AIR AND ITS PROPERTIES

Properties of AirConditioning air means changing the properties of the air. The air must be clean and contain the proper amount of heat and the proper amount of heat and the proper amount of moisture in order to create comfort conditions.

Since sheet metal workers install the equipment and systems that condition the air, they should understand the properties of air and how these properties can be changed for comfort and health.

Air has four important properties that you should know:

• Air is a mixture of different gases.• Air has weight.• Air expands when heated.• Air contains varying amounts of water vapour

Air is a Mixture of Gases

Earth’s atmosphere is like an ocean of air that surrounds us although we cannot see it. Ordinarily air is composed of approximately 78% nitrogen, 21% oxygen, and 1% miscellaneous gases. It also contains moisture.

Air Has Weight

Air is a combination of gases and has weight like any other material. Standard air is the condition of air that is used for many calculations and comparisons:

• Standard air is dry air at 70°F and a barometric pressure of 29.92 Hg (mercury) at sea level. It weighs about 0.075 pounds per cubic foot, or 1.2 kilograms per cubic meter. Dry air contains no moisture. (Ordinary air always contains some moisture.)

The weight of air under other conditions varies slightly from standard air. However, standard air conditions can be used for HVAC work if the temperature is between 40°F and 100°F and the elevation is less than 2000 feet. For other conditions, adjustments may be necessary.

One cubic foot of standard air weighs 0.075 pounds. Therefore one pound of air fills 13.33 cubic feet.

1 lb. of air = __1___0.075

1 lb. of air = 13.33 cubic feet

In metric units, one kilogram of air is 0.83 cubic meter of air.

Air Expands When Heated

Like most substances, air expands when it is heated and contracts when it is cooled. When a cubic foot of air is heated, it expands to a larger volume than one cubic foot. Since the original weight of one cubic foot of air has expanded, one cubic foot of the heated air weighs less than the original cubic foot of unheated air. For example, a cubic foot of standard air at 70°F weighs 0.075 pounds. A cubic foot of the same air at 100°F weighs 0.075 pounds.

Air Contains Water Vapor

All air contains some water vapor, but the amount varies. A dry climate has little water vapor in the air. On a muggy day, there is a large amount of water vapor in the air.

Hot air can hold much more water vapor, than cold air. This is another property of air that is important for HVAC systems.