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Chapter: Mechanical - Engineering Thermodynamics - Gas Mixtures and Psychrometry

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Important Questions and Answers: Gas Mixtures and Psychrometry

Mechanical - Engineering Thermodynamics - Gas Mixtures and Psychrometry

 

GAS MIXTURES AND PSYCHROMETRY

 

1.      What is difference between air conditioning and refrigeration?

 

Refrigeration is the process of providing and maintaining the temperature in space below atmospheric temperature.

 

Air conditioning is the process of supplying sufficient volume of clean air conditioning a specific amount of water vapor and maintaining the predetermined atmospheric condition with in a selected enclosure.

 

2.Define dry bulb temperature (DBT)?

                                                                 The temperature which is measured by an ordinary thermometer is known as dry bulb temperature. It is generally denoted by   .              

 

3.Define wet bulb temperature (WBT)?

                                                                 It is the temperature of air measured by a thermometer when its bulb is covered with wet cloth and exposed to a current rapidly moving air. It is denoted by   .                             

 

4.Define dew point temperature (DPT)?

                                                                 The temperature at which the water vapor present in air begins to condense when the air is cooled is known as dew point temperature. It is denoted by                                                             .

 

5.Define Relative Humidity (RH) and Specify humidity?

                                                                 Relative Humidity is defined as the ratio of the mass of water vapor (m(v)) in a certain volume of moist air at a given temperature to the mass of water vapor m(vs) in the same volume of saturated air at the same temperature.


Specific Humidity is defined as the ratio of mass of water vapor (m(v)) to the mass  of dry air in the given volume of mixture.            


                                                                                                         

6.  Differentiate between absolute and relative humidity?

Absolute humidity is defined as the ratio of the mass of water vapor  (m(v)) in a certain volume of moist sir at a given temperature to the mass of water vapor     at atmospheric conditions.                                                                                       

Relative Humidity is defined as the ratio of the mass of water vapor m(vs) in a certain volume of moist air at a given temperature to the mass of water vapor     in the same volume of saturated air at the same temperature.


 

 

7.Define degree of saturation?

Degree of saturation is the ratio of specific humidity of moist air to the specific humidity of saturated air at temperature.


 

 

8.What is dew point temperature? How is it related to dry bulb and wet bulb temperature at the saturation condition?

It is the temperature at which the water vapor present in air begins to condense when the air is cooled.

For saturated air, the dry bulb, wet bulb and dew point temperature are all same.

 

9.What is sensible heat factor?

The ratio of sensible heat load to the total heat loaded is known as sensible heat factor or sensible heat ratio.


 

 

10. What is humidification and dehumidification?

Humidification is defined as the process of adding moisture at constant dry bulb temperature. So, but . So, the humidity ratio increases from .

Dehumidification in this case, the humidity ratio decreases by removing moisture from air. But this process is carried out at constant dry bulb temperature , but. The heat involved in dehumidification is also called as latent heat load.

 

11. Explain the throttling process.

When a gas or vapor expands and flows through an aperture of small size, the process is called as throttling process.

 

12. What is psychrometry?

Psychrometry is a study of properties of moist air. It deals with the state of atmosphere with respect to moisture content. On the other hand psychometrics deals with the thermal properties of air and the control and measurement s of the moisture content in air in addition to study the effects of atmospheric moisture on commodities and the human comforts.

 

13. What is effective temperature?

Effective temperature is defined as that temperature of saturated air at which the subject would experience the same feeling of comfort as experienced in the actual unsaturated environment.

 

14. Describe the heating and humidification process.

In case of winter air conditioning heating and humidification are very common. The system consists of a coil for sensible heating of air from state1 to state 3 then along the wet bulb temperature line through state2. Thereafter the humidification occurs along the wet bulb line until the required moisture is added to reach state2. The amount of moisture addition can again be calculated by m w = m (w2-w1). if the ignores the internal functioning during heating and humidification, one can reach state 1 to 2 by adding appropriate amount of moisture and heat as shown by process 1-2, dotted line


 

15.Describe the Sensible Cooling and Heating process.

The energy transfer in these processes occurs with change in temperature only, the specific humidity remaining constant. The heat transfer for this process can be obtained by the energy balance for the control volume for the known enthalpies, h1 and h2 and for the steady mass flow rate

The magnitude of is less than during cooling. Hence there is heat transfer from the air flowing over the coil to the cooling fluid. On the other hand during heating the energy is transferred to the air flowing over the coil.

 

16. 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.

 

17. 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.

 


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