pshchrometry refrigeration and air conditioning BE Mechanical 6th sem ALA ppt GTU MSU various terms related to psychrometry and psychrometric chart with example

1. Refrigeration and Air Conditioning (2161908)
B.E. MECH – Sem VIth
Prepared by,
SHREEJIT S NAIR (170953119020)
PANDYA CHAITANYA (170953119023)
PATEL SHREY (170953119031)
RATHVAYOGESH (170953119036)
Group :- 5B2
Guided by,
Asst. Prof. Sushil Chaurasia
(Mechanical Deptt)

2. CONTENTS
• Intorduction
• Dalton’s Law Applied to Air
• Properties of Air/Psychrometric Terms
• Psychrometric chart
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3. INTRODUCTION
• It is a science of studying the thermodynamics properties of moist air.
• It is the science which deals with physical law of air – water – vapour mixture.
• 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.
• Psychrometry is the science of study of
– various properties of air,
– method of controlling its temperature
– moisture content or humidity and its effect on various materials and human beings.
• Studying of Psychrometry helps us for understanding different constituents of air and how
they affect each other.
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4. PSYCHROMETRICS IN DAILY LIFE
• Sea breeze and land breeze
– When and why do we get them?
• How do thunderstorms, hurricanes, and tornadoes form?
• What are dew, fog, mist, and frost and when do they form?
• When and why does the windshield of a car fog up?
– How do you de-fog it? Is it better to blow hot air or cold air?Why?
• Why do you feel dry in a heated room?
– Is the moisture content of hot air lower than that of cold air?
• How does a fan provide relief from sweating?
• How does an air conditioner provide relief from sweating?
• When does a soda can “sweat”?
• When and why do we “see” our breath?
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5. DO SAILBOATS PERFORM BETTER AT
LOW OR HIGH RH?
• Does dry air or moist air provide more thrust against the sail?
• Which is denser – humid air or dry air?
– Avogadro’s law:At the same temperature and pressure, the no. of molecules in a given volume is the
same, no matter what the gas is
– Air has ~ 80% N2 and ~ 20% O2(Ratio of N2:O2= 4:1)
– If 10 molecules of water are added to air, 8 molecules of N2 and 2 molecules of O2 are displaced
– Atomic weight gained by air = 10 x 18 = 180
– Atomic weight lost by air = 8 x 28 + 2 x 32 = 288
– Thus, there is a net decrease in weight and hence density
– So, humid air is less dense than dry air
• Thus, there is more thrust for the sail with dry air and hence sailboats perform better at low RH
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6. PSYCHROMETRICS IN THE FOOD INDUSTRY
• Heating or cooling of air
– To heat or cool a product
• Mixing different streams of air
• Drying a food product using hot and dry air
– Drier the air, better the drying efficiency
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7. DALTON’S LAW APPLIED TO AIR
• Let us consider an example. If there is mixture of five gases,A, B, C, D, and E, each gas will exert
its partial pressure say: PA, PB, PC, PD and PE which is same even if they would have occupied the
space separately.The total pressure P exerted by the mixture of gases is:
• P = PA+ PB + PC + PD + PE
• We have seen that air is the mixture of various gases like nitrogen, oxygen, water vapor and
other gases that don’t react chemically, but exists physically distinct, hence Dalton’s law of
partial pressure is applicable to it.
• If we consider the air made up of dry air (without considering individual constituents of air)
and water vapor, the total atmospheric pressure or the barometric pressure exerted by the air
is sum of the partial pressure of the dry air and partial pressure of the water vapor.
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8. • We have seen that air is the mixture of various gases like nitrogen, oxygen, water
vapor and other gases that don’t react chemically, but exists physically distinct, hence
Dalton’s law of partial pressure is applicable to it.
• If we consider the air made up of dry air (without considering individual constituents
of air) and water vapor, the total atmospheric pressure or the barometric pressure
exerted by the air is sum of the partial pressure of the dry air and partial pressure of
the water vapor.
• Let us suppose that the total barometric pressure exerted by air is P, and the partial
pressures exerted by the dry air and water vapor are Pd and Pv respectively, then:
• P = Pd + Pv
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9. • Since the percentage of dry air is higher in this mixture, it is quite obvious that the
partial pressure exerted by the dry air is much higher than that exerted by the water
vapor.
• However, during the cooling or heating of the air, the dry air does not undergoes any
change in phase, but the water vapor condenses on cooling and gets superheated on
heating.
• The major changes that occur in the air during air conditioning process of heating or
cooling the air occurs within the water vapor. Hence, though the quantity of water
vapor in the air is very small (much lesser than 1%) compared to the others gases, it
plays the major role in the psychrometric properties of the air.
• In fact most of the psychrometric studies are revolved around the water vapor
present in air.
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10. PROPERTIES OF AIR/PSYCHROMETRIC TERMS
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• Dry Air:
The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon
dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the
major portion of the combination.
• Moist air:
It is a mixture of dry air and water vapour. The amount of water vapour, present in
the air, depends upon the absolute pressure and temperature of the mixture.
• Saturated air:
It is a mixture of dry air and water vapour, when the air has diffused the maximum
amount of water vapour into it. The water vapours, usually, occur in the form of
superheated steam as an invisible gas. However, when the saturated air is cooled,
the water vapour in the air starts condensing, and the same may be visible in the
form of moist, fog or condensation on cold surfaces.

11. • Dry Air:
The pure dry air is a mixture of a number of gases such as nitrogen, oxygen, carbon
dioxide, hydrogen, argon, neon, helium etc. But the nitrogen and oxygen have the
major portion of the combination.
• Moist air:
It is a mixture of dry air and water vapour. The amount of water vapour, present in
the air, depends upon the absolute pressure and temperature of the mixture.
• Saturated air:
It is a mixture of dry air and water vapour, when the air has diffused the maximum
amount of water vapour into it. The water vapours, usually, occur in the form of
superheated steam as an invisible gas. However, when the saturated air is cooled,
the water vapour in the air starts condensing, and the same may be visible in the
form of moist, fog or condensation on cold surfaces.
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12. • Degree of saturation:
It is the ratio of actual mass of water vapour in a unit mass of dry air to the mass of
water vapour in the same mass of dry air when it is saturated at the same temperature.
• Humidity:
It is the mass of water vapour present in 1 kg of dry air, and is generally expressed in
terms of gram per kg of dry air (g / kg of dry air). It is also called specific humidity or
humidity ratio.
• Absolute humidity:
It is the mass of water vapour present in 1 m3 of dry air, and is generally expressed in
terms of gram per cubic-meter of dry air (g/m3 of dry air). It is also expressed in terms
of grains per cubic meter of dry air.
• Relative humidity:
It is the ratio of actual mass of water vapour in a given volume of moist air to the mass
of water vapour in the same volume of saturated air at the same temperature and
pressure. It is briefly written as RH.
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13. • Dry bulb temperature:
It is the temperature of air recorded by a thermometer, when it is not affected by
the moisture present in the air. The dry bulb temperature (briefly written as DBT) is
generally denoted by td or tdb.
• Wet bulb temperature:
It is the temperature of air recorded by a thermometer, when its bulb is surrounded
by a wet cloth exposed to the air. Such a thermometer is called wet bulb
thermometer. The wet bulb temperature (briefly written as WBT) is generally
denoted by tw or twb.
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14. • Wet bulb depression:
– It is the difference between dry bulb temperature and wet bulb temperature at
any point.The wet bulb depression indicates relative humidity of the air.
• Dew PointTemperature:
– It is the temperature of air recorded by a thermometer, when the moisture
(water vapour) present in it begins to condense. In other words, the dew point
temperature is the saturation temperature (tsat) corresponding to the partial
pressure of water vapour (P). It is, usually, denoted by tdp.
• Dew point depression:
– It is the difference between the dry bulb temperature and dew point temperature
of air.
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15. • PSYCHROMETRIC RATIO
– The psychrometric ratio is the ratio of the heat transfer coefficient to the
product of mass transfer coefficient and humid heat at a wetted surface. It may be
evaluated with the following equation:
𝑟 =
ℎ 𝑐
𝑘 𝑦
𝑐 𝑠
where:
𝑟 = Psychrometric ratio, dimensionless
ℎ 𝑐 = convective heat transfer coefficient,W m-2 K-1
𝑘 𝑦 = convective mass transfer coefficient, kg m-2 s-1
𝑐 𝑠 = humid heat, J kg-1 K-1
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16. PSYCHROMETRIC CHART
• A psychrometric chart is a graphical representation of the psychrometric processes of
air.
• Psychrometric processes include physical and thermodynamic properties such as dry
bulb temperature, wet bulb temperature, humidity, enthalpy, and air density.
• A psychrometric chart for a given location can tell you information about
temperature (wet bulb and dry bulb) and humidity (relative and absolute).
• A psychrometric chart can be used in two different ways. The first is done by plotting
multiple data points, that represent the air conditions at a specific time, on the chart.
Then, overlaying an area that identifies the “comfort zone.” The comfort zone is
defined as the range within occupants are satisfied with the surrounding thermal
conditions. After plotting the air conditions and overlaying the comfort zone, it
becomes possible to see how passive design strategies can extend the comfort zone.
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20. EXAMPLE
• Given thatT = 25 C andTw = 20 C Find (a) RH (b) Tdp (c) HR (d) v (e) h
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