First, we have to talk about metering devices. A metering device is essentially a restrictor in the liquid line just upstream of the evaporator. Its primary purpose is to create a pressure drop in the liquid refrigerant circuit so that the evaporator can be operated at a nominal pressure of 68.5 lb (only on paper), while the high side can be maintained at a much higher pressure. Without the metering device there would be no pressure drop, which would disable the refrigerant cycle completely.
An active metering device, such as a thermal expansion valve (TXV), actively regulates the superheat of the evaporator. TXVs control the rate of refrigerant flow to match the load conditions under which the system is operating.
A passive metering device, such as a capillary tube or an orifice, does not have direct control over the rate of refrigerant flow and so has no direct control over superheat.
When evaluating the refrigerant charge of an air conditioner or heat pump in cooling mode, there are two things you must know: what kind of refrigerant the machine uses, and what kind of cooling metering device it has. TXVs and fixed devices cause the system to behave in different ways, so knowing the type of metering device a system uses is absolutely necessary for an effective evaluation of the charge.
Let me address TXV systems first. Systems with TXV cooling metering devices do not lend themselves to charging by superheat. When the system is operating with a moderately incorrect charge, the expansion valve adjusts for this and continues to maintain the correct superheat (10o to 14o F). This means you have to use other symptoms, the most important of which is subcooling, to analyze the state of the refrigerant charge.
(Note: For those of you who asked for more clarification about the use of superheat in an expansion valve system, this topic will be discussed when the weather begins to warm up next spring.)
Even though TXV systems do not lend themselves to charging by superheat, capillary tube and orifice systems do. As a matter of fact, these fixed metering device systems by their nature are easy to charge by superheat. In a fixed metering device system, the superheat is controlled by the pressure differential across the metering device.
As the load changes on a system, the differential between the low-side pressure and the high-side pressure changes. The change in pressure differential across the fixed metering device forces the refrigerant into the evaporator at differing rates. Because of the variable flow rate of the refrigerant into the evaporator, the superheat constantly changes with varying load conditions. This also makes it an excellent indicator of refrigerant charge.
(By the way, because the pressure differential across the metering device is also affected by, among other things, coil cleanliness, evaporator and condenser airflow, and internal refrigerant system malfunctions, superheat is also sensitive to these things. This is a double-edged sword for the technician; in order to use superheat to evaluate a refrigerant charge effectively, the system must be otherwise trouble-free.)
Superheat is also an excellent tool for diagnosing these other problems, but that's for another article. To keep this article manageable, I will refer to a system with no problems other than refrigerant charge variables.
Now let's get down to the nitty gritty. How can you use superheat to charge or evaluate the charge of a fixed-orifice metering device system in the cooling mode?
1. Determine what the superheat is supposed to be at current load conditions.
2. Determine the actual superheat of the machine.
3. Compare the required superheat to the actual superheat and draw a conclusion as to what condition the system is in.
I'm going to use an R-22 system in this example. To determine the required superheat, you must know the three load conditions under which the system is operating. You must know the ambient temperature entering the outdoor coil.
The indoor drybulb temperature (IDDB) and indoor wetbulb temperature (IDWB) are indicators of how much moisture is in the indoor air. These readings must be taken while the system is operating. The indoor readings should be taken as close to the inlet of the evaporator coil as humanly possible.
If you haven't taken wetbulb temperatures before, it's easy. Place a small amount of tissue paper over the temperature probe of your thermometer and wet it. While operating the system in cooling, place the probe in the return airstream. As the water evaporates out of the wet tissue, the temperature of the probe will begin to drop. Within a couple of minutes, the temperature will quit dropping. Once the temperature stabilizes, this is the wetbulb temperature.
Let's say the load conditions under which our example system is operating are 90o ambient, 75o IDDB, 67o IDWB. Now all you have to do to determine the required superheat is to plug these three load conditions into a superheat chart. The chart would say that the machine should be operating at 13o superheat under those load conditions.
Now that you know what the superheat should be (how much refrigerant should be in the evaporator), you're ready to check the actual superheat on the system to see if it matches the required superheat.
You will need two pieces of information to determine the superheat of the system: the temperature at which the liquid refrigerant is boiling inside the evaporator, and the temperature of the refrigerant exiting the evaporator coil. The easiest way to determine the boiling temperature inside the evaporator is to look on your suction gauge.
As you can see in Figure 1, there are inner rings on the face of the gauge. One of these rings will tell you the boiling temperature of R-22 at any given pressure. For example, if the needle is pointing at a pressure of 70 lb, it will cross the R-22 temperature ring at about 41o. No matter what the suction pressure is on any system you are servicing, the corresponding boiling temperature is always available at a glance. To determine the temperature of the refrigerant as it exits the evaporator, attach a thermometer probe to the suction line.
On package and split system machines, I attach the probe downstream of all the components in the suction line but just upstream of the compressor (Figure 2). Let's say the suction pressure on our system is 75 lb. Looking at the suction gauge, we can tell that the boiling temperature of the liquid refrigerant in the evaporator is 44o. If the temperature of the gases in the suction line is 64o, that means the refrigerant gas has increased in temperature 20o. In other words, the actual superheat of the system is 20o. The chart says the system should be operating at 13o superheat.
The conclusion we can draw from this is that there is not enough liquid refrigerant in the evaporator, so after the liquid boils off into a gas, it picks up more heat than it is supposed to before leaving the evaporator because there is more evaporator for it to travel through.
Here are some general hints about charging by superheat:
If, when you get the system's superheat correct, the pressures are seriously out of whack, the machine has another problem.
Each manufacturer publishes its own superheat chart for use with its machines. There are slight differences between these charts. (Note: I'm conflicted as to whether to recommend using my chart or the factory charts. Normally I tend to recommend that technicians use factory everything; they don't pay those engineers good money for nothing. But in this case, it really is a judgment call.)
The problem with many (not all) factory superheat charts is that they assume a 50% indoor relative humidity. This makes them somewhat inaccurate if the actual indoor humidity is higher or lower than 50%. Which chart you use is probably not as important as the fact that you are aware of the discrepancies.
If you think this information can help you - and it can - apply it immediately. The biggest pitfall I see with my students is that they wait too long to start using the information. If you're like me, you have to "get your hands dirty" to cement the stuff into your brain.
Once you've got superheat under your belt, it will be time to move on to using subcooling measurements. More on that in my next article.