Over the years there are and have been a variety of compressor styles that have been used in ammonia refrigeration systems. Each of these compressor styles accomplish the same task of increasing the pressure of ammonia vapor. They provide the motivating force that moves the refrigerant through the system. So, what type of compressors do you have? Are they the best compressor(s) for your application? Are they the most efficient?
Arguably the most common compressor used in today’s ammonia systems are screw compressors. There are basically 2 different styles of screw compressors; single screw and twin screw. Both styles utilize a high-speed rotating “screw” or rotor to compress the refrigerant gas. In the case of a single screw, the main rotor meshes with gate rotors to close off a trapped volume of gas and then decreases the volume to increase pressure. With the twin screw, the main, or driven rotor meshes with a companion rotor to accomplish the compression.
Screw compressors have become popular for a variety of reasons, some of which include:
- Higher capacities in a relatively small footprint.
- Large, single unit capacities to meet the needs of very large systems.
- Variable capacity and volume control for operational stability and efficiency.
- Generally seen as requiring lower maintenance costs.
- Better oil retention. Less oil leaves the compressor and enters the piping system.
- High compression ratios allowing for lower temperature, single stage applications.
- Lower discharge gas temperatures because of the efficiency of the oil cooling systems.
Screw compressors do have a couple of less positive characteristics as well, including:
- High speed rotation, typically around 3600 rpm.
- No compressor handles liquid refrigerant well but screw compressors really don’t tolerate liquid at all.
- Difficult and time consuming to rebuild, especially in the field, resulting in high costs for repair when they do break down.
- Reciprocating Compressors
Probably the 2nd most common compressor in today’s ammonia refrigeration systems are reciprocating compressors. Reciprocating compressors use cylinders that travel up and down in a cylinder via a rotating crank shaft. The capacity of a machine is basically dictated by the number of cylinders.
Reciprocating compressors were once much more widely used but the many of the screw compressor advantages overshadowed them. Today, reciprocating compressors definitely have their place. In certain applications, reciprocating compressors have advantages:
Although unloading of reciprocating compressors is accomplished in increments instead of infinitely variable like screw compressors, they often unload more efficiently than screw compressors. So, if you have a system that runs at partial load a high percentage of the time, a reciprocating compressor can accomplish this without losing too much efficiency.
Reciprocating compressors seem to operate in cold weather, low load conditions a little bit better.
Reciprocating compressors can often be completely rebuilt, in the field, basically from the ground up.
Rotary Vane Compressors
Rotary Vane Compressors are very rarely applied in today’s applications. In general, rotary vane compressors were suited only as booster compressors because of the low compression ratios. There are still a few rotary vane compressors out there in operation, but I don’t ever hear about anyone installing them in new applications. I’m not even aware if anyone is still manufacturing them for ammonia applications.
Even less common today than the rotary vane compressors are centrifugal compressors. Centrifugal compressors were applied in their day as very high volume machines. Honestly, they were before my time so I can’t even really speak to the advantages and disadvantages.
The variety of ammonia compressors that are around today and those that have come and gone, certainly have and do fulfill a need to drive the ammonia refrigeration system. Today’s compressors are operationally and efficiently superior to the compressors of years past. However, in the end it comes down to the application. Any compressor that is applied improperly will cause problems. Likewise, those that are applied properly can provide you with years and years of high quality operation.
Feel free to give us a call if you have any questions about your compressors or the application of your compressors.