Btu Buddy 2: Charging A System After Loss Of Charge
In this second installment of the Btu Buddy series, Bob and Btu Buddy perform an emergency service call.
Bob receives a call from the company dispatcher that a 10-ton (R-22) air conditioning system cooling an insurance company is not functioning. There seems to have been some sort of accident with the condensing unit. The condensing unit sits outside the office and the evaporator section is just inside the building in an equipment room. When the customer called, she told the dispatcher that the lawn care company reported that they had damaged a pipe and it was spraying a white cloud of something with oil in it all over the place. The dispatcher advised the customer to shut off the unit until a service technician arrived.
When Bob arrives, he goes straight to the condensing unit and discovers the problem. The yard maintenance man cut a small gash in the liquid line between the condensing unit and the building. Refrigerant is still seeping from the gash when Bob arrives. He decides that he should cut the line and put a coupling in the 5/8-inch OD copper line using high-temperature solder. He is on his way to the truck to get out the torch and fittings when the insurance company manager comes out and asks how long the repair will take. It is 97 degrees F and the company auditors are in the office going over the books. They are already hot and the manager wants to make them as comfortable as possible. Bob says that it will take at least three hours to cut and repair the line, install a filter-drier, leak check, evacuate, and charge the system.
The Pressure Is On BobBtu Buddy steps in with some advice. “Bob, we all would prefer to go through the repair procedure that you mentioned, but there may be a more efficient way under the circumstances. Let’s go over some other possibilities,” he says.
“Here are some things that we know. Refrigerant is still boiling out of the oil in the system and will be for a while, so we know that no air is entering the system. There is no liquid refrigerant in the system, only the vapor refrigerant boiling out of the oil, so there is no chance of recovering any refrigerant without pulling air into the system,” reminds Btu Buddy.
Then he suggests, “Suppose you cut the line and flare both ends and install a flare union. You will be satisfied that no air is in the system, you can then pressure the system with R-22, leak check the flare connection, and charge the system. This way you can have the system back on and charging refrigerant into it within half an hour,” says Btu Buddy.
Making The RepairBob gets two flare nuts and a coupling for 5/8-inch tubing from his supply and gets started. He cuts the gash out, leaving enough space between the pipes to place the coupling. Bob then uses his flaring tool, which does not require the tubing to be reamed to remove any burr left from the cut. The tool has fluted sides on the flare cone and rolls the burr back as the flare is made (Figure 1).
He then fastens everything together but the last flare nut connection. Bob can see vapors slightly emitting from each fitting, so there is no air entering the system. The flare connection is made up and tightened. When the connections are tightened, Bob installs his gauge manifold and a cylinder of refrigerant and after purging the gauge lines, allows full tank pressure on the system and leak checks the flare. Since the system has been running fine for several months, it is assumed that the gash in the liquid line is the only leak.
Using The Proper Valve ArrangementsBob now has a system ready for a charge of refrigerant that is under full tank pressure. He will not be able to push liquid refrigerant into the system because the tank and the system are under the same pressure. He is trying to figure out how to get the charge into the system the quickest way.
Btu Buddy says, “Bob, if you start the system it will very quickly shut off because of low pressure, so the first thing you should do is place an electrical jumper on the low-pressure control. It is a good idea to use the one that is 3 feet long so you won’t accidentally go off and leave it.” So Bob jumps out the low-pressure control. Btu Buddy then advises that he shut the king valve on the liquid line and turn the refrigerant cylinder valve to the liquid opening. Then Bob starts the system. The low side gauge begins to drop and refrigerant feeds from the liquid connection on the refrigerant tank into the liquid line. This is a 10-ton system, so it is going to take several pounds of refrigerant. The liquid line gets very cold as liquid enters it. The system is actually using the refrigerant cylinder as the receiver and the system will start cooling very quickly.
Charging The System To The Correct ChargeWhen the system has run for a few minutes, Bob shuts off the refrigerant cylinder liquid valve and opens the king valve on the liquid line. The suction pressure is about 50 psig and holding, so the charge is fairly close to correct. The system will operate at this condition, so Bob removes the jumper wire from the low-pressure control. Bob then starts metering liquid refrigerant very slowly into the suction line. Barely opening the liquid line, he only lets the suction pressure rise to about 10 psig above the operating suction pressure. He lets refrigerant in very slowly until the sight glass is clear entering the expansion valve.
Bob goes inside and checks that the building is beginning to cool down. The manager is very thankful that the system is beginning to cool the office.
Bob starts to pack up his tools and Btu Buddy says, “Hold on, Bob, this job is not finished.” Bob says, “The system sight glass is full of liquid and the system is cooling, what else is there to do?” Btu Buddy points out, “The system will run and cool just like you have charged it, but it is not operating at its maximum capacity or efficiency. The only thing you have done up to now is furnish pure liquid to the expansion valve. What about subcooling for full efficiency?”
“Oh yeah,” says Bob, “I forgot about that.”
Bob goes to his truck and gets a temperature tester and fastens one of the leads to the liquid line and insulates it so the ambient air will not affect it. The ambient temperature is 97 degrees F so the system will be operating at about outside design conditions. The inside is still good and warm but will be down to about design temperature in a few minutes. Bob remembers that equipment is designed to operate at 95 degrees F dry bulb outside and 80 degrees F dry bulb with 50 percent relative humidity inside. Bob also knows that even though those are the conditions that the equipment is rated for, no one is comfortable at 80 degrees F dry bulb, so he and most others use 75 degrees F for their inside operating conditions.
Btu Buddy says, “The system should have about 10 to 15 degrees F of subcooling to be operating correctly. The system will gain about 1 percent capacity for each degree of subcooling and it will not cause any operating cost for this additional capacity.”
Bob checks the liquid line temperature and compares it to the condensing temperature on the gauges. The head pressure is 297 psig and the liquid line temperature is 132 degrees F. The system saturation or condensing temperature is about 130 degrees F, so there are 2 degrees of subcooling.
Bob adds refrigerant slowly to the suction line and watches the head pressure and the liquid line temperature. As he adds refrigerant, the liquid line temperature drops. Btu Buddy tells Bob, “To reach a subcooling of 10 degrees, the liquid line temperature needs to be 120 degrees F. For a subcooling of 15 degrees, the liquid line temperature needs to be 115 degrees F. As the refrigerant is added to a system, the head pressure should remain constant at 297 psig for this application. Once the saturation temperature is reached, it should not change while adding refrigerant until the system is overcharged. Adding refrigerant to obtain subcooling does not affect the condensing circuit because it is a circuit all its own (Figure 2). Since the pressures are the same with just a full sight glass as with a full sight glass and a 15-degree subcooling charge, the operating cost is the same. The system gains capacity by furnishing subcooled liquid to the expansion valve. The only cost is additional refrigerant.”
As Bob is loading his truck, the manager comes out and says, “I can’t thank you enough for getting this job done so efficiently. The office is cool again and the auditors attitudes have improved.”
While Bob is driving away, Btu Buddy says, “Being professional only takes a little more time, to learn what to do and use what you know. The difference between a professional and an amateur athlete is that one is dedicated to a career of high earning potential while the other just there to play around. This is much the same in our business, Bob, and you are a professional because you care.”
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 firstname.lastname@example.org.
Publication date: 05/19/2003