Tech method lesson 2A.txt
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- High superheat
- Superheat measurements are taken at the compressor inlet.
- Most compressors are refrigerant cooled so a steady source of cooled refrgerant vapor is essential to assure their survival.
- When you add heat to a liquid it boils or in other words evaporates.
- The evaporator must have a steady supply of liquid refrigerant from the condenser to do its job. this is why we check subcooling before measuring the superheat.
- The difference between the evaporating temperature and the compressor inlet temperature will tell us if there is not enough refrigerant in the low side (high superheat) or too much refrigerant (low superheat).
- The superheat must be low enough to flood the evaporator and cool the compressor but not low enough to flood the compressor.
Low head pressures (low condensing temperatures) will reduce the flow rate of the refrigerant through the metering device causing the superheat to be higher than normal. This is why we must raise the condensing temperature to 100 -110 degrees before taking superheat and subcooling measurement.
When the conditioned space is above design condition range then we must expect the superheat to be higher than normal (this is particularly true on fixed orifice systems).
- In general anything over 30 degrees superheat is considered excessive consult the manufactures specifications always.
- High superheat is caused by undercharge or restriction.
- Under charge is identified by high superheat and low or no subcooling.
- Unless you have reason to belive the system was not charged properly it is reasonable to assume you have a leak.
- Locate and repair the leak before continuing.
- Holding back the flow of refrigerant causes the refrigerant to accumulate in the high side while reducing the amount on the low side.
- Therefore a restriction is identified by a combination of high superheat and high to normal subcooling.
- The point of restriction must be found.
- By taking temperature measurements across the liquid line and liquid line devices we are able to find the restriction.
- A temperature drop across the liquid line of 5 degrees or less indicates the liquid line and devices are not restricted.
- A temperature drop across the liquid line of more than 5 degrees indicates the liquid line or its devices are restricted.
- Check across each device to find the point of restriction.
- If we cannot find a restriction in the liquid line then it is reasonable to assume the metering device is restricted.
- Replace it and never clean or adjust a metering device. If the metering device is a piston replace the entire assembly when able.
Never raise the superheat beyond the point where the discharge line of the compressor exceeds 215 degrees. exceeding this temperature may cause the compressor to overheat.
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