hvac5c psychrometry
TRANSCRIPT
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