A non-condensable gas (NCG) is a substance that exists in a gaseous state within a closed system but cannot be converted into a liquid under the system’s normal operating temperatures and pressures. These gases are contaminants because they do not participate in the intended phase change process, such as the condensation of steam or refrigerant vapor. Their presence disrupts the efficient operation of equipment like boilers, heat exchangers, and refrigeration units. NCGs are typically introduced accidentally or created through chemical reactions, while the primary working fluid is designed to cycle efficiently between liquid and vapor states.
The Difference Between Gases and Vapors
The distinction between a true gas (NCG) and a vapor lies in the critical temperature. The critical temperature is the highest temperature at which a substance can exist as a liquid, regardless of the pressure applied. Above this temperature, the substance exists only as a gas and cannot be condensed into a liquid by compression alone.
A vapor, such as the working fluid in a cooling system, is a substance operating below its critical temperature. This allows the vapor to be easily condensed back into a liquid state by extracting heat or increasing pressure, which is the principle behind condensation in industrial systems. The working fluid is engineered to operate in this region so it can readily change phase.
In contrast, a non-condensable gas has a critical temperature far below the operating temperature of the closed system. Common atmospheric gases like nitrogen and oxygen have extremely low critical temperatures. In a typical refrigeration or steam system, these gases are therefore above their critical temperature and behave as true, permanent gases. Because they are above this critical point, they cannot be liquefied within the system’s condenser and will not participate in the cycle of condensation and evaporation.
Common Sources and Examples
The most frequent NCG found in closed systems is air, primarily composed of nitrogen and oxygen. Air often infiltrates through leaks, particularly on the low-pressure side of refrigeration units or when a steam system develops a vacuum during shutdown. Improper evacuation procedures during installation or maintenance are also a major source, leaving residual air trapped inside the equipment.
Non-condensable gases can also be generated internally through chemical processes. For instance, in ammonia refrigeration systems, the slow decomposition of the refrigerant forms hydrogen and nitrogen gases over time. High operating temperatures can also cause the chemical breakdown of lubricating oils or refrigerants, yielding various non-condensable hydrocarbon gases and other decomposition products.
Gases may also be introduced during maintenance procedures. Nitrogen is often used to pressure-test a system for leaks before charging it with the working fluid. If this nitrogen is not completely removed afterward through a thorough vacuum process, it remains in the system as a non-condensable contaminant.
Why Non-Condensables Degrade System Performance
Non-condensable gases severely impair system efficiency by causing two main operational problems: increased operating pressure and reduced heat transfer. NCGs accumulate in the condenser, the section designed to convert the working fluid vapor back into a liquid. According to Dalton’s Law of Partial Pressures, the total pressure in the condenser is the sum of the partial pressure of the refrigerant vapor and the partial pressure of the non-condensable gas.
This added pressure, often called “head pressure,” forces the compressor to work against a higher load than intended. The compressor must expend more energy to achieve the required pressure for condensation, leading to increased power consumption. This higher pressure also elevates the system’s discharge temperature, which accelerates the degradation of lubricating oil and shortens the compressor’s lifespan.
Reduced Heat Transfer
NCGs reduce heat transfer efficiency by creating an insulating layer on the condenser’s heat exchange surfaces. Since the non-condensable gases do not condense, they accumulate as a stagnant film against the cool walls of the heat exchanger. This gas layer acts as a thermal barrier, preventing the hot working fluid from shedding heat efficiently to the external environment. This insulating effect results in incomplete condensation, reducing the system’s capacity to perform its intended function.