Air conditioning technicians 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.
Here is an example of a air conditioning line that had frozen up and some explanations about it.
First, turn the thermostat from the fan in the "Auto" poisition (where it should be normally set) to the "Fan-On" position. That turns the compressor off and the indoor fan on to melt the ice off of the coil as soon as possible. Of course, the filter should be checked to make sure it is not dirty and restricting airflow and the supply vents should be check to make sure they are all open. That is about all that can be done while the coil is thawing.
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. In this case, 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 when air is flowing, then a good look can be taken at the coil inlet to be sure the coil is not dirty. Then the unit will be started and the coil checked for performance. In the meanwhile, a look around to see if any oil can be found on the surface of the piping, a sure sign of a leak. There was oil around the liquid line service port. Checking with an electronic leak detector proved there was R-22 refrigerant leaking. Apparently the valve stem was not tightened. Tighten the valve stem and the refrigerant is no longer leaking.
Starting the unit and looking at the gauges it appeared the suction pressure had dropped to 30 psig. This is R-22, so the refrigerant is boiling in the coil at 7°F°, the superheat is 68° (75° - 7° = 68°).
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. There is a fixed bore orifice on this system. What superheat should the system have?
On this system, 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. You can use two rules of thumb for measuring the superheat at the condensing unit:
1. When the line set is 10 to 30 feet, we expect the superheat to be from 10° to 15° measured at the condensing unit.
2. When the line set is 30 to 50 feet, we 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.
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. The evaporator will have the correct charge and the condenser will have the correct level of liquid refrigerant in it.
What do you do if it is not a design temperature day?”
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. 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.
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. Sometimes referred to hunting equilibrium. Once it reaches equilibrium, the pressures will maintain a steady state.
When the system stabilized and the suction pressure was 65 psig and the line temperature was 60°. Then what was the superheat?
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°). More vapor refrigerant must be added carefully.
After a small amount of vapor was added, and after a few minutes, new readings of 70 psig and a line temperature of 52°. What is the superheat now?
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.
Let me tell you the steps that we took:
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°.
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.
The next step that will be covered is subcooling.
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