ARCH-432 Dew Point Saturation and Psychometrics
Mar 20, 2016
ARCH-432Dew Point Saturation and Psychometrics
AttendanceWho 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 KnowUnderstand 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
TermsDew 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
PsychrometryPsychrometry 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 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 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?
SaturationWhen 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 HumidityThe 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 SolvingAssume 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° FB. 35 ° FC. 42 ° FD. 56 ° FE. 60 ° F