Question

1) 20 kg/min water at 20º C is mixed adiabatically with 40 kg/min water at 80 ºC. What is the outlet water temperature?

Select one:

a. 80 °C

b. 60 °C

c. 50 °C

d. 70 °C

2)

Steam at 0.8 MPa and 500 ºC is throttled over a well insulated valve to 0.6 MPa, what is the outlet temperature

Select one:

a. 450 ºC

b. 500 ºC

c. DOF>0

d. 400 ºC

3)

Humid air at 70 C and 300 kPa contains 1.87 mol% water. What is the dew point under ideal conditions?

Select one:

a. 133.5 ºC

b. can't determine this because the temperature is too high

c. 35 ºC

d. 70 ºC

4)

Steam is accelerated by a nozzle steadily from a low velocity to a velocity of 210 m/s at a rate of 3.2 kg/s. If the temperature and pressure of the steam at the nozzle exit are 400 °C and 2 MPa, the exit area of the nozzle is:

Select one:

a. 8.4 cm2

b. 10.2 cm2

c. 152 cm2

d. 24.0 cm2

e. 23.0 cm2

5)

Steam expands in a turbine from 4 MPa and 500°C to 0.5 MPa and 250°C at a rate of 1350 kg/h. Heat is lost from the turbine at a rate of 25 kJ/s during the process. The power output of the turbine is:

Select one:

a. 182 kW

b. 246 kW

c. 207 kW

d. 157 kW

e. 287 kW

6)

Air at 20°C and 5 atm is throttled by a valve to 2 atm. If the valve is adiabatic and the change in kinetic energy is negligible, the exit temperature of the air will be:

Select one:

a. 24 °C

b. 20 °C

c. 17 °C

d. 10 °C

e. 14 °C

7)

Steam at 1 MPa and 300 °C is throttled adiabatically to a pressure of 0.4 MPa. If the change in kinetic energy is negligible, the specific volume of the steam after throttling is:

Select one:

a. 0.655 m3/kg

b. 0.233 m3/kg

c. 0.358 m3/kg

d. 0.646 m3/kg

e. 0.375 m3/kg

8)

Steam enters a diffuser steadily at 0.5 MPa, 300 °C and 122 m/s at a rate of 3 kg/s. The inlet area of the diffuser is:

Select one:

a. 50 cm2

b. 150 cm2

c. 15 cm2

d. 105 cm2

e. 190 cm2

9)

In a heating system, cold outdoor air at 10 °C flowing at a rate of 6 kg/min is mixed adiabatically with heated air at 70 °C flowing at a rate of 3 kg/min. The exit temperature of the mixture is:

Select one:

a. 40 °C

b. 55 °C

c. 45 °C

d. 85 °C

e. 30 °C

10)

Air is to be heated steadily by an 8 kW electric resistance heater as it flows through an insulated duct. If the air enters at 50°C at a rate of 2 kg/s, the exit temperature of air is:

Select one:

a. 55.4 °C

b. 50.0 °C

c. 58.0 °C

d. 46.0 °C

e. 54.0 °C

Answer #1

1) As the mixing is adiabatic,

heat in = heat out

Assuming Cp of water to be constant = 4.187 kJ/kg-K

Let the base temperature be 0 C

1------> 20 C stream of water

2 ---> 80 C stream of water

3 -----> T3 C stream of water (outlet)

By mass balance,

m3 = m1 + m2

.: m3 = 20 + 40 = 60 kg/min

m1Cp(T1-0) + m2Cp(T2-0) = m3Cp(T3-0)

.: m1(T1-0) + m2(T2-0) = m3(T3-0)

.: 20*(20-0) + 40*(80-0) = 60*(T3 - 0)

On solving, we get

**T3 = 60 C**

b) State 1:

From superheated steam table, at 0.8 MPa and 500 C,

specific enthalpy H1 = 3481.17 kJ/kg

Throttling is an isenthalpic process.

State 2 : P = 0.6 MPa

H2 = H1 ................(since isenthalpic)

From saturated steam table at P = 0.6 MPa,

specific enthalpy of satuarted vapor Hv = 2756.14 kJ/kg

As H2 > Hv , steam is superheated at the exit condition.

Now, look for H = 3481.17 kJ/kg and P = 0.6 MPa in the superheated steam table.

We get, **T = 500 C**

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