HVACR technicians who are proficient at systematic troubleshooting not only use proper instrumentation for measuring temperatures and pressures, but also usually have highly developed organizational skills. These skills will often help them quickly solve the toughest problems they may encounter in the service field.
However, troubleshooting modern refrigeration and air conditioning equipment also requires a thorough knowledge of the refrigeration systems and its functions. One of the best ways for service technicians to organize their thoughts is through the use of a service checklist, such as the one below:
|Compressor discharge temperature|
|Condenser outlet temperature|
|Evaporator outlet temperature|
|Compressor inlet temperature|
|Refrigerated space temperature|
|Low-side pressure (evaporating) in psig|
|High-side pressure (condensing) in psig|
|Calculated Values (°F)|
By simply measuring and recording the first six system temperatures listed in the Measured Values table above, taking the low- and high-side pressure readings from a gauge manifold set and converting them to temperature, and getting the compressor's voltage and amperage readings, a technician will be able to thoroughly troubleshoot any refrigeration or air conditioning system quickly.
With this information, the service technician can calculate the four values listed in the table above. Those calculations are as follows:
Condensing temperature - Ambient temperature = Condenser split
Condenser outlet temperature - Condenser temperature = Condenser subcooling
Evaporator outlet temperature - Evaporator temperature = Evaporator superheat
Compressor outlet temperature - Evaporator temperature = Compressor superheat
Let’s apply the system checklist to a refrigeration system that is not operating properly. Assume that this system has components that were originally sized properly and are still in operation. The system is a low-temperature, TXV/receiver system that uses R-134a as the refrigerant, and the desired box temperature is 50°F. On a service call, the technician records the following values:
|Compressor discharge temperature||250°F|
|Condenser outlet temperature||110°F|
|Evaporator outlet temperature||10°F|
|Compressor inlet temperature||25°F|
|Refrigerated space temperature||15°F|
|Low-side pressure (evaporating) in psig||6.2 psig (0°F)|
|High-side pressure (condensing) in psig||185.5 psig (125°F)|
|Calculated Values (°F)|
The symptoms for this system include:
- High discharge temperatures;
- High condensing pressures;
- High condenser splits;
- Normal-to-high condenser subcooling;
- Normal-to-high evaporator pressures;
- Normal superheats;
- High compression ratios; and
- High amp draw.
High discharge temperatures could indicate the outside of the condenser coil is plugged with dirt, grease, weeds, cottonwood fuzz, or dust, or a fan may be inoperable, causing poor airflow. The high discharge temperature is caused by the refrigeration system not being able to reject heat in the condenser: the condensing pressure rises and because of the pressure/temperature relationship, the condensing temperature will then rise. The high heat of compression from the high compression ratio also causes the discharge temperature to be high.
Since heat from the evaporator, suction line, motor, and heat of compression is rejected in the condenser, the condenser coil must be kept clean and have the proper amount of airflow going through it. A dirty condenser or restricted airflow across the condenser cannot reject this heat fast enough, so there will be higher condensing pressure and temperature. Once the temperature is elevated, the condenser split will become greater and heat will be rejected at the required rate. However, the system will be operating at elevated condensing temperatures and pressures, causing unwanted inefficiencies from high compression ratios
As the condensing temperature rises farther above the ambient, the temperature difference between the ambient and the condensing temperature will become greater. This is defined as a higher condenser split. At higher splits, heat can now be rejected because a greater temperature difference will enhance heat transfer. The system will suffer at these higher condensing pressures and temperatures because of the unwanted low volumetric inefficiencies from the higher compression ratios.
High condensing pressures cause high compression ratios, which in turn, cause low volumetric efficiencies that cause low refrigerant flow rates. Low flow rates will not form much liquid subcooling; however, what subcooling is formed in the condenser will be at an elevated temperature and will reject heat to the ambient faster because of the higher condenser split. Because of this faster heat rejection, the liquid in the condenser will cool faster and have a greater temperature difference when compared to the condensing temperature. This is one of the big differences between an overcharge of refrigerant and a blocked condenser. An overcharge of refrigerant can cause very high condenser subcooling, while a blocked condenser will have normal-to-high subcooling.
The TXV will try to maintain a constant amount of evaporator superheat, but because of the low refrigerant flows caused by low volumetric efficiencies, the evaporator may not keep up with the heat load. This could cause high refrigerated space temperatures and thus a bit higher evaporator pressures. Again, the TXV could be letting a little too much refrigerant out during the beginning of its opening stroke from the higher head pressures, causing a bit higher evaporator pressures. Otherwise, because the TXV is maintaining superheat and doing its job, there could be normal evaporator pressures — it all depends on the severity of the condenser's blocked condition.
The TXV will be maintaining the set evaporator superheat unless the condensing pressure exceeds the range of the valve. Each TXV has a pressure range that it can operate within, so consult the TXV manufacturer for more precise information on TXV temperature and pressure ranges.
The higher condensing pressures cause higher compression ratios, which lead to lower volumetric efficiencies.
Based on the system checklist and an analysis of the values, this system either has a dirty condenser or restricted airflow over the condenser. Hopefully this illustrates how using a system checklist can be an invaluable tool for service technicians to organize their thoughts along with recording and analyzing system temperatures and pressures for fast system diagnostics.