Btu Buddy 125: Superheat Explained - 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 on their way to a no cooling call at a residence. They were pretty sure that it was a low refrigerant call.
Bob said, “I hope that we can put the last lesson into practical practice. We have gone to a great deal of trouble to understand superheat. We said that the air conditioning technician must learn how to measure superheat for two big reasons:
1. To be able to prove coil performance.
2. To be sure to protect the compressor.”
They arrived at the residence and talked to the homeowner who said that the air conditioning line was frozen up. They explained to her that they would take care of it. They first went to the condensing unit and found the suction line was frozen and the side of the compressor was nearly covered with ice.
They went to the room thermostat and turned the system off and the fan to “fan-on.”
Bob said, “That turns the compressor off and the indoor fan on to melt the ice off of the coil as soon as possible. We will now check for obvious airflow problems to see if reduced airflow caused this. We can do this while the coil is defrosting.
“We will first check the air filter, and then check all of the supply registers to make sure they are open. That is about all we can do until the coil thaws and we can see the inlet side of the coil and see if it is dirty.”
Tim asked, “How will we know when the coil has thawed?”
Bob explained, “As a coil freezes the airflow becomes less and less until it is a solid block of ice and the only air that flows is around the coil ends. Eventually the ice pattern will travel down the suction line and on to the compressor. As air flows around the coil ends, it will start to melt. When I said it is solid block of ice, I meant the coil was frozen over. The ice quality is aerated ice. It has a lot of air in it because of the way it was formed. It will melt fairly quickly and we will know when it is close when air starts to flow out of the supply registers. When air is flowing, we will wait about 15 minutes and get a good look at the coil inlet to be sure the coil is not dirty. Then we will start the unit and check the coil for performance. Meanwhile, we need to take a look around and see if we can find any oil on the surface of the piping, a sure sign of a leak.”
Tim then said, “You have this all lined up in your mind, the sequence to finding out the trouble. How did you know what to do first?”
Bob said, “A very common sequence of events is for a low charge to cause the coil to begin to freeze due to a starved coil. I am assuming that is the case here, until I find out different. The other problem could be airflow. We found all of the registers open and the filter clean, so I am going to assume the coil is clean until we can see it. Meanwhile what did you find after inspection of the pipe?”
Tim responded, “There was an oil slick around the liquid line service port. I checked it with the electronic leak detector and there was a leak there. I removed the protective cap, which was not tight, and used my Schrader valve tool to tighten the valve stem. When I did that, there was no more sign of a leak.”
Bob said, “Good job, Tim. That was probably where the refrigerant went. I think we are on the right track. Go ahead and fasten gauges to the system and fasten a temperature lead on the suction line where it enters the condensing unit. We will be ready to check the unit charge when we can start the unit.”
It took about a ½ hour before the coil was thawed completely and they got a look at the coil inlet and it looked good. So now it was time to start the unit.
Bob said, “Start the unit and let’s see what we can find out.”
Tim started the unit and was looking at the gauges and said, “Look at that, the suction pressure has dropped to 30 psig. This is R-22, so the refrigerant is boiling in the coil at 7°F. It is no wonder it froze up.”
Bob asked what the temperature of the suction line was and Tim said, “75°.”
Bob then asked, “What is the superheat?”
Tim scratched his head and said, “Well, if the suction line temperature is 75° and the coil boiling temperature is 7°, I guess the superheat is 68° (75° - 7° = 68°).”
Bob said, “That is correct. If you will notice, the compressor does not have cool enough return suction gas to keep it cool and the coil cannot be performing well with so little refrigerant in it. You seem to have found the leak. Let’s charge some refrigerant into the system. What type of expansion device is in the system?”
Tim said, “I fooled you on that one. I looked and it is a fixed bore orifice. What superheat should the system have?”
Bob said, “The piping is very short. It is a split system and the suction line is only about 8 feet, so let’s say that the superheat should be 12° at the condensing unit. We are going to have to make some assumptions here. We cannot check the actual superheat at the coil, so we will assume that the tubing will gain about 2° along the way from the ambient air. The suction line is well insulated. If the suction line were longer, we would assume that it would gain more super heat due to conduction from the ambient air. I use two rules of thumb for measuring the superheat at the condensing unit:
1. When the line set is 10 to 30 feet, I expect the superheat to be from 10° to 15° measured at the condensing unit.
2. When the line set is 30 to 50 feet, I expect the superheat to be from 15° to 18° measured at the condensing unit.
“Please understand that there are some qualifications on these rules of thumb. If the unit has a charging chart furnished, you should use it. It is probably more accurate. The humidity in the conditioned space should not be excessively above or below 50 percent. Unless the unit has been off for a long time in a humid climate, you should be all right.
“Most importantly, the condensing temperature or the head pressure should be close to a design day. The outside temperature on a design day is 95° and the condensing temperature should be about 30° higher than the design outdoor temperature, or 125°. The head pressure should be about 278 psig for R-22 (Figure 1).
“The reason for this is because the air conditioning system is designed by the manufacturer to have an exact operating charge in each coil. With a fixed bore metering device, to ensure the correct amount of refrigerant in the evaporator is to have the correct pressure difference across the expansion device. With a suction pressure of about 70 psig and a head pressure of about 278 psig, the correct flow will occur across the expansion device (Figure 2). The evaporator will have the correct charge and the condenser will have the correct level of liquid refrigerant in it.”
Tim commented, “That is a lot to remember. What do you do if it is not a design temperature day?”
Bob said, “If it is not a design day, you can block the air to the condenser until you have the same head pressure as a design day and charge the unit until the evaporator has the correct superheat. The thermometer says that the outdoor temperature is 85° and we will want the head pressure to be about 275 psig to simulate a design day of 95°, so we will block some of the airflow to the condenser by either blocking the fan or putting some plastic around the condenser (Figure 3). At this time, there is not enough refrigerant in the unit to get the right head pressure. As we add refrigerant, watch the head pressure and adjust the airflow to maintain about 275 psig.”
Tim started adding refrigerant and said, “Look at the suction pressure rise and the superheat drop. The head pressure is getting about right also.”
Bob noted, “Don’t add refrigerant too fast. It is a lot easier to charge a system to the correct superheat while adding vapor than it is to get the superheat correct by removing refrigerant from the system. When it gets close, stop adding refrigerant and let the system run for a few minutes. Watch for the superheat to vary for a few minutes. With a fixed bore metering device the system will have too much refrigerant in the condenser for a few minutes, resulting in high superheat and then it will overfeed the evaporator for a few minutes. Until the system gets in balance, it will vary for a while. I call it hunting equilibrium. Once it reaches equilibrium, the pressures will maintain a steady state.”
They watched the pressures until the system stabilized and the suction pressure was 65 psig and the line temperature was 60°. Bob asked, “What is the superheat now?”
Tim said, “The line temperature is 60° and the suction pressure is 65 psig. That means the liquid in the evaporator is boiling at 38° corresponding to 65 psig. So the superheat is now 22° (60° - 38° = 22°). We must carefully add more vapor refrigerant.”
Bob agreed, “That is correct. Add a small amount of vapor and let it settle down again.”
Tim added a small amount of vapor and, after a few minutes, he recorded new readings of 70 psig and a line temperature of 52°. Bob said, “That is looking good. What is the superheat now?”
Tim said, “A reading of 70 psig corresponds to 41° and the line temperature is reading 52° so the superheat is 11° (52° - 41° = 11°). That seems perfect.”
Tim then said, “That seemed like a lot of talk to get to where we are at now, but when it came down to checking the superheat, it was not near as hard as it sounded. Let me tell you the steps that we took and see if I get them all correct:
1. Fastened gauges.
2. Fastened a temperature lead to the suction line close to where we took the suction line pressure.
3. Started the unit and observed the pressures.
4. Blocked the airflow to the condenser until we got the head pressure to correspond to the design conditions of 125° condensing temperature. For R-22 that was about 275 psig.
5. Then we added refrigerant until we had the desired superheat, which was supposed to be 10° to 15°, and we got 11°.
“Is that about right?”
Bob said, “I think that you are ready to go on this. You need to practice it without supervision and make sure you understand it. We were able to get the charge very close to the exact charge the manufacturer wanted by:
1. Creating the correct pressure drop across the orifice metering device so the correct refrigerant charge was in the evaporator and the condenser.
2. Verifying that the evaporator charge was correct.
3. Protecting the compressor.
“There is another verse to this song that we will go into next, called subcooling. This is enough for now.”
Tim said, “I agree. My head is spinning now with information.”
To see Part 1 of this series, go to Btu Buddy 124: Superheat Explained - Part 1.
To see the next article on subcooling, go to Btu Buddy 126: Gaining Efficiency with Subcooling.
Publication date: 8/19/2013