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AIR CONDITIONING. SCOPE AND USE OF AIR CONDITIONING. Not restricted to cooling only but might include: Control of temperature at all times by heating or cooling Control of air humidity by humidification or dehumidification Control of air movement at a desirable velocity

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  1. AIR CONDITIONING Air Conditioning

  2. SCOPE AND USE OF AIR CONDITIONING • Not restricted to cooling only but might include: • Control of temperature at all times by heating or cooling • Control of air humidity by humidification or dehumidification • Control of air movement at a desirable velocity • Introduction of outdoor air as required • Control of air quality by removal of dirt particles and odorous gases • Control of sound generated by the air conditioning equipment • Environmental control • Used for two purposes: • Comfort (people) • Process control (as required) Air Conditioning

  3. PSYCHROMETRICS • Study of air-water vapour (binary) mixtures • Content of water vapour can change • A/C processes may involve both sensible and latent heat transfer SOME IMPORTANT PARAMETERS IN PSYCHROMETRICS • Dry Bulb Temperature (TDB) – sensed with a normal thermometer bulb/sensor • Wet Bulb Temperature (TWB) – sensed by a thermometer whose bulb is wrapped with water soaked wick in rapidly moving air • Dew Point Temperature (TDP) – Temperature at which water vapour starts to condense at constant pressure • Humidity Ratio/Specific Humidity (W) – Mass of water vapour divided by the mass of dry air (mv/ma kgv/kga) • Relative Humidity ( or rh) – Ratio of actual water vapour pressure in the air to the water vapour pressure at saturation at the mixture temperature • va - volume of a mixture containing one kg of dry air (m3/kga) • h – enthalpy contained in a mixture containing 1 kga (kJ/kga) • va and hinvolve (1+W) kg of mixture Air Conditioning


  5. SOME IMPORTANT PSYCHROMETRIC PROCESSES Thermodynamic Wet Bulb Temperature (Adiabatic Saturator) Thermodynamic Saturator Air Conditioning

  6. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1) Adiabatic Saturator H2O Process Sling Psychrometer Air Conditioning

  7. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 2) hw = hf2, hv1 = hg1, hv2 - hw = hfg2, Air Conditioning

  8. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 3) IMPORTANT RELATIONSHIPS Specific Humidity Enthalpy or Air Conditioning

  9. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 4) • Datum: Dry saturated vapour at 0ºC, t in ºC • For A/C purposes, cpa 1.005, cpv  1.87 kJ/(kg.K), W  0.01 kgv/kga, hgo = 2500.8 kJ/kg, cp  1.024 kJ/(kga.K), and hence h 1.024t + 2500.8W kJ/kga  and W ; ; P = Pa + Pv ; and hence Solution for W1 From Adiabatic Saturator Air Conditioning

  10. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 5) • Cpa, t1, t2, hfg2, hg1, and hf2 from tables • Since at 2 air is saturated, 2 = 1 get W2 from where Ps2 from tables at t2 Heating and Cooling at Constant W (Sensible) Air Conditioning

  11. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 6) Cooling and Dehumidification or or Air Conditioning

  12. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 7) represents enthalpy carried away by the condensate ( 10ºC) which is negligible compared to the first term and hence where and Sensible Heat Factor (SHF) (Related to bypass factor) Important in A/C calculations. Air Conditioning

  13. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 8) Heating With Humidification or Equation of a straight line. For Q = 0, Air Conditioning

  14. hw = hgT1 – humidification at constant T1 (2’) hw > hgT1 – heating with humidification (2’’) hw < hgT1 - cooling with humidification (2) Spray with liquid water at air wet bulb temperature – Twb remains constant. Basis of evaporative cooling SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 9) Air Conditioning

  15. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 10) Adiabatic Mixing ; ; • Equation of a straight line (final state lies along this line) Air Conditioning


  17. SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1) • T4, Q, mao, 5, 6, 7, SHFroom and Qfan known. • Draw line from 5, 6, 7 to cross T4 (T5 – T4 10ºC) • Join 0 and 5 locate 1 – adiabatic mixing, i.e. • Hence • For known SHFcoil draw line 2-3, and hence 3-4 at constant W • Qcoil = ma1(h2 – h3), Qheater = ma3 (h4 – h3) Air Conditioning

  18. COMFORT AND HEALTH • Deep body temperature  36.9ºC • If body can easily maintain an energy balance, then feeling of comfort results • Body regulatory mechanisms: • Metabolism rate • Increase of the rate of cutaneous blood circulation (capillary dilation) • Sweating • Metabolism – depends on the level of activity • 1 MET (metabolic rate) = 58.2 W/m2 • Energy generated by an average sedentary MAN • Area (man)  1.8 m2 • 1 MET  105 W • Women  30% lower than men • Latent and sensible Comfort Conditions • Depends on activity and clothing • 1 clo  0.155 m2.K/W – heavy two piece suit with accessories • 0.05 clo  pair of shorts Air Conditioning

  19. COMFORT AND HEALTH (cont’d 1) Examples of Cooling Load Due to Occupancy Air Conditioning

  20. COMFORT AND HEALTH(Cont’d 2) ASHRAE Comfort Standard 55-81 (1981) (Sedentary) Air Conditioning

  21. COMFORT AND HEALTH(Cont’d 3) • Cooling T  24ºC • Heating T  22ºC • Humidity   40 – 50 % • Velocity in occupied zone V  0.15 m/s • For high activity – special charts (Fanger comfort Charts – ASHRAE HF) Air Conditioning


  23. COMFORT AND HEALTH(Cont’d 5) • Mean of annual extremes: Average of the lowest temp. recorded each year over 25-30 years • 99%: Temp. which has been equaled or exceeded 99% of the time during the three cold months (Ditto for 97.5%) • 1%: Temp. equaled or exceeded or equaled 1% of the time during the time during the cooling months • Daily range: Difference between average maximum and minimum temp. for the warmest month – has an effect on the energy storage of structures. Ventilation • Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) • Filtration, washing, scrubbing, adsorption, odour masking and counteraction • The smaller the particle, the more difficult to remove • Fibrous media (viscous impingement and straining), electronic air cleaners Air Conditioning

  24. COMFORT AND HEALTH(Cont’d 6) • Cooling T  24ºC • Heating T  22ºC • Humidity   40 – 50 % • Velocity in occupied zone V  0.15 m/s • For high activity – special charts Ventilation • Mainly to control odour – recommended standards for different spaces (minimum 2.5 l/s) • Filtration, washing, scrubbing, adsorption, odour masking and counteraction • The smaller the particle, the more difficult to remove • Fibrous media (viscous impingement and straining), electronic air cleaners Air Conditioning

  25. HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD Cooling Load • Temp. and humidity to be maintained at a comfortable level • Heat must be extracted – cooling load • Basis of equipment selection (cooling and dehumidification coil, heater, ducts, fans, piping, fans, pumps, etc.) Air Conditioning

  26. HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 1) • Heat gain: Rate at which heat is being received in the space at any time (solar radiation, lighting, conduction, convection, people, equipment, infiltration, etc.) • Storage effect:Heat does not immediately go into heating the room air. Radiant component first absorbed by room materials before being absorbed by room air. • Cooling load:Rate at which heat must be removed to maintain room design conditions (temperature and humidity) Air Conditioning

  27. HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Cont’d 2) Heat Gain/Cooling Load Components • Conduction through exterior walls, roof and fenestration (glazing/any light transmitting element) • Conduction through interior partitions, ceiling and floor • Solar radiation (short wave) through fenestration • Lighting and equipment • Occupancy • Infiltration • (Fans, duct heat gain, duct leakage) Air Conditioning

  28. ROOM AIR DISTRIBUTION • Good air distribution is necessary for comfort • Effective draft temp. difference from design condition between -1.7ºC and 1.1ºC within occupied zone (approx. < 1.75 m) • Air velocities  0.13 – 0.25 m/s (below or above cause discomfort) AIR FLOW PATTERNS The Horizontal Isothermal Jet Air Conditioning

  29. ROOM AIR DISTRIBUTION(cont’d) Air Conditioning

  30. ROOM AIR DISTRIBUTION(cont’d 1) • Zone I – Constant centerline velocity • Zone II – Transition zone • Zone III – Most important and the longest fully developed flow) - • Zone IV – Fast velocity decay – regarded as still air – very short • Throw – Distance to a specified velocity, e.g. 0.25 m/s Important Characteristics • Surface effects increase the throw and decrease the drop (c.f. free jet) • Jet parallel to a wall or ceiling tends to hug the surface (reduced entrainment –”ceiling effect” • Obstructions e.g. beams, columns etc. • Cold jet – drop • Warm jet - rise Air Conditioning

  31. ROOM AIR DISTRIBUTION(cont’d 2) • High sidewall diffuser – good for cooling Air Conditioning

  32. ROOM AIR DISTRIBUTION(cont’d 3) Ceiling Diffuser • Excellent for cooling • Large diffusion surface area • Handles large quantities of air Air Conditioning

  33. ROOM AIR DISTRIBUTION(cont’d 4) Slot Diffusers • Long strip-shaped with one or more narrow openings Plenum Ceilings • Hung ceilings with slots or perforations for air supply (specialized suppliers/installation) SELECTION CRITERIA FOR DIFFUSERS • Capacity – Volumetric flow rate • Throw – Axial distance (isothermal) jet travels till the maximum velocity is reduced to a specified level, e.g. 0.75, 0.5, 0.25 m/s • Noise Criterion (NC) • Tabulated • Standards for different spaces, ducts, applications, fittings • Pressure - Ps and Pv or Po Air Conditioning

  34. ROOM AIR DISTRIBUTION(cont’d 5) Room Characteristic Length (L) Air Conditioning

  35. ROOM AIR DISTRIBUTION(cont’d 6) Air Diffusion Performance Index (ADPI) • Effective Draft Temperature (EDT) • = (tx – tc) – a(vx – b) tx - local temp., ºC Tc – room average temp., ºC vx – local velocity, m/s a = 8, b = 0.15 • ComfortConditions:- 1.7   1.1˚C; vx < 0.35 m/s • ADPI – percentage of locations in occupied space of room which meet this criterion Air Conditioning

  36. ROOM AIR DISTRIBUTION(cont’d 7) Example Air Conditioning

  37. BUILDINGS AIR DISTRIBUTION FAN • Supply the required air to all conditioned space • Must provide the required pressure drop to cater for ducts, diffusers, filters, etc. • Types: • Axial : a) Vane axial - centerline of duct - guide vanes before and after wheel (rotor) to control rotation of stream - high speed (noisy) b) Tube axial - no guide vanes c) Propeller - low pressure applications - high mass flow rates • Centrifugal: a) Forward curved (blades) b) Radial c) Backward curved (airfoil) Most used in A/C – can move large or small quantities of air over wide ranges of pressure Air Conditioning

  38. BUILDINGS AIR DISTRIBUTION (Cont’d 1) Fan Selection • Fan characteristics • Capacity and total pressure • Efficiency • Reliability • Size • Weight • Speed • Noise • Cost Duct Design • Layout (supply and return) – related to supply diffusers and return grilles, location of machine room, and other structural and architectural considerations. • Selection of size is a compromise between capital and running costs. Air Conditioning

  39. HVAC SYSTEMS, EQUIPMENT & CONTROL • HVAC systems may conveniently be divided into two broad categories: • Equipment and systems which provide heating and cooling • Systems which provide ventilation (air distribution and diffusion) • It is important to understand the (initial) design of the installation, modifications, operation/performance, utilization hours of operation and even maintenance record (for energy management purposes) HVAC SYSTEMS • Related to system organization • Energy consumed depends on source of heating/cooling, air distribution, and whether working fluid is simultaneously cooled or heated. ALL AIR SYSTEMS • Most common • Moderate room air by providing conditioned air from a central source via ducts • Control by altering the amount of air supplied or its temperature • Provide best control of fresh outdoor air (quality) and humidity control Air Conditioning

  40. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 1) • Can be used to provide outside air for cooling interior spaces while providing heating for perimeter zones • Drawback – energy consumed in distribution Components of All Air Systems • Air Handling Unit (AHU) – fan, (heating and cooling) coils, filters, humidifier • (Supply and return) ducts circulate conditioned air. Sometimes plenum above suspended ceiling used as part of return path • Included in duct system is supplier of outdoor air and another for exhausting some of the return air Air Conditioning

  41. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 2) Single Zone Air Conditioning System Layout Air Conditioning

  42. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 3) • Can be used for all year round control • Can use 100% outdoor air – during intermediate cooling seasons – refrigeration equipment not used Control of proportion of outdoor air Air Conditioning

  43. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 4) • Pre-heat coil – in cold climates to prevent cooling coils from freezing • Face bypass – provides another method of controlling humidity – but not as good control as reheat coil • Single zone systems suitable for large open spaces with uniform load, e.g. stores, factories, arenas, auditoriums, exhibition halls, etc Air Conditioning

  44. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 5) Variable Air Volume (VAV) Systems • Same as single zone but individual thermostats control the amount of air supplied to room Air Conditioning

  45. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 6) • High degree of local temperature control • Moderate additional capital cost • AHU pressure increases (additional P for VAV) • AHU needs regulation to balance varying duct P requirements (fan inlet and outlet dampers) • Fan would operate off the optimum position – need variable speed drive • Supplementary heating may be necessary (minimum air to space must be supplied) • Single duct VAV systems most versatile and most widely used for large buildings (except where high degree of humidity control is required or high air exchange) Air Conditioning

  46. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 7) Reheat Systems Air Conditioning

  47. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 8) • Provides individual zone control of temp. and humidity • Wasteful – all air has to be cooled and then heated – double use (waste) of energy (cooling and then reheating) • Constant Air Volume (CAV) and VAV Reheat systems inefficient – highest level for all systems (CAV reheat systems most inefficient. VAV reheat inactive except when air modulation cannot meet minimum temp. requirements) • CAV and VAV systems with reheat can provide extremely tight control conditions (with humidity control) e.g. museums, printing plants, textile mills and other industrial process settings) Air Conditioning

  48. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 9) Multizone Systems Air Conditioning

  49. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 10) • A variation of the single duct CAV reheat system (NOT any system with thermostatically controlled zones – misconception) • Most common systems produce two streams at ~ 38C and ~ 13C • Streams blended with dampers to adjust room supply air temp. Dual Duct Systems • Air not blended in the fan room • Usually uses high velocity ducts (reduces size and cost of ducts but increased fan energy) with mixing boxes • Limited to buildings with strict temp. and humidity control requirements • Dual duct with VAV has efficient control (c.f. CAV) but requires a lot more distribution energy Air Conditioning

  50. HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 11) Air Conditioning

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