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Cork Institute of Technology
Bachelor of Engineering in Mechanical Engineering - Award
(NFQ – Level 7)
Summer 2006
Thermofluids & Systems
(Time: 3 Hours)
Instructions Answer any FIVE questions.
Examiners: Mr. C. Gibbons Mr. J. Connolly Mr. R. Simpson
Q1. Heat Transfer 1 (a) Explain the 3 radiative properties of a surface, which account for what happens to incident radiative energy. (4 marks)
(b) For each property, provide an example surface where the value is unity. (3 marks)
(c) Outline how an I.R. thermal imaging camera could be used to determine the emissivity of a copper container (5 marks)
(d) The rate of heat emitted from a surface depends on the surface emissivity Determine the rate of heat emitted from a ceramic heater unit of Є = 0.9, area = 100mm x 500mm with a surface temperature of 200°C. (5 marks)
(e) Explain why this is not the same as the nett heat leaving the surface. (3 marks)
Q2. Heat Transfer 2 (a) Outline the principle of thermal resistance and show how the equation for RT for convective and conductive heat may be deduced from the basic heat transfer rate equation for conduction or convection. (6 marks)
(b) Show how the thermal resistance method may be used to determine the overall heat rate between 2 fluids in a heat exchanger separated by a thin metal wall. (6 marks)
(c) Use the thermal resistance method to determine the thickness of insulation required to be added to the wall of a typical CIT lecture room to reduce the U-value to the correct building regulation level of 0.3 Wm-2^ K-1^ , given a construction of 100mm concrete block, 40mm polystyrene, 100mm concrete façade element. Assume external and internal h values of 20 and 10 resp Wm -2^ K-^. Take k concrete = 10 Wm-2^ K- k polystyrene = 0.1 Wm-2^ K- k insulation = 0.01 Wm-2^ K -1^ (8 marks)
Q3. Heat Exchangers 1 (a) Describe the operation of the following exchanger type: (i) Recuperative (ii) Regenerative (iii) Evaporative Give an example application in each case. (6 marks)
(b) Show from first principles that the LMTD of a counterflow heat exchanger is given by:-
θ m =
1
2
2 1
ln θθ
θ θ (14 marks)
Q6. Air Conditioning.
An air conditioning system is to be designed for a new conference/meeting room located in the Innovation Centre, CIT. The design is to satisfy the following requirements: Dimensions 20m x 20m x 3m Comfort Conditions 23°C 45% External Conditions 30°C 50% Inlet Air Temp 18°C Fresh Air 13 by mass Chiller Contact Factor = 0. Heat loads:- People 100 @ 100W sensible + 40W latent Equipment 300W Lighting 300W Neglect all other gains Fresh Air Requirement = 8 litre s -1^ / person
Use the psychometric chart to determine the following:- SHR, inlet RH, mass flow, ACR, fresh air supply Chiller duty, reheat duty.
Comment on your results. (20 marks)
Q7. IAQ/Clean Room Technology
(a) The recently introduced international standard for clean rooms indicates a number of distinct changes to the earlier ISO and American Standard.
- Briefly outline two of these changes and the effect of the change on clean room design selection. (4 marks)
(b) Briefly highlight the major differences between non-laminar and laminar flow clean room options, (6 marks)
(c) A clean room is required that will limit the concentration of 0.1μm particles and greater to 100 particles per cubic metre. Determine the ISO designation of this room. (4 marks)
(d) Explain the three main methods of particle capture of HEPA and ULPA high performance air filters. Indicate the variation of particle size with capture method. (6 marks)
