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A Trane Air Conditioning Clinic

Psychrometry

Air Conditioning Clinic TRG-TRC001-EN© American Standard Inc. 1999

Psychrometry

period one

The Psychrometric Chart


PsychrometricChart



Properties of Air

Dry-bulb temperature

Wet-bulb temperature

Dew-point temperature

Relative humidity

Humidity ratio


Dry-Bulb Thermometer


Wet-Bulb Thermometer


Condensation Occurs at Dew Point



Fog Occurs When Air Is Saturated



Relative Humidity

RelativeHumidity

Amount of moisture that a given

amount of air is holding=

Amount of moisture that a given

amount of air can hold


50% 100%(saturated)

Relative Humidity


Humidity Ratio


Properties of Air

Dry-bulb temperature

Wet-bulb temperature

Dew-point temperature

Relative humidity

Humidity ratio


Summer Design Conditions

95°F DB (dry bulb)

78°F WB (wet bulb)


Point of Intersection

95°F

78°F

72°F


25°F

30°F

35°F

40°F

45°F

50°F

55°F

19.14

24.19

29.94

36.51

44.34

53.63

64.63

60°F

65°F

70°F

75°F

80°F

85°F

90°F

77.61

92.89

110.82

131.83

156.38

185.03

218.42

dry-bulbtemp.

humidityratio

dry-bulb temp.

humidityratio

Properties of Saturated Air

030

Dry-Bulb Temperature (°F)

Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220

Plotting Saturation Points218.42

185.03

156.38

131.83

110.82

92.89

77.61

64.63

53.63

44.34

36.5129.94

24.1919.14


Saturation Curve

100% relative humidity curve

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Dry Bulb, Wet Bulb and Dew Point

60°Fdew point

60°Fwet bulb

A B C D

dry bulb 60°F 62°F 75°F 90°F

wet bulb 60°F 60.8°F 65.2°F 70°F

dew point 60°F 60°F 60°F 60°F

rel humidity 100% 92% 60% 37%

DCB

A

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Adding Sensible Heat

100% saturation

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220

60°Fdew point

60°Fwet bulb


Relative Humidity Curves

relative humidity

0%30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Properties of Air

humidity ratio

dry bulbrelative humidity

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Determining Wet-Bulb Lines

A

BI

CI

DI

B C D

AI

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Properties of Air

wet bulb

dew point

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220

humidity ratio



Specific Volume

specific volume lines

(cubic feet / pound of dry air)

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220


Effect of Adding Sensible Heat


Effect of Removing Sensible Heat


Effect of Adding Moisture


Effect of Removing Moisture


Removing Sensible Heat and Moisture


Removing Sensible Heat and Moisture

A


Psychrometry

period two

Air Mixtures



supplyfan

mixture outdoorair (OA)

recirculated air (RA)

A

C

B

coolingcoil

Determining Entering Air Conditions

80°F95°F


B

A

outdoor air

recirculated air


1,000 cfm

4,000 cfm = 0.25

OA

RA

mixture

=

=

=

25%

75%

100%

A

C B

4,000 cfmmixed air

1,000 cfmOA

3,000 cfmRA




95°F × 0.25 = 23.75°F

80°F × 0.75 = 60.00°F

mixture = 83.75°F


83.75°F

70°F


B

80°F

95°F

C

A


Psychrometry

period three

Sensible Heat Ratio


Effect of Removing Sensible Heat


Effect of Removing Latent Heat


Removing Sensible and Latent Heat


supply air

sensible

heat

latent

heat

return air

Heat and Moisture Transfer


Removing Sensible and Latent Heat

cooler & damper

warmer & drier



Heat Gain


SHRSensible Heat Gain

Sensible Heat Gain + Latent Heat Gain=

Sensible Heat Ratio (SHR)

78°F DB

65°F WB

index point

Sensible Heat Ratio Scale


Drawing an SHR Line

A

index point


A

SHR Dictates Supply Air Condition


C

D

B

80°F

Drawing an SHR Line

index point


Psychrometry

period four

Air Quantity


coil curves

Coil Curves



80,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

STEP 1: Calculate the sensible heat ratio (SHR)

Determining Supply Airflow

= 0.8080,000 Btu/hr

100,000 Btu/hrSHR =


room — 78°F DB, 50% RH

outdoor air (OA) — 95°F DB, 78°F WB

ventilation — 25% OA


95°F × 0.25 = 23.75°F

78°F × 0.75 = 58.50°F

mixture = 82.25°F

Determining Supply AirflowSTEP 2: Plot room, outdoor, and entering conditions

A

C

B


56.5°F 78°F

STEP 3: Identify supply air conditions

A

C

B

D




Sensible Heat Gain

1.085 × (Room DB – Supply DB)

SupplyAirflow

=

STEP 4: Solve the supply airflow equation



80,000 Btu/hr

1.085 × (78°F – 56.5°F)3,430 cfm=

STEP 4: Solve the supply airflow equation


D

A

C B

3,430 cfm

56.5°F DB

55.2°F WB

RA

OA

82.2°F DB

68.6°F WB

mixture


SA

56.5°F

DA

C

B



47,000 Btu/hr sensible heat gain

20,000 Btu/hr latent heat gain

= 0.7047,000 Btu/hr

67,000 Btu/hrSHR =

Arbitrarily Using 55°F Supply Air

A

C

B

D

55°F

DI

AI

Arbitrarily Using 55°F Supply Air

49°F


Psychrometry

period five

Tons of Refrigeration



What is Enthalpy?

Enthalpy (h) = Sensible Heat + Latent Heat

The total heat energy in one pound of air

(Btu/lb) at its present condition.

Determining Tons of RefrigerationSTEP 1:

Find enthalpies

entering and

leaving coil

A

B

C

D



Determining Tons of Refrigeration

STEP 2: Solve the total refrigeration load equation

4.5 × Supply Airflow × (h1 – h2)Refrigeration Load (Btu/hr)

=

h1 = enthalpy of air entering coil (Btu/lb)

h2 = enthalpy of air leaving coil (Btu/lb)


= 11.8 tons of refrigeration

Determining Tons of Refrigeration

STEP 2: Solve the total refrigeration load equation

4.5 × 3,430 cfm × (32.7 – 23.5) = 142,000 Btu/hr

142,000 Btu/hr

12,000 Btu/hr/ton

Sensible and Latent Coil Loads

A

B

C

D

sensibleload

latentload



4.5 × 3,430 cfm × (29.6 – 23.5) = 94,150 Btu/hr

7.8 tons of refrigeration (sensible)

Sensible and Latent Coil Loads

4.5 × 3,430 cfm × (32.7 – 29.6) = 47,850 Btu/hr

4.0 tons of refrigeration (latent)

Psychrometry

period six

Psychrometric Analyses


SHR at Full-Load Conditions

56.5°F 78°F

DA

B

C



80,000 Btu/hr

80,000 Btu/hr + 20,000 Btu/hr

Full-Load SHR:

= 0.80

47,000 Btu/hr

47,000 Btu/hr + 20,000 Btu/hr = 0.70

SHR Changes with Room Load

Part-Load SHR:

D

A

B

C

SHR at Part-Load Conditions


DA

B

CAI

DI

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

65.4°F

SHR at Part-Load Conditions



constant quantity of variable-temperature air

OA

RASA

B

D C

A

modulatingcooling coil

Constant-Volume System


reheat coil

coolingcoil

roomthermostat

adding reheat improves humidity control

B

E

DC

A

Constant-Volume with Reheat

A

C

B

47,000 Btu/hr

1.085 × (78°F – Supply DB)= 3,430 cfm

Supply DB = 65.4°F

Effect of Adding Reheat


D E

65.4°F56.5°F

D E

65.4°F56.5°F

Effect of Adding Reheat


A

C

B


mixed-air bypass improves humidity control

OA

RA

SA

B

D C

A

face-and-bypass

dampers

Mixed-Air Bypass

Effect of Mixed-Air Bypass

D

A

C

B

E

82.25°F × 0.455 = 37.4°F

51.40°F × 0.545 = 28.0°F

mixture = 65.4°F

65.4°F51.4°F


Effect of Mixed-Air Bypass

A

C

B

AI

65.4°F51.4°F

D

E



OA

RA

SA

B

D

A

C

variable quantity of constant-temperature air

variable-speed drive

Variable-Air-Volume (VAV) System

DA

C

B

47,000 Btu/hr

1.085 × (78°F – 56.5°F)= 2,015 cfm

AI

55.5°F

Calculating Part-Load Airflow



Software Tools

Psychrometry

period seven

Review


Properties of Air

30


Hum

idity

Rat

io (

grai

ns/lb

of d

ry a

ir)

25 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110

20

40

60

80

100

120

140

160

180

200

220

wet bulb

dew point

humidity ratio



80°F

95°F

83.75°F

70°F

C

B

A

Determining Mixed-Air Conditions

95°F × 0.25 = 23.75°F

80°F × 0.75 = 60.00°F

mixture = 83.75°F


A

Determining Sensible Heat Ratio

SHR = Sensible Heat Gain

Sensible Heat Gain + Latent Heat Gain


A

C

B

D

coil curve

Sensible Heat Gain

1.085 × (Room DB – Supply DB)

SupplyAirflow

Determining Coil Leaving Conditions

=

supply DB


A

C

B

D

Determining Tons of RefrigerationRefrigeration Load = 4.5 × Supply Airflow × (h1 – h2)


C

OA

RA

B

A

D

SA

PsychrometricAnalysis

A

BCD


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