Capacity - Using the Compressor to Find the Amount of Cooling

Imagine the system is running with the key pressures and temperatures are being recorded at set intervals, say 15 seconds. Each set of readings transmits enough information to enable the computer to plot the cycle on the P-h diagram. The compressor efficiency and COP of the process at each time interval is calculated by the computer. To find out how much cooling is being done, the mass flow rate of the refrigerant is needed. The term Capacity is used to denote the quantity of cooling. Other terminology includes Heat Extraction Rate, or Cooling Effect. It is measured in kW (Btu/hr in the old days).

Evaporator-Compressor-Condenser-Expansion Valve Circuit

Installation of a refrigerant flow meter would be an obvious method to find the mass flow rate, but this is costly, and intrusive. The compressor can be used as a flow meter. If the volume pumping rate of the compressor is known this will enable the mass flow rate to be found simply from the density of the fluid at the compressor inlet condition 2. Most compressors have a defined displacement (or swept volume), but the volumetric efficiency may not be known. Indeed loss of pumping by the compressor may be the object of the investigation. However there is one input which can be measured with good accuracy and at reasonable cost without disturbing the refrigeration system. This is the electrical power input to the compressor.

Compressor Heat Balance - Electrical Energy IN = Enthalpy Gain + Heat Loss

Mass Flow Rate
Measurement of the Electrical Energy input, E allows the mass flow rate to be determined from the simple relationship:

  

where h are the enthalpy values, per unit mass of the refrigerant. These values are known from the temperature and pressure measurements. The heat loss H, expressed as a percentage of E, is between 5 and 7% for most compressor types. Once the mass flow rate is known, the rate of heat flow at the evaporator and condenser can be calculated using the temperature and pressure measurements elsewhere in the ciricuit.

An explanation of measurement points, efficiency calculations, and heat loss estimation is in the Efficiency page.

An explanation of the P-h diagram, Practical Refrigeration Cycle, Evaporation and Condensation is in the the Understanding Refrigeration pages.



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