Recently, we’ve received a lot of questions in the field asking about key differences between maintaining equipment used for general air compression and equipment used for refrigeration compression. While on the surface it may seem like there aren’t many differences between these two applications, the truth is that there are some key considerations that do impact what operators need to do to maximize equipment performance.
Air compression versus refrigerant compression: The basics
Air compression can use several different types of compressors, including reciprocating, rotary, and axial compressors. In all cases, the compressors raise the pressure from atmospheric pressure to a desired level. Air compressors may have multiple stages, and the air is cooled between stages, dewatered, and collected at the receiver. The final product – compressed air – is then used to do work downstream from the receiver before it is then released to the atmosphere.
Refrigerant compression is identical to air compression at the compressor, though operating parameters (temperature) may be different. In addition, the oil used must be relatively inert to the gas compressed. If the gas used for refrigeration is compressed, bottled, and shipped off right away, then the refrigerant compression process is the same as for air compression. However, if the refrigerant is used to cool something and is then recaptured and reused, the process is completely different and involves additional components.
Refrigeration cycle basics
In the refrigeration cycle, things are not “cooled.” Rather, it would be more accurate to say that the “heat” is removed. Let’s look at the process:
At the compressor, gas is compressed from a low pressure to a high pressure. In doing so, the gas heats up.
From the compressor, the high-pressure gas enters a condenser where heat is transferred from the gas to the atmosphere. The gas undergoes a phase change to a high pressure liquid.
The high-pressure liquid enters the expansion valve, which allows the liquid to expand, lowering liquid pressure as it leaves the valve.
The liquid then enters the evaporator, where heat is removed from the inside air by the refrigerant, and in doing so the liquid again changes phase to a low-pressure gas.
The cycle is now complete and the low-pressure refrigerant is ready to repeat the process all over, again and again.
There are many different refrigerants that may be used in this cycle. These include:
Methane, Propane, Butane
Selection of the right refrigerant depends on the application, materials compatibility, chemical stability, temperatures desired, and environmental acceptability.
Selecting a lubricant that helps maximize performance and productivity
Making the decision on which lubricants to use for air compression is relatively simple. The choice is typically based on key operating parameters, such as compressor type, operating temperatures, water and air separability, and the application.
Lubricant selection for refrigerant and refrigeration compression is a little more complex, as the decision also factors in parameters such as the refrigerant fluid used, evaporator temperature, and compressor type. To make the right selection for these applications, consider these key guidelines:
Determine the refrigerant in use.
Determine evaporator type (oil carryover less than 15%) and temperature.
Compressor type and outlet temperature.
If mineral and synthetic lubricants are both indicated, synthetic lubricants will in general offer a higher level of performance than mineral oils with respect to equipment protection, particularly at high- and low-operating temperatures, compressor efficiency, and oil life.
Now, admittedly, this week’s tip covers the most simple of compressor systems and there are some complex components not covered. Our intent here is to cover the basics. If you’re looking for a more complete rundown of this application, leave your questions in the comments section below and I’ll do my best to answer them fully.
Why does the refrigeration effect occur in Refrigeration compression?
Because during compression, the refrigernt undergoes a phase change, gas to liquid, liquid to gas, and in doing so, heat is removed, providing the refrigeration effect. In air compression, there is no phase change.
Thanks for the information Rick Russo, in the doubt he had, greetings.
Thanks for the useful information! Available and understandable!
Interested in a more complete description of the process for these compressors. How is the oil department?
And The most important question: Why for refrigerating oils Mobil There is no suitable container, at least 20 L, whereas for these installations it is required a small volume of oil, and the oil from the opened packing because of its hygroscopicity should be immediately poured in the installation? This applies to both mobile refrigerators and stationary systems.
Dear Rick, good argument. Looking at refrigeration compressor lubricants can you say something more about miscibility and solubility of lubricant with the type of refrigerant, flocculation point, type of base oil (difference in behaviour and application betweeen PAO and esters). Thank you.
Some refrigeration oils are available in a 5 gallon or 20 liter pail. But as a practice I think that is a smallest size. If a package size is needed in your region, please discuss that with your Mobil representatives and if the need is sufficent, they should be able to make that happen.
I can say something more about miscibility and solubility of lubricant with the type of refrigerant, flocculation point, type of base oil (difference in behaviour and application betweeen PAO and esters). However - that would be a topic for a future tip. Thank you for the suggestion. I have added it to the list.
Good information, available and understandable.
Excellent and very complete.
Thank you for sharing.