MECH4880 Week 1 Unit 2 (1).pdf - MECH 4880 Refrigeration and Air Conditioning Psychrometrics Unit 2 Chris Menictas School of Mechanical and

MECH4880 Week 1 Unit 2 (1).pdf - MECH 4880 Refrigeration...

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MECH 4880 - Refrigeration and Air Conditioning Psychrometrics Unit 2 Chris Menictas School of Mechanical and Manufacturing Engineering
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Contents Introduction Psychrometric Properties Moisture Content Dew Point Temperature Percentage Saturation Relative Humidity Specific Enthalpy Specific Volume The Wet Bulb Temperature Properties of Moist Air
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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.
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Introduction One component is ‘dry air’ consisting of a mixture of nitrogen (78.09%), oxygen (20.95%), argon (0.93%), CO2 (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 .
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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.
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Psychrometric Properties Figure 1 Definition Sketch V - the volume of moist air T - dry bulb temperature m a – kilogram of dry air m v – kilogram of water vapour v – specific volume Pressure is taken as barometric unless specified as a different value p = 101.325 kPa How many properties needed to define the system ?
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Psychrometric Properties We can use Gibbs phase rule F = n + 2 – p To work out now many components needed to define the system. F = degrees of freedom n = number of components p = number of phases What are the components ? Dry air and water vapour - two components Number of phases ? One phase - gas Therefore F = 2 + 2 – 1 = 3
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Psychrometric Properties Three properties are required to define the system. Considering pressure is fixed (101.325 kPa) then 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, TDP d) percentage saturation, μ e) Relative humidity, f (or RH) f) specific enthalpy, h g) specific volume, v h) wet bulb temperature, TWB
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Moisture Content The moisture content (or humidity ratio, specific humidity or absolute humidity), denoted by the symbol w, is defined as: ‘the ratio of the mass of water vapour to the mass of dry air in a given volume of the mixture’.
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