One function of a metering device is to reduce the refrigerant’s liquid pressure from its high side to its low side. This sudden reduction in pressure causes a percentage of liquid refrigerant to instantly vaporize (flash) to a vapor.

The vapor that is formed is called flash gas. As the liquid refrigerant “flashes” to a vapor, it also cools the remaining percentage of liquid refrigerant down to the refrigerant’s temperature in the evaporator. Essentially the refrigerant cools itself down. Now the remaining liquid refrigerant is at the right temperature to effectively absorb heat energy from the fluid (air or water) passing across the evaporator coils.

The amount of liquid refrigerant that instantly flashes to a vapor is a necessary function of the basic cycle, but it does take away from the cycle’s ability to remove the most heat energy possible from the fluid crossing the evaporator coils. The more liquid refrigerant that flashes to a vapor, the less liquid is left to absorb this heat energy.

Less flash gas equals a higher net refrigerating effect and a more efficient system. Many designers reduce the amount of flash gas by additionally subcooling the liquid refrigerant entering the metering device. This lowers the refrigerant temperature; meaning less refrigerant will flash off to cool itself down to the evaporating temperature.

For example, let’s suppose liquid HFC-134a at 100°F is fed through a metering device and the temperature of refrigerant in the evaporator is at 20°. A portion of the 100° liquid refrigerant will boil off (flash) to cool itself down to 20°. However, if the entering liquid was cooled to 80° before it entered the metering device, less refrigerant would be needed to flash off and more liquid refrigerant would circulate through the evaporator. This would increase the amount of heat energy absorbed, increasing the system’s net refrigeration.

CAPACITY LOSS

A technician can calculate the Btu capacity loss by the effects of flash gas. This is done by knowing the refrigerant’s enthalpy value at its saturated liquid temperature entering the metering device and the enthalpy value of the saturated liquid refrigerant inside the evaporator. The difference between these two enthalpy values is the Btu loss per pound of refrigerant circulated through the evaporator.

For example, suppose a system using R-134a was operating with 100° liquid refrigerant entering its metering device and 20° evaporator temperature inside its evaporator. Using a saturated properties table provided by the refrigerant manufacturer, we determine that at a 100° saturated liquid temperature its enthalpy value is 45.1 Btu/lb and at 20° its enthalpy value is 18.4 Btu/lb. This gives us a Btu loss of 26.7 Btu per pound (45.1 – 18.4 = 26.7) of refrigerant circulated though the evaporator.

If the refrigerant temperature entering the metering device were reduced to 80°, the Btu loss would be reduced to 19.7 Btu per pound. At an 80° liquid temperature the new enthalpy value for the refrigerant is 38.1 Btu/lb, this gives a new difference of only 19.7 Btu (38.1 – 18.4 = 19.7). This results in an increase of 7 Btu/lb of additional heat energy that can be absorbed by the remaining liquid refrigerant.

Reducing the refrigerant liquid temperature entering its metering device is one means of increasing the overall capacity of any refrigeration system.

Publication date: 07/05/2010