The problem with screw compressors in Refrigeration is that should you not read the manual, you could easily screw up thousands of dollars of hardware and cause days' worth of labor to be spent to fix the problem.
Good day! Zak Ludick here from Cape Town, South Africa. I am working in the harbor at the moment on a mining vessel. I do not really expect this article to serve as a stern warning to many people who it may be relevant to, but I how someone that reads it may one day have this knowledge floating around in their brains and it might save some time, money and frustration!
A Screw compressor is generally used on large Chilled Water systems on systems that carry large condenser and evaporator shell and tube configurations. We are talking anything from 30 to 100kg of refrigerant in a single circuit.
In our case, we are dealing with a 45kg circuit. This does not matter though in this case as you can have this problem on any size system.
As the screw turns it causes a suction on the one side and a discharge on the other, thus a Low Pressure side and a High Pressure side. As the refrigerant passes through the screw, it is subjected to compression due to the tightening up of the space available for the refrigerant.
Here is a picture of the insides of a Bitzer compressor:
It is complicated and technical and all at once rather simple: There is a motor and it turns the screw and compression happens. Sure there is a whole world of dimensions and senors and safties involved, but that brings us to the actual problem that was caused here.
That screw is made of metal and metal expands when it heats up. Refrigerant from the LP side has just evaporated, thus is hot. This then gets put through a screw, adding mechanical energy to the refrigerant. This has been accounted for, but there is a limit!
Should the refrigarant be allowed to heat up too much, the crew will "grow" due to heat and then catch on to the sides of the screw casing. Now he have two things that happen here. The rotor could lock and then the motor burns out OR the rotor collides with the stator at high speed and the entire rotor and/or screw can break and snap off.
One of these causes burnt compressor oil to foul the refrigeration circuit and the other fouls it with pieces and shavings of metal.
Either way, your compressor is bust!
Now it must be removed and replaced. Sometimes the compressor specialists will use what is still viable and rebuild the compressor and this takes time.
In the case of having a compressor inside a ship... it is expensive. Recover refrigerant, disconnect piping and electrics, loosen mounting bolts. Then it must be rigged out of the plant and off the ship, taken away. A new one must be rigged in and pipes connected and then pressure tested, vacuumed and charged with refrigerant again.
Should there have been a motor burn out the system is contaminated and will need to be flushed and the compressor oil changed out from one to three times before it is non-acidic again.
Last laugh - How does this happen?
Well the flow of refrigerant will cause the compressor screw to heat up. If you have a blockage in the system and this causes low refrigerant flow, this build up of heat will happen!
But there are many safeties built into the compressor and chiller unit to prevent this! But it does not work when you reset the alarms and start the chiller over and over again. Then heat builds up too fast and boom. Broken.
Here are some pictures from site:
Thank you for reading!
Cheers!
