Would it be possible to answer 3 questions with fully working out or computer simulations as required. For Question 1, X=5, Question 2, Y=8
Condenser Design & Analysis Report Author 1 s3333333 and Author 2, s3334333 Semester , School of Engineering MIET - Applied Heat and Mass Transfer Introduction The objective of this assignment is to introduce students to the dynamics and interplay of heat transfer and fluid flow phenomena related to the design of a shell and tube condenser. The report contains three parts: 1. Analysis of the condenser and it’s role in the refrigeration cycle analysis 2. Heat transfer analysis inside the condenser heat exchanger 3. Heat and fluid flow visualisation of a shell and tube heat exchanger (CFD) Pressure vessel design of the condenser is not part of this assignment but in real life this aspect would have to be considered also. Students are to work in pairs and submit a joint report. This assignment is worth 20%. Question 1 A hermetically sealed electric motor compressor combination with isentropic efficiency of 60%, receives R134a vapour from an evaporator where the saturation temperature is -2◦C. The vapour leaves the evaporator with 2◦C of superheat and travels to the compressor in a well-insulated suction pipe. The cooling capacity achieved in the evaporator is 9.0 + 0.X kW where X is the last digit of one student number, e.g. student number is 3334568, so Q̇ = 8.8 kW. The condenser saturation temperature is 44◦C. Refrigerant (R134a) leaves the condenser as a liquid sub cooled by 4◦C, and travels directly to the throttle without change in temperature: a) (10 marks) Sketch the cycle on a pressure-enthalpy diagram with 60% isentropic efficiency compression. From your sketch provide the enthalpy values (read values from your plot) at each stage of the cycle. Using these values, determine the enthalpy change in the condenser, and evaporator? b) As a comparative study, use the thermophysical properties table for R134a, calculate the net enthalpy change for an isentropic compression (in the compressor stage). Show the steps on how you obtain the enthalpy values. c) For the remainder of the assignment, use the values obtained from the sketch/chart. Deter- mine the mass flow rate of refrigerant through the evaporator d) Determine the heat transfer rate rejected in the condenser, when receiving vapour from the compressor. Question 2 Taking into account the compromises between material costs, and compressor running costs, and water pump running costs, the company you work for has decided that a reasonable temperature difference between the saturation condensing temperature of the refrigerant, and the cooling water that enters the condenser is 11.0 + 0.Y ◦C, where Y is the last digit of the student number belonging to your partner (if individual then use your same number). A pump is used to transport the water 1 at a flow rate so that the water temperature increases by 6 ◦C, received from the hotter condensing refrigerant. The water passes through tubes which are pure copper, un-finned, and has 16mm-OD (outer di- ameter) and 1-mm-wall-thickness. For condensers associated with heat rejection capacities of the order of this one, tube plates of the pattern illustrated are used. The actual length of the tubes and hence shell is unknown and will depend on the capacity determined from Q1. a) Determine the water mass flow rate. b) Determine the lineal velocity of the water if the water travels through: (i) one tube per pass (e.g. 18 passes) (ii) two tubes per pass (e.g. 9 passes) and check if it falls within the ASHRAE recommendation of 1 to 3 m/se(This velocity range is intended to be sufficient for some self-cleaning of deposits to occur but not so great that tube metal is not eroded.) c) Sketch a temperature profile of the refrigerant and water in the condenser, then making the approximation that all the refrigerant vapour in the condenser is at saturation temperature, determine the LMTD and maximum permitted overall thermal resistance between the vapour and the water. d) Fouling occurs on the water side due to minerals found in water, but no fouling (i.e. non- condensables) occurs on the refrigerant vapour side. Determine and sketch the thermal resis- tance network associated with the heat transfer between the refrigerant vapour outside the pipe and the water flowing through the pipe. e) Using the permitted overall thermal resistance found in (2c) determine the length of tubing L required in the condenser, and then the individual tube length, for the situation where the water travels through two tubes per pass. You need to use the thermal resistance network and obtain expressions for each of the resistances in terms of the unknown L. Additional information is given as follows: - water is of a quality that produces a fouling factor of 0.000176 m2K/W - the refrigerant condensate film resistance requires a temperature difference which is un- known: Tsat − Tsurface and requires a guess and then iteration to improve the guess (only iterate once in this assignment). Use the relationship that this temperature difference as a fraction of the overall LMTD, will be the same ratio as the refrigerant condensate film resistance as a fraction of the overall resistance Tsat − Tsurface ∆Tlmtd == Rconden Rt - start your iteration guess with a fraction of 2/3, calculate through once, verify the results, and then refine it with one additional iteration. 2 f) Determine the water frictional pressure drop for the situation where the water travels through two tubes per pass. The company you work for would prefer that it not exceed 70kPa. Suggest how it might be reduced. Question 3 A smaller and simplified representative shell-tube heat exchanger was developed in CFD for analysis. While the heat exchanger model differs to the one analysed in Question 1 and 2, it serves as a powerful tool to understand the mechanisms of heat transfer taking place. The purpose of this question is to enable you to use a CFD tool to provide insightful visualisation; and familiarise yourself to the case which will be used in a later assignment. Therefore, while some questions appear rudimentary, it serves to create a strong foundation for the next assignment. The CFD model file, ’hx-2tubepass.cas.gz’ is shown in Figure . The fluid moving through the shell and tubes are both water. The settings for this question will be selected by you, with the following conditions. – The cold water enters through the tubes, and has a smaller heat capacity rate (i.e. Cc = ṁCp) than the hot water that flows through the shell. – Select a suitable mass flow rate such that the velocity through the tubes is between 1-3m/s. – The shell mass flow rate should be in the range of 0.5 − 2.0× of the tube mass flow rate. – The water temperatures should be in the range of 5 to 150◦C Set up the case, and run the simulation for 300 iterations. a) Extract the fluid properties of the water. Provide a summary of the fluid properties. Sum- marise the inlet conditions (flow rate, velocity, temperature). Show any calculations you used to determine the values used. Obtain the outlet temperatures b) Perform post-processing and create contours, streamlines at different parts of the heat ex- changer and use these images to describe the heat transfer, and flow phenomena occurring. Notes: The CFD model contains approximately 300,000 cells. This has been tested on an i7-6core CPU laptop which runs the simulation of 300 iterations in 20minutes. You may need to consider this when doing many parametrical studies. 3 Marking The report is out 100marks. The breakdown of marks is as follows: Overall Report (10-marks) Overall report format/structure/professionalism. Marks are based on is the structure, Intro, conclusion. Does it tell a flowing story or is it difficult to read. Figure captioning, reference citations, image quality, formatting, layout. Quality of figures, colours, font matching. Question 1 a) (10 marks) Correctly sketching the refrigeration cycle, labelling the processes, key points, and determining its values. b) (10 marks) Showed steps in calculations to arrive at the correct answer. c) (2 marks) Correct calculation to determine the mass flow rate. d) (2 marks) Correct calculation to determine the heat transfer rate in the condenser. Question 2 a) (3 marks) Correct calculation to determine water mass flow rate. b) (5 marks) Correct calculation to determine velocities for 1 tube and 2 tubes per pass. c) (5 marks) Correct calculation to determine the log mean temperature difference. d) (3 marks) Correct sketch of the thermal resistance network. e) (25 marks) Correct analysis of: equation set up of each resistance, resistance calculations, second iteration, and pipe length. f) (12 marks) Showed correct steps/assumptions in determining the pressure loss in the tubes. Question 3 a) (3 marks) Summary of conditions used, and simulated inlet and outlet velocities and temper- atures. b) (10 marks) Demonstrate an understanding of the flow behaviour in a shell tube heat ex- changer, use of post-processing images, relevant images to help describe the descriptions. 4 Saturated refrigerant-134a—Temperature table Specific volume, Internal energy, Enthalpy, Entropy, m3/kg kJ/kg kJ/kg kJ/kg·K Sat. Sat. Sat. Sat. Sat. Sat. Sat. Sat. Sat. Temp., press., liquid, vapor, liquid, Evap., vapor, liquid, Evap., vapor, liquid, Evap., vapor, T °C Psat kPa vf vg uf ufg ug hf hfg hg sf sfg sg �40 51.25 0.0007054 0.36081 �0.036 207.40 207.37 0.000 225.86 225.86 0.00000 0.96866 0.96866 �38 56.86 0.0007083 0.32732 2.475 206.04 208.51 2.515 224.61 227.12 0.01072 0.95511 0.96584 �36 62.95 0.0007112 0.29751 4.992 204.67 209.66 5.037 223.35 228.39 0.02138 0.94176 0.96315 �34 69.56 0.0007142 0.27090 7.517 203.29 210.81 7.566 222.09 229.65 0.03199 0.92859 0.96058 �32 76.71 0.0007172 0.24711 10.05 201.91 211.96 10.10 220.81 230.91 0.04253 0.91560 0.95813 �30 84.43 0.0007203 0.22580 12.59 200.52 213.11 12.65 219.52 232.17 0.05301 0.90278 0.95579 �28 92.76 0.0007234 0.20666 15.13 199.12 214.25 15.20 218.22 233.43 0.06344 0.89012 0.95356 �26 101.73 0.0007265 0.18946 17.69 197.72 215.40 17.76 216.92 234.68 0.07382 0.87762 0.95144 �24 111.37 0.0007297 0.17395 20.25 196.30 216.55 20.33 215.59 235.92 0.08414 0.86527 0.94941 �22 121.72 0.0007329 0.15995 22.82 194.88 217.70 22.91 214.26 s237.17 0.09441 0.85307 0.94748 �20 132.82 0.0007362 0.14729 25.39 193.45 218.84 25.49 212.91 238.41 0.10463 0.84101 0.94564 �18 144.69 0.0007396 0.13583 27.98 192.01 219.98 28.09 211.55 239.64