Troubleshooting techniques often require simultaneous knowledge of temperature, pressure, voltage, and current values in a system, which means that a single-function meter won't permit a complete analysis of the system. Frequently, multiple tools are required.
This article provides information on troubleshooting the refrigeration system while applying the principles of superheat and subcooling to HVACR equipment. It will also teach you the proper methods to tackle some typical troubleshooting tasks using thermometers, digital multimeters, pressure/vacuum modules, and HVACR accessories. Basic refrigeration principles are provided solely to illustrate how digital thermometers, multimeters, and accessories can make servicing and maintaining HVACR systems straightforward, fast, and accurate.
1. Compression of hot gas
7. Super heating
A basic vapor compression refrigeration system consists of four primary components: a metering device (e.g., a capillary tube, fixed orifice/piston, or a thermostatic expansion valve), evaporator, compressor, and condenser. (See Figure 1.)
Compression energy elevates the vapor pressure to a boiling point that is below the condensing mediums' temperature. In other words, the compressor elevates the boiling point of the refrigerant to a point at which the air (or water) moving across the condenser is low enough to condense the refrigerant to a liquid. Additional passes in the condenser coil cool the liquid refrigerant below its boiling point to ensure it remains a liquid as it experiences pressure drop in its journey to the evaporator. This cooling below the boiling point is called subcooling.
A metering device at the evaporator inlet acts as a "dam" to restrict flow and drop the refrigerant pressure to a new lower boiling point. This new boiling point is below the evaporator medium (air or water) temperature so that the air or water across the evaporator will cause the refrigerant to boil. After all of the refrigerant in the evaporator has boiled to a vapor, the vapor will pick up additional heat through extra passes in the evaporator. The amount of vapor temperature increase above the boiling temperature is called superheat.
The compressor reduces the gas to a high pressure while simultaneously raising the temperature of the gas. The hot gas is then delivered to the condenser where it is cooled, dissipating the heat and steadily converting the gas back to a liquid state.
Note: Liquid receivers are not typically used on refrigeration systems, which commonly rely on capillary tubes or fixed metering devices.
When the liquid under high pressure reaches the metering device, the cycle starts over.
While servicing most refrigeration systems, the technician will measure the temperature and pressure to determine system performance. Close monitoring of temperature and pressure to verify proper control and operation can ensure longer system life and reduce energy consumption.
Often, measuring temperatures or pressures at key points in a system can pinpoint trouble spots. Examples of such measurements follow.
Finding suction line superheat requires finding the suction pressure and two temperatures - the evaporator boiling temperature at a given pressure and the temperature of the refrigerant at the outlet of the evaporator on the suction line, commonly referred to as the superheat temperature/pressure method.
Finding the boiling temperature is determined by using a pressure-temperature (PT) chart. On older CFC and HCFC refrigerants, and some newer ozone-friendly refrigerants such as R-134a, boiling temperature remains constant during the saturation or boiling phase provided that the pressure remains the same within the evaporator.
On newer refrigerant blends, the temperature changes during the boiling or saturation phase. This is referred to as glide. Modern refrigerants with a temperature glide of 10Â°F (5Â°C) or higher use dew point (DP) temperature. This is the temperature of the refrigerant when the last of the liquid has boiled into a vapor. Any vapor temperature increase above the dew point temperature is called superheat. (See Figure 2.)
When measuring for superheat, remember to allow the system to run long enough for temperatures and pressures to stabilize while verifying normal airflow cross the evaporator. Using the pipe clamp or a Velcro pipe probe, find the suction line temperature by attaching the probe around a bare section of the pipe, at the outlet of the evaporator. Pipe temperature can be read at the inlet of the compressor on the suction line if the pipe is less than 15 feet from the evaporator and there is a minimum pressure drop between the two points. (See Figure 3.) Best results are obtained when the pipe is free of oxides or other foreign material.
The suction line temperature may also be taken by attaching a bead thermocouple to the suction line. Be careful to insulate the thermocouple and use heat-conducting compound to minimize errors due to heat loss to ambient air.
Note: Condensing temperature is derived from using the PT chart. On new refrigerant blends with high temperature glide, this is called the bubble point (BP) temperature. See Figure 2.
To measure subcooling with a pipe clamp temperature probe, or a Velcro pipe probe, allow the system to run long enough for temperatures and pressures to stabilize. Verify normal airflow and then find the liquid line temperature by clamping the pipe clamp around the liquid line. Attach the pressure/vacuum module to a service port on the liquid line (or discharge line at the compressor if a liquid line service valve port is not available). Make a note of the liquid line temperature and pressure. Convert the liquid line pressure to temperature using a PT chart for the refrigerant type being used. The difference of the two temperatures is the subcooling value.
Before making conclusions from the measured data, it is important to check external conditions that influence system performance. In particular, you should inspect and verify the proper airflow in cubic feet per minute (cfm) across coil surfaces and line voltage to the compressor motor and associated electrical loads. Remember to look for obvious problems at the coil surfaces such as dirty air filters upstream of the evaporator, or leaves and outside debris restricting airflow on the condenser.
A low or zero superheat reading indicates that the refrigerant did not pick up enough heat in the evaporator to completely boil into a vapor. Liquid refrigerant drawn into the compressor typically causes slugging, which can damage the compressor valves and/or internal mechanical components. Additionally, liquid refrigerant in the compressor, when mixed with oil, reduces lubrication and increases wear, causing premature failure.
On the other hand, if the superheat reading is excessive - above 20Â°F to 30Â°F - it indicates that the refrigerant has picked up more heat than normal, or that the evaporator is being starved of refrigerant. Possible causes of this condition include a metering device that is underfeeding, improperly adjusted, or simply broken. Additional problems with high superheat could indicate a system undercharge, a refrigerant restriction, moisture in the system, a blocked filter drier, or excessive evaporator heat loads.
For example, a very low reading between zero to 10Â°F subcooling indicates that the refrigerant did not lose the normal amount of heat in its travel through the condenser. Possible causes for this condition include insufficient airflow over the condenser, metering device problems such as overfeeding, misadjustment, or being stuck too far open, or the system may be undercharged. Oftentimes, the problem is simply that the condenser coil surface needs to be cleaned thoroughly to eliminate airflow restriction.
Excessive subcooling means the refrigerant was cooled more than normal. Possible explanations include an overcharged system, a restriction in the metering device, misadjusted (underfeeding), or faulty head pressure control during low ambient conditions.
Reprinted with permission from the Fluke Corp. Application Note "Troubleshooting HVAC/R systems using refrigerant superheat and subcooling." For more information, visit www.fluke.com.
Publication date: 08/07/2006