Humidity: is a general term used to refer to the water vapor (moisture) content of air. When this term is used, it is usually in reference to the sensation (or lack of) of moisture in the air. For purposes of heating and cooling conditions, the more narrowly defined terms of absolute humidy, relative humidity, and specific humidity are used.
Water vapor is actually steam at low termperatures and consequently, low pressure; hence its properties are those of steam. According to Dalton's law, in any mechanical mixture of gases each has has a partial pressure of its own, which is entirely independent of the partial pressures of the other gases of the mixture.
In all air conditioning calculations it should be understood that the dry air and water vapor composing the atmosphere are separate entities, each with its own characteristics. Water vapor is not dissolved in the air in the sense that it loses its own individuality and merely serves to moisten the air.
Cold air is saturated when it contains very small quantities of water vapor, wheras warm air is not saturated until it contains Fahrenheit is saturated when it contains but one-half of one grain when it contains 8 grains of vapor per cubic foot, while at 83 degrees, 12 grains per cubic foot are required to saturate.
Absolute Humidity: is the actual mass of water vapor in one cubic foot of air (the weight of water vapor per unit volume of air) and is experessed in grains or pounds per cubic food (1 lb = 7000 grains) or grams per cubic centimeter. Absolute humidity is equivalent to the dentistry of the air.
Specific Humidity: is the weight of water vapor per pound of dry air. Do not confuse specific humidity with relative humidity. Ther latter term indicates the percentage of water vapor, the former the weight.
Relative Humidity: is the ratio of absolute humidity to the maximum possible density of water vapor on the air at the same temperature. In other words, it is a percentage or ratio of water vapor in the mixture of dry air and water vapor at a certain termperature relatvie to the maximum quantity that the volume of air could possibly carry at that temperature. The relative humidity at any given temperature can be ovtained by first using a sling psychromter to determine the amount of moise actually present in the air and then dividing this figure by the amount of moisture that the air can hold at the temperature, and multiplying the result by 100 in order to obtain the percentage factor.
Drying Effect of Air: varies approximately inversely with its relative humidity. In other words, the drying effect decreases as the relative humidity increases. It should be noted that it is relative humidity that determines the drying effect of air and this effect depends both on the temperature and the water content of the air since relative humidity depends on both these factors.
Dew Point: is the temperature of saturation for a given atmospheric pressure. In other words, for a given atmospheric pressure, it is the temperature at which moisture behins to condense in the form of tiny droplets or dew.
Humidification: may be defined as the addition of moisture to the air. The conditioning machine that functions to add moisture to the air is called a humidifier. A humidifier is commondly a low pressure, low-temperature boiler in which the water is avaporated and vapor thus formed is caused to mixed with air.
As an Tucson air conditoining technician, humidity plays into a very small part of our job as the climate in southern Arizona has a low humidity. Our air conditioners can have more effect on our dry air than they could in a more humid climate such as Kansas or Minnesota. Our air conditioners do not have to work as hard. Today for instance, our humidity is 18% in Tucson and in Kansas it is 42%.
(Article content was taken from James Brumbaugh's book "HVAC Fundamentals: Air conditioning, Heat Pumps, and Distribution Systems (Volume 3)"
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