Compressor "Open-Drive" Shaft Seal Guide 

Belt-driven, open shaft compressor have been around for over 50 years.  Nevertheless, there is a surprising amount of confusion about how they work and how reliable they are.  Our hope is to try to clarify some of that confusion.  The image we use and some parts of the discussion are specific to Bitzer compressors.  However, the shaft seals used on all open-drive compressors are basically the same and the comments we make will be applicable to all of them.  Let's get started by taking a closer look at a "cut-away" view of the Bitzer Type II shaft seal.

The first thing to recognize is that refrigeration compressors DO NOT use rubber lip seals.  Instead, they use what are commonly called "face" seals.  A face seal is basically two finely machined surfaces which use the surface tension of some type of liquid (usually  oil or water) to create a barrier to gas and liquid flow.  On side of the seal is usually made of a softer material such as carbon or bronze and the other side is generally steel or iron.  The two surfaces fit together very tightly and, in the presence of liquid, make a very reliable and long-lasting seal.

It is important to know that face type seals have important advantages and disadvantages when compared to the better know rubber "lip" seals.  These are:

1.  Face seals require oil to seal -
Without a good coating of oil between the two faces, the seals will leak.  This is why it is a good idea to run your car air conditioner periodically even in winter.  By running the compressor a few minutes every month or so you re-establish that a coating of oil between the seals.  Without that oil, the seals will leak.  If you are going to leave you compressor unused for more that a month or two, make sure you either (a) have someone turn it on periodically) or, (b) close the compressor shutoff valves to prevent gas from leaking out of the rest of the system should a leak develop.

2.  A small amount of oil/gas leakage is normal -
Face seals do not create a perfect seal.  Realize that some leakage is normal.  One of the oldest tricks in the refrigeration technician "scam" book is to place an electronic leak detector right up against the shaft seal.  When the alarm goes off, the technician explains that the seal is "bad" and needs to be replaced.  The fact is that these seals very rarely fail and generally last the entire 20-30 years that the rest of the compressor will.  Also, realize that it is normal to have a small amount of oil "sling" out from around the shaft.  It does this in your car air conditioner as well.  You just haven't noticed it because car air conditioning compressor have a built-in "wick" to retain it in one area.  again, we are not talking about massive oil leaks here.  Just remember that a little oil does not mean that your seal is failing.

3.  Pressing in on the shaft will create leaks -
The end of the compressor crankshaft (opposite the shaft seal) is spring-loaded.  This spring tension serves to hold the two shaft seal faces tightly together at all times.  If you press in on the shaft, the seal faces will lift apart and leak.  You HAVE NOT permanently damaged anything.  However, if you did this when the system was charged with refrigerant you will have gotten a blast of gas and oil out from around the seal. 

NOTE:  When direct-coupling your motor and compressor, it is particularly important to be sure that they are not mounted so tightly that the motor shaft presses in on the compressor shaft.  The result will be leaking.

4.  New compressors often need to be "run in" before the shaft seals tightly -
When you first assemble your system the compressor shaft will be in much the same condition as it would if the compressor had just sat around unused for the past winter.  As a result, the shaft seal may not be too secure until it is run.  If you experience what seems to be excessive leakage from new compressor, first make sure that the shaft is not being pressed in (particularly important on direct-coupled systems).  After that, tilt the compressor towards the shaft seal to "wet" it with oil.  Then turn the compressor over several rotations by hand.  This should solve the problem.