# Psychrometry and Gas Mixtures | Interview Question and Answers

Psychrometry and Gas Mixtures | Interview viva ,oral  Question and Answers

1. What is humidification and dehumidification?

The addition of water vapour into air is humidification and the removal of water vapour from air is dehumidification.

2. Differentiate absolute humidity and relative humidity.

Absolute humidity is the mass of water vapour present in one kg of dry air.
Relative humidity is the ratio of the actual mass of water vapour present in one kg of dry air at the given temperature to the maximum mass of water vapour it can withhold at the same temperature. Absolute humidity is expressed in terms of kg/kg of dry air. Relative humidity is expressed in terms of percentage.

3. What is effective temperature?

The effective temperature is a measure of feeling warmth or cold to the human body in response to their temperature, moisture content and air motion. If the air at different DBT and RH condition carries the same amount of heat as the heat carried by the air at temperature T and 100% RH, then the temperature T is known as effective temperature.

4. Define Relative humidity.

It is defined as the ratio of partial pressure of water vapour (pw) in a mixture to the saturation pressure(ps) of pure water at the same temperature of mixture.

5. Define specific humidity.

It is defined as the ratio of the mass of water vapour (ms) in a given volume to the mass of dry air in a given volume (ma).

6. Define degree of saturation.

It is the ratio of the actual specific humidity and the saturated specific humidity at the same temperature of the mixture.

7. What is dew point temperature?

The temperature at which the vapour starts condensing is called dew point temperature. It is also equal to the saturation temperature at the partial pressure of water vapour in the mixture. The dew point temperature is an indication of specific humidity.

8. What is meant by dry bulb temperature (DBT)?

The temperature recorded by the thermometer with a dry bulb. The dry bulb thermometer cannot affect by the moisture present in the air. It is the measure of sensible heat of the air.

9. What is meant by wet bulb temperature (WBT)?

It is the temperature recorded by a thermometer whose bulb is covered with cotton wick (wet)saturated with water. The wet bulb temperature may be the measure of enthalpy of air. WBT is the lowest temperature recorded by moistened bulb.

10. Define dew point depression.

It is the difference between dry bulb temperature and dew point temperature of air vapour mixture.

11. What is meant by adiabatic saturation temperature (or) thermodynamic wet bulb? Temperature?

It is the temperature at which the outlet air can be brought into saturation state by passing through the water in the long insulated duct (adiabatic) by the evaporation of water due to latent heat of vaporization.

12.What is psychrometer?

Psychrometer is an instrument which measures both dry bulb temperature and wet bulb temperature.

13. What is Psychrometric chart?

It is the graphical plot with specific humidity and partial pressure of water vapour in y axis and dry bulb temperature along x axis. The specific volume of mixture, wet bulb temperature, relative humidity and enthalpy are the properties appeared in the Psychrometric chart.

14. Define sensible heat and latent heat.

Sensible heat is the heat that changes the temperature of the substance when added to it or when abstracted from it. Latent heat is the heat that does not affect the temperature but change of state occurred by adding the heat or by abstracting the heat.

15. What are the important Psychrometric processes?

1. Sensible heating and sensible cooling,
2. Cooling and dehumidification,
3. Heating and humidification,
4. Mixing of air streams,
5. Chemical dehumidification,

16. What is meant by adiabatic mixing?

The process of mixing two or more stream of air without any heat transfer to the surrounding is known as adiabatic mixing. It is happened in air conditioning system.

17. What are the assumptions made in Vander Waal’s equation of state?

1. There is no inter molecular forces between particles.
2. The volume of molecules is negligible in comparison with the gas.

18. Define coefficient of volume expansion.

The coefficient of volume expansion is defined as the change in volume with the change in temperature per unit volume.

19. State Helmholtz function.

Helmholtz function is the property of a system and is given by subtracting the product of absolute temperature (T) and entropy (S) from the internal energy (U).
Helmholtz function = U – TS

20. What are thermodynamic properties?

Thermodynamic properties are pressure (p), temperature (T), volume (V), internal energy (U), Enthalpy(H), entropy (S), Helmholtz function and Gibbs function

21. Define throttling process.

When a fluid expands through a minute orifice or slightly opened valve, the process is called as throttling process. During this process, pressure and velocity are reduced.

22. Define Molecular mass.

Molecular mass is defined as the ratio between total mass of the mixture to the total number of moles available in the mixture.

23. Define isothermal compressibility.

Isothermal compressibility is defined as the change in volume with change in pressure per unit volume keeping the temperature constant.

24. Define psychrometry.

The science which deals with the study of behavior of moist air (mixture of dry air and water vapour)is known as Psychrometry.

25. What is by-pass factor?

The ratio of the amount of air which does not contact the cooling coil (amount of bypassing air) to the amount of supply air is called BPF.

26. Define Apparatus Dew Point (ADP) of cooling coil?

For dehumidification, the cooling coil is to be kept at a mean temperature which is below the dew point temperature (DPT) of the entering. This temperature of the coil is called ADP Temperature.

27. State Dalton’s law of partial pressure

Dalton’s law of partial pressure states that “the pressure of a gas mixture is equal to the sum of pressures of its each components if each component is exerted alone of the temperature and volume of the mixture”.

28. What is the significance of compressibility factor?

The gases deviate from ideal gas behavior significantly at high pressures and low temperatures. This deviation from ideal gas behavior at a given temperature and pressure can be determined by the introduction of a correction factor called the compressibility factor.