L34 - The Psychrometric Chart

7/24/2019 L34 - The Psychrometric Chart

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2/17

Moist Air Properties

2

Using a combination of the ideal gas and real-fluid models

along ith ideal mi!ing e ha"e disco"ered#

( ) ( )# ,a w wa

Hh h T h T P

m= = +

$ote% The te!tboo& puts a '() sign on the enthalpy to remindus that its mass basis is per lbm of dry air in the mi!ture*

Applying this notation to the heating problem from +ecture ,#

( ) ( )2 2 2 1 1 1a a w a wQ m h h h h = + + & &

( )# #2 1aQ m h h= & &

Q

1

1

1

14 psia65 F0.50

300 cfm

P

T

V

=

=

=

=&

2

2

14 psia110 F

P

T

=

=


3/17

Moist Air Properties

,

Question% .s there an easier ay to determine h

(

/Answer% 0ES1

As ith pure fluids# a real-fluid mi!ture formulation has been

de"eloped for the dry air-ater "apor mi!ture* The

formulation uses the real-fluid beha"ior of the components

along ith real mi!ing* As such# the resulting formulation is

comple!* ortunately# it can be displayed graphically in a

series of charts &non as the psychrometric charts* The

psychrometric charts are published by AS34AE* They eredran at the Center for Applied Thermodynamic Studies

5CATS6 at the University o !daho* There are 7 .P 5English

unit6 charts# and S. charts*


4/17

Department ofMechanical Engineering

ME 322 Mechanical Engineering

Thermodynamics

"sychrometric "roperties

The Psychrometric Chart


5/17

Chart $o* 8 is a"ailable on the 9b+earn ebsite15


6/17

:et 9ulb and Dry 9ulb Temperatures

:et bulb and dry bulb temperatures are thermodynamic

properties of the moist air mi!ture* They are measured ith

a de"ice &non as a psychrometer*

;


7/17

Psychrometric Properties using the Chart

#iven% Moist air mi!ture in our classroom< T# Tbmeasured

ith a sling psychrometer*

$ind%The folloing psychrometric properties of the mi!ture

using the Psychrometric Chart

5a6 relati"e humidity

5b6 humidity ratio

5c6 specific enthalpy

5d6 specific "olume

5e6 de point temperature

T

#h

dpT

wbT

v

dpT


8/17

Psychrometric Properties in EES

=

#iven% Moist air at 8 atm#

=7># ?@ relati"e

humidity

$ind%The folloing

psychrometric propertiesof the mi!ture using EES

5a6 humidity ratio

5b6 specific enthalpy

5c6 specific "olume5d6 et bulb temperature

5e6 de point temperature


9/17

Department ofMechanical Engineering

ME 322 Mechanical Engineering

Thermodynamics

Air &onditioning "rocesses


10/17

The i"e 3BAC Processes

3eating :ith and ithout humidification

Adiabatic humidification

Cooling :ith and ithout dehumidification

Adiabatic mi!ing of moist air streams

E"aporati"e cooling

8@


11/17

3eating :ithout 3umidification

88

HQ&

1 2

1T 2T

1

2#

1h

#

2h

1

2

1 2 =

( )# #2 1H aQ m h h= & &

Constant humidity ratio 4elati"e humidity

decreases utlet relati"e humidity

may be too dry to becomfortable


12/17

3eating ith 3umidification

82

HQ&

1 2

water

1T 1'T

1

1'

#

1h

#

1'h

1 1'1

1'

2

#

2h

22

( ) ( )# #2 1 2 1H

water

a

Qh h h

m =

&

&

# #

1 2H a water water aQ m h m h m h+ + =&

& & &

1 2

2 1

2 1

w water w

water w w

a a a

m m m

m m m

m m m

+ =

= =

& & &

& & &

& & &

2T


13/17

Adiabatic 3umidification

8,

1 2

water

1T

1

1 1

2b

( ) ( )# #2 1 2 1 0waterh h h =

# #

1 2a water water am h m h m h+ =& & &

1 2

2 1

2 1

w water w

water w w

a a a

m m m

m m m

m m m

+ =

= =

& & &

& & &

& & &

2c2a

The location of state 2 52a#2b# or 2c6 depends on the

state of the ater being

inFected for humidification


14/17

Cooling :ithout Dehumidification

8?

CQ&

1 2

1T2T

1

2

#

1h

#

2h

1

2 1 2 =

( )# #1 2C aQ m h h= & &

This process is not "ery

common in 3BAC systems*

ften# the surface of the

heat e!changer is belo

the de point hich causesater to condense

1'T


15/17

Cooling :ith Dehumidification

87

CQ&

1 2

1T2T

1

2

#1h

#

2h

12

1

.f Tateris not gi"en# it iscommon to assume

that it is eGual to T2

water

dpT

2

# #

2 1c a water water aQ m h m h m h+ + =&

& & &

2 1

1 2

1 2

w water w

water w w

a a a

m m m

m m m

m m m

+ =

= =

& & &

& & &

& & &

( ) ( )# #1 2 1 2C

water

a

Qh h h

m =

&

&


16/17

Adiabatic Mi!ing

8;

1 (cold)

2 (hot)

#

1h

#

2

h

1

2

1

2

# # #

1 1 2 2 3 3a a am h m h m h+ =& & &

3 (warm)

3

#

3h

3

1 2 3a a am m m+ =& & &

( )# # #1 1 2 2 1 2 3# #

1 2 3

# #

2 3 1

a a a a

a

a

m h m h m m h

m h h

m h h

+ = +

=

& & & &

&

&

( )1 1 2 2 1 2 31 2 3

2 3 1

a a a a

a

a

m m m m

m

m

+ = +

=

& & & &

&

&

1 1 2 2 3 3a a am m m + =& & &

# #

2 3 2 3

# #

3 1 3 1

h h

h h

=

8-2-, are on a straight line1


17/17

E"aporati"e Cooling

8

1 2

water

1T

1

1

1

( ) ( )# #2 1 2 1 0waterh h h =

# #

1 2a water water am h m h m h+ =& & &

1 2

2 1

2 1

w water w

water w w

a a a

m m m

m m m

m m m

+ =

= =

& & &

& & &

& & &

2

2

2

2T

This is the adiabatic

humidificationprocess hen the

ater used for

humidification is

colder than T8

This system or&s "ery ell in hot#

dry climates* $otice that there canbe a significant increase in the

humidity le"els 5both relati"e

humidity and humidity ratio6*

L34 - The Psychrometric Chart

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