Operation below ambient pressure

For the design of commercial refrigeration installations, it is recommended not to operate below ambient pressure. This limits the evaporating pressure and by this the selection of refrigerants for a given refrigeration application.

Operating a part of the system below ambient pressure creates the risk to take ambient air into the system through small leaks. Ambient air always contains some amount of water vapour called humidity.

Operating below ambient pressure is possible if special measures are taken. In household refrigerators and freezers the low pressure side is significantly below ambient pressure. The appliances have hermetic systems that are intensely tested for leakage, so the risk for air entrance is very low. A small volume hermetic design of the suction side and the evaporator part can also be imagined for compact systems and some commercial installations. An intense leakage testing on installations built on site is a high effort, but not impossible. Some industrial installations with R717 (ammonia) also operate with parts below ambient pressure. Systems with R717 then contain air separation systems, called purger, and partly also water extraction systems. The water content in R717 does not freeze up at the expansion device because of the very good solubility. These technologies are often not transferable to other applications.

In this area, different parts of the refrigeration industry can learn from each other.

Impact of increased air content

When air has also come into the system, the circulating fluid is no longer the pure refrigerant. Thus, the thermodynamic properties resp. the operating conditions, like evaporating temperature or superheat, are wrong. An air buffer accumulates in the condenser and leads to increased high side pressure. The air part, the compressor pumps alongside the refrigerant, is missing in the refrigerant flow and thus in the refrigeration capacity. As air compression also needs power, but does not create refrigeration capacity, efficiency drops. Air has a much higher isentropic exponent, leading to significantly increased discharge temperature. Compressor and lubricant reach their limitations faster. The oil becomes hotter and is oxidised by the air. In combination with the water content of the air, it can form residues, like sludge, laquer or coking, which means carbon containing layers. The increased discharge temperature might be used for indication of air ingress by a specialised logic. When applying flammable refrigerants, ingress of air has to be taken into account in the risk assessment if operation below ambient pressure can not be safely avoided.

Problems due to water

With the most refrigerants, air ingress leads to chemical reactions and formation of ice at the expansion device outlet when operating with evaporation temperature below 0°C. With R717 (ammonia), water leads to a shift of the saturation pressures, at high water content, as ammonia is well miscible with water. Chemical reactions can be corrosion, detoriation of the lubricant – creation of sludge, hydrolysis of ester oil and so on – or reactions with the refrigerant. With more than 200 ppm water in the oil, the risk for increased wear in the compressor appears. Depending on refrigerant and evaporating temperature, formation of ice can already start at ca. 20 ppm of water. It can create blocking and thus lead to system operation failure. With hydrocarbon refrigerants, the water content can also form hydrates at the expansion device outlet, especially with capillary tubes. These look like gel or wax, blocking the flow.