Question

The atmospheric conditions are 250c dbt and specific humidity of 10 gm/kg of dry air. Determine the followings: a. Partial pressure of vapour b. Relative humidity

Answer 1

The specific objectives of this lecture are to:
1. Define psychrometry and the composition of moist air (Section 27.1)
2. Discuss the methods used for estimating properties of moist air (Section 27.2)
3. Present perfect gas law model for moist air (Section 27.2.1)
4. Define important psychrometric properties (Section 27.2.2)
5. Present graphical representation of psychrometric properties on a psychrometric
chart (Section 27.2.3)
6. Discuss measurement of psychrometric properties (Section 27.3)
7. Discuss straight-line law as applied to air-water mixtures (Section 27.3.1)
8. Discuss the concept of adiabatic saturation and thermodynamic wet bulb
temperature (Section 27.3.2)
9. Describe a wet-bulb thermometer (Section 27.3.3)
10. Discuss the procedure for calculating psychrometric properties from measured
values of barometric pressure, dry bulb and wet bulb temperatures (Section 27.4)
11. Describe a psychrometer and the precautions to be taken while using
psychrometers (Section 27.5)
At the end of the lecture, the student should be able to:
1. Define psychrometry and atmospheric air
2. Use perfect gas law model and find the total pressure of air from partial pressures
of dry air and water vapour
3. Define and estimate psychrometric properties
4. Draw the schematic of a psychrometric chart
5. Discuss the straight-line law and its usefulness in psychrometry
6. Explain the concepts of adiabatic saturation and thermodynamic wet bulb
temperature
7. Differentiate between thermodynamic WBT and WBT as measured by a wet bulb
thermometer
8. Estimate various psychrometric properties given any three independent properties
9. Describe a psychrometer
27.1. Introduction:
Atmospheric air makes up the environment in almost every type of air
conditioning system. Hence a thorough understanding of the properties of
atmospheric air and the ability to analyze various processes involving air is
fundamental to air conditioning design.
Psychrometry is the study of the properties of mixtures of air and water
vapour.
Atmospheric air is a mixture of many gases plus water vapour and a number
of pollutants (Fig.27.1). The amount of water vapour and pollutants vary from place
to place. The concentration of water vapour and pollutants decrease with altitude,
and above an altitude of about 10 km, atmospheric air consists of only dry air. The
pollutants have to be filtered out before processing the air. Hence, what we process
is essentially a mixture of various gases that constitute air and water vapour. This
mixture is known as moist air.

The moist air can be thought of as a mixture of dry air and moisture. For all
practical purposes, the composition of dry air can be considered as constant. In
1949, a standard composition of dry air was fixed by the International Joint
Committee on Psychrometric data. It is given in Table 27.1.
Constituent Molecular weight Mol fraction
Oxygen 32.000 0.2095
Nitrogen 28.016 0.7809
Argon 39.944 0.0093
Carbon dioxide 44.010 0.0003
Table 27.1: Composition of standard air
Based on the above composition the molecular weight of dry air is found to be
28.966 and the gas constant R is 287.035 J/kg.K.
As mentioned before the air to be processed in air conditioning systems is a
mixture of dry air and water vapour. While the composition of dry air is constant, the
amount of water vapour present in the air may vary from zero to a maximum
depending upon the temperature and pressure of the mixture (dry air + water
vapour).
At a given temperature and pressure the dry air can only hold a certain
maximum amount of moisture. When the moisture content is maximum, then the air
is known as saturated air, which is established by a neutral equilibrium between the
moist air and the liquid or solid phases of water.