Selecting the Right Compressors

For a refrigeration system to run at optimal energy efficiency, it’s important to carefully select the right type of compressors need to be selected, and the correct cooling method employed.

There are three primary types of refrigeration compressors – rotary screw, reciprocating, and rotary vane. Understanding how they work is critical in running an operation in the most effective way possible, said Tony Lundell, the director of standards and safety at IIAR.

First, we’ll take a look at the three types of compressors – how they’re are applied, configured and cooled – and then well explore common mistakes that are made in the industry regarding energy efficiencies using these different setups.

There are two main types of rotary screw compressors – single and twin. One of the main benefits of these compressors is their versatility. They can be used in nearly any refrigeration application, Lundell explained. They can accommodate compression ratios of up to 20:1 with ammonia and can be installed in a variety of configurations.

Twin-screw compressors have male and female rotors that draw refrigerant vapor in where it is compressed in the space between the two as they turn. The vapor is pushed through the compressor where it is pushed through a discharge port, Lundell said. These increase pressure and temperature significantly and very successfully. Single screw compressors are work similarly, but with only one turning element.

These compressors both use oil, which can be cooled in one of four ways.

  1. Liquid-injection cooling involves injecting high-pressure liquid refrigerant into the compressor where it flashes to a low-pressure temperature within the space between the rotors. That evaporation cools the oil in the system. This is a fairly low-cost option, but it comes with associated inefficiencies Lundell said.
    However, recent advancements in motorized expansion valves allow plants to lower their discharge pressure further than when using thermal expansion valves for liquid injection, Mike Reiner, director of engineering at GEA Systems North America, said.  
  2.  Thermosyphon cooling is another option. This is considered a passive method of cooling compressor oil. Thermosyphon cooling uses a heat exchanger, typically a shell-and-tube or a plate-and-shell heat exchanger mounted on the side of the compressor. High-pressure liquid ammonia is piped from an overhead pilot vessel into one side of the exchanger, and the heated oil passes through the other side, where it is cooled. Proper installation and refrigerant piping design is crucial for the thermosyphon system to properly function, Reiner said.
    There are three main advantages to using this method of cooling, Lundell explained. First, there is no capacity power penalty associated with it. Second, there’s no artificial lower limit to discharge pressure. Third, the heat rejected from the oil is routed directly to the condenser, providing energy savings especially in booster compressors and two-stage systems.
  3.   Water or glycol cooling is similar to thermosyphon cooling in that a heat exchanger is mounted to the side of the compressor unit, but water or glycol rather than ammonia is pumped through. This method makes the heat taken from the oil more readily available and useable. There is no limit to the discharge pressure of these systems, Reiner said. Another advantage of using water or glycol oil coolers is that the rejected heat can be used in other areas of the facility, such as for under-floor heating or preheating water.
  4.  Direct contact cooling between the refrigerant and oil is a relatively new method but works well with certain systems. A layer of liquid refrigerant is maintained on top of the oil within a separator, which boils off and constantly cools the oil. While this method is promising, Reiner said few if any manufactures are current use this cooling method.

Reciprocating compressors are widely used in high- or low-temperature environments. These systems can accommodate 8:1 compression ratios with ammonia and can be installed as boosters, high-stage, high-suction and single-stage compressors. Typically, they aren’t as large as screw compressors, Lundell said.

Reciprocating compressors use pistons – similar to a car engine – to compress refrigerant vapor within a cylinder. Most compressors have two to 16 cylinders. The pistons are driven by a crankshaft powered directly by an electric motor or indirectly using belts. An inlet valve is opened and lowpressure, low-temperature refrigerant vapor is drawn into the cylinder. The piston lowers, the valve is closed, and the piston rises to compress the vapor. During the compression process, heat is generated and must then be dissipated, Reiner said.

Typically, reciprocating compressors are cooled by water which is circulated through the heads and cylinder jackets, again, like an automobile engine. Some also have external oil coolers using water-cooled heat exchangers. Depending on location, water from natural sources can be circulated through the system at little to no cost in terms of energy expenditure and revenue. Some reciprocating compressors are air cooled, like old-fashioned Volkswagen engines or most motorcycles, and require no additional cooling, Riner said.

Finally, rotary-vane compressors are rarely used in new installations, but they remain abundant in older facilities. They are mostly used as booster compressors in low-temperature applications. They can accommodate a compression ratio of 5:1 with ammonia, according to Lundell.

Inside a rotary-vane compressor, there is an offset shaft with flat blades radiating from it. As the compressor turns, these blades thrust outwards and press on the vapor. They are fairly efficient, and they can move a tremendous flow of refrigerant without producing a lot of heat, Lundell said.

Rotary-vane compressors are cooled through liquid injection similar to screw compressors, or distilled oil or water can be pumped through the system’s jackets to take the heat away from the vanes.

While great many various combinations of compressors and cooling methods can be employed for any number of tasks, selecting the right combination and setting it up correctly is critical in ensuring the system runs as efficiently as possible.

Often multiple compressors are needed to keep a large load at a certain temperature. At a minimum, at least one compressor should have a variable frequency drive (VFD) installed. This way, the compressors without a VFD can remain working in a fully loaded condition and the VFD compressor can adjust to trim the load, Rainer said. This will save money, save energy and reduce maintenance costs.

Another problem some facilities run into is that when a facility’s production increases, the number of condensers doesn’t grow with it, Lundell said. Instead, engineers adjust the temperatures of the equipment, running at lower suction pressures and higher discharge pressures to keep up with increased demands on the equipment. This requires more energy and burns equipment out.

Finally, some facilities set condenser head pressures and never adjust them based on atmospheric conditions, leading to tremendous costs and inefficacies during times of temperature and humidity fluctuations. Lundell said that a wet-bulb approach can solve this issue, adjusting the pressure in the condenser and saving both money and energy.