MECH4880 Unit02 Wk1.pdf - Refrigeration and Air...

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Refrigeration and Air Conditioning Unit 2 Psychrometrics Revised by Dr Chris Menictas Original notes by Prof. Eddie Leonardi Refrigeration and Air Conditioning 2-1
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Contents Introduction .................................................................................................. 3 Psychrometric Properties ............................................................................ 4 Moisture Content ....................................................................................... 5 Dew Point Temperature ............................................................................. 6 Percentage Saturation ............................................................................... 9 Relative Humidity .................................................................................... 10 Specific Enthalpy ..................................................................................... 13 Specific Volume ........................................................................................ 17 The Wet Bulb Temperature ...................................................................... 21 Properties of Moist Air .............................................................................. 32 2-2 Refrigeration and Air Conditioning
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Introduction Psychrometry is the study of a mixture of air and fluid vapour, usually atmospheric air, although the principles have wider application to other gas/vapour mixtures. For the purposes of this study, air is treated as a binary (two-component) mixture. One component is ‘dry air’ consisting of a mixture of nitrogen (78.09%), oxygen (20.95%), argon (0.93%), CO 2 (0.025%), plus minute traces of neon and other gases. The other component is ‘water vapour’ . The water vapour is at a very low partial pressure. At this low pressure and atmospheric temperature, the water vapour behaves as a perfect gas. The following assumptions are therefore made without any noticeable loss in accuracy: (1) The vapour contains no dissolved gases. (2) The gaseous phases can be treated as a mixture of ideal gases. (3) There is no interaction between the components and all components are at the mixture temperature. 2-3 Refrigeration and Air Conditioning 1
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Psychrometric Properties In air-conditioning practice all calculations are based on the “dry-air part” since the water vapour part is continuously variable; doing so substantially simplifies calculations. For defining and calculating the relevant psychrometric properties, we consider a certain volume V of moist air at pressure p and temperature T , containing m a kilogram of dry air and m v kilogram of water vapour, as shown in the figure below. Dry air: m a , v a Water vapour: m V , m, p, T Figure 1. Definition Sketch The pressure is taken to be barometric pressure (and unless told otherwise) has a standard value of, kPa 101.325 p = (standard atmosphere) But now from Gibbs phase rule we note that, p 2 n F + = in which F is the degrees of freedom of the system, n is the number of components and p is the number of phases. Since for the air/water vapour mixture we have two components (dry air and water vapour) and only one phase (only gas) then the degrees of freedom for the system is given by, 3 1 2 2 F = + = . m v, v v 2-4 Refrigeration and Air Conditioning
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That is three properties are required to define the system. Since the pressure is fixed, two other properties are required to define the complete thermodynamic state of the mixture. These two may be chosen from any of the following: (a) dry bulb temperature, T (b) moisture content, w (c) dew point temperature, T DP (d) percentage saturation, m (e) Relative humidity, φ (or RH ) (f) specific enthalpy, h (g) specific volume, v (h) wet bulb temperature, T WB Moisture Content The moisture content (or humidity ratio, specific humidity or absolute humidity), denoted by the symbol w , is defined as:
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