Air Compression vs. Refrigeration Compression: What You Need to Know

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:

  1. At the compressor, gas is compressed from a low pressure to a high pressure. In doing so, the gas heats up.

  2. 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.

  3. The high-pressure liquid enters the expansion valve, which allows the liquid to expand, lowering liquid pressure as it leaves the valve.

  4. 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.

  5. 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:

  • Ammonia

  • CFC’s, HCFC’s

  • Methane, Propane, Butane

  • CO2

  • HFC’s

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:

  1. Determine the refrigerant in use.

  2. Determine evaporator type (oil carryover less than 15%) and temperature.

  3. 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.

  • Nathaniel:  This is a great question and one you must look at from a couple of perspectives.

    1.  Is the small oil volume of a Durhan-Bush refrigeration compressor worth the risk.  The risk being - if the transition for any reason causes an outage, are you willing to assume the risk?

    2.  If you want to do this - determine the composition of the Durhan-Bush oil and match it to a Mobil product.  If the Mobil product is an exact chemistry match, than the transition could be as simple as a drain, flush and refill.  Make sure the Mobil lubricant is compatible with the refrigeration gas being used.

    3.  If the Durhan-Bush and Mobil oils are not compatible then a drain, disassembly, cleaning, reassembly, and a double flush are required.  Pay close attention to small piping, filters and gaskets.  The specific procedure is on the compressor section of this web site.

    My personnel opinion - in my 30 year career in the field - I passed on small volumes like this due to the maintenance and risk involved.  The reward was too small for the risk taken.  Good luck

  • Hi Rick, Do you have suggestion on the flushing procedure for converting Durhan-Bush refrigeration oil to Mobil EAL Arctic. Please advise. Thank you.

  • Lawani - With propane I would use Mobil Glygoyle 22, a PAG synthetic.  Better solubility and less miscibility with propane.  This product should provide the performance you desire.  Switching from Peg 805 will require a complete and repeated flush as the two products are not compatible. The flush procedure is contained on the blog.  Good luck.

  • Our Refrigerant Compressor is an Ajax- Cameron Integral Engine and Compressor DPC-2803 we use Mobil pegassus 805, Part of our maintenance task as planned is to stop the unit and purged lube oil at the Propane Receiver and The Propane Chiller (evaporator) and add fresh propane so as to ensure integrity of the working fluid, the oil enters the refrigerant gas during lubrication of the compressor piston. This is really tasking, now my question is this.1. what synthetic lubricant can we use and how do we reduce oil carryover to the refrigerant gas. 

  • The Mobil products should work just fine but you should make sure they are compatible to determine if a flush is required.

  • I have a customer who is using Fusheng compressor and they are using Fusheng oil with the ODI 3000 hours, we want to convert for them to Mobil SHC 1025 with 8000 hours. Is it allright?

  • What is ODI of the Mobil SHC 1020 series for crew rotary compressor?

  • Excelente y muy completo.

    gracias por compartir.

  • Хорошая информация, доступно и понятно.

  • 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.

  • 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.

  • 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.

  • Спасибо за полезную информацию! Доступно и понятно!

    Интересует более полное описание процесса для этих компрессоров. Как происходит маслоотделение?

    И самый главный вопрос: почему для холодильных масел Mobil нет подходящей тары, минимум 20 l, тогда как для этих установок требуется небольшой объем масла, а масло из  вскрытой упаковки из-за своей гигроскопичности должно быть сразу залито в установку? Это касается как мобильных рефрижераторов, так и стационарных систем.

  • Gracias por la información Rick Russo, en la duda que tenia,saludos.

  • 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.