Nova Kool Manufacturing ULC

Basic Refrigeration Principles

Most users normally associate refrigeration with cold and cooling, yet the practice of refrigeration engineering deals almost entirely with the transfer of heat. This seeming paradox is one of the most fundamental concepts that must be grasped to understand the workings of a refrigeration system.

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

Absolute Pressure & Gauge Pressure

Absolute Pressure
Absolute pressure, normally expressed in terms of pounds per square inch absolute (psia) is defined as the pressure existing above a perfect vacuum. Therefore in the air around us, absolute pressure and atmospheric pressure are the same.

Gauge Pressure
A pressure gouge is calibrate to read 0 pounds per square inch when not connected to a pressure producing source. Therefore the absolute pressure of a closed system will always be gauge pressure plus atmospheric pressure. Pressures below 0 psig are actually negative readings on the gauge, and are referred to as inches of vacuum. A refrigeration compound gauge is calibrated in the equivalent of inches of mercury for negative readings. Since 14.7 psi is equivalent to 29.92 inches of mercury, 1 psi is approximately equal to 2 inches of mercury on the gauge dial.

It is important to remember that gauge pressures are only relative to absolute pressure. Table 1 shows relationships existing at various elevations assuming that standard atmospheric conditions prevail.

Table 1 - Pressure Relationships at Varying Altitudes

Pressure Relationships at Varying Altitudes.PNG

The absolute pressure in inches of mercury indicates the inches of mercury vacuum that a perfect vacuum pump would be able to reach. Therefore, at 5,000 feet, elevation under standard atmospheric conditions, a perfect vacuum would be 24.89 inches of mercury, as compared to 29.92 inches of mercury at sea level.

At very low pressures, it is necessary to use a smaller unit of measurement since even inches of mercury are too large for accurate reading. The micron, a metric unit of length, is used for this purpose, and when we speak of microns in evacuation, we are referring to absolute pressure in units of microns of mercury.

A micron is equal to 1/1000 of a millimetre and there are 25.4 millimetres per inch. One micron, therefore, equals 1/25,400 inch. Evacuation to 500 microns would be evacuating to an absolute pressure of approximately .02 inch of mercury, or of standard conditions, the equivalent of a vacuum reading of 29.90 inches mercury.

Katey Werner