Psychrometry and Air Conditioning Processes g
Transcription
Psychrometry and Air Conditioning Processes g
BBSE2005: Air Conditioning g and Refrigeration g I http://www.hku.hk/bse/bbse2005/ Psychrometry and Air Conditioning g Processes Dr. Sam C M Hui Department of Mechanical Engineering The University of Hong Kong E-mail: E mail: [email protected] cmhui@hku hk Dec 2010 Contents • Introduction to Psychrometry y y ((Handout Chapter p 1)* ) • Psychrometric Processes (Handout Chapter 2)* • Practical Use of Psychrometric Chart • Computer Software for Psychrometric Calculation and Analysis [* Acknowledgment: Denco Air Conditioning (UK)] Introduction to Psychrometry • Basics • The atmosphere • Water vapour • Saturated vapour pressure • Also, Appendix - Thermodynamic Basics • Perfect gas laws • 1st law of thermodynamics • Conservation of energy Introduction to Psychrometry • Psychrometry • The study of atmospheric air and its associated water vapour • Dryy air and moist air • Dalton’s law of partial pressures • Standard atmospheric pressure = 101.325 kPa • Saturated S t t d vapour pressure • Max. ppressure of water vapour p that can occur at any given temperature Introduction to Psychrometry • Psychrometric Chart (Theory) • • • • • Moisture content (g), or absolute humidity (w) Relative humidity (rh or RH) Percentage saturation (μ) Wet-bulb temperature (twb) Specific volume (v) • (See the illustration on psychrometric chart) Can y you read them from the chart? Enthalpy Psychrometric Chart Wet-bulb temperature Relative humidity Dew-point Dew point temperature Humidity ratio Dry-bulb temperature Specific volume Sling psychrometer Introduction to Psychrometry • The Psychrometric Equation • • • • Dew-point temperature (tdp) Specific enthalpy (h) Specific volume (v) Density (ρ) • Do you know how to find out the moist air properties using the psychrometric chart? Introduction to Psychrometry • Commonly used psychrometric charts • ASHRAE psychrometric chart • CIBSE psychrometric chart • Why are they slightly different? • Can yyou find out the differences? Psychrometric Processes • Common processes: • Sensible cooling / sensible heating • Cooling and dehumidification / heating and humidification • Humidification / dehumidification • Evaporative i cooling li / chemical h i l dehydration d h d i • Typical devices: • Cooling/heating coils • Humidifiers / dehumifiers Basic psychrometric processes 3 7 5 1 2 6 8 4 Process 0-1: Sensible heating P Process 00-2: 2 Sensible S ibl cooling li Process 0-3: Humidifying Process 0-4: Dehumidifying y g Process 0-5: Heating and humidifying Process 0-6: Cooling and dehumidifying P Process 00-7: 7 Cooling C li andd humidifying h idif i Process 0-8: Heating and dehumidifying Psychrometric processes Sensible cooling/heating Adiabatic dehumidification Cooling and dehumidification Evaporative cooling Cooling coil Cooling and dehumidification Cooling and dehumidification Simple air conditioning cycle C you draw Can d suchh a cycle l for f Hong H Kong K summer conditions? di i ? - Outdoor: DBT = 33 ºC; WBT = 28 ºC; flow = 20% of supply air - Indoor: I d DBT = 25 ºC; ºC %RH = 50% - Air leaving cooling coil: DBT = 13 ºC; %RH = 95% Psychrometric Processes • • • • • • • Sensible heating coils Cooling coils Humidifiers Water spray types Steam humidifier Room p psychrometric y process p Mixing air streams Psychrometric Processes • Calculations: • • • • • Sensible heat ratio (SHR) Space cooling load Cooling coil coil’ss load/capacity Humidification capacity Mixing processes • Principles of heat balance & conservation of mass Sensible and latent cooling loads Practical Use of Psych. Chart • Examples of psychrometric charts • • • • • ASHRAE CIBSE Carrier Mr. S K Wang (similar to Trane) The chart used in Mainland China (upside down) • [From Mollier diagram: many users in Scandinavia, Eastern Europe, and Russia] • Do you o know kno ho how to construct constr ct these charts? Psychrometric Software • ASHRAE Psychrometric Analysis CD-ROM CD ROM (2002) [AV 697 P97] • ArchiSci Software - PSYCHWIN (an evaluation version can be downloaded) • Psychrometric Chart (PSY) software (Free for download) • Daikin's D iki ' Free F Psychrometrics P h t i tool t l Analysis of Hong Kong climate on a psychrometric chart Analysis of HVAC operation strategy