How to Design a Shell and Tube Heat Exchanger

How to Design a Shell and Tube Heat Exchanger

Shell and Tube Heat exchangers are very commonly used in the oil and gas sectors. However, a lot of industrial factories also used shell and tube heat exchangers in their day to day operations.

In fact, some heat exchangers are even used to recover heat and hence have energy savings thus benefitting the factory as well.

Here is a step by step approach to design a shell and tube heat exchanger. We shall only focus on sensible heat transfer, and make extensive use of Chapter 11 in Perry’s Handbook (3). All the formulas and whatsoever described in the following section shall be in accordance to Perry’s Handbook.

Usually, the flow rates and the physical properties of the two stream involved are specified, and the temperatures of inlet/outlet of both the streams is also known. For example, inlet stream could be steam and the known temperature is X deg C and the outlet requirement is gas and the difference of temperature is also known.

If the outgoing temperature of one of the streams is not specified, usually a constraint is given. Thus, by an energy balance, the outgoing temperature of the second stream can be calculated along with the heat duty.

SIZE

1) The heat Duty, Q is usually specified by the requirement of the customer. The selected and design of the heat exchanger must at least meet or exceed (Even better) this requirement.

2) The next step is to make a guess on the Overall Heat Transfer Coefficient. If you are not sure, a standard OHTC is usually available on the standard books indicating what value to use between two streams.

1. For Example, A Water Medium on the Hot Fluid and a Water Medium on the Cold Fluid gives a OHTC of 800 -1500W/m2 C.
2. For Example, A Steam Medium on the Hot Fluid and a Water Medium on the Cold Fluid gives a OHTC of 1500 -4000W/m2 C.
3. For Example, A Steam Medium on the Hot Fluid and a Organic Solvent on the Cold Fluid gives a OHTC of 500 -1000W/m2 C.

Hence, you have to determine what fluid is passing through the shell and tube heat exchanger.

3) Select the stream that is to be used on the Tube Side. The tube side is more likely to be used for the fluid that is more likely to foul the wall, more corrosive, or for the fluid with the higher pressures. Another reason is because tube bundles are easily taken out for replacements rather than the shell side.

4) Gas or vapor is usually used on the shell side incoming stream. In addition, high viscosity fluids is also preferred on the shell side where pressure drop may be significantly large.

5) The next step is to determine the number of tubes N needed to do the job. Because we have an approximate figure for the heat transfer area, we can use the formula:-

A = N (3.14 X D) X L

Where D is the OD of the tubes designed, L is the overall length of the tube. Usually, for real cases application and as in most design casesd, OD ¾” to 1” is preferred over any other size and L is usually standard size of 6m. However, for minimum material waste, you can always design 3m length and divide the tubes into two. Realistically, a 6m heat exchanger is too bog for handling and also for operation purposes.

6. The next step is to check the Log Mean Temperature Difference (LMTD) of the fluids and ensuring that the velocity through a single tube does not exceed approximately 10 ft/s for liquids, ensuring that the pressure drop is under reasonable constraints to maintain turbulent flow and minimize fouling. If necessary, adjust the number of tube passes to get the velocity to fall within this range.

7.Determine the shell size is the following step. You have to decide what shape are the tubes are to be arranged. There are two types of arrangement, the triangular shape or the rectangular shape. The former is usually preferred over the latter because of pressure drop constraints. However, a square type has an advantage of having cleaning the tube bundle easily as compared to the triangular type.

8. A shell basically has two types of flow, a 1 pass flow or a 2 pass flow. If a two pass flow is designed, then the tubes are in the shape of U so that it pass to the end of the shell and return back to the supply at a lower level.

9. Once the size of shell is determined, the number of tubes inside the shell can be size up due to the constraints of the inner diameter of the shell.

10. No of baffles. You need to calculate next the number of baffles required and the spacing between each baffle. Normally, baffles are equal spaced and the minimum baffle spacing is usually one-fifth of the shell diameter. Bear in mind when you design a baffles inside a shell and tube heat exchanger on the pressure lost because the more baffles in the tube bundle will cause higher pressure lost, hence the performance of the heat transfer.

11. Finally, we are ready to check the thermal performance of the heat exchanger. Calculate the shell side and the tube side of the heat exchanger Heat Transfer Coefficients, Uo, the tube wall contributions to the resistance and the appropriate fouling resistances. Then check if this figure is almost similar to the estimated Uo that you have presume earlier. If it is within range, then the design is OK. If it is too small or too big value, you have to recalculate again and varying other datas, ie, type of flow, no of baffles, no of tubes etc etc.

12. Flow that is laminar or turbulent has different formulae and this can be obtained by the Heat Transfer Engineering book available in the market. Once all this has been determined and you have confirmed that everything is OK, then you have already design a shell and tube heat exchanger.

13. In the new technology world that we are living today, softwares are easily available to tabulate all the data requirement to design a shell and tube heat exchanger. However, if you would like to see how a standard design calculation of heat exchanger looks like, please contact us at marketing@extremevision-thai.com or you can visit our homepage www.extremevision-thai.com for more details and we shall happy to send you a copy of a sample design calculation of the shell and tube heat exchanger.

Below show a diagram of a U tube Shell and Tube Heat Exchanger

Mimic sample of a shell and tube heat exchanger.

Shell and ube Heat Exchanger