ACHRNEWS

The Professor: System Check Sheets

May 7, 2012
System Check Sheets 1
While my last series of columns explored liquid subcooling in a refrigeration system, this article will compare subcooling amounts in a refrigeration system incorporating an overcharge of refrigerant, a dirty condenser, and air in the system.

Note that all of the system check sheets used as samples in this article incorporate R-134a as a refrigerant. These systems are refrigeration systems with a thermostatic expansion valve (TXV) as a metering device with receivers.

Overcharged System

Table 1 shows an R-134a refrigeration system with an overcharge of refrigerant. Notice the 30 degrees of liquid subcooling backed up in the condenser. Because of the overcharge of refrigerant, the condenser will have too much liquid backed up in its bottom, causing high condenser subcooling. By overcharging a system with too much refrigerant, increased liquid subcooling amounts will be realized in the condenser.

However, just because a system has increased subcooling amounts in the condenser doesn’t necessarily mean the system is overcharged. This will be explained in the next two system checks. Remember, the condenser is where refrigerant vapor is condensed and liquid refrigerant is formed. This backed-up subcooled liquid at the condenser’s bottom will take up valuable condenser volume, leaving less volume for desuperheating and condensation of refrigerant vapors.

Too much liquid subcooling at the condenser’s bottom will cause unwanted inefficiencies by raising the head pressure and the compression ratio. Higher compression ratios cause lower volumetric efficiencies and lower mass flow rates of refrigerant through the refrigeration system. Higher superheated compressor discharge temperatures will also be realized from the higher heat of compression caused from the high compression ratio. (For a review of how much condenser subcooling is needed in a system, see “The Needed Amount of Condenser Subcooling,” in the April 2, 2012, edition of The NEWS.)

Remember, most conventional condensers’ functions are to:

• Desuperheat compressor discharge vapors,

• Condense these vapors to liquid, and

• Subcool refrigerant at its bottom.

System with Dirty Condenser

Table 2 shows a refrigeration system with a dirty condenser causing restricted airflow over the condenser. A similar condition would be a defective condenser fan motor starving the condenser of air. Both conditions caused the head pressure and thus condensing temperature to increase. Even the liquid at the condenser’s bottom will be hotter because of the elevated condensing temperatures. This creates a greater temperature difference between the liquid at the condenser’s bottom and the ambient (surrounding air) designed to cool the condenser and its liquid. This will cause the liquid at the condenser’s bottom to lose heat faster, causing more condenser subcooling. In this example, high condenser subcooling is not caused from an “amount” of liquid being backed up in the condenser, but from the liquid in the condenser’s bottom simply losing heat faster.

This phenomenon happens because the temperature difference between the liquid at the condenser’s bottom and the surrounding ambient is the driving potential for heat transfer to take place. As more and more air is restricted from flowing through the condenser, the amount of condenser subcooling will increase.

Notice that the system check sheet shows higher than normal condenser subcooling of 15˚. This system check sheet looks very similar to an overcharge of refrigerant because of the increased subcooling amounts, but do not be fooled by it. When a high head pressure and high condenser subcooling is experienced in a refrigeration system, the service technician must not assume an overcharge of refrigerant. The technician must first check to see if the condenser is dirty or a condenser fan is inoperative because of similarities of symptoms in both scenarios of an overcharge of refrigerant and restricted airflow over the condenser.

System Containing Air

Another similar scenario would be a refrigeration system containing air, as in Table 3. Air is a noncondensable and will get trapped in the top of the condenser. This will cause high head pressures and high condensing temperatures because of reduced condenser volume to desuperheat, condense, and subcool. Thus, the liquid at the condenser’s bottom will be hotter than normal and will lose heat faster to the ambient. This will result in an increase in condenser subcooling.

Table 3 shows 40˚ of condenser subcooling, but these amounts will vary depending on the amount of air in the system. Again, in this example, high condenser subcooling is not caused from an “amount” of liquid being backed up in the condenser, but from the liquid in the condenser’s bottom simply losing heat faster.

Publication date: 05/07/2012