MECHENG 715 – Semester Two 2020
Quiz B
Instructions:
• This is an open book quiz.
• This quiz is worth 10% of your total marks for the course.
• There are 12 multiple-choice questions worth a total of 10 points.
Different questions may have different points.
• You are to answer all 12 questions by selecting the correct answer to each
question.
• You have 90 minutes to complete this quiz. T
• The conditions for this quiz are the same as those for exams at the
University of Auckland. You are expected to comply with all the conditions.
You will already be aware that the University actively uses its resources to
check and take appropriate actions when conditions are breached.
• For any issues, email: a.subiantoro@auckland.ac.nz
Questions
1. Which of these conditions below is traditionally considered within the
thermally comfortable range?
(0.5 marks)
a. 31°C, 20% relative humidity
b. 23°C, 60% relative humidity
c. 5°C, 50% relative humidity
d. 24°C, 90% relative humidity
e. 40°C, 90% relative humidity
2. Determine the equivalent thermal resistance (in K/W) of a set of clothing with
an insulation of 0.5 clo worn by someone with body surface area of 1.7 m2.
Note: 1 clo is equal to 0.155 m2.K/W.
(0.5 marks)
a. 0.0132
b. 0.0456
c. 0.0775
d. 0.0912
e. None of the above
3. Calculate the enthalpy (in kJ/kg) of humid air at 1 atm, dry-bulb temperature
of 35°C and 80% relative humidity. The saturation pressure of water at 35°C
is 5.6291 kPa.
(1 mark)
a. 109
b. 298
c. 2,087
d. 2,759
e. None of the above
4. Determine the dry-bulb temperature of humid air with wet-bulb temperature
of 20°C and dew-point temperature of 15°C at 1 atm.
(0.5 mark)
a. 15°C
b. 20°C
c. 30°C
d. 40°C
e. None of the above
5. 1 kg of air at 1 atm, 5°C and 70% relative humidity is perfectly mixed with 2
kg of air at 1 atm, 30°C and 70% relative humidity. Determine the
temperature and relative humidity of the mixture.
(1 mark)
a. 17.5°C and 70%
b. 17.5°C and 90%
c. 21.7°C and 70%
d. 22.5°C and 80%
e. None of the above
6. Atmospheric air flowing at 1 m3/s and 1 atm is conditioned from 30°C and
90% relative humidity to 20°C and 60% relative humidity. Calculate the
cooling load (in kW) of this process.
(1 mark)
a. 45
b. 50
c. 53
d. 59
e. None of the above
7. The air in a certain room is to be maintained at 22°C. The room dimensions
are 3 m x 3 m x 3 m. The ambient air is at 18°C and 90% relative humidity.
The ventilation rate is 1 air change per hour. The room enclosure has an
effective area of 10 m2 with an effective R-value of 2.0. The convective heat
transfer coefficients indoor and outdoor are both 5 W/m2.K. The room is
occupied by 1 person who emits 100 W of heat. Determine the rate of
heat/cooling required.
(1 mark)
a. 20 W of heating
b. 20 W of cooling
c. 180 W of heating
d. 180 W of cooling
e. None of the above
8. Consider a refrigerator operating on the simple vapour compression
refrigeration cycle. The refrigerant is R-134a. The condensing and
evaporating temperatures are 50°C and 0°C, respectively. The refrigerant
enters the compressor at 10°C and leaves at 80°C. It enters the expansion
valve at 40°C and expands isenthalpically. Determine the coefficient of
performance (COP) of the refrigerator.
(1 mark)
a. 2.2
b. 2.6
c. 3.0
d. 4.0
e. None of the above
9. Which of these devices does NOT typically operate on vapour compression
refrigeration cycles?
(0.5 mark)
a. Boilers
b. Chillers
c. Heat pumps
d. Refrigerators
e. None of the above
10.The evaporator of a certain air conditioner absorbs 1.5 kW of heat from the
room. The COP of the system in cooling mode is 3.0. The condenser rejects
heat to atmospheric air flowing at 0.2 kg/s. Determine the air temperature
and relative humidity at the outlet of the condenser, if the air enters the
condenser at 31°C and 80% relative humidity.
(1 mark)
a. 31°C and 68% relative humidity
b. 31°C and 94% relative humidity
c. 41°C and 46% relative humidity
d. 41°C and 80% relative humidity
e. None of the above
11.Determine the payback period to replace a 1 kW electric heater with a heat
pump with a COP of 4.0 if the electricity tariff is $0.3/kWh, the usage is 4
hours per day and the replacement cost is $900.
(1 mark)
a. 100 days
b. 300 days
c. 1000 days
d. 3600 days
e. None of the above
12.(Bonus question) Which of these pairs below taught the MECHENG 715 –
Building Services course in the Department of Mechanical Engineering at the
University of Auckland, New Zealand in Semester Two 2020?
(1 mark)
a. Clark Kent - Bruce Wayne
b. Jacinda Ardern – Judith Collins
c. Krishnan Jayaraman – Stuart Norris
d. Jaynie Ng – Muizz Shah
e. Rajnish Sharma – Alison Subiantoro