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View Subscription Details: Letter ( 08/09/2007 ) - HTML Format

Air in ammonia system

Air may enter a system through leaks in seals, gaskets, or uncapped valves when suction pressure of refrigeration plant is negative (lower than atmospheric pressure). Another way that air gains access to the system is when refrigeration system was opened.

Usually, compressor manufacturers recommend using vacuum pump to evacuate a compressor after opening. I would not suggest this step because the mechanical seal of this compressor can start leaking after evacuation. Air drawn into the system is eventually pumped to condenser where the liquid seal prevents it from traveling further.

The presence of air in condensers penalizes the system performance through the artificial elevation of the condensing pressure. The air adds partial pressure to that of the refrigerant pressure and thus increases the pressure against which the compressor must work. Energy use of this compressor will increase. A further penalty is the reduction in the heat-transfer coefficient by requiring the refrigerant to diffuse through the air on its way to the tube surface where it condenses.  

A test of the need for purging is to compare the actual head pressure and saturation pressure at the temperature of ammonia condensate flowing from condenser to high pressure receiver or to thermosyphon receiver. If the actual pressure is significantly higher than the saturated pressure corresponding to condensate temperature, purging should be done.

Usually, the refrigeration plants with positive suction pressures do not have air purgers. Manual purging can be done from time to time based on comparison of actual head pressure and saturated pressure at the condensate temperature.

Example. A refrigeration plant operates at head pressure of 140 psig. Temperature of condensate flowing from condenser is 75 °F. Corresponding pressure to that temperature is 125 psig.  140 - 125 = 15 psig can be purged from this refrigeration plant and new head pressure will be around 125 psig.  

Automatic purgers are a necessity for refrigeration plants with negative suction pressures. These purgers proceed from one purge point to another to extract the air. Usually, the purge points are located above the liquid ammonia surfaces (condensers, thermosyphone receivers, high pressure receivers), where air concentration is maximal. To minimize air leaks into the system, the suction side of these plants should be pressurized (pressure above atmospheric) from time to time and tested for ammonia leaks.

 

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