CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2014/15
Module Title: Thermofluids 3
Module Code:
INTR7009
School:
Mechanical and Process Engineering
Programme Title:
Bachelor of Engineering in Mechanical Engineering – Award
Bachelor of Engineering (Hons) in Sustainable Energy – Year 3
Programme Code:
EMECH_7_Y3
ESENT_8_Y3
External Examiner(s):
Internal Examiner(s):
Dr Garrett Byrne, Dr Gerry Woods
Prof. John Currie, Mr Edmund Collins
Dr Keith McMullan, Mr Michael P O’Mahony
Instructions:
Answer Question 1 and TWO other questions.
Duration:
2 Hours
Sitting:
Autumn 2015
Requirements for this examination:
Steam tables
Log book
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you
are attempting the correct examination.
If in doubt please contact an Invigilator.
Page 1 of 4
Q1
(a)
List FOUR desirable properties for a Refrigerant
(4 marks)
(b)
In the context of a refrigerant, explain the meaning of ODP and GWP
(4 Marks)
(c)
A cold storage plant is used to cool 10,000 litres of milk per hour from 30oC to 4oC and the heat
leakage into the plant is estimated to be 60kW. The refrigerant used is ammonia and the
temperature required in the evaporator is -8oC. The compressor delivery pressure is 10.34bar and
the condenser liquid is undercooled to 24oC before throttling. The plant has a brine circulating
system and the rise in temperature of the brine is to be limited to 5K. Assuming that the vapour is
dry saturated on leaving the evaporator and that the compression process is isentropic,
Draw the P-h and T-s diagrams for this process
(6 Marks)
Calculate
(i)
The power input required taking the mechanical efficiency of the compressor as
90%
(20 Marks)
(ii)
The rate at which the brine must be circulated in litres per second
(6 Marks)
For milk: specific heat capacity, 3.77kJ/kgK; density, 1030 kg/m3
For Brine: specific heat capacity, 2.93 kJ/kgK; density 1190kg/m3
Q2
Derive an expression for the thermal resistance of a cylindrical layer to heat conduction.
(8 marks)
With regard to insulated pipes, explain what is meant by the Mean Critical Thickness of
Insulation
(4 marks)
A cylindrical refrigeration chamber 0.9 m inside height is made from a layer of stainless steel
1 mm thick lagged on the outside with plastic foam 190 mm thick. The surface heat transfer
coefficient on the inside is 50 W/m²K and on the outside 75 W/m²K. The inner diameter of the
refrigerator is 0.7 m. The thermal conductivity of stainless steel is 41 W/mK and of plastic
foam is 0.5 W/mK. Calculate the heat transfer rate from the chamber when the internal
temperature is 190 K and the ambient temperature is 16degC. (Ignore the problems of the
corners.)
(18 marks)
Page 2 of 4
Q3
(a)
In Heat Exchanger analysis when is LMTD method suitable and when is the
Effectiveness/NTU method suitable
(10 marks)
(b)
A 4 kg/s product stream from a distillation column is to be cooled by a 3 kg/s water stream
in a counterflow exchanger. The hot and cold stream inlet temperatures are 400 and 300 K,
respectively, and the heat transfer area of the exchanger is 30 m2.
(i)
Sketch the flow direction and known temperatures for this counter flow heat
exchanger.
(4 marks)
(ii)
If the overall heat transfer coefficient is estimated to be 820 W/m2 K, determine
using the NTU effectiveness method, the product and cooling stream outlet
temperatures.
The specific heat of the product stream can be taken to be 2500 J/kg K and that of
the cold stream as 4180 J/kg K.
(16 marks)
Q4
A workshop, 20m x 10m x 8m high, is heated by the use of electric heaters. The total
window area is 60m2 and the door area is 5m2. The workshop which must be maintained at
20oC when the outside temperature is -2oC, has the following construction
U (W/m2K)
Walls, plastered cavity brick
1.8
Windows, single glazed
5
Door, wood 25mm thick
2.5
Floor, wood on joists
1.98
Flat roof, concrete
2.5
(i)
(ii)
(iii)
Calculate the max power required to heat the building which has 2 air changes
per hour
(10 marks)
Targeting the TWO areas with the biggest losses, discuss ways to reduce the
power requirements
(8 marks)
The workshop is operational 8 hrs/day, 5 days a week. It is estimated that the
heating is required 35 wks of the year. There is a proposal to insulate the walls
at a cost of €5000. New U value for the walls would be 0.8W/m2K. Power is
charged at €0.19/kWhr. Should this proposal be implemented, giving reasons.
(12 marks)
Specific heat capacity of air(constant pressure) 1.01kJ/kgK, density of air 1.2kg/m3
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