psychrometric properties of humid air calculation procedures

18
ELSEVIER Applied Energy 48 (1994) I lg ~: 1994 Elsevier Science Limited Printed in Great Britain. All rights reserved 0306-2619/94/$7.00 Psychrometric Properties of Humid Air: Calculation Procedures Y. O. Devres TUBITAK Marmara Research Centre, Food and Refrigeration Technology Department, P.O.Box 21, 41470 Gebze-Kocaeli,Turkey A BSTRA CT Knowledge of the psychrometric properties is essential during the designing of air conditioning, cold storage, and drying processes where humid air is a working fluid. In this study detailed procedures Jor calculating psychro- metric properties are given. Seven main properties of the psvchrometrics, namely dry-bulb, wet-bulb and dew-point temperatures, atmo~sphericpressure, humidity ratio, relative humidity and enthalpy can be calculated using the given procedures. According to the Gibbs Phase Rule, in the humid air case, any three intensive properties will be sufficient to evaluate the remaining properties. Therefore the combination of three out of seven properties gives a total of 35 different sets. Computer soJhvare has been developed and utilised to obtain the psychrometric properties q/" humid air. It was Jound that given three input parameters, the remaining four parameters could, except in three cases, can be calculated with neglLgible error. NOTATION h Enthalpy of humid air (kJ/kg) hs* Enthalpy of humid air at saturation at the thermodynamic wet-bulb temperature (kJ/kg) hw* Specific enthalpy of condensed water at the thermodynamic wet- bulb temperature and a pressure of 101 325 Pa (kJ/kg) Pw Partial pressure of water vapour in humid air (Pa) Pws Pressure of saturated pure water (Pa) P Total pressure of humid air (Pa) Ra Gas constant for dry air (J/kg K) T Dry-bulb temperature (°C) 1

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Page 1: Psychrometric Properties of Humid Air Calculation Procedures

ELSEVIER

Applied Energy 48 (1994) I lg ~: 1994 Elsevier Science Limited

Printed in Great Britain. All rights reserved 0306-2619/94/$7.00

Psychrometric Properties of Humid Air: Calculation Procedures

Y. O. Devres

T U B I T A K Marmara Research Centre, Food and Refr igera t ion Technology Department ,

P.O.Box 21, 41470 Gebze-Kocaeli ,Turkey

A BSTRA CT

Knowledge of the psychrometric properties is essential during the designing of air conditioning, cold storage, and drying processes where humid air is a working fluid. In this study detailed procedures Jor calculating psychro- metric properties are given. Seven main properties of the psvchrometrics, namely dry-bulb, wet-bulb and dew-point temperatures, atmo~spheric pressure, humidity ratio, relative humidity and enthalpy can be calculated using the given procedures. According to the Gibbs Phase Rule, in the humid air case, any three intensive properties will be sufficient to evaluate the remaining properties. Therefore the combination of three out of seven properties gives a total of 35 different sets. Computer soJhvare has been developed and utilised to obtain the psychrometric properties q/" humid air. It was Jound that given three input parameters, the remaining four parameters could, except in three cases, can be calculated with neglLgible error.

NOTATION

h Enthalpy of humid air (kJ/kg) hs* Enthalpy of humid air at saturation at the thermodynamic wet-bulb

temperature (kJ/kg) hw* Specific enthalpy of condensed water at the thermodynamic wet-

bulb temperature and a pressure of 101 325 Pa (kJ/kg) Pw Partial pressure of water vapour in humid air (Pa) Pws Pressure of saturated pure water (Pa) P Total pressure of humid air (Pa) Ra Gas constant for dry air (J/kg K) T Dry-bulb temperature (°C)

1

Page 2: Psychrometric Properties of Humid Air Calculation Procedures

2 Y .o . Devres

7* Wet-bulb temperature (°C) TD Dew-point temperature (°C) v Specific volume of humid air (m3/kg) W Humidity ratio of humid air (kg/kg) W S Humidity ratio of humid air at saturation (kg/kg) Ws* Humidity ratio of humid air at saturation at thermodynamic wet-

bulb temperature (kg/kg)

o~ Parameter defined in Table 2 /3 Parameter defined in Table 3 p. Degree of saturation ~b Relative humidity

INTRODUCTION

In the study of the processes of air conditioning, cold storage and drying, a knowledge of the psychrometric properties of the working fluid, that is humid air, is essential. In general, the psychrometric properties of a medium can be predicted either analytically by recourse to the laws for gases or by consulting specially prepared charts and tables. Using these charts, if the atmospheric pressure or altitude is known, it is easy to find the psychrometric properties using two other known properties. However, during the designing, for instance, of a cold store, in most cases its altitude is different from sea level. And yet, most of the charts available in the references are based on sea level properties. Therefore, finding psychrometric properties from zero-altitude charts can create errors. Although the best method for finding these properties is to calculate them analytically using the perfect gas relationships, in many situations, it is often neither convenient nor practical. A realistic solution for these problems is to employ a computer for the numerical solution of the relevant equations: the psychrometric properties being defined in the software.

There are seven main different properties in the psychrometric charts, namely dry-bulb, wet-bulb and dew-point temperatures, atmospheric pressure, humidity ratio, relative humidity and enthalpy. ~ According to the Gibbs Phase Rule, there are four degrees of freedom for a system consisting of humid air, assuming it to be a mixture of nitrogen, oxygen, and water vapour. For all practical purposes the ratio of the masses of oxygen and nitrogen in air is constant, hence the degrees of freedom are reduced to three. Thus any three intensive properties will be sufficient to evaluate the remaining properties. 2 Therefore the combination of three out of seven properties gives a total of 35 different sets. These are shown

Page 3: Psychrometric Properties of Humid Air Calculation Procedures

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Page 4: Psychrometric Properties of Humid Air Calculation Procedures

4 Y . o . Devres

in Table 1. In this study, each set is used separately in an attempt to find solutions for the remaining unknown variables. In some cases analytical solutions were not found and numerical methods had to be employed.

METHOD

Psychrometric equations

For the determination of psychrometric properties, a knowledge of the water-vapour saturation-pressure is essential. The equations for calculating saturation pressures for the temperature ranges -100°C to 200°C are given in Table 2. The solutions for the saturations vapour-pressure as a function of temperatures were found with REGAN, 3 using polynomial REGression ANalysis on data obtained from ASHRAE. 4 In some cases however, the saturation pressure was known allowing the temperature to be calculated from the data. In such cases, temperatures calculated using saturation pressures where derived from regression analysis are shown in Table 3.

The definition of psychrometric properties can be given very easily where perfect gas relations are employed. ~ As a result, only equations (in total 28 equations) are given in Table 4. In every equation, in order to obtain the property given in the second column, the known properties given in the third column must be replaced in the equation given in the fourth column. The units of each property are shown in the last column.

Recommended procedures for the calculation of psychrometric properties

Calculation procedures are shown in Table 5. With the following steps, it is easy to calculate the unknown properties. In some cases, however, numerical analyses must be employed for solution.

RESULTS AND DISCUSSION

In the computer program, 32 out of the 35 combinations have been solved successfully. In combinations 9, 11 and 26 (see Table 5), the equa- tions could not be solved. In each of these cases, two of the equations differed only by a numerical factor, effectively reducing the number of free parameters and not permitting non trivial solutions.

A computer program was developed in F O R T R A N for calculating the psychrometric properties. Nine subroutines were written for numerical solution. For 32 cases, all the properties were calculated with only small

Page 5: Psychrometric Properties of Humid Air Calculation Procedures

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per

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re

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Page 6: Psychrometric Properties of Humid Air Calculation Procedures

TA

BL

E 4

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rom

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c P

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s

Eq.

No.

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o ob

tain

K

now

n(s)

E

quat

ion

Rem

arks

1 Pw

s T

a=

A

. T

2 +

B.

T+

C

+D

. •l

Pw

s =

10

00

.ex

p(a

)

T i

n K

, Pw

s in

Pa

for

con

stan

ts s

ee T

able

2

2 T

Pw

~ T

=E

. /3

4+

F.

/33+

G.

/32

+ H

. /3

+ K

/3

=ln

(Pw

~)

T i

n K

, Pw

s in

Pa

for

con

stan

ts s

ee T

able

3

3 P

w

To

a=A

. T

t~+

B.

T o

+

C+

D.

To

I Pw

=

10

00

.ex

p(a

)

T O

in K

, Pw

in

Pa

for

con

stan

ts s

ee T

able

2

4 T

O

Pw

T

D=

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34

+F

./3

3+

G./

32

+H

./3

+K

/3

= I

n (p

w)

T D

inK

,pw

inP

a fo

r co

nst

ants

see

Tab

le 3

5 p*

s T

* o

t=A

. T

.2+

B

. T

* +

C+

D

. T

* i

P*s

= 1

00

0.e

xp

(a)

T*

in K

, P

*s i

n P

a fo

r co

nst

ants

see

Tab

le 2

6 h

T,

W

h =

T+

W

. (2

501

+ 1-

805.

T

) T

in °

C,

hin

kJ

/kg

h -

25

01

. W

7

T

h,W

T

- 1

+ 1-

805.

W

T

in

°C,

h in

kJ/

kg

e~

h-T

8

W

h, T

W

=

T i

n °C

, h

in k

J/k

g

2501

+

1-80

5.

T

9 W

P

, Pw

W

= 0

.621

98

- Pw

P

and

Pw

in

Pa

P -

Pw

Page 7: Psychrometric Properties of Humid Air Calculation Procedures

10

P

Pw

, H

/ P

= 0.

621

98

• ~

"+

p,,

P an

d p

,~ i

n P

a W

11

p~

P,

W

12

W~

P,

Pw~

13

P

Pw

~, W

~

14

Pw~

P,

W~

15

~7

P, P

*~

16

p *

14'*

P

,,vs

~

.

17

p~

P

, W

~

18

W*

h,

W,

T*

P.W

P

w

W

+ 0.

621

98

W,

= 0-

621

98

. P

'~

P -

Pw~

P =

0-62

1 98

py~

+ P

w~

w~

p,,~

=

0.62

1 98

e W

~ +

0.62

1 98

p*~

14 7

= 0-

621

98

• --

--

p -

p*

P 0.

621

98

• p*

~ t

p*~

w*

P.

W*

p*

~-

W*

+

0.62

1 98

w,*

= h

*-h

+

W

h*

Pan

dp

~in

P

a

P an

d P

w~

in P

a

P an

d p

,~.~

in P

a

P an

d p

,,,~

in P

a

P an

d p

{~ i

n P

a

P an

d p

*~ i

n P

a

P an

d p

*~ i

n P

a

/7":

T

* +

(250

1 +

1.8

05

. T

*).

W

*

h*

=4

.18

6.

T*

h, h

* h

* i

n k

J/k

g,

T*

in °

C

ca

7

Page 8: Psychrometric Properties of Humid Air Calculation Procedures

TA

BL

E

4--

-co

ntd

Eq.

No.

T

o ob

tain

K

no

wn

(s)

Equ

atio

n R

ema

rks

19

W

W.*

,h,

T*

W-

h-h

*+

W

*

h*

20

W

Ws*

, T

, T

* W

=

(250

1 -

2.38

1 .

T*

).

W*

-

(T-

T*

)

25

01

+

1.80

5 .

T-

4.1

86

. T

*

21

T*

W~

, W

, T

T

* =

22

W*

W

, T

, T

*

23

T

W,

T*,

W

~*

24

th

Pw,

Pw~

25

pw

4,, p

w~

26

Pws

4~, P

w

27

/.z

IV,

W s

28

v T

,P,

W

25

01

. (W

s* -

W

) -

T.

(1

+ 1

-80

5.

W)

2.3

81

. W

*

-4.1

86

. W

- 1

Ws ,

_- (

2501

+

1.80

5 .

T

- 2-

381

. T

*).

W

+

(T

- T

*)

25

01

-

2.38

1 .

T*

T=

(2

501

- 2.

381

. T

*).

W

*

- (2

501

- 4-

186

. T

*).

W

+

T*

~b =

PJP

ws

P~

= 4

). P

ws

Pw

s =

Pw

/q~

R a

. T

Y

z

__

1 +

1.8

05

. W

- (1

+

1-60

78.

W)

P

h*

=

T*

+

(2 5

01

+ 1

.80

5.

T*

).

IV*

hw*

-- 4

.18

6

. T

*

h, h

* *

T*

°C

s,

hw

in

kJ/

kg

, in

T a

nd

T

* i

n °C

T a

nd

T

* i

n °C

T a

nd

T

* i

n °C

Tan

d

T*

in°C

Pw a

nd

Pw

s in

Pa

Pw a

nd

Pw

~ in

Pa

Pw a

nd

Pw

s in

Pa

R a

--

287-

055

J/k

g.

K

v in

m3/

kg,

T i

n K

, P

in P

a

e~

Page 9: Psychrometric Properties of Humid Air Calculation Procedures

Psychrometric properties 0[" humid air 9

TABLE 5 Recommended Procedures for the Calculation of Psychrometric Properties

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

1 T, T*, TD 1 Pws 1 2 p*s 5 3 Pw 3 4 P 9, 15, 18 Replace eqns (9) and (15) in eqn (18), then solve

the resulting equation using numerical analysis

2 T, TD, P

3 T , W , P

4 T , W , d )

5 W 9 6 W s 12 7 w* 15 8 h 6 9 ~b 24

1 Pws 1 2 Pw 3 3 W 9 4 W S 12 5 T* 15, 21

6 P*s 5 7 Ws* 15 8 h 6 9 ~ 24

1 Pw~ 1 2 Pw 11 3 TD 4 4 W s 12 5 T* 15, 21

6 P*s 5 7 W* 15 8 h 6 9 4~ 24

1 Pws 1 2 Pw 25 3 P 10 4 TD 4 5 W s 12 6 T* 15, 21

7 P*s 5 8 ~ 15 9 h 6

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

I C~mtinued)

Page 10: Psychrometric Properties of Humid Air Calculation Procedures

10 E O. Devres

TABLE 5--contd

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

5 T , (o ,h 1 Pw~ 1 2 Pw 25 3 W 8 4 P I0 5 T D 4 6 W s 12 7 T* 15, 21 Replace eqn (15) in eqn (21), then solve the

resulting equation using numerical analysis

6 T , T * , P

7 T, TD, W

8 T ,c~ ,P

8 P*s 5 9 W* 15

1 Pws 1 2 P*s 5 3 W* 15 4 W 20 5 W~ 12 6 Pw 11 7 T D 4 8 h 6 9 ~b 24

1 Pw~ 1 2 Pw 3 3 P 10 4 W, 12 5 T* 15, 21

6 p*~ 5 7 W* 15 8 h 6 9 cb 24

1 Pws 1 2 Pw 25 3 W 9 4 W~ 12 5 T o 4 6 T* 15, 21

7 Pw*~ 5 8 W* 15 9 h 6

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Page 11: Psychrometric Properties of Humid Air Calculation Procedures

Psychrometric properties o f humid air 11

TABLE 5---contd.

Proc. Known Step To Use Remarks no. properties no. obtain eqn ( s )

9 T, IV, h UNRESOLVABLE

10 T, T*, W 1 p,,, 1 2 p*~ 5 3 w* 22 4 P 16 5 w~ 12 6 p,, 11 7 T D 4 8 h 6 9 4' 24

11 T, TD, 05 UNRESOLVABLE

12 T, h, P 1 p,,~ 1 2 W 8 3 p,,. 11 4 T D 4 5 w~ 12 6 T* 15, 21

13 T, T*,05

14 T, tl, T D

7 p*.~ 5 8 W* 15

s

9 4' 24

1 p,,,~ 1 2 p,, 25 3 T D 4 4 p*~ 5 5 P 9, 20

6 W 9 7 w~ 12 8 w~ 15 9 h 6

1 Pws 1 2 W 8 3 Pw 3 4 P 10 5 w, 12 6 T* 15, 21

7 p*~ 5 8 w~* 15 9 4' 24

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (9) in eqn (20), then solve the resulting quadratic equation

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

{ ' iO l l f t l l l i , d ,

Page 12: Psychrometric Properties of Humid Air Calculation Procedures

12 Y.O. Devres

TABLE ~-contd.

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

15 T, T*, h 1 Pws 1 2 W 8 3 P*s 5 4 W* 22 5 P 16 6 p~ 11 7 T D 4 8 W s 12 9 ~ 24

16 T*, T D, P 1 Pw 3 2 p*~ 5 3 W* 15 4 W 9 5 T 23 6 P,,,s 1 7 W s 12 8 h 6 9 ~b 24

17 T*, P, W 1 Pw 11 2 TD 4 3 p*s 5 4 W* 15 5 T 23 6 Pw~ 1 7 Ws 12 8 h 6 9 ~b 24

18 T*, W, 1 p*~ 2 7"

3 Pws 4 P 5 ws 6 w* 7 Pw 8 TD 9 h

5 1, 10, 20, Replace eqn (25) in eqn (10), then solve the

25 resulting equation with eqns (1) and (25) using numerical analysis

1 10, 25

12 15 11 4 6

Page 13: Psychrometric Properties of Humid Air Calculation Procedures

Psychrometric properties o f humid air 13

TABLE 5---contd.

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

19 T*, ~b, h 1 P*s 5 2 T 8, 9, 15, Replace eqns (9) and (25) in eqn (8) and solve

18, 25 for P(T), then substitute P(T) in eqn (18) with eqn (15) and solve the resulting equation using numerical analysis Use P(T) found in step 2

20 T*, TD, W

21 T*, 4', P

22 T*, W, h

3 P 4 Pw~ 1 5 w, 12 6 Pw 25 7 T D 4 8 W 9 9 W* 15

1 Pw 3 2 P 10 3 p*~ 5 4 W* 15 5 T 23

6 Pws 1 7 W~ 12 8 h 6 9 4~ 24

1 p*~ 5 2 w* 15 3 T 9, 20, 25

4 Pw~ 1 5 W~ 12 6 W 9 7 Pw 25 8 T D 4 9 h 6

1 P*s 5 2 T 7

3 Pws 1 4 W* 22 5 P 16 6 Pw 11 7 T D 4 8 w, 12 9 ¢b 24

Replace eqns (9) and (25) in eqn (20) and solve the resulting equation using numerical analysis

; ( )mlmued)

Page 14: Psychrometric Properties of Humid Air Calculation Procedures

14 Y.O. Devres

TABLE 5 contd.

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

23 T*, T D,4~ 1 Pw 3 2 p*s 5 3 Pws 26 4 T 2 5 P 9, 15, 20 Replace eqns (9) and (15) in eqn (20) and solve

the resulting quadratic equation

24 T*, P, h

25 T*, T D, h

26 TD, P, W

27 T D, W , ~

6 W 9 7 W* 15 8 W~ 12 9 h 6

1 p*~ 5 2 w* 15 3 W 19 4 Pw 11 5 TD 4 6 T 7 7 Pw~ 1 8 W~ 12 9 ~ 24

1 P*s 5 2 Pw 3 3 P 9, 15, 18

4 W 9 5 T 7 6 Pws 1 7 W~ 12 8 W* 15 9 4~ 24

UNRESOLVABLE

1 Pw 3 2 Pw~ 26 3 T 2 4 P l0 5 W~ 12 6 T* 15, 21

7 P*s 5 8 w * t5 9 h 6

Replace eqns (9) and (15) in eqn (18) and solve the resulting quadratic equation

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Page 15: Psychrometric Properties of Humid Air Calculation Procedures

Psychrometric properties q f humid air 15

TABLE ~ c o n t d

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

28 T D, oh, h 1 Pw 3 2 Pws 26 3 T 4 W 8 5 P 10 6 W~ 12 7 T* 15, 21 Replace eqn (15) in eqn (21), then solve the

resulting equation using numerical analysis

29 TD, P, c~

30 T D, W, h

31 TD, P , h

8 p*~ 5 9 W* 15

1 Pw 3 2 Pw~ 26 3 T 2 4 W 9 5 w~ 12 6 T* 15, 21

7 P*s 5 8 w* 15 9 h 6

1 Pw 3 2 P 10 3 T 7 4 Pws 1 5 W~ 12 6 T* 15, 21

7 P*s 5 8 W* 15 9 4~ 24

1 Pw 3 2 W 9 3 T 7 4 Pw~ 1 5 w~ 12 6 T* 15, 21

7 P*s 5 8 w* 15 9 ~h 24

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

r ~ o H l i t i l l c d !

Page 16: Psychrometric Properties of Humid Air Calculation Procedures

16 Y.O. Devres

TABLE 5~contd.

Proc. Known Step To Use Remarks no. properties no. obtain eqn(s)

32 P, W, ~h 1 Pw 11 2 T D 4 3 Pws 26 4 T 2 5 Ws 12 6 T* 15, 21 Replace eqn (15) in eqn (21), then solve the

resulting equation using numerical analysis 7 P*s 5 8 W* 15 9 h 6

33 P, ~, h 1 T 6, 9, 25

34 P, W,h

35 w, 4~,h

2 Pws 1 3 W~ 12 4 Pw 25 5 T D 4 6 W 9 7 T* 15, 21

8 P*s 5 9 W* 15

1 T 7 2 Pws 1 3 Pw 11 4 T D 4 5 Ws 12 6 T* 15, 21

7 P*s 5 8 w * 15 9 q~ 24

1 T 7 2 Pws 1 3 Pw 25 4 T D 4 5 P 10 6 W s 12 7 T* 15, 21

8 p*~ 5 9 W* 15

Replace eqns (9) and (25) in eqn (6) and solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Replace eqn (15) in eqn (21), then solve the resulting equation using numerical analysis

Page 17: Psychrometric Properties of Humid Air Calculation Procedures

TA

BL

E 6

R

esul

ts o

f P

rope

rtie

s fo

r D

iffe

rent

Com

bina

tion

Set

s

Pro

per

ty

Ca

lcu

lati

on

s

Co

mb

ina

tio

n

Co

mb

ina

tio

n

Co

mb

ina

tio

n

Co

mb

ina

tio

n

Co

mb

ina

tio

n

set

no.

6 se

t no

. 12

se

t no

. 19

se

t no

. 2

7

set

no.

33

T (

°C)

20-0

00*

20.0

00*

19-9

99

20.0

00

20.0

00

T*

(°C

) 15

.000

" 15

.000

15

.000

" 15

.000

15

-000

T

D (

°C)

11.7

55

11.7

55

11.7

55

11.7

55"

11.7

55

P (P

a)

101

325"

000*

10

1 32

5"00

0*

101

323"

823

101

324-

057

101

325"

000*

W

(g/

kg)

8-58

9 8"

589

8"58

9 8"

589*

8"

589

~"

4~ (%

) 59

"008

59

"009

59

"008

* 59

"008

* 59

"008

* "4

h

(kJ/

kg)

41 "7

91

41 "

791"

41

"79

1 *

41 "7

91

41-7

91 *

~

p,~

(Pa)

2

338.

879

2 33

8-87

9 2

338.

879

2 33

8.90

0 2

338-

894

p*~

(Pa)

1

705.

505

1 70

5.51

8 1

705.

505

1 70

5.52

4 1 7

05.5

18

,~.

p,~.

(Pa)

1

380.

122

1 38

0.14

2 1

380.

126

1 38

0.13

8 I 3

80.1

35

W~

(g/k

g)

14.6

96

14.6

96

14.6

97

14-6

97

14.6

96

W*

(g/k

g)

10.6

48

10.6

48

10.6

49

10.6

49

10.6

48

* K

now

ns.

-7,

Page 18: Psychrometric Properties of Humid Air Calculation Procedures

18 Y.O. Devres

errors. In some cases, however, especially when P was not known, a maximum + 0.5% error occurred. In the other cases, errors considerably smaller than this were obtained.

The results of different combination sets are given in Table 6. In com- bination set no. 6, the properties are calculated without any numerical analysis procedures. In combination sets 12, 19, 27 and 33, however, numerical analysis techniques were employed (see Table 5). In the columns of Table 6, '*' following the data indicates the known psychrometric properties. The calculated values are shown with three digits for comparison. However, no significant differences between any combination sets were found.

CONCLUSION

When humid air is used as a working fluid, it is essential to use reliable data for any necessary calculations. A good method to perform these calculations is by the use of computer software embodying the properties of perfect gases. In this study such software has been developed and utilised to obtain the psychrometric properties of humid air. It was found that given three input parameters, the remaining four parameters could, except in three cases, be calculated with negligible error.

R E F E R E N C E S

1. ASHRAE, Fundamentals Handbook, Psychrometrics. American Society of Heating, Refrigeration and Air-Conditioning Engineers, New York, 1981, Chapter 5, pp.l-10.

2. Agrawal, K.K. & Rao, H.V., A computer model of psychrometric properties of air. ASAE Transactions, 17 (1) (1974) 67-9.

3. Devres, Y.O., Curve-fitting (regression analyses) VAX package computer programme using least-square methods. TI[IBITAK Marmara Research Centre, Food and Refrigeration Technology Publications No.121, Gebze-Kocaeli, 40 p.

4. ASHRAE, Fundamentals Handbook, Psychrometric Tables. American Society of Heating, Refrigeration and Air-Conditioning Engineers, New York, 1981, Chapter 6, pp.l-16.