Cycles Contributions by: John L. Falconer & Will Medlin Department of Chemical and Biological Engineering University of Colorado Boulder, CO 80309-0424.

CyclesContributions by:

John L. Falconer & Will MedlinDepartment of Chemical and Biological Engineering

University of ColoradoBoulder, CO 80309-0424

Supported by the National Science Foundation

Consider the best possible heat engine working in air at 25°C. The engine continuously converts heat from a source at 300°C to work, and heat is continuously transferred into the engine at a rate of 100 kJ/s. The maximum possible rate at which the engine can continuously produce work is ___________ 100 kJ/s.

A. greater than  

B. equal to  

C. almost 

D. significantly less than  

For a Carnot heat engine, which statement must be incorrect?

A. |Qc| > |W|

B. |QH| < |W|

C. |QH| > |QC|

A Carnot heat engine is run in reverse so for every 100 kJ of electrical work added to the pump, 700 kJ of heat is added to the building. Is this consistent with the 1st and 2nd laws?

A. Yes

B. No

C. Cannot be determined

Which diagram corresponds to a Carnot engine in which both adiabatic steps in the cycle are irreversible?

T

S

1 2

4 3

A BT

S

1 2

4 3

D

S

T

1 2

4 3

T

S

1 2

4 3

C

Which diagram corresponds to a Carnot heat pump in which both adiabatic steps in the cycle are irreversible?

T

S

1 4

2 3

T

S

1 4

2 3

T

S

1 4

2 3

T

S

1 4

4 3

A

C

B

D

Which diagram represents a Rankine cycle?

P

H

P

P

P

H

H

H

A B

C D

Which diagram represents a Rankine cycle?

P

H

PP

P

H

P

H H

H

A B C

D E

Which diagrams are not possible for a vapor compression refrigeration cycle using a throttle valve?

A. 2 & 5

B. 1 & 4

C. 2, 3, & 4

D. 1, 2, 4, & 5

E. 1, 4, & 5

T T

S

T

SS

S S

#1 #2 #3

#4 #5TT

Which diagrams do not represent a vapor compression cycle? The compressor is adiabatic, the expansion valve is a throttle process, the evaporator and condenser are each at constant pressure.

A. 1 B. 1 & 2 C. 2 D. 2 & 3 E. 3

P

H

PP

H H

#1 #2 #3

Which diagrams do not represent a vapor compression cycle? The compressor is adiabatic, the expansion valve is a throttle process, the evaporator and condenser are each at constant pressure.

A. 1 B. 1 & 2 C. 1 & 3 D. 2 & 3 E. 3

P

H

PP

H H

#1 #2 #3

Indicate, in the correct order, which P-H diagram represents a vapor compression cycle and which represents a Rankine power cycle.

A. 1& 2

B. 2 & 3

C. 3 & 4

D. 4 & 2

E. 1 & 3 PP

H H

#3 #4

PP

H H

#1 #2

AB

C

D

Pre

ssure

Enthalpy (kJ/kg)

Which line is most likely to represent a reversible turbine in a Rankine cycle?

A

B

C

DPre

ssure

Enthalpy (kJ/kg)

Which line is most likely to represent a liquid pump?

A well-insulated water heater heats water from 10°C to 50°C. The efficiency of the process is _______________ .

A. much greater than 1

B. much less than 1

C. almost equal to 1

D. not possible to tell

Which will require more work to compress from 1 atm to 100 atm?

A. Liquid

B. Gas

C. Solid

Liquid water at 1.5 MPa and 180°C expands continuously through a throttling valve to 100 kPa. What can you definitely say about this system?

A. All the liquid evaporates

B. Some of the liquid evaporates

C. The temperature increases

D. The temperature does not change

Liquid water at 1.5 MPa and 180°C expands continuously through a throttling valve to 100 kPa. What can you definitely say about this system?

A. All the liquid evaporates

B. The temperature increases

C. The temperature decreases

D. The temperature does not change

1,000 J/s electrical energy is supplied to a heat pump for a house. Which is the most likely amount of heat that is provided to the house?

A. 35,000 J/s

B. 3,000 J/s

C. 1,000 J/s

D. 400 J/s

Rank the following types of energy in terms of quality:

1) hot combustion gas

2) hot water

3) electricity

4) chemical energy

A. 4 > 3 > 1 > 2

B. 3 > 4 > 1 > 2

C. 3 > 1 > 4 > 2

D. 4 > 1 > 3 > 2

E. 4 > 3 > 2 > 1

The quality of steam from an irreversible turbine, compared to a reversible turbine, at the same final pressure is _______.

A. higher

B. lower

C. the same

Liquid water at 10°C flows into an insulated tank where an electric heating element heats it to its exit temperature of 50°C. The efficiency of this process is ____________ 100%.

A. much greater than  

B. about equal to  

C. much less than 

Which cannot be done continuously?

A. Convert 100 kJ work to 100 kJ heat

B. Convert 100 kJ electricity to 200 kJ heat

C. Convert 100 kJ heat to 100 kJ work

D. Both A and B

Which statement is incorrect?

A. Work can be continuously converted to heat with efficiency close to 100%

B. Heat can be continuously converted to work with efficiency close to 80%

C. Electricity can be continuously converted to work with efficiency greater than 50%

D. Electricity can be converted to heat with efficiency close to 100%

Suppose both adiabatic steps in a Carnot cycle are irreversible. Which is the correct statement about the entropy change of the Carnot engine?

A. ΔSengine > 0

B. ΔSengine = 0

C. ΔSengine < 0

D. None of the above