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.

B.

C.

D.

E.

Willis Carrier Karl Trane Bill Lennox Dr. Frederick Glock 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 chart Be able to apply psychrometrics to predict and solve simple HVAC problems

Terms

Dew point Wet bulb temperature Dry bulb temperature Enthalpy Relative humidity Sensible heat Latent heat Mixing 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 – water mixtures.

 To obtain comfort, and select an appropriate HVAC system, the psychrometric conditions must be known.

Properties of Air on a Psychrometric Chart

Dry-bulb temperature Wet-bulb temperature Dew-point temperature Relative humidity Humidity ratio

Air Conditions on the Psychrometric Chart

12 0 10 0 80 18 0 16 0 14 0 22 0 20 0 60 40 20 dew point humidity ratio 25 30 35 40 wet bulb relative humidity dry bulb 45 50 55 60 65 70 75 80 Dry-Bulb Temperature (°F) 85 90 95 100 105 110

Dew Point

Condensation occurs at the Dew Point Temperature

( o F)

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

Amount of moisture that a given amount of air is holding Relative Humidity

(percentage)

= Amount of moisture that a given amount of air can hold

100% RH Saturated 50% RH

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% RH B.

75  F/45% RH C.

D.

75  78  F/55% RH 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