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PRESSURE, TEMPERATURE, AND VOLUME

We said earlier that it is important that you understand some of the ways in which pressure affects liquids and gases and some of the relation-ships between pressure, temperature, and volume in gases.

The boiling point of any liquid varies according to the pressure on the liquid-the higher the pressure, the higher the boiling point. You should remember that condensing a gas to a liquid is just the reverse process of boiling a liquid until it vaporizes. The same pressure and temperature relationship is required to produce either change of state.

Water boils at 212°F at the atmospheric pressure of 14.7 psia, at 80°F under a vacuum of 29 inches of mercury, and at 489°F at a pressure of 600 psig. Refrigerants used in vapor com-pressor cycle equipment usually have much lower boiling points than water under any given pressure. However, these boiling points also vary according to pressure. At atmospheric pressure, for example, the refrigerant R-12 boils at - 21.6°F. At 30 psig, R-12 boils at 32°F, which is the freezing point of water. You should see that R-12 cannot exist as a liquid at ordinary temperatures. It must be confined within a container or closed space.

When the temperature of a liquid is raised to the boiling point corresponding to its pressure and if application of heat is continued, the liquid will begin to boil and vaporize. The vapor that is formed will remain at the same temperature as the boiling liquid as long as it is in contact with the liquid. A vapor CANNOT be superheated as long as it is in contact with the liquid from which it is being generated.

The pressure-temperature-volume relation-ships of gases are expressed by Boyle’s law, Charles’s law, and the general gas law or equation. We will briefly discuss each of these laws.

BOYLE’S LAW states that the volume of any dry gas varies inversely with its absolute pressure, provided the temperature remains constant. This law may also be expressed as the formula wherein V1 is the original volume of the gas, P1 is its original absolute pressure, V2 is its new volume, and P2 is its new absolute pressure.

CHARLES’S LAW states that the volume of a gas is directly proportional to its absolute temperature, provided the pressure is kept constant. The equation for Charles’s law is

The GENERAL GAS EQUATION combines Boyle’s law and Charles’s law. It expresses the interrelationship of the volume, the absolute pressure, and the absolute temperature of gases. The general gas law is expressed by the formula

In Boyle’s law, Charles’s law, and the general gas law, the equations indicate the nature of the interrelationship of the pressure, the volume, and the temperature of any gas. You probably will not find it necessary to use the equations themselves, but you should have a thorough understanding of the principles they express. Let’s summarize them:

1. When TEMPERATURE is held constant, increasing the pressure on a gas causes a proportional decrease in volume. Decreasing the pressure causes a proportional increase in volume.

2. When PRESSURE is held constant, increasing the temperature of a gas causes a proportional increase in volume. Decreasing the temperature causes a proportional decrease in volume.

3. When the VOLUME is held constant, increasing the temperature of a gas causes a proportional increase in pressure. Decreasing the temperature causes a proportional decrease in pressure.

In discussing the effects of pressure on a gas, we have pointed out that the volume and the temperature of gas are different AFTER the pressure has been changed. It is important to note, however, that a temperature change normally occurs in a gas WHILE the pressure is being changed. Compressing a gas raises its temperature; allowing a gas to expand lowers its temperature. As you will see, these two facts are important to your understanding of the operating principles of the refrigeration cycle.



 


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