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Subcooling

Subcooling is the difference between condensing temperature and the actual temperature of the liquid line.  It is the process of cooling refrigerant below its condensing temperature at a given pressure.  In other words, subcooling is refrigerating the refrigerant.

To determine how much subcooling an operating system has:

  • First, find the condensing temperature.  This is found by taking the head pressure, and by using a T & P chart, converting head pressure to temperature.
  • Second, the actual temperature of the liquid line needs to be taken as near to the inlet of the expansion device as possible.  A good electronic thermometer probe securely attached to a clean portion of the liquid line, insulated to prevent ambient influence, should be used to get the temperature measurement.  Subtract the actual liquid line temperature from the condensing temperature.  The answer is the degrees of subcooling the system has.

Systems that have no receiver, or any special devices to increase subcooling, should have about 10°F subcooled liquid refrigerant.  Systems that use regular receivers will have little or no subcooling.  Since receivers have liquid and vapor present together in them, subcooling will be lost in receivers.

One reason for subcooling the refrigerant is to give it more heat removal capacity.

A compressor is a fixed displacement pump.  It pumps a certain number of pounds of refrigerant through the system and doesn’t care how many BTU’s are in a pound.  Subcooling increases the number of BTU’s each pound of refrigerant can absorb.  Increasing the number of BTU’s per pound increases the capacity of the system.

Another reason to subcool the system refrigerant is to eliminate flash gas.  Flash gas, that is, vapor mixed in with liquid refrigerant that enters an expansion device, can cut system capacity.  All expansion devices rely on a solid head of liquid at their inlet to perform properly.

Occasionally a liquid line is improperly sized, or extended, creating a large pressure drop that causes flash gas to form.  A system is slightly undersized.  Perhaps a load was added to equipment that was marginal to begin with.  Whatever the reason, a subcooling heat exchanger may save the system.  These inexpensive devices really consist of simply welding the liquid line to the suction line.  See Figure 1. Because of system variables, the degrees of subcooling one of these devices will produce when installed can only be empirically determined.

Figure 1.

Another way to subcool is called “ambient subcooling.”  Additional passes may be added to a condenser and called a special subcooling circuit.  An entire ambient subcooler may be added to a system.  These can be effective, but have one major drawback.  When the ambient gets warm, they simply don’t work!

The most effective, but most expensive, way to subcool a liquid line is with another refrigeration system.  Typically, an air conditioning high temperature system is used to subcool the liquid line of a low temperature system.

In the 1970’s and early 1980’s, because of the energy crisis, these systems showed great promise of large energy savings by transferring part of the load of a low temperature system to a high temperature system through a subcooler, which essentially is the high temperature’s system evaporator.  On paper, these systems showed great payback.  Unfortunately, problems arose, such as leaking subcoolers which then let two different refrigerants mix, ruining both systems, high initial installed cost, rooftop A/C units that can’t have subcoolers added to them, and other problems, too many to detail.  Not many of these systems were installed or are being installed.   Figure 2 shows a typical system schematic of such a system.

Figure 2.

Other methods of increasing efficiency through subcooling have and are being tried.  Receivers have been used as surge tanks.  Surge tanks with solenoid and check valve systems have been tested by alternating the tank’s use as a surge tank or receiver based on ambient temperature.  This can be used to take advantage of increased subcooling when applicable.

Suffice it to say that subcooled liquid refrigerant at the entrance to an expansion device is a necessity for proper operation of a refrigeration system.  Manufacturers will note the amount of subcooling needed to get the published capacity of a unit.

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