Questions regarding the heat exchange tubes

Use a moderate size; the more commonly used ones are those with an outer diameter of 19 and 25

This is a very good topic; such choices are often encountered when performing thermal calculations. I originally planned to discuss these aspects in detail in my post on systematic knowledge of heat exchanger design: http://bbs.hcbbs.com/thread-437212-1-1.html. But since there’s already a discussion on this, I’ll cover the most basic principles here first. As long as the pressure drop across the tubes is acceptable, it’s better to use tubes with smaller diameters but in larger numbers, as this improves the heat exchange efficiency. Additionally, to save costs and reduce pressure drops, we can increase the length of the tubes while using those with smaller diameters; this results in a smaller overall diameter for the equipment. Statistics show that costs can be reduced by more than 25% in this way. Slender-type coolers are actually quite popular abroad, as they offer good performance along with cost savings. However, there are limitations regarding the manufacturing process. These are some of the factors to consider. There are many other aspects to take into account in both thermal design and structural design, and it’s necessary to consider all factors together in order to achieve an optimal design. I will discuss these aspects in detail in my post on systematic knowledge of heat exchanger design: http://bbs.hcbbs.com/thread-437212-1-1.html. Please pay close attention and engage in active discussions. Once again, please make sure to focus on the key points of the discussion when replying. This post was last edited by vivo1314 on 2009-4-9 at 23:40

I believe that, provided the heat exchange area meets the requirements, it is advisable to choose tubes with a larger diameter and fewer tubes in total, as this approach results in a lower pressure drop and makes it easier to clean the shell side. Last edited by Houfei Benniao on 2009-4-9 23:45.]

When the heat exchange area remains constant, there is a significant difference among the various pipe specifications available. One factor to consider is the flow velocity inside the heat exchange tubes, which must not exceed the maximum allowable flow velocity for that type of material； Secondly, pressure drop must be taken into account; high flow rates can result in excessive pressure drop ； Thirdly, space needs to be taken into consideration; when the diameter of the piping is too large, it must not exceed the area where the heat exchanger can be placed

Each has its advantages and disadvantages, and the choice should be made depending on the specific situation. The main factors to consider are: cost, size, resistance, and scaling； Generally, small tubes are preferred as they are compact and take up less space, making them suitable for applications where there are restrictions on the size of the structure. However, their resistance is relatively higher, and they tend to scale more easily; therefore, a higher flow rate is necessary. Regarding costs, it is necessary to consider the proportion of material and processing costs; too high or too low a proportion will increase costs, so it is important to choose pipes with a moderate cost level.

Using a small tube diameter increases the heat transfer area per unit area, results in a more compact structure, reduces metal consumption, and improves heat transfer efficiency. However, fluids with small diameters experience high flow resistance, are difficult to clean, and tend to scale. Large-diameter pipes are suitable for viscous or dirty fluids, while smaller-diameter pipes can be used for cleaner fluids

From a cost perspective, if pressure drop is acceptable, smaller-diameter pipes should be used and the pipes made longer. But this also brings about problems such as high resistance, difficulty in cleaning, and scale removal. From a maintainability perspective, using larger-diameter pipes facilitates cleaning and descaling; however, for the same heat exchange area, they are more expensive than pipes with smaller diameters.

Small-diameter pipes offer good heat transfer performance, but they also result in higher pressure losses, and cleaning them inside is difficult. Additionally, the flow space on the shell side is limited, which makes it easy for problems such as corrosion or blockages to occur if the quality of the cooling water is poor. Large-diameter pipes are easier to handle in terms of pressure losses and cleaning, but their heat transfer efficiency is not as good. Therefore, pipes with a diameter of 19 or 25 mm are usually chosen; the specific type of pipe to use should be determined based on specific considerations

We use HTRI for selection; when finer heat exchange tubes are chosen, the overall heat transfer coefficient in the REPORT increases. Thus, under the same process conditions, using finer heat exchange tubes allows for a reduction in the heat exchange area. Additionally, finer heat exchange tubes generally have thinner walls as well. So the total cost is also low. It depends on the specific circumstances, such as pressure drop and flow rate requirements. I used to use heat exchange tubes with a diameter of around 25 as well; nowadays, most of the systems I work on involve heat exchange between water and water, or between oil and water. The heat exchange tubes can even be 8mm in diameter. In terms of actual performance, it’s also quite good.

I’ve heard foreigners give lectures, saying that elongated heat exchangers are better than those that are short and stout. When conditions permit, for optimization, an elongated shape should be used. But we all know that in the vast majority of cases, the conditions do not allow one to do as they please; not only in terms of manufacturing processes and on-site installation, but even when it comes to manufacturing the components themselves, there are always problems with those elongated shapes. So, as for how to optimize it, it still needs to be considered comprehensively based on actual conditions. In terms of process design, flow rate and pressure drop need to be considered; on-site conditions relate to available space, while manufacturing requires attention to deformation issues.

Following the answer from the third floor: pipes with a small diameter but in large numbers can **increase the heat exchange area**. thereby improving heat exchange efficiency

Of course, it’s better to choose tubes with a small diameter but in large numbers – this improves heat transfer efficiency, allows the equipment to be more compact, reduces the diameter of the housing, and thus lowers costs. After all, energy conservation and emission reduction are being promoted these days!

If the heat exchange area is sufficient, it’s better to choose tubes with a larger diameter! Low fluid resistance, anti-stall!

When making a choice, one should not consider only the heat exchange area; it is also crucial to take into account the properties of the two fluids involved. Using smaller pipe diameters can increase the heat exchange area, thereby improving the efficiency of heat transfer. However, fluids with small diameters experience high flow resistance, are prone to clogging, and are difficult to clean. Large-diameter pipes are suitable for viscous or dirty fluids, while smaller-diameter pipes can be used for cleaner fluids

Based on economic considerations, go with larger diameters and fewer numbers

What everyone has said makes a lot of sense; in fact, there are no absolute answers to any problems. As everyone has said, although thin and long pipes offer good heat transfer performance, various other factors such as the properties of the medium, the difficulty of obtaining such pipes, and pressure drops must also be taken into consideration. Therefore, when dealing with problems, it is necessary to conduct more analysis and discussion; once the issues that need attention are resolved, the equipment can basically be finalized.

In current heat exchanger design, the heat exchange tubes are often constrained by process conditions; in other words, these process conditions typically include the diameter of the heat exchange tubes.

To summarize, the main concerns regarding small diameters revolve around the following aspects: some people consider the pressure drop, believing that smaller heat exchange tubes result in a greater pressure drop; however, it is sufficient as long as the pressure drop meets the design requirements； Some people consider cleaning and descaling, believing that small diameters make cleaning difficult. Generally speaking, in special environments where the process water is quite clean, excessive concern is given to dirt and fouling factors in China; it is assumed that scaling will always occur. In fact, if the material permits, increasing the flow velocity in the tube side to keep the flow in a turbulent state can basically prevent scaling. As long as the flow condition is well controlled, not only will there be no dirt inside the tubes, but the tube banks and shell side will also remain free of fouling. We have disassembled several shell-and-tube heat exchangers produced by renowned foreign manufacturers that had been in use for nearly 10 years, and the inner walls of the tube banks and shells were still smooth, without even any rust. Of course, this requires very detailed analysis during the design phase. Additionally, we tend to assume higher fouling coefficients when making calculations, which actually tends to be detrimental to heat exchange efficiency. I have raised this issue for further discussion among everyone. However, in device design, the design of any individual component must take the overall structure into account; to optimize the whole, certain advantages of individual parts have to be sacrificed, and each situation must be considered on its own

What everyone has said is correct, but when determining the required heat exchange area, people focus more on pressure drop, scaling, and the diameter of the heat exchanger. What isn’t taken into account is the issue of vibration that occurs when the tubes become longer and thinner. I admit that, under conditions that meet the process requirements, slender tubes are superior to thicker ones, but vibration needs to be carefully considered when the tubes are made slender. Our company’s previous evaporators aimed for high heat transfer coefficients, which led to several cases of the copper tubes in those heat exchangers cracking. I hope everyone can weigh the options carefully.

Currently, most shell-and-tube heat exchangers in China are available in 3M and 6M sizes. When selecting the tube size for design, it is necessary to consider not only the working medium but also fatigue failure. If the medium tends to form scale, the pipe diameter cannot be too small, as this makes cleaning difficult; moreover, if the flow pattern of the medium is unstable, the pipes cannot be too long, as excessive length can lead to vibrations that damage the welds between the tube sheet and the pipes. At the same time, for the same heat exchange area and identical shell diameter, the tube diameter should be smaller; the reason is that smaller openings and a larger distance between tubes result in less impact on the weld stress in the tube sheet.