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Department of Mechanical Engineering ME 322 Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart Psychrometric Properties in EES Air Conditioning Processes

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Page 1: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Department of Mechanical Engineering

ME 322 – Mechanical Engineering

Thermodynamics

Lecture 34

The Psychrometric Chart

Psychrometric Properties in EES

Air Conditioning Processes

Page 2: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Moist Air Properties

2

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

along with ideal mixing we have discovered,

# ,a w w

a

Hh h T h T P

m

Note: The textbook puts a ‘#’ sign on the enthalpy to remind

us that its mass basis is per lbm of dry air in the mixture.

Applying this notation to the heating problem from Lecture 37,

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

PT

V

2

2

14 psia110 F

PT

Page 3: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Moist Air Properties

3

Question: Is there an easier way to determine h# ?

Answer: YES!

As with pure fluids, a real-fluid mixture formulation has been

developed for the dry air-water vapor mixture. The

formulation uses the real-fluid behavior of the components

along with real mixing. As such, the resulting formulation is

complex. Fortunately, it can be displayed graphically in a

series of charts known as the psychrometric charts. The

psychrometric charts are published by ASHRAE. They were

drawn at the Center for Applied Thermodynamic Studies

(CATS) at the University of Idaho. There are 5 IP (English

unit) charts, and 7 SI charts.

Page 4: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Department of Mechanical Engineering

ME 322 – Mechanical Engineering

Thermodynamics

Psychrometric Properties

The Psychrometric Chart

Page 5: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Chart No. 1 is available on the BbLearn website! 5

Page 6: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Wet Bulb and Dry Bulb Temperatures

Wet bulb and dry bulb temperatures are thermodynamic

properties of the moist air mixture. They are measured with a

device known as a psychrometer.

6

Page 7: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Psychrometric Properties using the Chart

7

Given: Moist air mixture in our classroom; T, Twb measured

with a sling psychrometer.

Find: The following psychrometric properties of the mixture

using the Psychrometric Chart

(a) relative humidity

(b) humidity ratio

(c) specific enthalpy

(d) specific volume

(e) dew point temperature

T

#h

dpT

wbT

v

dpT

Page 8: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Psychrometric Properties in EES

8

Given: Moist air at 1 atm,

85°F, 40% relative

humidity

Find: The following

psychrometric properties

of the mixture using EES

(a) humidity ratio

(b) specific enthalpy

(c) specific volume

(d) wet bulb temperature

(e) dew point temperature

Page 9: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Department of Mechanical Engineering

ME 322 – Mechanical Engineering

Thermodynamics

Air Conditioning Processes

Page 10: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

The Five HVAC Processes

• Heating

– With and without humidification

• Adiabatic humidification

• Cooling

– With and without dehumidification

• Adiabatic mixing of moist air streams

• Evaporative cooling

10

Page 11: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Heating Without Humidification

11

HQ

1 2

1T 2T

1

2#

1h

#

2h

12

1 2

# #

2 1H aQ m h h

• Constant humidity ratio

• Relative humidity

decreases

• Outlet relative humidity

may be too dry to be

comfortable

Page 12: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Heating with Humidification

12

HQ

1 2

water

1T 1'T

1

1'

#

1h

#

1'h

11'

1

1'

2

#

2h

22

# #

2 1 2 1

H

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

Page 13: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Adiabatic Humidification

13

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 (2a,

2b, or 2c) depends on the

state of the water being

injected for humidification

Page 14: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Cooling Without Dehumidification

14

CQ

1 2

1T2T

1

2

#

1h

#

2h

12 1 2

# #

1 2C aQ m h h

This process is not very

common in HVAC systems.

Often, the surface of the

heat exchanger is below

the dew point which causes

water to condense

1'T

Page 15: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Cooling With Dehumidification

15

CQ

1 2

1T2T

1

2

#

1h

#

2h

121

If Twater is not given, it is

common to assume

that it is equal 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 2

C

water

a

Qh h h

m

Page 16: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Adiabatic Mixing

16

1 (cold)

2 (hot)

#

1h

#

2h

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 3

1 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

1-2-3 are on a straight line!

Page 17: The Psychrometric Chart Psychrometric Properties in · PDF fileDepartment of Mechanical Engineering ME 322 – Mechanical Engineering Thermodynamics Lecture 34 The Psychrometric Chart

Evaporative Cooling

17

1 2

water

1T

1

11

# #

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

humidification

process when the

water used for

humidification is

colder than T1

This system works very well in hot,

dry climates. Notice that there can

be a significant increase in the

humidity levels (both relative

humidity and humidity ratio).