Water Contamination in Ammonia Refrigeration Systems
Water contamination has many effects on an ammonia refrigeration system. The pressure-temperature relationship becomes impaired. The compressor oil begins forming organic acids and sludge from a complex chemical reaction. Pressure drops increase through piping. Pump and evaporator performance are adversely affected. The system must be operated at a lower suction pressure to maintain the desired room temperatures or to handle the same processing system loads. As the suction pressure is lowered, this increases the BHP/ton, causing less compressor capacity and increasing power consumption.
But, how does water even enter the ammonia refrigeration system? Water can gain entrance into the system in many ways. Systems that operate with the suction pressure in a vacuum are the most common source. Leaks from valve stem packings, screwed and flanged piping joints, threaded and welded pipe connections, leaking safety relief valves, pump seals, booster compressor seals and deteriorated piping and evaporator coils become sources of infiltration.
Other sources result from inadequate evacuation procedures on startup or following the opening of the system after a maintenance service or repair. The original source may be from moisture in new vessels that were not properly drained or dried after the completion of the ASME hydrostatic test. It could have entered during construction as vapor through open piping or weld joints that were only tacked in place and later condensed to liquid.
A lack of adequate purging or no purging at all can keep any non-condensables that made it into the system from being removed, resulting as a contaminant. A typical automatic purger removes non-condensable gases from the ammonia by chilling the refrigerant to condense it into a liquid and separate it from the non-condensable gases, which are then isolated and purged out. A typical automatic purger does not remove water, although some new designs are now able to do so. To remove the water, the refrigerant is heated to boil off the ammonia leaving the water. This water is isolated and then drained out.
Draining oil from vessels or bleeding equipment down with a hose into a container of water prior to service or repair could result in unwanted infiltration. A ruptured tube or tubes in a shell and tube heat exchanger, such as a chiller or oil cooler, can also be a contamination source.
It’s advisable to measure an estimated amount of the water content of an ammonia system by obtaining a sample from the system. The connection point needs to be determined where the highest water content buildup exists. The driving force of the water separating from the ammonia is due to heat being absorbed. The separation and buildup of water content in the system driven by the heat absorption is due to the large difference in vapor pressure between the water and the ammonia. In 2-stage systems, the point will be located in vessels and evaporators serving the low side of the system. In recirculating systems, the point will be at the vessel that supplies liquid to the evaporators. In a pump system, the point will be at the pump receiver. In a gas pressure system, the point will be at the controlled pressure receiver. In flooded systems, the point will be at the evaporator and surge drum.
Once a 100 ml sample is obtained and evaporated using a graduated sampling container, the residual remaining will be a mixture of water, oil, non-volatile impurities and approximately 30% ammonia. Then the percentage of water present and the cost of additional operations can be estimated using specific developed charts. If necessary, a distiller unit or regenerator unit can be connected to remove the excess water content in the system.
The effects of the water contamination may take years to detect before the problem is truly recognized. During this time, room temperatures may have been compromised from progressively deteriorated evaporator performances, suction pressures may have been lowered, more compressors may have been operating and additional electrical energy was consumed to meet the same requirements.
In summary, preventing excess water contamination in an ammonia refrigeration system prevents higher energy operating costs and unnecessary maintenance service costs.