Air ConditioningFundamentals Prepared by PM Muhammad Abd Razak FKM, UiTMPP 1 Long Time Ago … 2 The Need for Air Conditioning Comfort Comfort is the sensation of well being HVAC systems provide thermal comfort Our body is a thermal machine. It produces heat by burning food. The heat may be sensible and latent. Generated body heat is released to the surrounding area by: -Radiation to lower temperature system. -Convection through the air to the environment. -Evaporation, namely by elimination of water through perspiration. In general the more heat that you give off, the cooler and drier 3 the space must be for you to be comfortable and visa versa. Comfort and Productivity Why is comfort so important in the work place? ☺ An air-conditioned environment of 75°F [24°C].5% on average Errors decrease by 0. 55%RH has a positive effect on people Performance increases 9.S.9% Fewer absences ☺ Improved morale Survey was conducted by the U. Government Administration–General Services 4 . Comfort and Productivity An environment that is not comfortable reduces performance Cold ambient with stagnant air Warm ambient with distributed air Warm ambient with stagnant air Warm ambient with high humidity -9% -15% -23% -28% 5 . . Return Air Heating Coil Cooling Coil Supply Fan Diffuser Supply Duct Outside Air Filter Section Cooling Coil Thermostat Room Pressure Reducing Device Boiler Condenser Fan Pump Outside Air Condenser Compressor 6 .A Simple HVAC System Return Duct Exhaust Air Connects to Controls.. 25 m/s air cleanliness ~ ## ppm of a given space. Air Conditioning System Consists of a group of equipment connected in series to control the environmental parameters...Air Conditioning Process For an average person it means “the cooling of air” By Definition : a process of treating the air to establish and maintain required standards of the.15 – 0... It is a direct application of the refrigeration cycle 7 ... air temperature ~ 25˚C Humidity ~ 50 – 55 % RH ventilation & air movement and ~ 0. Human Comfort Office Hospital College Temperature Humidity Humidifying Dehumidifying Cleanliness Ventilation Washing Filtration Odour Removal or Process Control Printing Electronics Medical Heating Cooling Distribution 8 . air-conditioning application is partly about controlling the water vapour content in air.Atmospheric air: Air in the atmosphere containing some water vapour (or moisture). The energy content is Atmospheric air contains about 2% water vapour. Dry air: Air that contains no water vapor. but it plays a major role in human comfort. 9 . Therefore. Then. we can treat the atmospheric air as an ideal-gas mixture (dry air + vapour).Water vapour in air is in the superheated form behaves as if it existed alone obeys the ideal-gas relation pv = RT. Pa and Pv are the partial pressures of each component 10 . Thus. 82 kJ/kg · °C at −10 to 50°C range Thus 11 .9 kJ/kg at 0°C cp.avg = 1. the lines of constant enthalpy h coincide with the lines of constant temperature T (in the superheated vapor region (Fig 14-2) h = h(T ) since water vapour is an ideal gas and thus In the temperature range of 10 to 50°C.Below 50°C. the hg of water can be determined from this equation with negligible error For water hg = 2500. The amount of water vapour in air varies & can be expressed in many ways. Ra = 0. 12 .4619 kJ/kg Saturated air: The air saturated with moisture.2871 kJ/kg Rv = 0. Specific humidity (humidity ratio): The ratio of mass of water vapour present in a unit mass of dry air. at 50% RH Then at 100% RH 13 . What does that means? Say.Relative humidity φ: The ratio of the amount of moisture the air holds (mv) to the maximum amount of moisture the air can hold at the same temperature (mg). Combining the two terms or 14 . The atmospheric pressure is 101. the specific humidity iii. the dew point temperature (tdp) (ie the temperature at which the vapour will start to condense) 2. the specific humidity ii. Moist air in a room is at 25°C with relative humidity of 55%. the mass of air iv. Find i. the dew point temperature iii. 35°C and 70% relative humidity.Example 1. the mass vapour 15 . the partial pressure of water vapour ii.325 kPa. 100 m3 of atmospheric air is 1 bar. Find i. The specific enthalpy of atmospheric air is expressed per unit mass of dry air.The amount of dry air in the air–water-vapor mixture remains constant. 16 . but it is the amount of water vapor that changes. Thus h = Cpa(t) + ω[2500.82(t)] [kJ/kga] t is the Dry-bulb temperature (tdb) i. 17 .9 + 1.9 + Cpv(t)] [kJ/kga] or h = 1.005(t) + ω[2500.e. the ordinary temperature of atmospheric air. ADIABATIC SATURATION AND WET-BULB TEMPERATURE The specific humidity (and relative humidity) of air can be determined from the pressure inlet and the exit temperatures of an adiabatic saturator. 18 . 622 Pg . w hg . w d – dry bulb temp w – wet bulb temp 19 . w P − Pg .Simplified to ω1 = C p (Tw − Td ) + ω ' h fg .d − h f .w where ω' = 0. td and tw are obtained and cross referenced with a wet bulb depression table to find the value of RH. many calculations and tabulations are involved. A more practical approach is using a Psychrometric Chart 20 . If this method is to be employed in Air Conditioning. Using a sling psychrometer.The above method is tedious and not practical. 21 . Also the chart serves as a valuable aid in visualizing HVAC processes such as heating cooling humidification and dehumidification.Psychrometric charts: Present moist air properties in a convenient form. tw=15°C specific volume enthalpy φ tdp ω state point 22 .For td = 25°C. Sometimes two or more of these processes are needed to bring the air to the comfort zone (temp (20 – 30°C). φ (40 – 60%) Humidifying Cooling Heating Dehumidifying 23 .AIR-CONDITIONING PROCESSES Air-conditioning processes include simple heating (raising the temperature) simple cooling (lowering the temperature) humidifying (adding moisture) and dehumidifying (removing moisture). In practice. both the dry-bulb temperature and moisture content generally change simultaneously Heating & Humidifying Sensible heat causes a change in the dry bulb temp but not moisture content Cooling & Dehumidifying Latent heat causes a change in moisture content but not in the dry bulb temp 24 . Air-conditioning processes are modeled as steady-flow processes with general mass and energy balances. Mass balance Energy balance 25 . heat pump etc. In simple heating. but relative humidity increases.Simple Heating and Cooling (ω = constant) “Sensible” heating by means of a electric resistance heater. the cooling coil is in place. Specific humidity remains constant. a hot water coil. but relative humidity decreases and In simple cooling. specific humidity remains constant. 26 . Dry air Water/vapour ma1 = ma2 = ma mv1 = mv2 ma ω1 = ma ω2 ω 1 = ω2 Energy ma1h1 + Qin = ma2h2 (heating) Qin = ma(h2 .h1) 27 . φ = 100% When the air is cooled below its dew-point temperature.Cooling with Dehumidification When the relative humidity is too high. Vapour is saturated. 28 . some moisture is removed from the air or to dehumidify it. condensation will start to form. ω2) Energy ma1h1 = ma2h2 + mwhw + Qout Qout = ma(h1 .Dry air ma1 = ma2 = ma Water/vapour mv1 = mv2 + mw maω1 = maω2 + mw mw = ma (ω1 .mwhw 29 .h2) . Steam injection 30 . The air is passed through a heating section and then through a humidifying section.Heating with Humidification Low RH resulting from simple (sensible) heating can be eliminated by humidifying the air. In the humidifying section Dry air ma2 = ma3 = ma Water/vapour mv2 + mw = mv3 maω2 + mw = maω3 mw = ma (ω3 .h2) 31 .ω1) Energy mah2 + mwhw = mah3 mwhw = ma(h3 . The return air is mixed with a small fraction of fresh outside air (±10%) before it is routed into the living space.(Adiabatic) Mixing of Airstreams AC applications may require mixing of airstreams. Insulation 32 . Dry air ma1 + ma2 = ma3 Water/vapour mv1 + mv2 = mv3 ma1ω1 + ma2ω2= ma3ω3 Energy ma1h1 + ma2h2 = ma3h3 33 .
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