How To Measure Superheat

Understanding superheat and its relation to a refrigeration system can help determine if the system is operating properly

BY ALAN B. CLAY JR.


Superheat: the heat added to refrigerant vapor after the vapor has changed state. Simple as this may seem, many hvacr technicians don't fully understand superheat or it's importance in relation to a refrigeration system.

Ordinarily, service technicians are concerned with superheat readings taken at two different places in the refrigerant system: at the outlet of the evaporator coil and at the compressor, 8 to 12 inches from compressor on the suction line (see Figure 1).

Finding superheat at the evaporator coil or at the compressor is relatively easy. First, the technician must use his or her compound (low side) gauge to get the boiling or evaporating pressure of the coil. Next, we have to convert this pressure to its corresponding temperature with the use of a pressure/temperature chart.

(Note: All pressure temperature charts are not the same. Some have the psig reading in the column to the far left and you have to look under the correct refrigerant to find the corresponding temperature. Others have the temperature reading in the far left column and you have to look under the correct refrigerant to find the right pressure.)

On an R-12 system, if the evaporating pressure is 21 psig, converting this to temperature you get 20° F(see Figure 2). You now have to find the temperature of the suction line at the outlet of the evaporator coil. To do this, you should use a good strap-on thermometer. Most wholesalers carry thermometers designed specifically to take temperature measurements from refrigerant lines. Be sure to properly insulate the temperature sensing element to insure an accurate reading.

As an example, let's say the suction line temperature at the evaporator outlet is 27° F. You then subtract the temperature you converted from the pressure reading at A, which is 20° F, from the temperature of the line at B, which is 27° F (see Figure 3). On this particular sys-tem the superheat at the evaporator coil is 7° F: that is, 27° F ­ 20° F = 7° F superheat.

The procedure for calculating compressor superheat is similar. Using your thermometer, find the temperature of the suction line 8 to 12 inches from the compressor (location C in Figure 3). In our example, this temperature is 47° F. From this subtract the temperature you found from the pressure/temperature chart at location A in Figure 3. The superheat at our compressor is 27° F: 47° F ­ 20° F = 27° F superheat.

Superheat Applications

Finding the correct superheat is not enough. Hvacr mechanics and technicians must be aware of the proper superheat on any given type of equipment. Generally, tables that give approximate superheat temperatures are referring to superheat readings that are taken when the equipment is at the design conditioned load or space temperature.

Under normal operating conditions, superheat measurements that are taken when the box is hot are high and don't give an accurate picture of what's happening inside the coil. Although manufacturers differ slightly on what they consider "correct" superheat, there are some general rules a technician can use.

For air-conditioning applications, the superheat at the coil should be between 10° F and 15° F depending on the ambient temperature and load conditions. For medium-temperature applications, 6° F to 10° F superheat is normal. On low-temperature boxes, 4° F to 8° F is generally recommended. Remember, these figures are meant to be used only as a guide and the manufacturers' recommended superheat settings should be used at all times when possible.

You may wonder why this is important? Having the correct superheat at the evaporator is important. Superheat is a technician's window to how the evaporator is performing. Too much superheat (see Figure 4) means that the evaporator is not receiving enough refrigerant and, therefore, its capacity is diminished.

Too little superheat (see Figure 5) means that the evaporator is being over-fed with refrigerant. When the superheat is correct we know that the proper amount of refrigerant is inside the evaporator and the coil is operating at its maximum capacity.

Superheat readings taken at the compressor allow the technician to determine if liquid refrigerant is flooding back to the compressor. The temperature you found in the evaporator is the saturated temperature of the refrigerant being used at a certain pressure.

In our example, the pressure is 21 psig and the saturation temperature is 20° F. This is the temperature at which the refrigerant is "boiling" or evaporating, but you have to remember that some liquid is still present. To ensure that the remaining liquid will boil off, additional heat above the saturation temperature must be added to the refrigerant, thus the term superheat.

So, if there is superheat at the compressor, we know that vapor is entering the compressor. However, it's not enough to know that vapor is entering the compressor; equally important is the temperature of the vapor.

Remember, reciprocating compressors use suction gas as a method of cooling the motor windings. If the suction gas is too hot, the temperature of the windings also will be too hot. Proper insulating of the suction line and correct sizing of heat exchangers, along with correct evaporator superheat, will assist in achieving the correct return gas temperature. Most compressor manufacturers recommend return gas temperatures to the compressor no higher than 65° F or 70° F to guarantee proper cooling of the motor windings.

Ensuring Correct Superheat

For most service technicians, with the exception of those who work on chillers, soft-serve ice cream machines and other specialized equipment, the vast majority of equipment encountered in the field will use a thermostatic expansion valve (TEV), a capillary tube or some other type of fixed bore metering device.

The TEV, or TXV as some technicians call it, maintains a constant superheat at the outlet of the evaporator. If you are working on a system with a TEV metering device, be sure to check the entire system operation before adjusting the valve.

Improper line size, over-charging, undercharging, improper positioning of the sensing bulb, too great of a load, improperly sized equipment and flashing off in the liquid line all can and will cause the superheat to be incorrect. If it's determined that the incorrect superheat is a result of the valve not being set correctly, make the necessary adjustments according to the valve manufacturer's instructions.

The superheat on capillary tube and other systems that use some type of fixed bore restriction as a metering device can't be adjusted without adding or removing refrigerant. If you are sure the system has the proper charge but the wrong superheat, normally the metering device isn't the problem unless it was changed and the wrong size installed - a larger fixed bore restriction will decrease superheat while a smaller fixed bore will increase superheat - or it may have a full or partial restriction.

Frequently in the field, a replacement compressor may be undersized or oversized. The same may be true of other components that have been replaced in the system such as evaporators. In either case, the superheat will be affected.