Book - School of Science and Technology

Psychrometry 515whereM ˆ mass flow of entering air (kg/s)H ˆ sensible heat gains (kW)c ˆ specific heat capacity of air (kJ/kg K)t ˆ design temperature rise (K)Latent heat gainsThese do not affect the quantity of conditioned air required since they do not cause a risein temperature. The latent gains are treated quite separately from the sensible gains. Themass flow of air required to deal with the latter will usually be found to produce no morethan a small increment in humidity but in an extreme case, limitation of that increment toan acceptable figure may require that the mass be increased and the design temperaturerise reduced in consequence.PsychrometryPsychrometry is a subject concerned with the behaviour of mixtures of air and watervapour and a knowledge of it is necessary in order to perform any air-conditioningcalculations. Some of the general principles were referred to in Chapter 1 but a completestudy is outside the scope of this book and is dealt with in many textbooks on thermodynamicsand several excellent specialist works.*Most of the terms which relate to mixtures of air and water vapour were defined inChapter 1 of this book but a brief list recapitulating those items which are particularlyrelevant here would include:Dry bulb temperature, DB ( C)Wet bulb temperature, WB ( C)Dew-point temperature, DP ( C)Vapour pressure (kPa)Relative humidity, RH, and percentage saturation (%)Absolute humidity or moisture content (kg/kg of dry air)Total heat, TH, or specific enthalpy (kg/kg of dry air)Specific volume (m 3 /kg of dry air)Barometric pressureThe standard level of atmospheric pressure is 101.325 kPa exactly, corresponding to760 mm of mercury at 0 C and standard gravity (9:80665 m/s 2 ). This level, equating to1.01325 bar, was the barometric pressure used in calculation of the psychrometric propertiesof air and water vapour presented in the Guide Section C1, and referred to later in thischapter. For practical purposes in air-conditioning design, the data so presented are* Goodman, W., Air Conditioning Analysis. Macmillan, New York, 1947. Jones, W. P., Air ConditioningEngineering. Butterworth Heinemann, 2001.