Specifying Shell-and-Tube Heat Exchangers

1 Specifying Shell-and-Tube Heat Exchangers Understand what heat exchanger design specialists need to know and remember, you know your process best Asif Raza Shell-and-Tube heat Exchangers are one of the most important and commonly used process equipment items in the chemical process industries (CPI). If you are working on a project during either the basic or the detailed engineering phase, there is a good chance that you will need to specify one or more Shell-and-Tube Exchangers and perhaps many of them. While the actual design will likely be done by a specialist at an equipment vendor or within your own company, you still need to fill out a process datasheet for each heat exchanger and in due course, review the vendor s detailed proposal. You know your process best, and it is a bad idea to rely on the vendor always to make the right decisions.

2 This article shows you the basics of Specifying and selecting Shell-and-Tube heat Exchangers : the process information and preliminary design decisions needed to fill out the datasheet, and how to check any corresponding assumptions made by the vendor. Although it does not go into detail on the design procedure, the article is also a good starting point if you intend to design the heat exchanger yourself. Figure 1. Which fluid goes on the shellside and which on the tubeside? There is no straightforward answer, but the guidelines presented here will help you decide Figure 1. Which fluid goes on the shellside and which on the tubeside? There is no straightforwardanswer, but the guidelines presented here will help you decide Datasheet information Though every company is likely to have its own heat exchanger datasheet, most of them look much like the sample shown in Figure 2 (p.)

3 49). To complete the datasheet you will need to know: 1. The composition and normal flowrate of the process fluid(s), and the temperature change required. Refer to heat and material balances. 2. Process fluid properties density, viscosity and thermal conductivity at the operating temperature and pressure. Figure 2. A typical datasheet for a Shell-and-Tube heat exchanger lists all the information required for a detailed design Source: TEMA (Tubular Exchanger Manufacturers Association, Inc.; Tarrytown, ; ). Which fluid on which side? Next comes your first design decision: Which fluid goes on the shellside and which on the tubeside (Figure 1)? There is no straightforward answer, but some considerations and rules of thumb outlined in an online reference ( ) and incorporating the author s experience are summarized here: Corrosive fluids are best kept to the tubeside.

4 Since the tubeside has less metal than the shellside, this will minimize the use of expensive metals that may be needed to withstand the fluids corrosive properties. Fluids at extreme pressures and temperatures are preferably kept to the tubeside, because they are likely to require a greater metal thickness, or more expensive materials of construction. The tubes, being smaller in diameter than the shell , withstand higher pressures. Fluids that need to be kept at a high velocity, such as water or propylene glycol for cooling, should be kept on the tubeside. Dirty fluids, or streams that are otherwise likely to cause fouling, should go on the tubeside. This is because the tubes are easier to clean than the shell . For instance, it is often possible to clean the tubes by water jetting, having simply opened the head of the exchanger, without needing to remove the tube bundle.

5 The shell and the outside of the tube bundle, on the other hand, are harder to clean mechanically, and chemical cleaning is often the only option. The shellside offers a larger cross-section for vapor flow, and hence lower pressure drops. Process vapors to be condensed are therefore normally placed on the shellside, though the tubeside is generally used for condensing steam. The baffles on the shellside help to ensure good mixing, which reduces the effects of laminar flow and therefore tends to increase heat-transfer coefficients. Hence you will get better heat transfer if viscous fluids are kept on the shellside I confirmed this recently on a project involving a very viscous polymer. Twisted tubes, static mixers or tube inserts increase turbulence and thus heat-transfer coefficients on the tubeside by reducing the effects of laminar flow.

6 Because these are usually proprietary technologies, however, your ability to check the vendor s performance claims may be limited. If you think you would benefit from one of these technologies, work closely with the vendor and be sure to evaluate all the options. In heat exchanger designs that feature gaskets or floating heads, the shellside typically is not a suitable location for fluids that are hazardous, corrosive or especially valuable, because the risk of leaks is too high. Such fluids should therefore normally go on the tubeside. Exchangers featuring all-welded construction can safely carry hazardous fluids on the shellside, though you should remember the difficulty of cleaning the shellside. Thermal expansion may be an issue if one of the fluids undergoes a temperature change of more than 150 200 C (300 400 F).

7 In this case you would normally put the high-temperature-change fluid on the shellside, which is better able to handle large temperature changes in certain exchanger designs. In summary, the fluids preferred on the tubeside are the following: Cooling water The more-fouling, erosive or corrosive fluid The less-viscous fluid The fluid at higher pressure The hotter fluid The smaller volumetric flowrate. Remember, however, that none of the suggestions above is definitive. Use them as a starting point, but if they indicate a different fluid arrangement from what has been used in the past in your plant or industry, you may find that there is a good reason. If two suggestions conflict, or the performance of your initial configuration looks unsatisfactory because the predicted pressure drop or heat-transfer performance does not meet your requirements do not be afraid to reverse the arrangement of the two fluids and see whether that improves matters.

8 More key decisions Allowable pressure drop. You will have to understand the process thoroughly before you can attempt to specify the pressure drop on each side of the heat exchanger. As a rule of thumb, start with 10 psi on both the shellside and the tubeside. If there is a pump upstream of the heat exchanger, there probably will be no concern about pressure drop as long as the pump can handle this. For gases, if there is a compressor upstream, check with your equipment-design engineer that it can provide the necessary pressure drop. For cooling water, check for constraints on the allowable return pressure at the battery limit of the unit. Sometimes the need to optimize the heat exchanger means that you will have to take a higher pressure drop than originally specified. A higher pressure drop means higher velocity, which in turns gives a higher Reynolds number and a higher heat-transfer coefficient.

9 Give the heat exchanger vendor an allowable pressure drop as high as realistically possible to allow flexibility in optimizing the design. Once the designer has confirmed the calculated pressure drop, pass this value on to your rotary equipment engineer, who will need it for sizing pumps and compressors. Fouling factors. These are very important in sizing the heat exchanger. Do not expect the vendor to provide you with fouling factors. A higher fouling factor translates to a lower design heat-transfer coefficient ( U d) and a larger required surface area. Fouling factors can often be taken from existing plant data. If these are not available, you will have to assume a value taken from company guidelines or published sources (Table 1). Make sure that your customer whether internal or external is in agreement with your assumed fouling factor.

10 Designing with a too-high fouling factor will result in an oversized heat exchanger that will cost you more and probably will not work as intended. Table 1. typical fouling factors Fluid Typical fouling factor (ft2 F h/Btu) Fuel oil Steam (clean) Exhaust steam (oil bearing) Refrigerant vapors (oil bearing) Compressed air Industrial organic heat-transfer media Refrigerant liquids Hydraulic fluid Molten heat-transfer salts Acid gas Solvent vapors MEA and DEA solutions DEG and TEG solutions Caustic solutions Vegetable oils Lean oil Cooling water Natural gas Atmospheric tower overhead vapors Vacuum overhead vapors Specifying appropriate fouling factors is important but not always easy. In the absence of operating experience, pick figures from reliable published sources. Source: TEMA Excess area.