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FME 706/FML 2007- 08Air Conditioning1 AIR CONDITIONING.

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Presentation on theme: "FME 706/FML 2007- 08Air Conditioning1 AIR CONDITIONING."— Presentation transcript:

1 FME 706/FML Air Conditioning1 AIR CONDITIONING

2 FME 706/FML Air Conditioning2 SCOPE AND USE OF AIR CONDITIONING Not restricted to cooling only but might include: oControl of temperature at all times by heating or cooling oControl of air humidity by humidification or dehumidification oControl of air movement at a desirable velocity oIntroduction of outdoor air as required oControl of air quality by removal of dirt particles and odorous gases oControl of sound generated by the air conditioning equipment Environmental control Used for two purposes: oComfort (people) oProcess control (as required)

3 FME 706/FML Air Conditioning3 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 (T DB ) – sensed with a normal thermometer bulb/sensor Wet Bulb Temperature (T WB ) – sensed by a thermometer whose bulb is wrapped with water soaked wick in rapidly moving air Dew Point Temperature (T DP ) – 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 (m v /m a 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 v a - volume of a mixture containing one kg of dry air (m 3 /kga) h – enthalpy contained in a mixture containing 1 kga (kJ/kga) v a and h involve (1+W) kg of mixture


5 FME 706/FML Air Conditioning5 SOME IMPORTANT PSYCHROMETRIC PROCESSES Thermodynamic Wet Bulb Temperature (Adiabatic Saturator) Thermodynamic Saturator

6 FME 706/FML Air Conditioning6 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 1) Adiabatic Saturator H 2 O Process Sling Psychrometer

7 FME 706/FML Air Conditioning7 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 2) h w = h f2, h v1 = h g1, h v2 - h w = h fg2,


9 FME 706/FML Air Conditioning9 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 4) Datum: Dry saturated vapour at 0ºC, t in ºC For A/C purposes, c pa 1.005, c pv 1.87 kJ/(kg.K), W 0.01 kg v /kg a, h go = kJ/kg, c p kJ/(kg a.K), and hence h 1.024t W kJ/kg a and W ; ; P = Pa + Pv ; and hence Solution for W 1 From Adiabatic Saturator

10 FME 706/FML Air Conditioning10 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 5) C pa, t 1, t 2, h fg2, h g1, and h f2 from tables Since at 2 air is saturated, 2 = 1 get W 2 from where P s2 from tables at t 2 Heating and Cooling at Constant W (Sensible)

11 FME 706/FML Air Conditioning11 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 6) Cooling and Dehumidification or

12 FME 706/FML Air Conditioning 12 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 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.

13 FME 706/FML Air Conditioning13 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 8) Heating With Humidification or Equation of a straight line. For Q = 0,

14 FME 706/FML Air Conditioning14 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 9) h w = h gT1 – humidification at constant T 1 (2) h w > h gT1 – heating with humidification (2) h w < h gT1 - cooling with humidification (2) Spray with liquid water at air wet bulb temperature – T wb remains constant. Basis of evaporative cooling

15 FME 706/FML Air Conditioning15 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 10) Adiabatic Mixing ; ; Equation of a straight line (final state lies along this line)


17 FME 706/FML Air Conditioning17 SOME IMPORTANT PSYCHROMETRIC PROCESSES (contd 1) T 4, Q, m ao, 5, 6, 7, SHF room and Q fan known. Draw line from 5, 6, 7 to cross T 4 (T 5 – T 4 10ºC) Join 0 and 5 locate 1 – adiabatic mixing, i.e. Hence For known SHF coil draw line 2-3, and hence 3-4 at constant W Q coil = m a1 (h 2 – h 3 ), Q heater = m a3 (h 4 – h 3 )

18 FME 706/FML Air Conditioning18 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/m 2 Energy generated by an average sedentary MAN Area (man) 1.8 m 2 1 MET 105 W Women 30% lower than men Latent and sensible Comfort Conditions Depends on activity and clothing 1 clo m 2.K/W – heavy two piece suit with accessories 0.05 clo pair of shorts

19 FME 706/FML Air Conditioning19 COMFORT AND HEALTH (contd 1) Examples of Cooling Load Due to Occupancy ActivityExampleMale Adult Total Watts Total Adjusted Watts Sensible Watts Latent Watts Seated at restTheatre, movie Seated, very light work, writing Offices, hotels, apartments Standing, light work or walking slowly Retail store, bank Light Bench workFactory Heavy work, heavy machine work, lifting Factory Heavy work, athletics Gymnasium

20 FME 706/FML Air Conditioning20 COMFORT AND HEALTH (Contd 2) ASHRAE Comfort Standard (1981) (Sedentary)

21 FME 706/FML Air Conditioning21 COMFORT AND HEALTH (Contd 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)

22 FME 706/FML Air Conditioning22 COMFORT AND HEALTH (Contd 4) OUTDOOR DESIGN CONDITIONS WinterSummer Station (Elevation) Mean Annual Extrem es 99% C 97.5% C Design Dry Bulb C Outdoor Daily Range C Design Wet Bulb C 1%2.5%5%1%2.5%5% Nairobi (1820 m) Addis (2363 m) Lagos (3 m) Dar es Salaam (14 m)

23 FME 706/FML Air Conditioning23 COMFORT AND HEALTH (Contd 5) Mean of annual extremes:Average of the lowest temp. recorded each year over 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

24 FME 706/FML Air Conditioning24 COMFORT AND HEALTH (Contd 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

25 FME 706/FML Air Conditioning25 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.)

26 FME 706/FML Air Conditioning26 HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Contd 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)

27 FME 706/FML Air Conditioning27 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) HEAT TRANSMISSION IN BUILDINGS AND COOLING LOAD (Contd 2)

28 FME 706/FML Air Conditioning28 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

29 FME 706/FML Air Conditioning29 ROOM AIR DISTRIBUTION (contd)

30 FME 706/FML Air Conditioning30 ROOM AIR DISTRIBUTION (contd 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 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

31 FME 706/FML Air Conditioning31 ROOM AIR DISTRIBUTION (contd 2) High sidewall diffuser – good for cooling

32 FME 706/FML Air Conditioning32 ROOM AIR DISTRIBUTION (contd 3) Ceiling Diffuser Excellent for cooling Large diffusion surface area Handles large quantities of air

33 FME 706/FML Air Conditioning33 ROOM AIR DISTRIBUTION (contd 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 - P s and P v or P o

34 FME 706/FML Air Conditioning34 ROOM AIR DISTRIBUTION (contd 5) Room Characteristic Length (L)

35 FME 706/FML Air Conditioning35 ROOM AIR DISTRIBUTION (contd 6) Air Diffusion Performance Index (ADPI) Effective Draft Temperature (EDT) = (t x – t c ) – a(v x – b) t x - local temp., ºC T c – room average temp., ºC v x – local velocity, m/s a = 8, b = 0.15 Comfort Conditions: ˚C; v x < 0.35 m/s ADPI – percentage of locations in occupied space of room which meet this criterion

36 FME 706/FML Air Conditioning36 ROOM AIR DISTRIBUTION (contd 7) Example Terminal Device Room Load (W/m 2 ) T 0.25 /L for max ADPI Max ADPIFor ADPI greater than Range of T 0.25 /L Circular – 1.3 Ceiling – 1.2 Diffuser – 1.3

37 FME 706/FML Air Conditioning37 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

38 FME 706/FML Air Conditioning38 BUILDINGS AIR DISTRIBUTION (Contd 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.

39 FME 706/FML Air Conditioning39 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

40 FME 706/FML Air Conditioning40 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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

41 FME 706/FML Air Conditioning41 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 2) Single Zone Air Conditioning System Layout

42 FME 706/FML Air Conditioning42 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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

43 FME 706/FML Air Conditioning43 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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

44 FME 706/FML Air Conditioning44 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 5) Variable Air Volume (VAV) Systems Same as single zone but individual thermostats control the amount of air supplied to room

45 FME 706/FML Air Conditioning45 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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)

46 FME 706/FML Air Conditioning46 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 7) Reheat Systems

47 FME 706/FML Air Conditioning47 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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)

48 FME 706/FML Air Conditioning48 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 9) Multizone Systems

49 FME 706/FML Air Conditioning49 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 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

50 FME 706/FML Air Conditioning50 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 11)

51 FME 706/FML Air Conditioning51 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 12) ALL WATER (HYDRONIC) SYSTEMS Distribute hot or cold water from central plant Terminal units heat or cool room air Ventilation brought in through external wall directly to room or via terminal unit Lower capital cost and requires less space than all air system – H 2 O has higher density and specific heat Useful when space is limited e.g. existing building not originally conditioned Disadvantages Many units – maintenance Control of ventilation air quantities not precise Humidity control limited Popular for low cost central systems in multi-room high-rise applications Water heated to C or chilled to C and piped to devices – finned heaters or coolers Steam also used Latent heat 50 times more effective as water ( T ~ 20 C) But higher volume (~ 1600 times) Per m 3 water requires less piping space

52 FME 706/FML Air Conditioning52 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 13) PIPING CONFIGURATIONS Single Pipe Series System Least piping Maintenance of any unit necessitates shutdown of entire system Individual unit control not possible T diminishes with distance

53 FME 706/FML Air Conditioning53 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 14) One Pipe Main Offers individual control Special diverting tee – directs some of the water to the tee

54 FME 706/FML Air Conditioning54 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 15) Two Pipe - Direct Return Facilitates individual control

55 FME 706/FML Air Conditioning55 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 16) Two Pipe – Reverse Return Balanced – provides nearly equal flow path Central unit Terminal units Pump

56 FME 706/FML Air Conditioning56 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 17) Three Pipe System Separate heating and cooling supply pipes but common return with appropriate 3 way valves Possible to heat some rooms while cooling others Return can be direct or reverse

57 FME 706/FML Air Conditioning57 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 18) Four Pipe System Two separate pipe systems – one for cooling and one for heating

58 FME 706/FML Air Conditioning58 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 19) HYDRONIC TERMINAL DEVICES Radiators Hollow cast iron sections through which hot water flows – free convection Convectors Heaters – free convection Unit Heaters etc

59 FME 706/FML Air Conditioning59 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 20) Fan Coil Units Small air handling unit No outside air provision (usually) Hot or cold water supply Can be placed anywhere – cooling near ceiling, heating near floor If with outdoor air, known as unit ventilators

60 FME 706/FML Air Conditioning60 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 21) AIR-WATER SYSTEMS Water and conditioned air from central system to individual terminal units Utilize best features of all air and all water systems Water carries most of the energy Usually distributed air only enough for ventilation – usually by high velocity ducts Supplied air distributed via fan coil units, or directly to rooms Most systems use induction units Central air – known as primary air. As it flows through unit at high velocity it inducts room air (secondary air) – no fan required – minimizes maintenance Induction units popular with high rises Initial cost relatively high Primary air as low as 25% of all air system – not adequate for outside air cooling even for mild climates – hence chilled water supplied to unit coils

61 FME 706/FML Air Conditioning61 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 22)

62 FME 706/FML Air Conditioning62 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 23) UNITARY SYSTEMS Refrigeration and air conditioning packaged together, i.e. refrig. equipment, fan, fan coils, filters, dampers and control Usually in or close to air conditioned space Can be all air, all water or air – water. Generally all air and largely inclined to the more simple such as single zone with or without reheat, or multizone. Categorized as: Room units Unitary conditioners Roof units Room Units Dampers adjustable to allow outdoor air through cooling coil Low cost and simplicity Ideal for existing building – electrical power upgrading may be necessary No flexibility to handle high latent heat or changing sensible heat ratio – no good humidity control High sound levels Air cleaning quality marginal (only large particles)

63 FME 706/FML Air Conditioning63 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 24) Maintenance for large number of units Energy wasteful Up to (approx) 3 tons (~ 10 kW )

64 FME 706/FML Air Conditioning64 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 25) UNITARY A/C UNITS In or near space Heating sometimes included Available in vertical or horizontal package Limited ductwork can be connected if air distribution is desired Popular in small commercial application s Normally only condenser not packaged Split system Condenser and compressor one package and cooling coil (with fan) inside (popular for residential heat pump) Same advantages and disadvantages as room units Large units have multiple compressors Available up to ~ 50 tons (175 kW)

65 FME 706/FML Air Conditioning65 HVAC SYSTEMS, EQUIPMENT & CONTROL (contd 26) ROOFTOP UNITS (DIRECT EXPANSION – DX) Outdoor installation All components packaged together or compressor and condenser may be remote Heating may be incorporated May be used with ductwork Do not use building space Relatively low cost Available with multizone arrangement Humidity control limited Popular in low cost one floor buildings (e.g. supermarkets and suburban commercial buildings)

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