ASHRAE Journal - January 2013 - 77

refrigeration applications

Problem of Excess Gas
By Andy Pearson, Ph.D., C.Eng., Member ASHRAE

This article is the eleventh in a series exploring refrigeration and heat pump concepts without using jargon.

ast month’s jargon buster included a definition for non-condensable gas, sometimes abbreviated to NCG (fridge guys love three-letter

acronyms, or TLAs as we like to call them).

The definition mentioned that other at the very beginning of the nineteenth gases can sometimes get mixed in with century or his analysis of color blindness, the refrigerant. The most common of which is still known as Daltonism in these is air, which can leak into systems some places. However, we need to think where the evaporator operates at a pres- about his “law of partial pressures” that sure lower than atmospheric pressure, or describes how mixtures of gases behave. might be left in the system after service Simply put, Dalton realized that gases or maintenance. Nitrogen leaking into are fundamentally antisocial; each one part of the system during a pressure test in a mixture behaves as if it is the only will also act as a non-condensable gas. one there. So if there were three gases So why is this a bad thing? A system mixed in equal quantities with a total that has non-condenspressure of 150 psi, each able gas trapped in the one would behave as if condenser will need to the pressure was only 50 run harder to achieve the psi and it was filling the same cooling effect and space. Doesn’t this strike therefore will be less efyou as a bit odd? ficient than it should be. Imagine an ice skatIn this case “run harder” ing rink with a bunch of means the compressor has There’s only so much gas that hockey players knocking to pump the gas up to a you can tolerate. Photo credit: seven bells out of each higher pressure than ought English Wikipedia user Froggydarb. other. Mixed among them to be necessary in order to reliquefy it. are a group of skaters, some holding There are several analogies used to hands, some doing the conga and some describe the effect; some of them are bet- just gliding round in circles. Also on the ter than others, and some are downright same rink there are a couple of figure wrong so we need to go a bit carefully skaters practicing their high lifts and here. The effect has nothing to do with the triple axels. It seems unlikely that these air “covering part of the heat exchange three groups would not exert some kind surface” or “insulating the inside of the of effect on each other, but for gases it is condenser.” different. They are oblivious. To get to the real reason we need to In the condenser each gas behaves as look to the work of a self-taught scientist if the others were not there, and so will from the northwest of England called condense at the temperature that matches John Dalton. He is perhaps most famous the pressure that they alone exert on the for his pioneering work on atomic theory system. For R-134a in an air-cooled
January 2013

condenser when the air temperature is 95°F (35°C) the condensation will happen at about 120°F (about 49°C) if the condenser is reasonably sized, and liquid would come out the condenser at about 115°F (46°C). If there is no air in the system then the compressor needs to deliver the gas to the condenser at 171 psi (1179 kPa) so that it will turn to liquid at the right temperature. If 10% of the gas in the condenser is nitrogen that was left in the system after a pressure test, then the compressor still has to deliver the R-134a at 171 psi (1179 kPa), but this is only 90% of the total pressure, so the actual discharge pressure of the compressor would need to be 192 psi (1323 kPa). If you attempt to recreate this feat of calculation beware! The pressures given here are gage values but the partial pressure calculation is done with absolute values. So for this system, where 10% of the molecules in the condenser are nitrogen, not R-134a, the pressure gage on the compressor discharge would suggest that the condensing is happening at 128°F, not 120°F (53°C, not 49°C) as intended. Usually a high condensing temperature indicates a condenser problem—dirty fins or blocked airflow or some such hindrance. However, the liquid coming out the condenser, which is always slightly colder than the actual condensing temperature, would still be about 115°F (46°C). This is the key to diagnosing excess gas in the system. The condenser is apparently underperforming and yet is still able to make the liquid as cold as before even though the pressure is higher. It helps to have figures for good performance as a comparison when checking for excess gas.

Andy Pearson, Ph.D., C.Eng., is group engineering director at Star Refrigeration in Glasgow, U.K.
ASHRAE Journal 77

ASHRAE Journal - January 2013

Table of Contents for the Digital Edition of ASHRAE Journal - January 2013