Dew Point - Tdp The Dew Point is the temperature at which watervapor starts to condense out of the air, thetemperature at which air becomes completelysaturated. Above this temperature the moisture willstay in the air. If the dew-point temperature is close to the airtemperature, the relative humidity is high, and ifthe dew point is well below the air temperature, therelative humidity is low. The Dew Point temperature can be measured byfilling a metal can with water and ice cubes. Stir bya thermometer and watch the outside of the can.When the vapor in the air starts to condensate onthe outside of the can, the temperature on thethermometer is pretty close to the dew point of theactual air. The dew point temperature can be read byfollowing a vertical line from the state-point to thesaturation line. Dew point is represented along the100% relative humidity line in the Mollier diagram.
Dry-Bulb Temperature - Tdb Dry bulb temperature is usually referred to as airtemperature, is the air property that is most commonused. When people refer to the temperature of the air,they are normally referring to its dry bulb temperature.Dry-bulb temperature - Tdb, can be measured by using anormal thermometer. The dry-bulb temperature is anindicator of heat content and is shown along the left axisof the Mollier diagram. The horizontal lines extendingfrom this axis are constant-temperature lines.Wet-Bulb Temperature - Twb Wet bulb temperature is associated with the moisturecontent of the air. Wet bulb temperature can be measuredwith a thermometer that has the bulb covered with awater-moistened bandage with air flowing over thethermometer. Wet bulb temperatures are always lowerthan dry bulb temperatures but they will be identical with100% relative humidity in the air (the air is at thesaturation line). On the Mollier diagram, the wet-bulblines slope a little upward to the left (dotted lines).
Heating of Air
Cooling and Dehumidfying Air
Mixing of Air of different ConditionsThe heat balance for the mix can be expressed as:LA hA LC hC (LA LC)hBwhereL mixing rateh enthalpy of the airThe moisture balance for the mix can beexpressed as:LA xA LC xC (LA LC) xBwherex water content in the airCalculating the mixture variables xB and hB makesit possible to calculate the mixing temperature tB.
Humidifying ,Adding Steam or Water (liquid)
Psychrometric ChartThe psychrometric chart is a variant of the Mollier diagram used insome parts of the world.The process transforming a Mollier diagram to a psychrometricchart is shown below. First it has to be reflected in a vertical mirror,then rotated 90 degrees.
Evaporation from WaterSurfacesThe amount of evaporated water can be expressed as: m mevapevapA hc( xs x ) cp amount of evaporated water (kg/s)A water surface area (m2)h c heat transfer coefficient (W/m2 K)c p mean specific heat for moist air (J/kg K)x humidity ratio in the air (kg/kg)x s humidity ratio in saturated air at the same temperature as the water surface (kg/kg)
Problem 6 (page 22)An indoor pool evaporates a certain amount of water, which is removed by adehumidifier to maintain 25ºC, φ 70% RH in the room (state 1 in figure).The dehumidifier, shown in figure, is a refrigeration cycle in which moist airflowing over the evaporator cools such that liquid water drops out, and the aircontinues flowing over the condenser. The air after the evaporator (state 2)has a temperature of 14ºC. For an air flow of 0,10 kg/s dry air the unit has acoefficient of performance COPR 3,0.Total pressure in the room is constant 101325 Pa.Calculatea) the amount of water that evaporates from the pool ( steady state)b) the compressor work inputc) the absolute humidity and enthalpy (kJ/kg of dry air) for the air as it returnsto the room (state 3 in figure)1CondenserEvaporator2Water liquid3
Problem dryerHeating coilOutdoor airAT 14 CΦ 60% RHBC Wood dryerCapacity: 500 kg/hDT 40 CΦ 90% RHVolume flow of moist air: 20100 m3/h
TA 14o C , ϕ A 60 % RHpws 1599 Pa,pw 0.60 1599 959.4 Pa959.4 0.00595 kg / kg101325 959.4hA 1.01 14 0.00595(2502 1.84 14) 29.17 kJ / kgx A 0.622TD 40o C , ϕ D 90 % RHpws 7375 Pa,pw 0.90 7375 6637.5 Pa6637.5 0.04360 kg / kg101325 6637.5hD 1.01 40 0.04360(2502 1.84 40) 152.70 kJ / kgxD 0.6228314.51Ru(273.15 40)T 0.0436018.02V M 0.94914 m3p6637.5m 1.0 0.04360ρ 1.09952 kg / m3V0.94914m m moist air V ρ 20100 1.09952 22100.3 kg / h m dry air m moist air22100.3 21177.0 kg / h1.0 x 1.0 0.04360adiabatic conditions for the dryer hC hD m water in dryer m dry air ( xD xC ) xC xD m water in dryer 0.04360 m dry airhC hD 152.70 kJ / kg500 0.01999 kg / kg21177.0Heating coil xB xC 0.01999 kg / kgMixing rate of outdoor air (mix)xB (mix) x A (1 mix) xDmix xB xD 0.01999 0.04360 0.627x A xD 0.00595 0.04360Amount of outdoor air : 62.7% m dry air , A 0.627 21177.0 13280.2 kg / hhB (mix) hA (1 mix) hDhB 0.627 29.17 (1 0.627) 152.70 75.23 kJ / kg Q heating coil m dry air (hC hB ) 21177.0(152.70 75.23) 455.7 456 kW3600
Problem dryerwith heat pumpCooling coilOutdoor airAT 14 CΦ 60% RHBCondenserCD-Wood dryerCapacity: 500 kg/hFT 28ºCEEvaporatorT 40 CΦ 90% RHVolume flow of moist air: 20100 m3/h
TA 14o C , ϕ A 60 % RHpws 1599 Pa,pw 0.60 1599 959.4 Pa959.4 0.00595 kg / kg101325 959.4hA 1.01 14 0.00595(2502 1.84 14) 29.17 kJ / kgxA 0.622TE 40o C , ϕ E 90 % RHpws 7375 Pa,pw 0.90 7375 6637.5 Pa6637.5 0.04360 kg / kg101325 6637.5hE 1.01 40 0.04360(2502 1.84 40) 152.70 kJ / kgxE 0.622TF 28o C , ϕ F 100 % RH (assumed )pws 3780 Pa,pw 3780 Pa ( 6637.5 Pa condensation occur )3780 0.02410 kg / kg101325 3780hF 1.01 28 0.02410(2502 1.84 28) 89.82 kJ / kgxF 0.6228314.51Ru(273.15 40)T 0.0436018.02V M 0.94914 m3p6637.5m 1.0 0.04360ρ 1.09952 kg / m3V0.94914m m moist air V ρ 20100 1.09952 22100.3 kg / h m moist air22100.3 21177.0 kg / h1.0 x 1.0 0.04360m dry airadiabatic conditions for the dryer hD hE m water in dryer m dry air ( xE xD ) xD xE m water in dryer 0.04360 m dry airhD hE 152.70 kJ / kg500 0.01999 kg / kg21177.0Cooling coil xC xD 0.01999 kg / kg (no condensation occur )Heat transfer to heat pump (evaporator ) 21177.0(152.70 89.82) 369.89 kW3600Condenser xB xC 0.01999 kg / kgQ L m dry air (hE hF )
Mixing rate of outdoor air (mix)xB (mix) x A (1 mix) xFxB xF 0.01999 0.02410 0.226x A xF 0.00595 0.02410mix Amount of outdoor air : 22.6% m dry air , A 0.226 21177.0 4795.5 kg / hhB (mix) hA (1 mix) hFhB 0.226 29.17 (1 0.226) 89.82 76.11 kJ / kg Q condenser m dry air (hC hB ) need to calculate hCFrom (log P h) diagram for R717 we readafter evaporator : h1 1780 kJ / kg(1460 kJ / kg from CATT 2)after compressor : h2 2100 kJ / kg(1776 kJ / kg from CATT 2)after condenser : h3 1010 kJ / kg(700.6 kJ / kg from CATT 2)before evaporator : h4 h3 1010 kJ / kg (700.6 kJ / kg from CATT 2) QL369.89 0.4804 kg / s(h1 h4 ) (1780 1010)Q L m R 717 (h1 h4 )m R 717 Work input to compressor : W comp m R 717 (h2 h1 ) 0.4804(2100 1780) 153.72 154 kWHeat transfer from condenser : Q H m R 717 (h2 h3 ) 0.4804(2100 1010) 523.61 kWthis heat will heat the mixed air flow Q H m dry air (hC hB ) QH523.61 76.11 165.12 kJ / kg21177.0m dry air3600Adiabatic conditions in dryer hD hE 152.70 kJ / kghC hB Q cooling coil m dry air (hC hD ) 21177.0(165.12 152.70) 73.07 73 kW3600Wood dryer using only outdoor air consumes about 456 kW of heatWood dryer with a mechanical heat pump consumeselectricity 154 kW and deliviers 75 kW of heatps: no efficiency have been included.