Post on 10-Mar-2018
CHAPTER NO.4 Psychrometry
C605.4-Explain Psychometric properties & calculate various
parameters.
Necessity of Air conditioning
Purpose of air conditioning is
1) To provide comfort conditions for human comfort.
2) To provide comfort in commercial places like office, restaurant ,shopping complex , banks etc.
3) To provide controlled conditions for industrial process.
4) To provide ultra clean atmosphere for precision work.
Dalton’s low of partial pressure
• Dalton’s law of partial pressure statures that ‘the total pressure of mixture of gases equal to the sum of the partial pressures exerted by each gas when it occupies the mixture volume at there temperature of mixture’.
• According to Dalton’s law of partial pressure, • Pt = Pa + Pb + Pc.
Psychometrics properties of air
1. Dry air : It is a mixture of oxygen (20.91%) and nitrogen (79.09 %) by volume.
2. Moist air: It is mixture of dry air and water vapour.
3. Saturated air : Air which have maximum amount of water vapor.
4. Dry bulb temp.(DBT) : It is temperature of air recorded by ordinary thermometer with clean and dry sensing elements.(td or tdb)
5. Wet bulb temp.(WBT) : It is temperature of air recorded by thermometer when its bulb is covered with a wet cloth.(tw or twb)
Psychometric properties of air
6. Wet bulb depression : It is the difference between dry bulb temp. and wet bulb temp.
7. Dew point temp.: It is the temperature of air recorded by thermometer when the moisture present
in the air begins to condensed.
8. Dew point depression : It is difference between dry bulb temp. and dew point temp. of air.
9. Specific humidity or Humidity ratio (W) : It is defined as mass of water vapour in unit mass of dry air.
w= mv/ma
10. Degree of saturation or Percentage humidity ( ) :
It is the ratio of mass of water vapour in unit mass
Psychometrics properties of air
of dry air to the mass of water vapour in the same mass of dry air when it is saturated at the same temperature.
= w/ws
11. Relative humidity : It is the ratio of actual mass of water vapour (mv) in a given volume of moist air to mass of water vapour (ms) in the same volume of saturated air at the same temp. and pressure.
= mv/ms
12. Enthalpy of moist air : (H) It is total heat contained in moist air. It is sum of sensible heat and latent heat of moist air.
Psychrometric processes
The process of changing and affecting the psychometric properties of the moist air are called psychometric processes.
The processes are :
1. Sensible heating 2. Sensible cooling
3. Humidification 4. Dehumidification
5. Heating and humidification
6. Heating and dehumidification
Psychometric processes
7. Cooling and dehumidification
8. Cooling with adiabatic humidification.
9. Adiabatic mixing of two air streams.
Properties Of Air
Dry-bulb temperature
Wet-bulb temperature
Dew-point temperature
Relative humidity
Humidity ratio
Dry-Bulb Thermometer
Wet-Bulb Thermometer
Condensation Occurs At Dew Point
Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc. 1999
Fog Occurs When Air Is Saturated
Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc. 1999
Relative Humidity …describes the degree of saturation
Relative Humidity
Amount of moisture that a given
amount of air is holding
= Amount of moisture that a given
amount of air can hold
50% 100% (saturated)
Relative Humidity …compares moisture content to saturation
Humidity Ratio …compares water vapor to dry air, by weight
Knowing Two Properties …lets you determine the remaining three
Dry-bulb temperature
Wet-bulb temperature
Dew-point temperature
Relative humidity
Humidity ratio
Summer Design Conditions
95°F DB (dry bulb)
78°F WB (wet bulb)
“X Marks The Spot”…
95°F
78°F
72°F
Psychrometric Chart
Air Conditioning Clinic TRG-TRC001-EN © American Standard Inc. 1999
Basic Concepts
0 5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 95 100 105 110 115 120
DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Staturation Line
Saturation Line
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DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Dry Bulb Temperature
Constant Dry Bulb Temperature
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DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Humidity Ratio
Constant Humidity Ratio
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DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Humidity Ratio
Constant Humidity Ratio
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DRY BULB TEMPERATURE - °F
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10% RELATIVE HUMIDITY
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Relative Humidity
Constant Relative Humidity
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DRY BULB TEMPERATURE - °F
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12.5
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PE
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15.0
15.5
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Specific Volume
Constant Specific Volume
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DRY BULB TEMPERATURE - °F
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85 WET BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Web Bulb Temperature
Constant Wet Bulb Temperature
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DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Enthalpy
Constant Enthalpy
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DRY BULB TEMPERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
Constant Enthalpy
and
Constant Web Bulb
Constant Enthalpy and Web Bulb
Typical Chart With Enthalpy Lines
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DRY BULB TEMPERATURE - °F
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10% RELATIVE HUMIDITY
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
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PERATURE - °F
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PSYCHROMETRIC CHARTLexington, Kentucky USABAROMETRIC PRESSURE 28.874 inches of Mercury
Linric Company Psychrometric Chart, www.linric.com
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Typical Chart Without Enthalpy Lines
Reading a Psychrometric Chart Practice
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
State Point
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DRY BULB TEMPERATURE - °F
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Linric Company Psychrometric Chart, www.linric.com
Constant Dry Bulb Temperature
70 Dry Bulb
State Point 70 F
State Point 60%
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Wet-bulb temperature = ? F
Wet-bulb
Wet-bulb
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Wet-bulb temperature = 61 F
Wet-bulb
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Dew point = ?
Dew Point
Dew Point
Dew Point
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Dew point = 55.5 F
Dew Point
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Specific volume = ?
Specific Volume
Specific Volume
Sea Level Chart
Dry-bulb temperature = 70 F
Relative Humidity = 60%
Specific volume = 13.6 ft3 / lb dry air
Specific Volume
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Humidity ratio = ?
Humidity Ratio
Humidity Ratio
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Humidity ratio = 0.0094 lb water / lb dry air
or
7000 grains = 1 lb water
7000 x 0.0094 = 65.8
Humidity Ratio
Sea Level Chart
Dry-bulb temperature = 70 F
Relative humidity = 60%
Enthalpy = ?
Enthalpy
Enthalpy
Sensible Heating
Mixing Air Streams
ma1
h1
w1
ma2
h2
w2
ma3
h3
w3
Energy Balance
ma1 h1 + ma2 h2 = ma3 h3
Mass Balance
ma1 w1 + ma2 w2 = ma3 w3
By Algebra
h2 – h3 = w2 – w3 = ma1
h3 – h1 w3 – w1 ma2
Cooling
Cooling Systems
Cooling Systems
Evaporative Cooling