Most users typically associate refrigeration through cold and also cooling, however the exercise of refrigeration engineering deals practically entirely v the transfer of heat. This seeming paradox is among the most fundamental concepts that need to be grasped to understand the functions of a refrigeration system.

Cold is really just the lack of heat, simply as darkness is the lack of light, and dryness is the absence of moisture.

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Absolute PressureAbsolute pressure, normally expressed in regards to pounds every square inch pure (psia) is identified as the press existing over a perfect vacuum. As such in the air around us, absolute pressure and atmospheric push are the same.

Gauge PressureA push gouge is calibrate to read 0 pounds per square inch when not associated to a pressure producing source. Because of this the absolute press of a closed system will always be gauge press plus atmospheric pressure. Pressures below 0 psig are actually an adverse readings on the gauge, and also are described as customs of vacuum. A refrigeration compound gauge is calibrated in the tantamount of inch of mercury for negative readings. Since 14.7 psi is identical to 29.92 inches of mercury, 1 psi is roughly equal to 2 inches of mercury ~ above the gauge dial.It is vital to remember the gauge pressures are only relative to absolute pressure. Table 1 mirrors relationships currently at various elevations suspect that standard atmospheric conditions prevail.

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Table 1 - pressure Relationships at differing Altitudes


The absolute push in inches of mercury indicates the inch of mercury vacuum that a perfect vacuum pump would be able to reach. Therefore, at 5,000 feet, elevation under conventional atmospheric conditions, a perfect vacuum would be 24.89 inches of mercury, as compared to 29.92 inch of mercury in ~ sea level.At an extremely low pressures, that is important to usage a smaller unit that measurement due to the fact that even inches of mercury are too big for accurate reading. The micron, a metric unit that length, is offered for this purpose, and also when we speak that microns in evacuation, we space referring to absolute press in units of microns the mercury.A micron is equal to 1/1000 of a millimetre and also there space 25.4 millimetres per inch. One micron, therefore, equals 1/25,400 inch. Evacuation come 500 microns would be evacuating to an absolute push of roughly .02 customs of mercury, or of typical conditions, the identical of a vacuum reading of 29.90 inch mercury.