ARCH-432

Dew Point Saturation and Psychometrics

Attendance

Who is typically referred to as the “Father of Cool”? (hint: generally credited with developing the science of psychometrics)

A. Willis CarrierB. Karl TraneC. Bill LennoxD. Dr. Frederick GlockE. Prof. Max Kirk

What You Need to Know

Understand the underlying principles of psychrometrics

What You Need To Be Able To Do

Read a Psychrometric chartBe able to apply psychrometrics to predict and solve simple HVAC problems

Terms

Dew pointWet bulb temperatureDry bulb temperatureEnthalpy

Relative humiditySensible heatLatent heatMixing line

Enthalpy

To understand in a simple way, Enthalpy means change of heat. Enthalpy can be positive or negative. For water when heated to a very high temp changes its state into vapor. Here heat change takes place.

Enthalpy

Enthalpy

Psychrometry

Psychrometry is the science dealing with the physical laws of air – watermixtures. To obtain comfort, and select an

appropriate HVAC system, the psychrometric conditions must be known.

Properties of Air on a Psychrometric Chart

Dry-bulb temperatureWet-bulb temperatureDew-point temperatureRelative humidityHumidity ratio

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

Dew Point Condensation occurs at the Dew Point Temperature

(oF)

Saturation

You get sheep?No you get Fog!

What happens when the DP, WB, and DB are the same?

Saturation

When the dry-bulb, wet-bulb, and dew-point temperatures are the same, the air is saturated. It can hold no more moisture. When air is at a saturated condition, moisture entering the air displaces moisture within the air. The displaced moisture leaves the air in the form of fine droplets. When this condition occurs in nature, it is called fog.

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)

Relative Humidity

The fourth property RH, is therefore a comparison of the amount of moisture that a given amount of air is holding, to the amount of moisture that the same amount of air can hold, at the same dry-bulb temperature.

Zones with ASHRAE Standard 55

Team Problem Solving

Based upon your estimates of occupancy, met and clo value, what DB/RH condition point will create the most comfort in the first floor of Carpenter Hall in summer?A. 73 F/50% RHB. 75 F/45% RHC. 75 F/55% RHD. 78 F/40% RH

Plot this point on your Psychrometric chart.

Team Problem Solving

Assume that the gym is kept at 72° F and 35% RH in winter. At what temperature does the inside surface of the glass have to reach in order for water to condense?

A. 24° F

B. 35 ° F

C. 42 ° F

D. 56 ° F

E. 60 ° F