Non-condensable gases (NCGs) are gases present within a steam system that do not condense into a liquid when cooled under normal operating conditions. Unlike steam, which readily condenses to release latent heat, NCGs remain in their gaseous state, acting as undesirable contaminants. Their presence severely compromises the efficiency and reliability of processes that depend on rapid, effective heat transfer, such as sterilization or industrial heating.
The Physical Nature of Non-Condensable Gases
Steam is a condensable vapor designed to release latent heat when it changes phase from gas to liquid on a cooler surface. Non-condensable gases, such as nitrogen, oxygen, and carbon dioxide, remain gaseous across the typical operational temperature range of a steam system. This fundamental difference dictates the problems they cause. When steam mixes with NCGs, Dalton’s Law of Partial Pressures applies: the total pressure measured is the sum of the partial pressures of all gases present. The temperature of the saturated steam is directly tied to its partial pressure, not the total pressure reading displayed on a gauge.
The inclusion of NCGs lowers the steam’s partial pressure, even if the total system pressure remains constant. For example, if the total pressure corresponds to \(180^\circ \text{C}\) for pure steam, the presence of NCGs means the actual steam temperature could be significantly lower, such as \(160^\circ \text{C}\) with 50% air present. This temperature depression causes the process to run colder than the pressure gauge indicates, leading to ineffective heating or incomplete sterilization.
Common Sources and Entry Points in Closed Systems
NCGs enter steam systems through several pathways related to operation and maintenance. The most common source is the air that naturally fills the system during shutdown or before startup. When a steam line is depressurized or steam condenses and creates a vacuum, air is drawn in through seals, valve packings, vacuum breakers, and minor leaks in the piping.
Another significant source is the boiler feed water, which contains dissolved gases, primarily oxygen and carbon dioxide. When this water is heated, these dissolved gases are released into the vapor phase and carried throughout the distribution network. Oxygen is highly corrosive, while carbon dioxide frequently originates from the breakdown of bicarbonates in the feed water.
The chemical reaction involving bicarbonates produces carbon dioxide, which mixes with condensed water to form carbonic acid. This acidic condensate attacks the steel components of the pipes and equipment, accelerating corrosion and compromising system integrity.
Impeding Heat Transfer and Process Effectiveness
The primary concern with NCGs is their dramatic reduction of heat transfer efficiency due to their insulating nature. When steam enters a heat exchanger, it moves toward the cooler surface and condenses, releasing latent heat. As the steam condenses, the NCGs are left behind and accumulate directly against the heat transfer surface. This accumulation forms a stationary, insulating film or boundary layer that acts as a thermal barrier. This layer prevents the steam from making direct contact with the metal surface, which is necessary for effective condensation.
The thermal conductivity of air is extremely low, meaning even a thin film can create thermal resistance equivalent to a thick metal wall. This insulating blanket severely reduces the heat transfer rate, often by 21% or more depending on gas concentration. In processes like sterilization, this leads to “cold spots” where the required temperature is never reached. For industrial heating, reduced efficiency means the process takes longer, requires higher steam pressure to compensate, and consumes more fuel.
Detection and Management
Detecting NCGs relies on recognizing the discrepancy between the system’s measured pressure and its corresponding temperature. In a pure saturated steam system, a specific pressure correlates to a specific temperature. The presence of NCGs, however, causes a measurable temperature depression relative to the pressure gauge reading. Specialized NCG sensors can monitor this difference, providing real-time data on gas concentration.
Management Strategies
Effective management begins with mechanical removal strategies. This includes using vacuum systems to evacuate air from the vessel before steam is introduced, a common practice in sterilization. Continuous or intermittent venting of NCGs from the highest points of the system and at the ends of heat exchangers is also necessary, as they are less dense than steam and collect in these locations. Finally, maintaining the system’s physical integrity through regular leak checks and utilizing deaerators minimizes the introduction of contaminants from the boiler feed water.