4.Psychrometric Chart

( )

: 1 week

Composition of Humid Air ()

Dry Air ()

and

Water Vapor ()

T (/)

W250C, 50%

W = 0.01 kg H2O/kg dry air

Humidity Ratio(, 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)]

Note: pV = (m/M)RuT = m(Ru/M)T = mRTRu = 8.3143 (J/gmole-K) = Ma = 28.97 (g/gmole) = Ms = 18.015 (g/gmole) =pt = =pa + ps=101.325 kPa

(ps)

t

(0.622)( ) (0.622)( )101.325

where p = 101.325 kPa = atmospheri

( )( ) ( )(101.325)0.622 0.6

c pressure

, 2

2

s s

t

s t

s s

W Wp

p pWp p

pW W

p

Hence

(Table A-1)

= =+

+

= =

= = / = ps / ps,sat: W = (0.622) [ps / (pt - ps)]

Wsat = (0.622) [ps,sat / (pt - ps,sat)]

= = [W (pt-ps)] / [Wsat (pt - ps,sat)]

(W / Wsat)

Note : pt >> ps,sat

Relative Humidity (, )

Specific Enthalpy of Humid Air (h)h = cp t + W hgwhere hg = specific enthalpy of saturated water vapor at t

cp = 1.0 kJ/kg-K dry airt = dry-bulb temperature ()

!

h

s

T

WLines of constant specific enthalpy

Specific Volume (v)-

101.325a a a

a a t s s

R T R T R Tp

Vvm p p p

= = = =

(V)ma = Pa = Ra = Ru / Ma

Wet-Bulb temperature ()

sump

Sump ( = t2)

(Based on ):

h2 = h1 + hf (W2 - W1)

hf (t2) = (h2 - h1) / (W2 - W1)

hf(t2)(t2 : state 1

Psychrometric Chart

(Based on ):

hf (t2) = (h2,sat - h1) / (W2,sat - W1)

hf(t2)(t2 : state 1

Note: ( t2) h1W1 PsychrometricChart

(Wet-Bulb Temperature)(Dew-Point Temperature)

36oC 50%

=24oC =27oC

Straight-Line Law When air is transferring heat and mass (water) to or from a wetted surface, the condition of the air shown on a psy-chrometric chart drives toward the saturation line at the temperature of the wetted surface.

wetted surface

!

!

Lewis relationship

Note:

Wet-Bulb Thermometer ()

T

W

Wetted-surface temperature

: h2 = h1 + hf (W2 - W1) wet-bulb temperature ! wetted-surface temperature wet-bulb temperature !

Approach II: h1 = h2 - hf (W2 - W1) = 89.7 (117.3)(0.0241-0.019)

= 89.1 (kJ/kg)

at T = 40oC, = 41%

Approach I: 3.1()h1 = 89 (kJ/kg)

at 28oC; Table A-1

:

89 (kJ/kg)

89.7 (kJ/kg)

(28oC)

1

2 0.0241

0.019

Processes Sensible Heating or Cooling ()

Processes Humidification ()

Processes Cooling and Dehumidification

Processes Chemical Dehumidification ()

Processes Mixing ()

:: w1h1 + w2h2 = (w2 + w1) h3

:: w1W1 + w2W2 = (w2 + w1) W3

kg/s Dry Air

Homework 2 (2)

3-43-63-8