This invention relates generally to refrigerated containers with controlled atmosphere apparatus and, more particularly, to a method and apparatus for preventing the corrosion of an air compressor therein.
In cargo containers used for transporting food and other perishables it is necessary to maintain an oxygen poor atmosphere in order to prevent food spoilage, and to prevent damage from insects and similar vermin. Limiting the amount of oxygen in the container, while maintaining the internal temperature within a desired cool range inhibits metabolic processes within the food, and can eventually kill organisms that would attack valuable food cargo. In such containers the need to maximize payload volume limits the space that is available for refrigeration and environmental control equipment, including the system for maintaining the controlled internal atmosphere. This system typically includes an air compressor, heater, and filter that feeds a gas separator that discharges oxygen poor effluent into the chamber, and returns the remainder into the ambient. The gas separator is typically a semipermeable membrane. In order to conserve space, and to protect the system from a harsh marine environment it is standard practice to dispose its air compressor inside the container, within the controlled atmosphere. The protection from salt water and particulates afforded by internal placement of the compressor, and the cool internal temperatures of the operating environment extend the useful life of the air compressor and other components of the system.
Because the air compressor is located within the refrigerated space and the suction port is located outside the container, the ingested air is at a different temperature and humidity ratio then the conditioned space in which the compressor resides. This condition causes condensation to form in the suction line tubing, and the condensation is then drawn into the compressor to cause moisture related problems that reduce bearing and compressor ring life.
This problem is exacerbated by other design considerations. In order to prevent ring blow-by gases from altering container conditioned atmospheres, the applicants have adopted the practice of back venting the air compressor crank case gases into the suction ports. With reciprocating positive displacement compressors, this practice causes pressure pulsations inside the crank case, and these pressure fluctuations create a continuous flow of warm, and sometimes moist, air into the crankcase by way of the breather tube. A crank case condensation problem is encountered when the compressor is running at low discharge pressures, with a perishable set point, in moderately moist environments. Briefly stated, the heat generated by the fundamental compression and motor work is not sufficient to raise the aluminum crank case above the compressor suction dew points. Continuous running at these conditions causes moisture to collect and corrode on internal components. When the equipment is stored during the down season, this water causes corrosive damage to the compressor.