IIAR

In many closed-circuit ammonia refrigeration systems, water contamination can occur over a period of time and the effects of that contamination can easily go unnoticed. One thing water contamination can do is cause an aqueous ammonia solution to be formed, which then replaces anhydrous ammonia refrigerant. This problem can become continuous if it isn’t resolved, and will increase over time if the source is not identified.

Water contamination can result in many problems for an ammonia refrigeration system. The pressure-temperature relationship can become impaired, the compressor oil may start to form organic acids, and sludge may develop from a complex chemical reaction. Other changes might include pressure drops that increase through piping and pump and evaporator performance that are adversely affected.

Sometimes, the system must be operated at a lower suction pressure in order to maintain the desired room temperatures or to handle the same processing system loads. And as the suction pressure is lowered, this increases the BHP/ton causing less compressor capacity while at the same time causing an increase in power consumption, a two-fold penalty.

So how do we identify the cause of water contamination? First we must look at how and why it is occurring. Water can gain entrance to a system in many ways. Systems which 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, deteriorated piping, and deteriorated evaporator coils become sources of infiltration with the system operating in a vacuum.

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 which were not properly drained or dried after the completion of the ASME hydrostatic test. In another scenario, for example during construction, water can enter a system as vapor through open piping or weld joints that were only tacked in place and later condensed to liquid. Condensation could have occurred in the system if air was used as the medium for the final pressure testing.

A lack of adequate purging or no purging at all can keep any non-condensables that have made it into the system from being removed, resulting in the introduction of a contaminant.

While draining oil from vessels or bleeding equipment down with a hose into a container of water prior to service or a repair in which the pressure may still be in a vacuum range could result in unwanted infiltration. An inadequate oil draining procedure in itself could be the root cause of infiltration.

A ruptured tube or tubes in a shelland-tube heat exchanger, such as a chiller or oil cooler, can also be a contamination source.

Performing adequate evacuation procedures on startup or following the opening of the system after a maintenance service or repair is crucial for contamination prevention.

There will be a continuous increase in water content of the ammonia in the system if steps are not taken to control the amount of infiltration. The effects of the water contamination in a system may take years to detect before the problem is truly recognized. During the non-detected time and until the water is removed and stopped, 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 very likely had to be consumed to meet the same requirements.

Meanwhile, the estimated amount of the water content of an ammonia system can be measured. Obtaining a sample from the system to test the water content must be done by a qualified person or persons familiar with the system using the appropriately developed and approved procedure to do so. The connection point on the system from which to take the sample should be determined to be the point where the highest water content buildup exists. The buildup of water is due to the large difference in vapor pressure between the water and the ammonia. In two-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 which 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 percent ammonia (in the water residue). Once the ml residue amount is determined, the percent water present and the cost of additional operations can be estimated using specific developed charts.

Finally, a regenerator, distiller, dehydrator can be connected to remove the water in the system.

In summary, preventing water contamination in an ammonia system prevents higher energy operating costs and unnecessary maintenance service costs.