Non-Condensable Gases

Non-Condensable gases in a freeze dryer (lyophilizer) refrigeration system can degrade system performance and cause premature failure of mechanical components.

A basic law of Chemistry is that in a mixture of gases, each gas exerts its own pressure independently of the others, and the total pressure of the system is the sum of the individual pressures.

Another basic characteristic of a gas is that if the space in which it is enclosed remains constant so it cannot expand, its pressure will vary directly with its temperature. Each gas in a mixture experiences the same volume and temperature, and each gas will exert its own pressure as dictated by its gas constant (R) and the Ideal Gas Law: PV=nRT.

Air is a mixture of approximately 78% Nitrogen, 21% Oxygen, 1% Argon, and trace amounts of other gases. Any pressure above the normal refrigerant condensing pressure will be due to the amount of air trapped in the refrigeration system. This over-pressure can easily reach 40 to 50 psig or greater.

Whenever a system is running with abnormally high compressor head pressure, air (non-condensables) in the system is a prime suspect. An abnormally high operating pressure puts a greater strain on the compressor's mechanical components, the oil, and the motor, and it also affects the refrigeration system efficiency by altering the compression ratio.

The design condensing temperature on water cooled refrigeration systems is normally determined by the temperature of the water supply, usually from 70 °F to 80 °F, and the water flow rate available.

To calculate the pressure of any non-condensable gases present you need a refrigerant vapor pressure chart of the gas that is in your system.

To remove the non-condensables follow good refrigeration practices or your company's SOP.