Psychrometric Chart 中文讲义

Note : pt >> ps,sat
Specific Enthalpy of Humid Air (h)
h = cp t + W h g
where hg = specific enthalpy of saturated water vapor at t cp = 1.0 kJ/kg-K dry air t = dry-bulb temperature ( 度
度
W
)
律 T Wetted-surface temperature
量
:
h2 = h1 + hf (W2 - W1)
wet-bulb temperature ! Wettedsurface temperature wet-bulb temperature !
Processes – Sensible Heating or Cooling
!
冷
wetted surface
!
Lewis relationship
度
異
Approach I: h1 = 89 (kJ/kg)
Approach II: h1 = h2 - hf (W2 - W1) = 89.7 – (117.3)(0.0241-0.019) = 89.1 (kJ/kg)
Wet-Bulb Thermometer (
Humidity Ratio(
度
度
, W)
= W = ms / ma = kg of water vapor / kg of dry air = (psV/RsT) / (paV/RaT) = (ps/pa) / (Rs/Ra) = [ps/(pt-ps)] / (Rs/Ra) = [ps/(pt-ps)] / (Ma/Ms) = (0.622) [ps/(pt-ps)] 理 pV = (m/M)RuT = m(Ru/M)T = mRT 數 Ru = 8.3143 (J/gmole-K) = 量 Ma = 28.97 (g/gmole) = 量 Ms = 18.015 (g/gmole) = = 101.325 kpa pt =
4. Psychrometric Chart
: 1 week
Composition of Humid Air (
ห้องสมุดไป่ตู้
)
Dry Air ( and Water Vapor (
)
)
250C, 50%
度 W
W = 0.01 kg H2O/kg dry air
T
250C, 50%
度
W = 0.01 kg H2O/kg dry air
)
不
!
Lines of constant specific enthalpy
W
T
Specific Volume (v)RaT RaT RaT = = v= pa pt − ps 101.325 − ps
Wet-Bulb temperature (
度)
度 sump
Sump (
度 - t2)
量
(Based on
)
量 : 量 :
: w1h1 + w2h2 = (w2 + w1) h3 : w1W1 + w2W2 = (w2 + w1) W3
Homework 2 ( 2)
3-4 3-6 3-8
Note:
(ps)
ps ps W = (0.622)( ) = (0.622)( ) pt − ps 101.325 − ps where p t = 101.325 kpa = atmospheric pressure Hence, W W )( pt ) = ( )(101.325) ps = ( W + 0.622 W + 0.622
( 冷 )
冷
Processes – Humidification (
)
Processes – Cooling and Dehumidification
Processes – Chemical Dehumidification
( )
Processes – Mixing (
kg/s Dry Air 量
度 冷
Relative Humidity (
度=φ = = ps / ps,sat / 度
度, φ)
力
: W = (0.622) [ps / (pt - ps)] Wsat = (0.622) [ps,sat / (pt - ps,sat)] 度=φ = [W (pt-ps)] / [Wsat (pt - ps,sat)] ≈ (W / Wsat)
):
h2 = h1 + hf (W2 - W1)
hf = (h2 - h1) / (W2 - W1)
hf (state ”1” 度(t2) 度
Straight-Line Law –
律
When air is transferring heat and mass (water) to or from a wetted surface, the condition of the air shown on a psychrometric chart drives toward the saturation line at the temperature of the wetted surface.