10/28/97 A F Emery 1

PSYCHROMETRICSand

ELEMENTARY PROCESSES

(English Units)

Ashley F. EmeryUniversity of Washington

10/28/97 A F Emery 2

Psychrometrics

The study of a mixture of dry air and water vapor

Although precise thermodynamic relations areavailable for moist air, we will treat moist air as a mixture of ideal gases

10/28/97 A F Emery 3

Why study psychrometrics?

The degree of moisture has a strong effect on

1) heating, cooling, and comfort2) insulation, roofing, stability and deformation of building materials3) sound absorption, odor levels, ventilation4) industry and agriculture

10/28/97 A F Emery 4

Dry Air and Water Vapor

Nitrogen 78.084 28.0134Oxygen 20.448 31.9988Argon .934 39.9430Carbon Dioxide .031 44.0100

Dry AirComponent % by vol MW

Effective MW 28.9645

Water Vapor 18.0153

10/28/97 A F Emery 5

IDEAL GAS

PV=mRTP = pressure lbf/sq. ft.V= volume cu. ft.m=mass lbmR=gas constantT=temperature R =F+460

352.53daR

778.85wR

778.85wRRLbm

FtLbf

10/28/97 A F Emery 6

wVdaVV

wTdaTT

wPdaPP

wMdaMM

Dalton’s Law

wPdaP ,

partial pressureswPdaP ,

10/28/97 A F Emery 7

Mixture of Gases

TdaRdaMWdanVdaP

TwRwMWwnVwP letting wndann

universalRdaRdaMW and remembering that

we obtain

Pnwn

wP

10/28/97 A F Emery 8

IMPORTANT PROPERTIES

Humidity ratio, W

wPPwP

danwn

daMWwMW

damwmW

622.0

Humidity ratio is the mass of water vapor per unit massof dry air. Units are Lbm/Lbm, grams/grams, orgrains/lbm (7000 grains=1Lbm)

10/28/97 A F Emery 9

IMPORTANT PROPERTIES

Saturated Humidity ratio,

wsPPwsP

sW 622.0

Saturation is when the air contains the maximum amountof water vapor at its current temperature. The saturationpressure is taken from the steam tables at the moist airtemperature.

sW

10/28/97 A F Emery 10

IMPORTANT PROPERTIES

Relative humidity,

wsPwP

Relative humidity is defined as the ratio of the partial pressure of the water vapor to the saturation pressureat the same temperature

10/28/97 A F Emery 11

IMPORTANT PROPERTIES

Degree of saturation,

sW

W

Degree of saturation is the ratio of the amount of water contained in the moist air to that which would be contained if the air were saturated

10/28/97 A F Emery 12

IMPORTANT TEMPERATURES

Dry Bulb

Dew Point

bT

dpT= temperature of moist air at rest

= temperature at which the water vapor will condense out of the moist air. It is the temperature for which W is the saturated humidity ratio

WdpTsW )(

10/28/97 A F Emery 13

IMPORTANT TEMPERATURES

Adiabatic Saturation Temperature,*T

**)( shlhWsWh

*lh

*,,*shsWTWhbT ,,

it is the temperature at which liquid water wouldevaporate into the moist air without any heat additionto the system

*T satisfies

10/28/97 A F Emery 14

IMPORTANT TEMPERATURES

Wet Bulb Temperature,

Is the temperature reached by evaporative cooling.

A cotton sock is wrapped around a thermometer, saturated with distilled water. The water evaporatesand the resulting temperature is called the wet bulbtemperature.

It is a close approximation to *T

wbT

10/28/97 A F Emery 15

Thermodynamic Properties

whWdahah enthalpy, BTU/unit weight of dry air

=0.24 T +W(0.45 T +1061.1)

wPP

TdaRav

specific volume, cu. ft. /unit weight of dry air

10/28/97 A F Emery 16

Example

moist air at 80F dry bulb, 65F dew point, 14.696psia?,,,,, vhsWW

10/28/97 A F Emery 17

Example (continued)Example

a) degree of saturation

595.00222.0

0132.0

sW

W

d) relative humidity

604.0507.0

306.0

PwswP

e) enthalpy

h=0.24*80 +0.0132*(0.45*80 + 1061.1) = 1153.8 BTU/lbm-da

f) volume

144*)306.0696.14

)46080(*35.53

dav = 13.90 cu. Ft. /lbm-da

10/28/97 A F Emery 18

bT

W

h

*T

rh

Psychrometric Chart

saturation line

10/28/97 A F Emery 19

Simple Heating

bT

W

1 2

)12( hhdamq FTFTFT 702,40*

1,501

dam q

10/28/97 A F Emery 20

Simple Heating, solution

)12( hhdamq

FTFTFT 702,40*1,501

2h

0030.021 WW

1h 0.24*50+0.003*(0.45*50+1061.)=15.25

0.24*70+0.003*(0.45*70+1061.)=20.08

)12(/ hhdamq = 4.83 BTU/Lbm-da

10/28/97 A F Emery 21

Simple Heating and Humidification

bT

W

1

)13( hhdamwhwmq

FwTFTFTFTFT 50,63*3,703,40*

1,501

3

)13( wwdamwm

amq

wm

10/28/97 A F Emery 22

Simple Heating and Humidification,Solution

bT

W

1

30108.03 W0030.01 W

1h

3h

15.25

0.24*70+0.0108*(0.45*70+1061.1)=28.60

)13(/ wwdamwm =0.0108-0.003=0.0078Lbm/Lbm

wh

FwTFTFTFTFT 50,63*3,703,40*

1,501

18.06

10/28/97 A F Emery 23

Simple Heating and Humidification,Solution

bT

W

1

30108.03 W0030.01 W

1h

3h

15.25

0.24*70+0.0108*(0.45*70+1061.1)=28.60

wh

FwTFTFTFTFT 50,63*3,703,40*

1,501

18.06

whdamwmhh

dam

q

)13( =28.60-15.25-0.0078*18.06=13.21 BTU/Lbm-da

10/28/97 A F Emery 24

Dehumidification and Cooling

bT

W

2

10030.03 W0108.01 W

1h

2h

28.60

15.25

FTFTFTFT 40*2,502,63*

1,701

The answer is the same as for the previous problem

since the end points are the same BUT how can we actually go from point 1 to point 2??

10/28/97 A F Emery 25

Dehumidification and Cooling, solution

bT

W

2

11’

2’

1 to 1’ by cooling1’ to 2’ by cooling and dehumidification2’ to 2 by heating

10/28/97 A F Emery 26

Dehumidification and Cooling, solution

bT

W

2

11 to 1’ by cooling

1h

'1T 59.2F

28.60

'1h 0.24*59.2+0.0108*(0.45*59.2+1061.1)=25.96

1w 0.0108

dam

q '11 (25.96-28.60)=-2.64

1’

2’

10/28/97 A F Emery 27

Dehumidification and Cooling, solution

bT

W

2

1

1’ to 2’ by cooling and dehumidification

1’

2’

amq

wm

0108.01 W

0030.02 W27'2 Tassume that the water leaves at

10/28/97 A F Emery 28

Dehumidification and Cooling, solution

bT

W

2

11’ to 2’ by cooling and dehumidification

1’

2’

'2,'2'1 whwmhdamqhdam

'2,'1'2 whdamwmhh

dam

q

96.25'1 h

5'2, wh80.6'2 h27'2 T

20.19'2'1 dam

q

0078.0damwm

10/28/97 A F Emery 29

Dehumidification and Cooling, solution

bT

W

2

12’ to 2 by heating

1’

2’

2'2 hdamqhdam

'22 hhdam

q

25.152 h 80.6'2 h

45.82'2 dam

q

10/28/97 A F Emery 30

Dehumidification and Cooling, solution

bT

W

2

11’

2’

45.82'2 dam

q

20.19'2'1 dam

q

64.2'11 dam

q

39.1321 dam

q

10/28/97 A F Emery 31

Difference between Humidification and Dehumidification

bT

W

1

2

bT

W

2

11’

2’

Water is injectedat 50F

Water is rejected at 27F

39.1321 dam

q

21.1321

dam

q

10/28/97 A F Emery 32

Adiabatic Mixing of 2 Streams

1

2

3

33,22,11, wdamwdamwdam

33,22,11, hdamhdamhdam

10/28/97 A F Emery 33

Adiabatic Mixing of 2 Streams

1

2

3

23

2,1

3

1,3 h

mdam

hmdam

h

23

2,1

3

1,3 w

mdam

wmdam

w

bT

W

1

23

10/28/97 A F Emery 34

Adiabatic Mixing of 2 Streams, example

1

2

3500 cfm at 60F dry bulb and rh=50%is mixed with 250 cfm at 80F dry bulband 60F wet bulb.

min85.3721.13/5001/5001,Lbm

vdam

195.1874.13/2502/2502, vdam

10/28/97 A F Emery 35

Adiabatic Mixing of 2 Streams, example

1

2

3

0067.02 w 0059.03 w38.201 h 58.262 h

055.01 w

83.223 h

68F db, 40% rh, 43.4F dp