What do you know about the fixed displacement piston compressor?

By Richard Hawkins, MACS contributor

Editor’s Note:  For those following these blogs as a series, last week we focused on operation of the compressor which was used on the vehicle in the case study from my class at the Training Event & Trade Show.  It was a pressure controlled variable displacement unit.  There were other compressors which were covered in the class, so this week and in the following weeks we will turn our attention to some of the other units.

What do you know about the fixed displacement piston compressor?  Please see picture #1.

Picture #1:  Fixed displacement piston compressors were the most widely used units in mobile air conditioning for years.  The trend in recent years with OEMs has been toward the use of variable compressors.

The fourth bullet point in the picture above states: “Build pressure when rotated by hand” and is noted in red. This contrasts with variable displacement piston compressors which do not build pressure when rotated by hand.  If a fixed displacement compressor is encountered which will not build pressure when rotated by hand, that would indicate a problem with it.

Fixed displacement piston compressors function in exactly the manner which their name suggests.  When running, they pump at only one displacement which is 100%. There is nothing inside of them to change their output.  The only thing that changes output is varying the speed of the vehicle’s engine.

To contrast this with the operation of the pressure controlled variable displacement compressor, we will revisit the case study then pose a “What If” question.  Please see pictures #2 through #4 to recap the case study, case study questions and correct answer.

Picture #2: Last week’s case study. 

Picture #3:  The case study questions and multiple-choice answers.

Picture #4:  The correct answer.

Now “what if” we were to take that restricted expansion valve and install it on a comparable system that used a fixed displacement compressor?  Please see picture #5.

Picture #5: This scenario brings up some interesting questions.

As indicated, this brings up some interesting questions.  Please see picture #6 for the answers to the questions in picture #5.

Picture #6:  Answers to the questions in picture #5.

There is a big why in the picture above so that needs to be addressed. The short answer to it is: Because the compressor is pumping at 100%, but let’s look at the “why” for each individual item.

High side:  Likely somewhere around 125 psi (at an idle) & 135 psi at 1500 rpm.  Why?  With a fixed displacement compressor which is pumping at 100% (verses a very low displacement) for a variable unit, the hi side pressure is going be higher.  However, it would not be as high as it would with an expansion valve which was functioning properly.  That is because the volume of refrigerant which is being pumped is low.  Note: On an 80° F Day, the high side pressure of a properly functioning expansion valve system could be expected to be in a range of about 160 to 200 psi.

Low side: Probably somewhere around 0 (at an idle) & 5 inches of vacuum at 1500 rpm.  Why?  With a fixed displacement compressor which is pumping at 100% (verses a very low displacement for a variable unit), more refrigerant will be pulled out of the low side resulting in the lower pressure level.

Vent temperature:  About the same (70° F).  Why?   The vent temperature should be about the same or might be slightly less because the low side pressure would be noticeably lower.  However, the liquid level in the evaporator will still be extremely low.   This would result in a lot of refrigerant superheating and a vent temperature which will not be a great deal different.

Suction line temperature:  About the same (65° F).  Why?  The vent temperature and suction line temperature are closely related because they are a function of evaporator temperature.  As a result, it should be about the same or maybe slightly lower, but not a great deal different.

Note:  There is some speculation involved in these answers.  You never know for sure how a system is going to operate with a defective part unless you install it and run the system.  However, this provides a rough idea of what could be expected.

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