chapter no.4 psychrometry - pimpri chinchwad · pdf filenecessity of air conditioning ¾...

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


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


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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Constant Dry Bulb Temperature


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Constant Humidity Ratio

Constant Humidity Ratio

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10% RELATIVE HUMIDITY

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Constant Relative Humidity

Constant Relative Humidity

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


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Constant Specific Volume

Constant Specific Volume

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85 WET BULB TEMPERATURE - °F

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Constant Web Bulb Temperature

Constant Wet Bulb Temperature

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Constant Enthalpy

Constant Enthalpy

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


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Constant Enthalpy

and

Constant Web Bulb

Constant Enthalpy and Web Bulb

Typical Chart With Enthalpy Lines

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PERATURE - °F

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DE

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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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70 Dry Bulb

State Point 70 F

State Point 60%

Sea Level Chart



Wet-bulb temperature = ? F

Wet-bulb

Wet-bulb

Sea Level Chart



Wet-bulb temperature = 61 F

Wet-bulb

Sea Level Chart



Dew point = ?

Dew Point

Dew Point

Dew Point

Sea Level Chart



Dew point = 55.5 F

Dew Point

Sea Level Chart



Specific volume = ?

Specific Volume

Specific Volume

Sea Level Chart


Relative Humidity = 60%

Specific volume = 13.6 ft3 / lb dry air

Specific Volume

Sea Level Chart



Humidity ratio = ?

Humidity Ratio

Humidity Ratio

Sea Level Chart



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



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

Evaporative Cooling

chapter no.4 psychrometry - pimpri chinchwad · pdf filenecessity of air conditioning ¾...

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