Presentation on theme: "Heat Exchanger Design ME 414 Design Project Created and Designed by: Michael Stark Joshua Keith Billy Burdette Brandon Mullen Joseph Listerman."— Presentation transcript:
Heat Exchanger Design ME 414 Design Project Created and Designed by: Michael Stark Joshua Keith Billy Burdette Brandon Mullen Joseph Listerman
Project Goals Design Heat Exchanger Create a light weight heat exchanger Heat exchange must be as efficient as possible Cost must be kept low as possible The size of the heat exchanger must be under design constraint
Project Guidelines During the process of a liquid chemical product, its temperature needs to be reduced by 20 degrees Celsius. Mass flow rate is 220,000 kg/hr Fluid enters the heat exchanger at 45 C and should leave at 25 C Material properties of this chemical product can be approximated as water Cooling of the chemical product will be achieved by using treated city water City water is available at 20 C Mass flow rate is adjustable and one of the design parameters to be selected Exit temperature of city water from the heat exchanger is a function of the selected mass flow rate Professor Toksoy
Project Optimization Must cool the chemical from 45 C to 25 C Heat exchanger length can not exceed 7 meters Heat exchanger shell diameter can not exceed 2 meters Minimize heat exchanger shell and tube weight hence the cost Minimize heat exchanger pressure drop Professor Toksoy
Heat Exchanger Design Inputs for MATLAB Chemical to be cooled was set as Shell side liquid Mass flow rate of cooling water = 220 kg/sec Shell ID =.889 m Shell thickness = 5 mm Tube OD = 6.35 mm Tube thickness =.457 mm Tube Length = 2.88 m Baffle space =.6 m Helical Baffles Counter flow One shell pass and one tube pass Aluminum was used for both shell and tube materials Gnielinski equation used for tube side Nusselt correlation Square tube pitch
Heat Exchanger Design Inputs for MATLAB Explained Chemical to be cooled was set as shell side liquid – In order to keep shell side pressure drop to a minimum we needed to keep the mass flow rate in the shell low. The only way we found of doing this and getting the desired Q was to push the chemical to be cooled through the shell. Mass flow rate of cooling water = 220 kg/sec - For these inputs this calculates out to an average tube side fluid velocity of ~1 m/s which falls within the recommended range of.9 – 2.4 m/s. Tube OD = 6.35 mm - The small OD was needed to increase the surface area for heat transfer for a given shell ID. Tube thickness =.457 mm - The small tube thickness was needed to increase the heat transfer coefficient and also reduced the total material weight and cost.
Tube Length = 2.88 m - The tube length was increased to increase the calculated Q. Baffle space =.6 m - Although slightly larger then the recommended value of 40-60% of shell ID,.6 m worked well. Helical Baffles – A helical baffle will increase the heat transfer coefficient considerably without dramatically increasing pressure drop due to the nature of the flows. Counter flow - Because of the narrow band of temperatures between the two fluids, a counter flow arrangement was used in order to increase the log mean temperature difference between the two fluids without having to increase the mass flow rate of the water to very high levels. Heat Exchanger Design Inputs for MATLAB Explained Cont.
One shell pass and one tube pass - One pass was used for both the shell and tube because currently the program does not calculate pressure drop due to multiple passes correctly. We discovered this late into the project and did not have time to fix the issue. If the pressures were calculated properly the water output temperature for one shell pass and two tube passes must stay below 28.33 deg C in order to keep the log mean temperature difference correction factor valid for the given temperature requirements. Aluminum was used for both shell and tube materials - Aluminum was chosen for its excellent heat transfer properties and its reduced weight. Gnielinski’s equation used for tube side Nusselt correlation – For the calculated Reynolds number of 5800 this correlation is most applicable. Petuhkov – Krillov’s correlation is used for Reynold’s number larger then 104. Heat Exchanger Design Inputs for MATLAB Explained Cont.
D.O.E. Run 1
Factors Shell mass flow rate Tube length Shell internal diameter The most significant affect on heat transfer was tube length, a result of increased surface area. Shell I.D. and tube length had the greatest affect on weight, the larger the shell the more tubes can fit inside. Shell side pressure drop increases with tube length and mass flow rate. Dramatic decrease as shell ID increases. The only factor affecting the tube side pressure drop was tube length.
D.O.E. Run 2
Factors Baffle Space Tube Thickness Baffle Cut Baffle spacing has a large affect on q and shell side pressure drop. Tube thickness was the only factor affect HE weight in this DOE. Baffle cut doesn’t seem to have any affect on other parameters. We fixed baffle spacing because it heavily influenced shell side pressure drop.
Final optimization factors Mass flow rate of the shell fluid fixed to 220 kg/s Tube length Shell internal diameter Tube outer diameter We adjusted the ranges of our chosen factors and ran the DOE again. The mass flow rate only affected the shell side pressure drop at this stage of the design. We chose the shell side mass flow rate based on what we decided would yield reasonable shell outlet temperature using counter flow.
Factorial Design Analysis – Heat Rate Tube length has the largest affect on the heat rate. Shell ID has the smallest relative affect on heat rate. Shell ID had a negative affect on heat rate. This was a result of more tubes decreasing the velocity in the tube. The result is laminar flow inside the tube.
Factorial Design Analysis - ∆P Tube We can see that tube length has the largest affect on tube side pressure drop. Shell ID has no affect on tube pressure drop. We expected tube OD to have a larger affect on tube side pressure drop.
Factorial Design Analysis - ∆P Shell Shell ID had the largest affect on shell side pressure drop. The affect of tube OD on the pressure drop was surprising. We attribute this affect to the 60° triangular pitch tube arrangement. As tube OD grows larger there is more pressure drop in the shell.
Factorial Design Analysis – HE Weight The shell inside diameter has the largest affect on weight. The larger the shell diameter the more tubes we could fit inside, thus increasing weight. Because tube length determines the length of the heat exchanger, it too has a large affect on heat exchanger weight.
Design Optimization - 1 The design optimized to our original design. We expected our final tube diameter to be 6.35 mm with a mass flow rate of 220 kg/s. Optimal Tube OD was 8.3mm The tube length was longer than our original design called for, which was a result of maximizing the q calculated. We set target values for the shell and tube side pressure drops. We set a target range for total weight between 900-1100 kg.
Design Optimization - 2 The design optimized to our original design. We expected our final tube diameter to be 6.35 mm with a mass flow rate of 220 kg/s. Optimal Tube OD was 8.3mm, adjusted it to 9.525 mm to coincide with standard tube dimensions. The tube length was longer than our original design called for, which was a result of maximizing the q calculated. We set target values for the shell and tube side pressure drops. We set a target range for total weight between 900-1100 kg.
Heat Exchanger Design Output from MATLAB
Heat Exchanger Final Design Tube side mass flow rate of 220 kg/sec Tube OD set to 9.525 mm, thickness 0.889 mm Shell ID set to.889 meters, thickness 5 mm Heat exchanger is a one pass counter flow tube arrangement with helical baffles and optimized tube length of 2.6 m. The ratio between desired and calculated heat rate is 1.00.
Further Analysis We believe that cost could be decreased by over-designing the HE and reducing the number of tubes until we got the desired heat ratio. The tube mass flow rate was an important design consideration because the outlet temperature of the shell fluid was completely dependent on it. After performing a macroscopic heat balance, counter flow was chosen because the cold fluid outlet temp was expected to be higher than the hot fluid outlet temp.
Matlab Program Improvements Create program checks in order to eliminate unrealistic designs. If multiple tube passes are used with parallel flow it is possible to calculate a LMTD_CF that is an imaginary number. Provide the operator more detailed information regarding the Nusselt correlations.
Cost Summary Heat Exchanger Dry Weight 730 Kg Heat Exchanger & Fluid Weight 2287 Kg Cost OnlineMetals.com $37.00 per 8ft length of aluminum tubing Total estimated aluminum tubing cost $337,000.00 $11.00 per 8ft length of mild steel tubing Total estimated mild steel tubing cost $100,000.00 Instillation and Manufacturing