The amount of liquid subcooling a refrigeration or air conditioning contains not only affects system capacity but also the effectiveness and capacity of expansion type metering devices. In today's competitive service market, every conscientious service technician should understand why a refrigeration or air conditioning system must have the proper amount of liquid subcooling.
This article will compare subcooling amounts in a refrigeration system for three scenarios: an overcharge of refrigerant, a dirty condenser, and air in the system.
Before diving into these scenarios, it is first important to understand the definition of liquid subcooling. Any sensible heat taken away from the 100% saturated liquid point in the condenser can be defined as liquid subcooling. Subcooling is defined as the difference between the measured liquid temperature and the liquid saturation temperature at a given pressure. Total liquid subcooling occurs from the start of the 100% saturated liquid point in the condenser to the metering device.
The saturated liquid temperature can be obtained from a pressure/temperature relationship chart using the condensing pressure. This means that as soon as all of the saturated vapor in the condenser changes phase to saturated liquid, subcooling will start to occur if any more heat is taken away. Remember, this is now a temperature change or sensible heat change. So, any drop in temperature of the liquid below the saturation temperature for the pressure at that point will be considered liquid subcooling.
Condenser subcooling is the liquid subcooling present in the last passes of the condenser’s bottom. It can be measured by subtracting the actual condenser liquid out temperature from the saturation temperature measured at the condenser outlet:
Condensing temperature (saturated) - Condenser liquid out temperature = Condenser subcooling
When subcooled, the refrigerant is not generating or losing any vapor pressure, so there is no pressure/temperature relationship, and a pressure/temperature chart cannot be used. Thus, the condenser outlet temperature has to be measured with some sort of temperature measuring device fastened to the condenser outlet.
The saturated temperature, on the other hand, can be acquired from the condensing pressure read from a pressure gauge located near the condenser outlet. This phenomenon happens because a pressure/temperature relationship does exist in a saturated condition. If pressure drops exist in the system, the pressure must be measured where the temperature was taken to get an accurate liquid subcooling amount.
The system check below shows an R-134a refrigeration system with an overcharge of refrigerant.
|Compressor discharge temperature||240°F|
|Condenser outlet temperature||90°F|
|Evaporator outlet temperature||30°F|
|Compressor in temperature||40°F|
|Evaporating (low-side) pressure||8.8 psig (20°F)|
|Condensing (high-side) pressure||172 psig (120°F)|
Notice the 30°F of liquid subcooling backed up in the condenser in this overcharged system. Because of this overcharge, the condenser will have too much liquid backed up in its bottom, causing high condenser subcooling. With an overcharge, 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.
Remember, most conventional condensers are designed to:
- Desuperheat compressor discharge vapors;
- Condense these vapors to liquid; and
- Subcool refrigerant at its bottom.
The system check below shows a refrigeration system with a dirty condenser, causing restricted airflow over the condenser.
|Compressor discharge temperature||250°F|
|Condenser outlet temperature||110°F|
|Evaporator outlet temperature||10°F|
|Compressor inlet temperature||25°F|
|Refrigerated space temperature||15°F|
|Evaporating (low-side) pressure||6.2 psig (0°F)|
|Condensing (high-side) pressure||186.5 psig (125°F)|
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 in the above system check for a dirty condenser, there is a higher-than-normal condenser subcooling of 15°F. This system check sheet looks somewhat similar to an overcharge of refrigerant because of the increased subcooling amounts, but do not be fooled by it. When 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.
A similar condition would be a defective condenser fan motor starving the condenser of air. Both conditions would cause the head pressure, 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.
Air in the System
Another similar scenario would be a refrigeration system containing air or other non-condensables. Air is a non-condensable gas and will get trapped in the top of the condenser and not condense to a liquid. 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.
The system check below for air in the system shows 40°F of condenser subcooling, but these amounts will vary depending on the amount of air in the system.
|Compressor discharge temperature||235°F|
|Condenser outlet temperature||85°F|
|Evaporator outlet temperature||17°F|
|Compressor in temperature||40°F|
|Evaporating (low-side) pressure||8.8 psig (5°F)|
|Condensing (high-side) pressure||185.5 psig (125°F)|
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.
Air can enter a refrigeration system through a variety of ways, including:
- Leaks of tubing, gaskets, or flanges;
- Poor charging procedures;
- Poor recovery or recycling procedures; and/or
- Technicians forgetting to purge hoses when accessing the system.
When air gets into a system, it will collect in the top of the condenser and be trapped. Air is a non-condensable and cannot be condensed like refrigerant vapors. The liquid seal (subcooled liquid) at the condenser’s bottom will prevent air from leaving the condenser. Air will cause a reduction of condensing surface area and cause high condensing (head) pressures.
Air can enter the system through a leak in the low side of the refrigeration system, and these leaks will eventually lead to undercharged system. Severely undercharged systems will run vacuums in the low side of refrigeration system. These vacuums will suck in air from the atmosphere because of the system's low-side pressure being lower than the atmospheric pressure.
In conclusion, when a refrigeration system has high head pressure and high condenser subcooling, the service technician must not always assume an overcharge of refrigerant. Many times the liquid at the condenser’s bottom is simply losing heat faster, causing more condenser subcooling. Often the 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 being hotter and simply losing heat faster.