Btu Buddy 3: Charging A System With A Fixed Bore Orifice
In this installment of the Btu Buddy series, Bob gets a service call from the dispatcher who says that a residence with a very sick lady is getting very hot. The homeowner needs to get his air conditioning system working again right away. It is 96 degrees F and the lady's bedroom is upstairs, where the heat has a tendency to migrate.
Bob arrives at the job and asks the owner what the “sequence of events” was. He says that the unit was cooling yesterday, but is not doing well today.
Bob goes to the outdoor unit. The fan is running, but the compressor is not running. He shuts off the disconnect switch and removes the compressor compartment door. The compressor is very hot. There is oil in the compressor compartment and he can hear the refrigerant leaking. Bob looks around and finds that the discharge line has been rubbing on the cabinet and there is a very small leak there. Refrigerant is still leaking at a pretty good rate, so he decides to recover it.
Recovering The RefrigerantBob connects his recovery machine and when the manifold gauge starts to get close to 0 psig, Btu Buddy makes an appearance and asks, "When are you going to stop the recovery machine?" Bob says, "When it gets to about 28 inches of mercury on my manifold gauge."
"That may not be the best idea," says Btu Buddy. "You know you have a leak and if you allow the system to go into a vacuum, air will enter the system." Bob says, "Not much air will get in through that small hole."
Btu Buddy reminds him, "No air should knowingly be allowed into the system. There is a better way. Stop the recovery at 0 psig, repair the leak, and charge the system. No air enters, the repair is made in a timely fashion, and the system is back on-line in a minimum of time." Bob stops the recovery at 0 psig on the manifold gauge as Btu Buddy suggested.
Bob uses 15 percent silver solder and closes the hole in the discharge line. He gently moves the line away from the cabinet where it will not rub again. It is time to start the system. Bob tells Btu Buddy, "This system uses R-22 and has an orifice type of metering device, and we do not know what the system charge is supposed to be." Btu Buddy explains, "We can charge the system using the superheat method. The ambient temperature is 96 degrees F and this is an ideal day for charging using the superheat method. We need to know the approximate length of the suction line."
Bob measures the line length to be about 40 feet. The air handler is under the house on the far side from the condensing unit. "Why do we need to know the line length?" asks Bob.
"We really need to be able to check the superheat at the air handler by checking the suction line temperature and the suction pressure at the air handler," says Btu Buddy. "But we cannot readily check the suction pressure there and it is not convenient to check the temperature at the evaporator, so we are going to take these measurements at the condensing unit and approximate the temperature rise in the suction line by taking the temperature at the condensing unit. We can take the pressure at the same place as the temperature and get an accurate superheat reading. The reading is just at the condensing unit, not the evaporator.
"The superheat in the suction line is greater at the condensing unit than at the evaporator because it picks up heat all along the way, so we will make some assumptions in order to charge the unit as close to the factory charge as possible. We could use three different line lengths for determining what superheat to use: up to 10 feet, 10 feet to 30 feet, and 30 feet to 50 feet. For line lengths up to 10 feet, we will use 10 degrees F of superheat. For line lengths over 10 feet to 30 feet, we will use 10 to 15 degrees F of superheat. For line lengths of 30 feet to 50 feet, we would use 15 to 18 degrees F of superheat. This is probably as accurate as we can charge a unit with an unknown charge in the field."
Starting The UnitBob has made the repair and is ready to start the unit. He connects the cylinder of R-22 to the system to charge liquid refrigerant into the liquid line. The system pressure is 0 psig, so he opens the manifold gauge to allow pure liquid into the liquid line connection. The liquid refrigerant will flow both ways, toward the evaporator and toward the condenser. When the liquid stops entering, he shuts off the liquid and prepares to charge into the suction line. He turns on the system and the outdoor fan starts, but the compressor does not start. Bob touches the compressor and it is still hot. Bob says to Btu Buddy, "I hope the compressor is not burned. It is very hot."
Btu Buddy tells Bob, "It is not likely, unless it burned just before we arrived. We could find out for sure by turning off the power and testing the compressor electrically, or we can cool the compressor and try it again. There are three ways we can cool the compressor:
1. Wait until tomorrow. But, remember, the sick lady needs cooling.
2. Leave the compressor compartment door open and run the fan. This only works when the compressor compartment is open to the outdoor fan airflow and it may take several hours. Some units have isolated compressor compartments and air will not flow over the compressor.
3. Cool the compressor with water. The compressor is not touching the compressor shell; it is suspended in the vapor space inside. So this takes a few minutes, but it is the quickest of the three ways.
Since it is a long drive back to town and back out here tomorrow, and the customer really needs the system to be working, let's cool it with water".
Bob asks, "How do we do that without danger of electrical shock?" Btu Buddy tells Bob, "Shut off the power to the unit and lock the box so no one else can turn it on. You have a personal lock and key. Now, get a water hose and lay the nozzle on top of the compressor and let water trickle over the compressor dome (Figure 1). Now this will take about 15 minutes. Let’s use the time to the customer’s advantage and change the air filters and oil the indoor fan motor."
Bob starts the unit and the compressor starts up this time. Btu Buddy tells Bob, "Meter liquid refrigerant slowly into the suction line keeping the pressure about 10 psig above the actual suction pressure. That motor is still hot enough to vaporize any liquid that reaches it."
Bob notices that the actual suction pressure is 50 psig, so he allows the entering liquid to raise the suction pressure to about 60 psig (Figure 2). Bob fastens his temperature tester to the suction line and insulates it from the ambient air so he will get a true suction line gas temperature.
Bob shuts off the refrigerant flow into the system for a superheat check. The suction line temperature is 70 degrees F and the suction pressure is 61 psig, which corresponds to about 35 degrees F refrigerant boiling temperature. The superheat is 35 degrees F (70 degrees F suction line temperature minus 35 degrees F boiling temperature taken from the low side gauge pressure reading converted to temperature), so there is not enough refrigerant in the system yet. So Bob starts to add liquid again.
Btu Buddy tells Bob, "I think it would be a good idea to change over to vapor before you overcharge the unit. It is much easier to charge a unit using superheat by adding vapor than to overcharge it and recover refrigerant to the correct level of superheat."
Bob keeps charging the system until the suction pressure is 69 psig, which corresponds to approximately 40 degrees F. The suction line temperature was reading 57 degrees F, so the superheat is 17 degrees F, which is very good (Figure 3). He then shuts the valve on his liquid line gauge hose and pulls the liquid refrigerant from the gauge line into the low side of the system, minimizing the amount of refrigerant lost as all that was left was vapor at the low side pressure. A final check of superheat shows it to be 16 degrees F, well within the range for a correct charge. The final amount of refrigerant from the liquid line lowered the superheat a small amount.
"Well, Bob," says Btu Buddy, "you learned a valuable lesson from this call and you got the system back on in record time. The homeowner always appreciates good service."
Bill Johnson has been active in the HVACR industry since the 1950s. He graduated in gas fuel technology and refrigeration from the Southern Technical Institute, a branch of Georgia Tech (now known as Southern Polytechnic Institute). He taught HVAC classes at Coosa Valley Vocational & Technical Institute for four years. He moved on to become service manager for Layne Trane, Charlotte, N.C. He taught for 15 years at Central Piedmont Community College, part of this time as program director. He had his own business for five years doing installation and service work. Now retired, he is the author of Practical Heating Technology and Practical Cooling Technology, and continues as a co-author of Refrigeration & Air Conditioning Technology, 5th Edition, all published by Delmar Publishers. For more information, he can be reached at 704-553-0087, 704-643-3928 (fax), or email@example.com.
Publication date: 06/16/2003