The superheat thermometer is used to check for the correct temperature differential of the refrigerator gas. The in- let and outlet sides of the evaporator coil have to be measured to obtain the two temperatures. The difference is obtained by subtracting.
Test thermometers are available in boxes. See Fig. 1-30. The box protects the thermometer. It is important to keep the thermometer in operating condition. Several guidelines must be followed. Figure 1-31 illustrates how to keep the test thermometer in good working condition.
Fig. 1-30 Test thermometer. (Marsh)
Preventing kinks in the capillary is important. Keep the capillary clean by removing grease and oil. Clean the case and crystal with a mild detergent.
SUPERHEAT MEASUREMENT INSTRUMENTS
Superheat plays an important role in refrigeration and air-conditioning service. For example, the thermostatic expansion valve operates on the principle of superheat. In charging capillary tube systems, the superheat measurement must be carefully watched. The suction line superheat is an indication of whether the liquid refrigerant is flooding the compressor from the suction side. A measurement of zero superheat is a definite indicator that liquid is reaching the compressor. A measurement of 6 to 10°F (-14.4 to -12.2°C) for the expansion valve system and 20°F (6.7°C) for capillary tube system indicates that all refrigerant is vaporized before entering the compressor.
The superheat at any point in a refrigeration system is found by first measuring the actual refrigerant temperature at that point using an electronic thermometer. Then the boiling point temperature of the refrigerant is found by connecting a compound pressure gage to the system and reading the boiling temperature from the center of the pressure gage. The difference be- tween the actual temperature and the boiling point temperature is superheat. If the superheat is zero, the refrigerant must be boiling inside. Then, there is a good chance that some of the refrigerant is still liquid. If the superheat is greater than zero, by at least 5°F or better, then the refrigerant is probably past the boiling point stage and is all vapor.
The method of measuring superheat described here has obvious faults. If there is no attachment for a pressure gage at the point in the system where you are measuring superheat, the hypothetical boiling temperature cannot be found. To determine the superheat at such a point, the following method can be used. This method is particularly useful for measuring the refrigerant superheat in the suction line.
Instead of using a pressure gage, the boiling point of the refrigerant in the evaporator can be determined by measuring the temperature in the line just after the expansion valve where the boiling is vigorous. This can be done with any electronic thermometer. See Fig. 1-32. As the refrigerant heats up through the evap- orator and the suction line, the actual temperature of the refrigerant can be measured at any point along the suction line. Comparison of these two temperatures gives a superheat measurement sufficient for field service
Fig. 1-31 How to take care of the thermometer? (Marsh)
Fig. 1-32 Hand-held electronic thermometer. (Amprobe)
unless a distributor-metering device is used or the evaporator is very large with a great amount of pressure drop across the evaporator.
By using the meter shown in Fig. 1-33, it is possible to read superheat directly, using the temperature differential feature. Strap one end of the differential probe to the outlet of the metering de- vice. Strap the other end to the point on the suction line where the superheat measure is to be taken. Turn the meter to temperature differential and the superheat will be directly read on the meter.
Figure 1-34 illustrates the way superheat works. The bulb “opening” force (F-l) is caused by bulb temperature. This force is balanced against the system back-pressure (F-2) and the valve spring force (F-3). The force holds the evaporator pressure within a range that will vaporize the entire refrigerant just before it reaches the upper part or end of the evaporator.
The method of checking superheat is shown in Fig. 1-35. The procedure is as follows:
Fig. 1-33 Electronic thermometer for measuring superheat. The probes are made of thermo-couple wire. They can be strapped on anywhere with total contact with the sur- face. This thermometer covers temperatures from –50° to 1500°F on four scales. The temperature difference between any two points directly means it can read superheat di- rectly. It is battery operated and has a ±2 percent accuracy on all ranges. Celsius scales are available. (Thermal Engineering)
Fig. 1-34 How superheat works. (Parker-Hannefin)
Fig. 1-35 Where and how to check superheat? (Parker-Hannefin)
- Measure the temperature of the suction line at the bulb In the example, the temperature is 37°F.
- Measure the suction line In the example, the suction line pressure is 27 psi.
- Convert the suction line pressure to the equivalent saturated (or liquid) evaporator temperature by using a standard temperature-pressure chart (27 psi = 28°F).
- Subtract the two The difference is superheat. In this case, superheat is found by the formula: 37°F – 28°F = 9°F Suction pressure at the bulb may be obtained by either of the following methods:
- If the valve has an external equalizer line, the gage in this line may be read
- If the valve is internally equalized, take a pressure gage reading at the compressor base Add to this the estimated pressure drop between the gage and the bulb location. The sum will approximate the pressure at the bulb. The system should be operating normally when the superheat is between 6 and 10°F (-14.4 and -12.2°C).
