valve Capacity = 1 ton X 1.12 = 1.12 tons

From the preceding example, we find that the addition of the liquid pump pressure will increase the capacity of the TXV. It not only eliminates the development of "flash gas" prior to the TXV, but it increases the TXV capacity through the increased pressure at the valve inlet.

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The chart at the right illustrates the capacity increase as result of a 20 psi increase provided by the liquid refrigerant pump. Although an increase in capacity is achieved at all condensing temperatures, notice the greatest increase is shown at the lower condensing temperatures. It is at the lower condensing temperature where the increased valve capacity is needed most.

An under feeding TXV will result in inferior heat exchange in the evaporator and poor oil return. The increase in the TXV capacity due to pumping will greatly enhance the oil return to the compressor. Oil return through and from the evaporator is dependent upon the velocity of the vapor refrigerant to carry the oil back to the compressor. A full feeding TXV helps assure that this velocity is at its peak.

In the semi-hermetic or hermetic compressor the motor cooling is dependent upon the cool returning vapors to dissipate heat from motor windings. The underfeeding of the TXV can result in highly superheated vapor leaving the evaporator. This superheated vapor entering the compressor adds directly to the high discharge temperatures.This condition at the TXV will also result in a loss of capacity at the compressor and force the compressor to work at a diminished Saturated Suction Temperature. As an example, compare the compressor capacity rating at 42 F SST and 40 F SST.

6DH3-3500 @ 40 SST=390365 BTU/H with an EER of 14.2
6DH3-3500 @ 42 SST=406503 BTU/H with an EER of 14.7

As the above values illustrate, under feeding of the TXV in this case would also result in an additional loss of compressor capacity by the reduced inlet pressure of the vapor.

There are many types of subcooling which will be discussed, but we now have learned that true energy reduction can best be obtained by making sure that we also maintain the mass flow through the expansion valve. Boosting the liquid refrigerant pressure with a pump will result in maintaining the valve capacity and the overall efficiency of the system.