Introduction to Compression Ratio:
The compression ratio of a compressor is calculated by dividing the head pressure (discharge pressure) by the suction pressure.
These pressures must be in absolute pressure (psia). To convert gauge readings (psig) to psia, simply add 14.7 (or 15) to each value.
Impact of Compression Ratio on Temperature:
An excessive compression ratio leads to high discharge temperatures, which can affect the thermal stability of the oil inside the compressor.
Compressor oil is generally stable up to about 300 degrees Fahrenheit. Keeping the compression ratio within acceptable limits helps maintain safe operating temperatures.
Example of Compression Ratio Calculation:
With a suction pressure of 83 psia and a discharge pressure of 275 psia, the compression ratio is 3.3:1, resulting in a discharge vapor temperature of about 175°F.
If the suction pressure drops to 25 psia while the discharge pressure remains at 275 psia, the compression ratio increases to 11:1, raising the discharge temperature to about 280°F.
Pressure-Enthalpy Diagram:
The relationship between suction pressure, discharge temperature, and compression ratio can be visualized using a pressure-enthalpy diagram.
For an air conditioning application with a 40°F evaporator temperature (83 psia), the compression ratio is 3.3:1, producing a discharge temperature of 175°F.
For a freezer application, lowering the evaporator temperature to -20°F (25 psia) increases the compression ratio to 11:1 and the discharge temperature to 280°F.
Risks of High Compression Ratios:
In ultra-low temperature systems, such as a glass freezer with an evaporator temperature of -40°F (15 psia), the compression ratio can reach 18:1, resulting in a dangerously high discharge temperature of 320°F.
Such high temperatures can overheat the oil at the discharge valve, leading to potential compressor failure.
No comments:
Post a Comment