Thermodyanmics Question bank

ENGINEERING THERMODYNAMICS

                                                                       UNIT  5      PSYCHROMETRY

                                                                      PROBLEMS

Simple Problems

1)      What is the lowest  temperature that air can attain in an evaporate cooler ,if it enter

at  1 atm, 302 k, and  40% relative humidity?

2)      Consider a room that contains air at 1 atm, 308k, and 40% relative humidity. Using the

Psychrometric chart , determine the  specific humidity, the enthalpy, the wet bulb temperature,

The dew point temperature and the specific volume of the air.

3)      Atmospheric air at 1.0132 bar has a DBT of 32®C and a WBT of 25®C. compute (1) The partial

Pressure of water vapour, (2) The specific humidity, (3) the dew point temperature 

(4) the relative humidity ,(5) the degree of saturation, (6) the density of air in the mixture,

(7) the density of vapour in the mixture and (8) the enthalpy of the mixture.

       4)   In a laboratory test a sling psychrometer recorded dry bulb and wet bulb temperatures are

              303K and 298 k respectively. Calculate (1) vapour pressure (2) relative humidity (3) specific

              humidity (4) degree of saturation (5) dew point temperature (6) enthalpy of mixture

5)  A room 7m × 4m × 4m is occupied by an air water vapour mixture at 38°C. The atmospheric pressure is  1bar and the relative humidity is 70%. Determine humidity ratio, dew point temperature, mass of dry air and mass of water vapour. If the mixture of air –water vapour is further cooled at constant pressure until the temperature is 10°C. Find the amount of water vapour condensed.

Adiabatic Mixing

6)    30 m³/min of moist air at 15®C DBT and 13®C WBT are mixed with  12m³/min of moist air  a25°C              DBT and 18°C WBT. Determine DBT and WBT of the mixture assuming  the barometricpressure is  1 atm.

7)   Air at 20°C, 40% relative humidity is mixed adiabatically with air at 40°C, 40% RH in the ratio of                              1Kg  of former with 2 Kg of latter. Find the final condition( humidity and enthalpy ) of air.

8) A steam of air at 101.32 kpa, 18°C, and a relative humidity of 30% is flowing at a rate of               14.15m³/min. A second steam at 101.32 kpa, 38°C and a relative humidity of 50% is flowing at the rate of 8.5 m³/ min. The two streams are mixed adiabatically to form a third stream at 101.32 KPa. Determine the specific humidity, the relative humidity and the temperature of the third stream.

9) 1 kg of air at 313K dry bulb temperature and 50% relative humidity is mixed with 2 Kg of air at 293K DBT and 203K dew point temperature. Calculate the temperature and specific humidity of the mixture.

10)  Saturated air at 20°C at a rate of 1.167 m³/ sec is mixed adiabatically with the outside air at 35°C and 50% RH at a rate of 0.5 m³/ sec. Assuming adiabatic mixing condition at 1 atm, determine specific humidity , the relative humidity , DBT and volume flow rate of the mixture.

Psychometric Process Problem

11)An air conditioning system is to take in outdoor air at 283 k and 30% relative humidity at a

Steady rate of  45 m³/min and to condition it to 298 k and 60% relative humidity. The outdoor

air is first heated to 295 k in the heating section and then  humidified by the injection of hot

steam in the humidifying section. Assuming the entire process takes place at a pressure of 100 Kpa, determine  (1) the rate of heat supply in the heating section and  (2) the mass flow rate of the steam required in the humidifying section

12) Air at 16°C and 25% relative humidity passes through a heater and then through a humidifier to reach final DBT of 30°C and 50% relative humidity. Calculate the heat and moisture added to the air. What is the sensible heat factor?

                                                     THEORY PART

1)      Differentiate between

a)      Dry bulb temperature and Wet bulb temperature.

b)      Wet bulb depression and Dew point depression.

2)      In an adiabatic mixing of two streams , derive the relationship among the ratio of mass of streams, ratio of enthalpy change and ratio of specific humidity change.

3)      Show the following process on skeleton psychrometric chart

a)      Dehumidification and Cooling

b)      Heating and humidification.

4)      Describe the adiabatic cooling process and deduce the expression for its enthalpy.

5)      Draw the cooling and dehumidification process and explain sensible heat factor, Bypass factor and effectiveness of coil with respect to it.

Unit-IV (Big Questions)

1.Using the chaperon equation, estimate the value of the enthalpy of vaporization r-234a, 293k, and compare it with the tabulated value
2. Show that C_P-C_V=Ý for an ideal gas
3.  Show that the joule-Thomson coefficient of an ideal gas is zero (Or) Derive joule-Thomson coefficient of an ideal gas
4. Using the cycle relation and the first Maxwell relation, derive the other three Maxwell relations.

               (Or)Derive Maxwell relations.
5. Write a short note on generalized compressibility chart
6.  Derive wan der waal equation in terms of reduce parameter
7. Derive TdS equation taking temperature, volume and temperature, pressure as independent properties.
8. Prove that the total pressure is a sum of partial pressures
9.  A closed vessel has a capacity of  0.5m³ .l it contains 20% nitrogen and 20%oxygen 60% carbon dioxide by volume at 20%c and impa .Calculate the  equivalent molecular mass, gas constant, mass percentage and the mass of mixture
10.. A certain gas has C_p=0.913kj/kg and C_v=0.653kj/kgk. Find the molecular weight and the specific gas constant r of the gas

UNIT IV

1. What are the unique features of vanderwaal’s equation?

(i)                            Inter molecular attractive study is made.

(ii)                          Shape factor is considered.

(i)                            Inter molecular attractive study is made.

(iii)                        Shape factor is considered

(iv)                         

2. What is compressibility factor? What does it signify? What is its value for vanderwaal’s gas at critical point?

We know that, the perfect gas equation is PV=RT. But for real gas, a correction factor has to be introduced in the perfect gas equation to take into account the deviation of real gas from the perfect gas equation. This factor is known as compressibility factor (Z) and is defined by Z= PV/RT

At critical point, the vander waal’s equation                                    

                        P_CV_C/RT_C= 1 for ideal gases.

3. State Avogadro’s law and its significance.

Avogadro’s law states,” Equal volumes of different perfect gases at the same temperature and pressure, contain equal number of molecules”.

4. Write Maxwell’s equation.

(ðT/ðV) _s= - (ðP/ðS) _v

(ðT/ðP) _s= (ðV/ðS)_p

(ðP/ðT) _v= (ðS/ðV)_T

(ðV/ðT) _p= - (ðS/ðP)_T

5. What do you mean by equation of state?

PV=mRT is the equation of state which is obeyed by all ideal gases.

6. State Dalton’s law of partial pressure.

The total pressure of a mixture of gases is equal to sum of the partial pressure of individual gases if each gas exists in the total volume at mixture temperature.

P=P₁+P₂+P₃+…………+P_K

P=Total pressure

P₁&P₂,……… P_N= Partial pressure of each gas

7. Have you ever encountered ideal gas? If so where?

No. There is no ideal gas which obeys equation of state strictly. But gases can behave almost as ideal gases when they are in low densities and high temperatures.

8. What is meant by coefficient of expansion?

It is defined as the ratio between changes in volume to change in temperature per unit volume when the pressure is kept constant.

9. Explain (a) mass fraction & (b) mole fraction.

(a) Mass fraction:

             X_i=m_i = Mass of individual gas component

                   M_i    Mass of gaseous mixture

(b) Mole fraction:

             Y_i=N_i = No.Of moles of individual gas component

                   N     No. of moles of gaseous mixture

10. What does joule Thomson coefficient represent?

µ = (ðT/ðP) _h

Joule Thomson coefficient=   change in temperature         

                                                     Change in pressure                 when enthalpy is constant

11. What is meant by virtual expansion?

Viral or virtual expansions are only applicable to gases of low and medium densities.

The equation state of a substance is given by

P= RT + a (T) + b (T) + c (T) + d (T)

      V        v²        v³          v⁴         v⁵

       The coefficient of a(T),b(T),c(T),d(T)… are virial coefficients. The virial coefficient will vanish when the pressure becomes zero. Finally, the equation of state reduces to the ideal gas equation.

12. Difference between ideal gas and real gas.

An ideal gas is one which strictly follows the gas laws under all conditions of temperature and pressure.

In actual practice, there is no real gas which strictly follows the gas laws over the entire range of temperature and pressure. However hydrogen, oxygen, nitrogen and air behave as an ideal gas under certain temperature and pressure limits.

13. State Boyle’s law:

Boyle’s law states that, “the volume of a given mass of a gas varies inversely as its absolute temperature, when the temperature remains constant”.

V α 1/ p

14. State Charles law:

Charles’s law states, “The volume of a given mass of a gas varies directly as its absolute temperature, when the pressure remains constant”.

V α T

15. State joule’s law:

Joule’s law states, “The internal energy of a given quantity of a gas depends only on the temperature”.

16. What is clausius clapeyron equation?

It gives the relationship between saturation pressure & saturation temperature, the enthalpy of vaporization and specific volume of two phases.

     dp   = h_fg

      dt      T_fg

UNIT V

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 containing a specific amount of water vapour and maintaining the predetermined atmospheric condition with in a selected enclosure.

2. Define psychrometry.

            It is the study of behavior of moisture content in dry air.

3. Name some psychrometric processes.

Ø Sensible heating

Ø Sensible cooling

Ø Humidification

Ø Dehumidification

Ø Heating and humidification

Ø Cooling and dehumidification

Ø Heating and dehumidification

Ø Cooling and humidification

4. Define dry bulb temperature (DBT-t_d).

            Temperature measured by ordinary thermometer is known as dry bulb temperature.

5. Define wet bulb temperature (WBT-t_w).

            Temperature measured by a thermometer whose bulb is covered by wet cotton is known as wet bulb temperature.

6. Define dew point temperature (DPT- t_dp).

            The temperature at which the water vapour condenses as water in atmospheric air is known as dew point temperature.

7. Define relative humidity and specific humidity.

            Relative humidity is defied as the ratio of mass of water vapour in the given volume of air at given temperature to mass of water vapour in the same volume of air at same temperature when the air is saturated.          

Φ = mass of water vapour in the given volume of air at given temperature

       mass of water vapour in the same volume of air at same temperature when the         air is saturated

                Mass of water vapour present in 1kg of dry air is known as specific humidity.

            w= mass of water vapour in the given volume of air at given temperature

                        Mass of same volume of air at same temperature

8. Define degree of saturation.

            Degree of saturation is the ratio of specific humidity of dry air to the specific humidity of saturated air of same mass.

            µ= specific humidity of dry air

                 Specific humidity of saturated air of same mass.

9. How is dew point temperature related to dry bulb and wet bulb temperature?

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

10. 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.

11. Define bypass factor (BPF) of a coil.

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

                    BPF = Amount of air bypassing the coil

                                    Total amount of air passed