1

Mixture Composition

Dalton’s Model Amagat’s Model

ii

i

mn

M

Describing mixture composition

1

j

ii

m m

1

1j

fii

m

ii

ny

n

1

j

ii

n n

1

1j

ii

y

1

j

i ii

M y M

For Gases:

ii

n RTp

V

1

j

ii

p p

ii

n RTV

p

1

j

ii

V V

Chapter 12 Ideal Gas Mixture and Psychrometric

Applications

2

Ideal Gas Properties for Mixtures

1

j

i ii

u y u

1

j

i ii

h y h

Evaluated at the mixture temperature

,1

j

v i v ii

c y c

,1

j

p i p ii

c y c

p

v

ck

c

1

j

i ii

s y s

Evaluated at the mixture temperature, and appropriate partial pressure, pi

u Mu h Mh s Ms v vc Mcp pc Mc

i i iu M u i i ih M h i i is M s , ,v i v ic Mc, ,p i p ic Mc

3

Mixture Analysis (non-reacting)

2 11

j

i iii

u y u T u T

2 11

j

i iii

h y h T h T

2 2 1 11

, ,j

i iii

s y s T p s T p

4

Ideal Gas Cases

For Ideal Gases using Tables A-22, A-23:

o o2

2 11

lni ip

s s T s T Rp

Assuming Constant Specific Heats:

2 1vu c T T , 2 1i v iu c T T

2 1ph c T T , 2 1i p ih c T T

2 2

1 1

ln lnp

T ps c R

T p 2 2

,1 1

ln lni p i

T ps c R

T p

5

Psychrometric Principles

Dry air + Water vapor = Moist Air

/a a

a a

m R M Tp y p

V

/v v

v v

m R M Tp y p

V

0.622v v

a v

m p

m p p

Humidity Ratio

,

v

g T p

p

p

Relative Humidity

6

Psychrometric Properties

a va

Hh h

m v gh h T

Mixture Enthalpy Water Vapor Enthalpy

, lnv v gs T p s T R Water Vapor Entropy

Dew Point Temperature

Sling Psychrometer picture courtesy of USA Today

7Dry Bulb Temp.

Humidity Ratio

Wet

Bul

b Te

mp.

Relative Humidity

Mix

ture

Ent

halp

y

ASHRAE “Comfort Zone”

DRY

HUMID

HOT

COLD

8

Dehumidification

1 2w v vm m m

1 2

w

a

m

m

1 2 2 1 2r mix mix f

e ia

h h hm

h hm

9

Humidification

2 1w v vm m m

2 1

w

a

m

m

2 1 2 2 10 a mix mix fm h h h

10

Evaporative Cooling

2 1

w

a

m

m

2 2 1 2 1f mix mixh h h

11

Adiabatic Mixing

3 1 2a a am m m

1 21 23

3

a amix mixmix

a

m h m hh

m

1 21 23

3

a a

a

m m

m

12

Cooling Towers