Btu Buddy 123: Devastated 20-Ton Condensing Unit - Part 2
Now, Bob’s company has promoted him to help train a new employee, right out of a school specializing in HVAC, just like Bob was. Bob is now Tim’s Btu Buddy. Tim is anxious to travel with Bob. Tim realizes that he is right out of school, with the theory and lab work that he accomplished in school, but still needs help. He knows that he worked with many of the components of the systems in the school, under ideal conditions with good light and air conditioning. Now it is into the field, sometimes under the house with poor lighting, or out on the rooftop in the sun, where the real action is. He is naturally and normally reluctant, but he has Bob to help guide him.
Bob and Tim were driving to the job they left yesterday and talking over what needs to be done today, when Tim asked, “How are we going to move the condensing unit we disconnected to the new location so we can connect it to the system where we disconnected the defective condensing unit? You said you had a plan.”
Bob explained, “There are some short lengths of pipe in the back of the truck that I found and loaded before you got to work this morning. I saw several sheets of 1/2-inch plywood that I spotted at the job site. We will use the lever truck to raise the defective unit (Figure 1) and slide a plywood sheet under the unit. Then we will put some short lengths of pipe under the unit. Then we will roll the unit onto the next sheet of plywood and keep the unit moving across the roof by laying the plywood we just rolled over and the pipe lengths we just used ahead of the unit (Figure 2). With a couple of men from the maintenance department to help, in just a few minutes, we can position the unit to the edge of the roof where a small crane can reach it and set it to the ground for disposal. We will then go to the replacement unit and roll it across the roof to its new location to be connected and started up. That will allow us to move some heavy equipment around by hand without a large crane; they are expensive.”
Tim said, “That is a good idea. Use simple tools already on the job.”
When they arrived at the job site, they moved the defective condensing unit to the edge of the roof and the replacement unit into place where they could connect it and started to work.
Tim asked, “Are we going to install a new refrigerant filter-drier in the system?”
Bob said, “Yes, anytime you open a system up for very long, a drier is necessary. If fact, I brought a suction line filter also for additional protection. This is a filter only and will stop any particles from entering the compressor (Figure 3). These filters are typically fitted with Schrader valve connections on each side where pressure drop across the filter can be measured. The maximum recommended pressure drop would be 3 psig. If the pressure drop becomes more than 3 psig, the filter is becoming stopped up and will need to be removed or changed. It is not likely that there will be any debris in this system, but we are installing it to last and we do not know what other service people may have introduced into the system.”
Tim said, “I like the fact that we are doing a really professional job. This is a manufacturing plant and they rely on the work being done correctly.”
They connected the replacement condensing unit to the evaporator section and put in a small amount of R-22 for a trace refrigerant and then pressurized the system to 150 psig with nitrogen and each took an electronic leak detector and went over the whole system and found no signs of a leak. They then connected the vacuum pump and started it running and went to lunch.
While eating lunch, Tim asked, “How long do you think this replacement unit will perform? It is about 10 years old already.”
Bob answered, “We took all precautions with the installation and the replacement unit seemed in great shape. I have seen systems that were installed before I was born that are still operating. The stress points of a system are the moving parts, the fans and compressor. If they are maintained, there seems to be no limit to a unit’s lifetime. For example, if the coils are kept clean and the metering device works correctly, the compressor will not be stressed except on startup. The biggest stress on a compressor should be startup. At startup each time the thermostat calls for cooling, the compressor crankshaft starts turning with minimum lubrication until the oil pressure is raised. The fewer startups a compressor has, the longer it will last. This is one reason that it is best to size the condenser as close as possible to the load, to keep the compressor running. Running times are not as hard on the compressor as the number of startups.”
Tim asked, “What about the motor? Won’t it wear out?”
Bob explained, “The motor has bearings the same as the compressor and gets its lubrication from the same oil pump and operates under the same rules as the compressor. The motor windings are a different case. The windings are subject to flexing when the compressor starts up. There is a lot of torque in the windings. If the motor was manufactured with any windings that are not held tight, they will rub together and wear the varnish off of some of the windings after many startups. They will then short together and the motor will burn. That’s not a good thing. It all gets back to the fewer startups a compressor has, the longer it will live.”
Tim then asked, “Don’t the electrical system, the coils, and the cabinet come into the picture also?”
Bob said, “The coils do not normally have any wear on the inside or the outside. If the outside, the air side, is kept clean, the compressor will have less strain. If the evaporator coil gets dirty, low superheats can occur. If the outdoor coil gets dirty, high head pressures will occur. Low superheat and high head pressure together can be hard on the compressor. The cabinet on the outdoor coil is subject to the outdoor conditions. If the cabinet is in a chemical or salty atmosphere, it may deteriorate from corrosion. Otherwise, the technician should always replace every screw that is removed after service. Every screw is essential to long cabinet life. The manufacturers do not install more screws than are needed.”
Bob and Tim returned to the job and found the vacuum pump had evacuated the system to 500 microns. They valved the vacuum pump off and let the vacuum stand for 15 minutes and it stayed the same, so they allowed liquid refrigerant to enter the king valve until the refrigerant stopped flowing, then started the unit and completed the charge using vapor refrigerant until they had 12°F of subcooling.
Tim then checked the superheat at the evaporator and found it to be 12° superheat. The system was operating correctly. As they rode away, Tim said, “I wish you would review superheat and subcooling for me someday. I would like to hear your explanation. I studied it in school, but now that I am in the field, I would like a refresher.”
Bob said, “Sure, it is always a pleasure to explain something to a person that is really interested.”
Publication date: 6/24/2013