Air Conditioning Fundamentals PDF

___________________________________________________________________________ 1. AIRCONDITIONING FUNDAMENTALS 1.0. BASIC TERMINOLOGIES 1.0.1 Kilo-Calorie It is defined as the amount of heat to be added (removed) to raise (lower) the temperature of one Kg of water by one degree...

___________________________________________________________________________ 1. AIRCONDITIONING FUNDAMENTALS 1.0. BASIC TERMINOLOGIES 1.0.1 Kilo-Calorie It is defined as the amount of heat to be added (removed) to raise (lower) the temperature of one Kg of water by one degree celesius. 1.0.2 BTU It is defined as the amount of heat to be added (removed) to raise (lower) the temperature of one pound of water by one degree Fahrenheit. 1 Kilo - calorie = 3.97 BTU 1.0.3 Sensible heat It is that heat which when applied to a body, results in a rise of its temperature. It is the heat which is sensed by a thermometer. 1.0.4 Latent heat It is that heat which when applied merely changes the state of substance, whether solid, liquid or, gas, without causing any change in its temperature. Latent heat of fusion of ice 80 k.cal/kg. (144 BTUs/Ib) Latent heat of evaporation of water 538.75 k.cal/kg (970 BTUs/lb) 1.0.5 Enthalpy It is a calculated property of vapour which is defined as "Total heat content". It is the sum of the sensible heat" and latent heat. ' 1.0.6 Critical temperature (Tc) There is a certain temperature for every liquid or gas, which is called its critical temperature. When a gas is above its critical temperature, any amount of increase in pressure cannot liquify it. When the temperature is below its critical point, the gas can be liquified without lowering its temperature, by merely increasing the pressure. 1.1 TEMPERATURE AND PRESSURE RELATIONSHIP Water boils at 100°C when the pressure on it is atmospheric. If the pressure is increased to above atmospheric, the boiling point increases and if the pressure is decreased to below atmospheric, the boiling point decreases. Boiling water does not necessarily have to be hot, because if there is vacuum over the surface of the water, it will boil at a very low temperature. The same thing is true in regard to other liquids such as, the various refrigerants which are used to produce low temperatures. These refrigerants have the same properties as ________________________________________________________________________ ___________________________________________________________________________ water except that the range of their boiling points is lower 1.2 PSYCHROMETRY The measurement of moisture content in air is known as Psychrometry. The air is mainly a mixture of dry air and water vapour. The relative amount of water vapour in the atmosphere is very small, variations in this amount as well as changes in temperature are very significant and important in airconditionmg. The following terms are in use in study of air-vapour mixture. 1.2.1 Dry Bulb Temperature (DBT) It is the temperature of air as measured by an ordinary thermometer. 1.2.2 Wet Bulb Temperature (WBT) It is the temperature of air as measured by an ordinary thermometer, whose glass bulb is covered by a thin cotton sleeve soaked in water. When the sleeve is wetted, mercury column of the thermometer will be observed to start dropping down, until it finally settles down at a definite minimum value. This is because the water in the WB takes away its latent heat, as it gets vapourised from the mercury in the bulb as a result of which the bulb gets cooled and the mercury column shows a low temperature. The drop in the temperature depends upon the relative humidity of the surrounding air. When air is heated, the DB increases; WB also increases, but at a slower rate and therefore, the difference between DB and WB widens indicating a lower humidity. When air is cooled, it becomes more and more humid, until it becomes fully saturated and the difference between DB and WB goes on getting reduced until it is zero at saturation. At saturation DB and WB are equal. If cooling is continued further some of the moisture contained in the air gets thrown out and will condense, since air has only a limited holding capacity for water vapour. 1.2.3 Dew Point Temperature (DPT) It is the temperature of air at which the moisture of air starts condensing on the surface. The relative humidity at dew point is 100%. 1.2.4 Effective Temperature (ET) It may be defined as an arbitrary index that combines into single factor the effect of temperature, humidity and air movement on human comfort in a noise free environment. This corresponds to a DB temperature of saturated air at which a given percentage of people feel comfortable. Thus an effective temperature of 21°C can be expressed as 23°C DB at 70% RH or 25°C DB at 30% RH. ________________________________________________________________________