Refrigeration Rack Overview w/ Sped up Oil Change
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The refrigeration circuit of a rack has the same main parts as an HVAC refrigeration circuit, but there are a few extra parts. These racks have a liquid receiver on the liquid line; the liquid line feeds into the receiver from the top via a ball valve and resumes out of the bottom. The receiver holds extra liquid refrigerant. (You can see if you’re overfilling the receiver by checking a dial indicator or the display.) Then, the liquid line continues into the pressure differential valve and into the filter drier. This rack has a sight glass after the drier to ensure that we have a full column of liquid refrigerant before that refrigerant goes to the liquid header. The compressors on this rack have fans mounted to them, which assist with cooling the compressors.
Refrigeration rack systems often have hot gas defrost, which sends hot discharge gas backward through the suction line and the coils to melt ice off the coil. As the hot gas moves through the coil, it will lose heat and eventually condense back to a liquid. The liquid will go through the distributor tubes and come out just before the metering device and go through the liquid return line. A check valve allows that liquid refrigerant to merge back into the main liquid line past the differential valve.
The compressors are all indoors on refrigeration racks. So, the discharge line runs out of the rack room to the outdoor condenser. Then, the liquid line runs from the outdoor condenser back to the receiver inside the rack room. The outdoor condensers do the same thing as they do in HVAC units; they turn gas into liquid via desuperheating, changing state, and subcooling.
As with HVAC units, signs of oil are important leak indicators on refrigeration racks. The suction lines and suction header (which the suction lines feed into) should all be free of oil. Bryan also likes to check that the fans are all running as they should.
The compressors all individually discharge into individual discharge lines that tie into a single discharge header; the discharge gas comes together at the header and goes back to the condensing unit in a single line. Racks also have suction and liquid core driers, which have replaceable cores (which may have activated carbon in them). The core drier screens can become restricted, which can cause flashing to occur in the sight glass; these can be cleaned quite easily with electrical contact cleaner.
We are using alkylbenzene (AB) oil for the oil change, which is better for oil return than mineral oil. We’re also replacing the cores with high-acid driers because the system has tested positive for acid. After some time, we’ll replace the cores and do another acid test. Whenever we install a new drier, we pull a vacuum on it to make sure there is no air in the line; otherwise, you could end up with non-condensables, high head pressure, and high humidity.
The oil separator takes the discharge gas and separates the oil from the refrigerant. Oil separates from the refrigerant through velocity changes or screens, and it drains out of the bottom while the discharge gas continues through the circuit. The oil then runs back to the reservoir. From there, the reservoir would send oil to the oil filter and then to the compressor oil level controllers.