Psychrometry

HVAC

CNST 305 Environmental Systems 1Dr. Berryman

5c Indoor Air Quality, Psychometrics; Dry and Wet Bulb; Enthalpy;

Condensation; Dew Point Temp.

Chapter 2.1 – 2.3 (Toa)

2.11 Mathematics and Science - Physics (1hr); 4.11 Analysis and Design of Construction Systems - Mechanical (2 hrs)

Psychrometry Psychrometry is the science dealing with

the physical laws of air – watermixtures.

When designing an air conditioning system, the temperature and moisture content of the air to be conditioned, and the same properties of the air needed to produce the desired air conditioning effect, must be known.

Once these properties are known, the air conditioning task can be determined.

Psychrometric Chart

Properties of Air Dry-bulb temperature Wet-bulb temperature Dew-point temperature Relative humidity Humidity ratio

Thermometers

Dry Bulb

Wet Bulb

Measured in degrees Fahrenheit (oF)

Dew Point Condensation occurs at the Dew Point Temperature

(oF)

Saturation

Fog• DP, WB, DB same

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% RH

100% RH - Saturated

(percentage)

Humidity Ratio Actual weight of water in

an air – water vapor mixture

Pounds of moisture per pound of dry air

7000 grains of water in a pound

at sea level one pound of 70°F air occupies approximately 13.5 cubic feet, and one grain of water in that air weighs about two-thousandths (0.002) of an ounce

wet bulb

dew pointdew point

30

Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)

Hum

idity

Rat

io (g

rain

s/lb

of d

ry a

ir)Hu

mid

ity R

atio

(gra

ins/

lb o

f dry

air)

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 ratiohumidity ratio

dry bulbrelative humidity

Air Conditions on the Psychrometric Chart

Specific Volume

14.013.0

13.5

14.5

specific volume lines (cubic feet / pound of dry air)specific volume lines (cubic feet / pound of dry air)

30

Dry-Bulb Temperature (°F)Dry-Bulb Temperature (°F)

Hum

idity

Rat

io (g

rain

s/lb

of d

ry a

ir)Hu

mid

ity R

atio

(gra

ins/

lb o

f dry

air)

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

Enthalpy The total heat energy in one pound of air

(Btu/lb) at its present condition

Enthalpy (h) = Sensible Heat + Latent Heat

EXAMPLEUsing the Chart Summer Design Conditions

95°F DB (dry bulb) 78°F WB (wet bulb)

Find:

Relative Humidity

Humidity Ratio

Dew Point

Answer

95°F95°F

78°F78°F

50%

50%

40%

40%

72°F72°F

Condensation

EXAMPLEBeach Conditions 92oF 60% RH

Beverage 45oF

Condensation (cont.)

EXAMPLEWinter: Glass surface temperature = 40oF

Inside Design: 77oF 30% RH

Comfort Zone

dry-bulb temperature

humidity ratio

wet-bulb temperature

80°F[26.7°C]

70°F[21.2°C]

60 %

RH

30 % RHA

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

Effect of Adding Sensible Heat

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

Effect of Removing Sensible Heat

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

Effect of Adding Moisture

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

Effect of Removing Moisture

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

Removing Sensible Heat and Moisture

AA

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

Removing Sensible Heat and Moisture

AA

BB

CC

DD

sensible load

sensible load

latent load

latent load

32.732.7

23.523.5

29.629.6

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

Sensible and Latent Coil Loads

Next Time Calculating Heating Loads

• Section 2.5 Toa

Heat Transmission