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

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

1 AIR CONDITIONING FME 706/FML 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) FME 706/FML Air Conditioning

3 PSYCHROMETRICS SOME IMPORTANT PARAMETERS IN 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 h involve (1+W) kg of mixture FME 706/FML Air Conditioning

4 PSYCHROMETRICS (Cont’d 1)
PSYCHROMETRIC CHART FME 706/FML Air Conditioning

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

6 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 1)
Adiabatic Saturator H2O Process Sling Psychrometer FME 706/FML Air Conditioning

7 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 2)
hw = hf2, hv1 = hg1, hv2 - hw = hfg2, FME 706/FML Air Conditioning

8 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 3)
IMPORTANT RELATIONSHIPS Specific Humidity Enthalpy or FME 706/FML 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 = kJ/kg, cp  kJ/(kga.K), and hence h 1.024t W kJ/kga  and W ; ; P = Pa + Pv ; and hence Solution for W1 From Adiabatic Saturator FME 706/FML 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) FME 706/FML Air Conditioning

11 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 6)
Cooling and Dehumidification or FME 706/FML 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. FME 706/FML Air Conditioning

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

14 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 9)
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 FME 706/FML Air Conditioning

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

16 SOME IMPORTANT PSYCHROMETRIC PROCESSES (cont’d 11)
EXAMPLE OF A SIMPLE CENTRAL AIR-CONDITIONING SYSTEM FME 706/FML 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) FME 706/FML Air Conditioning

18 COMFORT AND HEALTH Metabolism – depends on the level of activity
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  m2.K/W – heavy two piece suit with accessories 0.05 clo  pair of shorts FME 706/FML Air Conditioning

19 COMFORT AND HEALTH (cont’d 1)
Examples of Cooling Load Due to Occupancy Activity Example Male Adult Total Watts Total Adjusted Watts Sensible Watts Latent Seated at rest Theatre, movie 115 100 60 40 Seated, very light work, writing Offices, hotels, apartments 140 120 65 55 Standing, light work or walking slowly Retail store, bank 235 185 90 95 Light Bench work Factory 255 230 130 Heavy work, heavy machine work, lifting 470 165 300 Heavy work, athletics Gymnasium 585 525 340 FME 706/FML Air Conditioning

20 COMFORT AND HEALTH (Cont’d 2)
ASHRAE Comfort Standard (1981) (Sedentary) FME 706/FML 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) FME 706/FML Air Conditioning

22 COMFORT AND HEALTH (Cont’d 4)
OUTDOOR DESIGN CONDITIONS Winter Summer Station (Elevation) Mean Annual Extremes 99% C 97.5% Design Dry Bulb C Outdoor Daily Range C Design Wet Bulb C 1% 2.5% 5% Nairobi (1820 m) 7 9 10 27 26 13 19 18 Addis (2363 m) 2 4 5 29 28 16 Lagos (3 m) 21 22 33 32 Dar es Salaam (14 m) 17 31 FME 706/FML Air Conditioning

23 COMFORT AND HEALTH (Cont’d 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 FME 706/FML 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 FME 706/FML Air Conditioning

25 HEAT TRANSMISSION IN BUILDINGS AND 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.) FME 706/FML 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) FME 706/FML 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) FME 706/FML Air Conditioning

28 ROOM AIR DISTRIBUTION AIR FLOW PATTERNS FME 706/FML 2007- 08
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 FME 706/FML Air Conditioning

29 ROOM AIR DISTRIBUTION (cont’d)
FME 706/FML 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 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 FME 706/FML Air Conditioning

31 ROOM AIR DISTRIBUTION (cont’d 2)
High sidewall diffuser – good for cooling FME 706/FML Air Conditioning

32 ROOM AIR DISTRIBUTION (cont’d 3)
Ceiling Diffuser Excellent for cooling Large diffusion surface area Handles large quantities of air FME 706/FML 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 , 0.5, 0.25 m/s Noise Criterion (NC) Tabulated Standards for different spaces, ducts, applications, fittings Pressure - Ps and Pv or Po FME 706/FML Air Conditioning

34 ROOM AIR DISTRIBUTION (cont’d 5)
Room Characteristic Length (L) FME 706/FML 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 Comfort Conditions:    1.1˚C; vx < 0.35 m/s ADPI – percentage of locations in occupied space of room which meet this criterion FME 706/FML Air Conditioning

36 ROOM AIR DISTRIBUTION (cont’d 7)
Example Terminal Device Room Load (W/m2) T0.25/L for max ADPI Max ADPI For ADPI greater than Range of T0.25/L Circular 250 1.8 76 70 0.7 – 1.3 Ceiling 190 83 80 0.7 – 1.2 Diffuser 120 1.6 88 65 1.5 93 90 FME 706/FML 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 FME 706/FML 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. FME 706/FML 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 FME 706/FML 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 FME 706/FML Air Conditioning

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

46 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 7)
Reheat Systems FME 706/FML 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) FME 706/FML Air Conditioning

48 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 9)
Multizone Systems FME 706/FML 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 FME 706/FML Air Conditioning

50 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 11)
FME 706/FML Air Conditioning

51 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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 – H2O 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 m3 water requires less piping space FME 706/FML Air Conditioning

52 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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 FME 706/FML Air Conditioning

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

54 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 15)
Two Pipe - Direct Return Facilitates individual control FME 706/FML Air Conditioning

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

56 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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 FME 706/FML Air Conditioning

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

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

59 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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 FME 706/FML Air Conditioning

60 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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 FME 706/FML Air Conditioning

61 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 22)
FME 706/FML Air Conditioning

62 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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) FME 706/FML Air Conditioning

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

64 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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) FME 706/FML Air Conditioning

65 HVAC SYSTEMS, EQUIPMENT & CONTROL (cont’d 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) FME 706/FML Air Conditioning


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