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Category Archive: Heat Exchanger Design

Tag Archive: Heat Exchanger Design

  1. Three Types of Mechanical Failures in Shell & Tube Heat Exchangers and How to Prevent Them

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    Shell and tube heat exchangers are well favored by plant engineers for their long life spans and minimal maintenance needs in addition to their ability to efficiently transfer heat. While stainless steel heat exchangers are reliable and durable for the most part, there are times when stress and extended use take their tolls on the equipment’s components.

    Mechanical failure can appear in several different forms, each with its own symptoms and consequences. When you know what to look for, you can work to prevent mechanical failure in your exchangers or identify an issue quickly so you can solve it promptly. Here are three types of mechanical failure that shell and tube heat exchangers can sustain in time:

    1. Fatigue

    Repeated thermal cycling can lead to fatigue in shell and tube heat exchangers. Fatigue can cause cracking in the tubes beginning with small, hard-to-see striations that can quickly grow larger. Eventually, fatigue cracks can span the diameter of the tube and completely sever it.

    There are several factors that can increase the risk of fatigue or cause it to escalate more quickly. A report on failure analysis of shell and tube heat exchangers from the College of Engineering Pune in Maharashtra, India, explained that the stress ratio can play a big role in contributing to thermal fatigue. As may be expected, when stress increases, so does the risk of failure due to fatigue.

    “Small weld defects can lead to fatigue and severe damage.”

    Another factor that can lead to fatigue faster is poor welding practices, as well as other fabrication shortcomings. In one failure analysis published in Case Studies in Engineering Failure Analysis, engineers determined that a faulty weld joint where the tube met the tubesheet was the catalyst that resulted in failure. After careful review, the team identified a small welding defect, just 0.4 millimeters long, which was the first crack that would eventually lead to dozens of small fractures throughout the tube. With use, the cracks grew and propagated.

    In addition to the welding defect, issues with thermal expansion also caused serious stress on the tube-tubesheet joint. The report noted that it’s best to have expansion positions 15 or more millimeters away from the tube end to lessen the stress expansion would have on the tubesheet. In the failed exchanger, the expansion position was very close to the tube end, which likely caused even greater stress on the already faulty weld joint.

    Avoiding fatigue must begin with fabrication. Even a tiny 0.4-millimeter mistake can lead to severe damage to the exchanger. Additionally, understanding how to adjust your process to reduce stress within the equipment can help.

    2. Metal erosion

    Metal erosion in stainless steel shell and tube heat exchangers negatively impacts the quality of the equipment in two ways. First, as metal erodes away from the tubes, they become weaker and more susceptible to damage. Second, as the protective outer layer of the tubes wears down, the tubes face a higher risk of corrosion. Any corrosion already forming will only get worse as metal erosion takes place, according to Plant Engineering.

    Metal erosion can be caused by excessive speed of flow within the exchanger and by abrasive solids suspended in slurry streams. When a high-velocity stream is divided into thin, sharp jets of fluid upon entering the heat exchanger, it can also lead to metal erosion. In cases like this, the erosion pattern is horseshoe shaped and very localized. Additionally, high temperatures, such as those that allow for flash steam to occur, can also increase the risk of metal erosion. Plant engineers are most likely to see metal erosion occur on the bends of U-tubes or at the tube entrances.

    To prevent metal erosion, it’s important to understand the maximum velocity fluid within the exchanger can reach without causing harm to the components. This largely depends on the materials of construction, the fluid type and temperature, among other factors. Stainless steel fabrication, for example, can handle much higher velocities than a copper shell and tube heat exchanger. Other alloys that contain steel, stainless steel and copper-nickel combinations are also sturdy and can handle higher flow speeds.

    3. Thermal expansion

    As the fluids within the shell and tube heat exchanger transfer heat, the tubes and shell begin to heat up. Depending on the materials of construction, the temperature change, how fast it occurs and other factors, either the tubes, the shell or both may swell, a process called thermal expansion.

    Thermal expansion is fine, as long as you know how to prepare for it. If your exchanger is not equipped or built to handle thermal pressures that cause the metal alloys to widen, it can sustain extensive damage. Plant engineers will most commonly see thermal expansion in exchangers where the cold fluid that’s being heated is valved off, most often in steam-heated exchangers.

    In fixed tube heat exchangers, thermal expansion of the tubes can cause them to grow too large for the tubesheet, causing pull out, warped tubes or a damaged tubesheet. A U-tube shell and tube heat exchanger is a popular deterrent for the potential damages of thermal expansion because the side of the tubes with the U-bend is expected to absorb the heat that leads to swelling, thus saving the fixed side from the stress. However, that doesn’t mean this is an infallible strategy, Mechanical Design of Heat Exchangers explained. In process dealing with very high temperature changes, it’s important to consider all possibilities of thermal expansion and ensure the tubes can handle it.

    Thermal expansion can also be a problem when the fabrication material for the tubes is subject to faster expansion than the fabrication material for the shell. When the tubes swell but the shell doesn’t, it can cause severe damage to both the tubes and the shell.

    Preventing damage due to thermal expansion must begin with the heat exchanger design. First, it’s important to understand the properties of various alloys and how they react in certain conditions. As such, engineers building the exchanger must have a thorough idea of how the exchanger will be used, including information about planned temperatures and the types of fluids that will be introduced to the equipment. If it’s determined that thermal expansion is highly likely, the exchanger can be built to include an expansion joint.

    When designing and fabricating shell and tube heat exchangers, it’s important to have a full idea of what types of mechanical failure the equipment will be at risk for, and to take measures to prevent it. The engineers at Enerquip are well-versed in the many different design options for heat exchangers and can construct a high-quality exchanger for your operation. If you’re looking for the right exchanger for your process, reach out to Enerquip.

  2. 7 Shell Configurations to Consider When Designing a Shell and Tube Heat Exchanger

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    The shell is one of the most important components of a shell and tube heat exchanger, but it’s also one of the most complex elements to design, according to Chemical Engineering Progress. While the tubeside design typically deals with one flow stream, the shell has multiple instances of cross-flow and bypass streams.

    Choosing the right shell for your process will help optimize the rate of heat transfer. The Tubular Exchanger Manufacturers Association determines standards for fabricating shell and tube heat exchangers, and classifies shell styles into seven different types:

    TEMA E Shells

    TEMA E Shells
    The E-type shell is by far the most common design, included in more than half of all shell and tube heat exchangers. With this style, the shellside fluid enters the exchanger at the top of one end and exits through the bottom of the other. This one-pass design suits the majority of processes with a few exceptions, Thermopedia explained.

    “Most shell and tube heat exchangers use a single-pass E type shell.”

    TEMA F Shells

    TEMA F Shells

    When a process requires a countercurrent stream or a major temperature cross (when the cold stream exits the exchanger at a higher temperature than the hot stream’s outlet temperature), an F type shell can be the answer.

    This is a two-pass exchanger where fluid enters through one side, travels the entire length of the exchanger and doubles back around, leaving through a different outlet on the same side. The two passes are separated by a longitudinal baffle that spans the majority of the length of the shell, but ends before it reaches the far tubesheet. This allows the streams to make the double-pass.

    One common concern with F-type shells is leaking around the baffle. However, there can be steps taken in the design stage to make this less likely to occur.

    TEMA G and H Shells

    TEMA G and H Shells

    When the pressure drop needs to be kept low, as in horizontal thermosyphon reboilers, a split-shell design, or a G-type shell, is often employed. There are no baffles in a G-type exchanger and a single support plate is installed in the center of the shell.

    TEMA has specific rules about how long a shell can span unsupported in a shell and tube heat exchanger. The association states that the longest a shell can be unsupported is 1.5 meters, according to CEP Magazine. With just one support plate in a G-type shell, this means that these shells can’t be longer than 3 meters. However, there are applications where a split-shell design is needed at greater lengths. In these cases, engineers can turn to the H-type shell, also called the double-split flow exchanger. Essentially, an H-type shell is two G type shells put together. This means there are two areas where the flow is divided and then reunited, as well as two support plates.

    TEMA J Shells

    TEMA J Shells

    When the pressure drop within an E type shell is too large for the exchanger to handle, there are a few remedies engineers can try. First, adding double segmental baffles could help ease the pressure drop. If this doesn’t work, engineers often turn to the J type shell, also called the divided-flow shell.

    In a J type shell, the fluid enters at the center, splits into two streams (one going right and the other left) and ultimately leaves through two separate outlets. The streams are typically combined again outside the exchanger through a system of pipes. This model is also called a J 1-2 shell. In other models, the fluid can enter at two points, combine and ultimately leave in a single outlet nozzle. These may be referred to as a J 2-1 shell or an I type shell.

    Large pressure drops can cause tube vibration, a problem that could cause damage to the tubes, tubesheets and shell. Because of this, J-type shells are useful for reducing the risk of damage due to vibration.

    TEMA X Shells

    TEMA X Shells

    When an application calls for a pure cross flow design or one that has very low or virtually no pressure drop, the X type is generally best. These are most often used for condensers and gas coolers.

    “X-type shells are best when an application doesn’t permit any pressure drop.”

    With an X-type shell, fluid enters from one side and exits directly opposite. There may be as many inlet nozzles as needed. Having multiple inlets carefully spaced along the length of the exchanger more evenly distributes the fluid across the tubes. X-type shells can also include as many support plates as needed because these run parallel to the flow and won’t interfere with the heat transfer capabilities.

    TEMA K Shells

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    The final type of shell that TEMA defines is the K type, which stands for “kettle.” This style is almost exclusively used for kettle reboilers, though it could also be used for chillers. Similar to X-type shells, K models are cross-flow exchangers and allow for as many support plates as needed. They are built with an enlarged shell to allow for vapor disengagement so the shellside liquid carryover is minimized. When K type shells are used in chillers, the shellside fluid is boiled while the tubeside fluid cools.

    When designing the right shell and tube heat exchanger for your process, it’s important to know how different shell configurations will affect the rate of heat transfer, pressure drop, the risk of vibration and more. While the majority of processes can use the E type shell, there’s a chance that one of the other six designs suits your needs better. If you’re unsure, don’t worry: The helpful engineers at Enerquip can help determine exactly what you need.

  3. Tubeside or Shellside: Comparing Fluid Allocation Options for Your Shell and Tube Heat Exchanger

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    Every shell and tube heat exchanger will have two fluids: one on the tubeside and the other on the shellside. Chances are, you carefully considered your options for fluids and came to a conclusion based on the needs of your process and the properties of your choices.

    You should take the same thoughtful deliberation when deciding which fluid to put on the shellside and which to the tubeside. The location for each makes a big difference in the effectiveness of heat transfer, maintenance needs and the cost of the exchanger and any replacements.

    Tubeside or shellside? Ask yourself these five questions.

    1. Which fluid is more likely to foul?

    Preventing fouling is one of every shell and tube heat exchanger engineer’s main goals. Measures are taken in the design, materials of construction, fluid selection and cleaning schedule to reduce the chances of fouling. The same should go in decisions surrounding fluid allocation.

    Though fouling should be avoided, it will inevitably happen sooner or later. When it does, you want to make sure cleaning the exchanger is as easy as possible. In most shell and tube heat exchangers, cleaning the tubes is easier than the shell. Therefore, putting your fluid that’s more likely to foul on the tubeside is generally a smart idea.

    Of course, this depends on the heat exchanger design. U-tubes are typically more difficult to clean than straight tubes. However, it’s often easier to reach the shell’s interior for cleaning in these models. So, in exchangers with this configuration, the fouling fluid may be better off on the shellside.

    2. Which fluid is more corrosive?

    Corroded tubes are easier to replace than a corroded shell.If you’re working with corrosive fluids, it’s important to minimize their effect on the shell and tube heat exchanger. This starts with materials of construction – stainless steel fabrication tends to hold up better to common corrosive fluids than carbon steels.

    However, when working with these types of fluids, they will eventually take a toll on the metal. When that happens, you may need to replace the corroded parts of the exchanger. It’s generally easier and less expensive to replace the tubes. Replacing the shell often means replacing the whole exchanger. Because of this, putting the more corrosive fluid on the tubeside can help minimize damage and costs over time.

    Smart Process Design, a chemical engineering blog, pointed out that this also allows fabricators to use different materials of construction for the tubes and the shell, with the more resistant metal used for the tubes.

    3. What states are the fluids in?

    Fluids that are or will become vapors inside the heat exchanger should generally be allocated to the shellside because they have a larger volume and a lower heat-transfer coefficient. This will maximize its heat-transfer coefficient and lower the pressure drop.

    However, vapors that won’t condense may be better off on the tubeside so they won’t become lost or stagnant in the shell’s recesses, which can bring down heat transfer efficiency.

    For fluids that could solidify, it’s best to keep them on the tubeside. That way, should the fluid freeze up, the tubes can be mechanically cleaned, or removed and replaced if necessary. If the frozen fluid were on the shellside, it’d be very challenging to remove the tubes and could require an all-out replacement of the equipment.

    4. Which fluid has the highest pressure?

    High-pressure fluids generally need to be contained by thicker metal walls. Building thicker tubes will require less metal than building a thicker shell, and therefore is less expensive. As such, allocating your highest pressure fluid to tubeside can help keep exchanger costs down.

    5. How viscous are the fluids?

    Generally speaking, more viscous fluids should go on the shellside because these fluids typically have a low heat-transfer coefficient. Putting them on the shellside can maximize heat exchange efficiency. When using this strategy a staggered tube configuration can further improve heat transfer capabilities, as this will promote greater turbulence.

    Viscous fluids also tend to have a higher pressure drop. This further supports putting them on the shellside as a way to minimize the drop, but also necessitates some careful considerations. A fluid with a high pressure drop is more likely to bypass the baffles, which will hurt the heat transfer rate. It can also increase the probability for vibration issues. It’s important to get in front of vibration issues as early as possible, even making adjustments in the design phase as a preventative measure.

    Fluid allocation begins with design

    The right fluid allocation isn’t always obvious. There are multiple considerations to take into account, and some fluid combinations might pose conflicting factors; if both fluids would be better served on the shellside, you’ll still need to choose which one to put through the tubes.

    Knowing which fluids you’ll be using in your shell and tube heat exchanger ahead of the design phase can help immensely in fabricating a custom piece of equipment specifically made for your processes. This will help identify potential fouling, vibration or corrosion issues and work to prevent them from the very beginning.

    When you work with the engineers at Enerquip, you’ll be partnering with a knowledgeable team who loves to solve problems. Reach out to request a quote today.

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  4. Freezing Fouling in Shell and Tube Heat Exchangers: What you need to know

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    Fouling is a natural part of heat exchange. However, not all fouling is alike. Some types are more common but less damaging, and many can be anticipated long before installing an exchanger.

    One type of fouling that is relatively less common but potentially very damaging is freezing fouling. It’s important to understand this phenomenon, what causes it, how to prevent it and which measures to take when it does happen.

    What is freezing fouling?

    Freezing fouling, also called solidification fouling, occurs when the fluid inside the shell and tube heat exchanger seizes up and creates a block of substance that is difficult to remove. There are a number of reasons why this might happen.

    Every fluid has a freezing point, and it’s critical that those who work with heat exchangers and the fluids that go inside understand at which temperatures materials will freeze. Intuitively enough, one primary cause of freezing fouling is when the heat transfer surface falls below a fluid’s freezing point, Thermodedia explained.

    This might be the case when using a shell and tube heat exchanger to chill water. If the heat exchanger surface that is in contact with the water (i.e. the tubes if the water was entered on the tubeside) falls below 32 degrees Fahrenheit, the water will begin to freeze. How much it will freeze depends largely on the temperature difference between the tubeside and shellside fluids, among other factors. It could be a matter of a thin sheet of ice on the surface of the tubes (or shell, if the water is on that side), or a thicker mass of ice.

    Moist air can also freeze when coming into contact with a cold surface. Keep this in mind if you’ll be working with low temperatures and anticipate evaporation or mist to result from your process.

    Freezing fouling doesn’t necessarily mean the entire fluid will solidify. When using a solution, there may be various components with differing freezing points. Those ingredients that have relatively higher melting points could be challenging to keep liquid in certain processes. The solution may separate as a result, resulting not only in a partially frozen slurry, but also a liquid with different proportions of components than expected.

    Freezing fouling and crystallization fouling

    Crystallization fouling occurs when some solutes within a solution solidify and begin to accumulate on the heat transfer surface, wrote InTechOpen, an open access science, medical and technology book publisher. Depending on the solute and the conditions, people who work with shell and tube heat exchangers may refer to this phenomena with different terms, like:

    • Scaling, one of the most common, describes solid deposits that are very difficult to remove.
    • Sludge, softscale or powdery deposit describe softer, more porous, mushy or slimy deposits.

    Crystallization fouling and freezing fouling are two different events, but they do have a Venn diagram-like overlap when it comes to waxy deposits. When waxy hydrocarbons from a hotstream come in contact with the cold surface, waxy deposits can form on the heat transfer surface. These types of deposits may technically be crystallization fouling, but many people identify it as freezing fouling.

    Paraffin is one substance in particular that commonly results in a waxy precipitate, according to the Society of Petroleum Engineers’ PetroWiki. Naphthenic hydrocarbons, which like paraffin are found in crude oil, also causes wax-like deposits, but are much softer and referred to as “microcrystalline wax,” often accumulating at the the bottom of the vessel in a sludge-like substance. Since waxes have a high melting point – paraffin’s is generally between 104 and 158 degrees Fahrenheit – these deposits are often seen at ambient temperatures.

    Preventing freezing fouling in a shell and tube heat exchanger

    To avoid freezing fouling in your shell and tube heat exchanger, you must begin with understanding the fluids you’re using and how they respond to different environmental conditions, including temperature and pressure levels. Additionally, when working with solutions that contain solutes with varying freezing points, it’s critical to understand the properties of all components.

    When you know which substances you’re working with, their properties and expected behaviors, you can prevent freezing fouling by not creating the conditions in which they’ll solidify.

    With more complex substances, like crude oil, it’ll be more difficult to determine exactly which conditions will lead to the formation of solid materials. In the case of paraffin, engineers need to know the wax appearance temperature, also called the “cloud point” or “WAT.” This depends on many factors, including the weight and size of paraffin molecules, the ratio of water to oil, the composition of the oil and the presence of other substances that aid in solidification, among others.

    In some cases, freezing fouling could result from a malfunctioning component or an incorrect setting, according to an article for Plant Engineering. This may be the case if your chiller, condenser or evaporator freezes up when it’s not supposed to. In these cases, you may be unprepared for the event, and the ice formation can cause considerable damage if allowed to continue. Much like a pipe bursting during a cold winter, your tubes or shell could pop open with the pressure of the expanding ice.

    If all components are set and behaving as they should, taking precautions ahead of time could prevent a freeze-up. If you’re using antifreeze to prevent it but ice forms anyway, you may need to readjust your concentration of antifreeze. A thermal protection device or control system can also be advantageous. Finally, if you’re preparing your equipment for a seasonal shut-down in the winter, not properly and thoroughly draining your equipment can lead to freezing.

    Responding to freezing fouling

    In some cases, a certain process requires engineers to use substances that may solidify, risking the occurrence of freezing fouling. In these instances, it’s important to be prepared for if and when freezing fouling occurs so you can prevent further damage to the equipment.

    If your freezing fluid is on the shellside, you may be able to warm up the equipment using electric tracing, Chemical Processing noted. Heat exchangers exposed to cold environmental conditions can also be insulated to help prevent damage from the elements.

    However, if you know the fluid is one that would be very challenging to remove in this way, it may be better to allocate it to the tubeside. If it permanently solidifies with little or no hope of rescuing it from the exchanger, you’ll at least be able to remove the tube bundle for replacement; if an unmovable substance sets on the shellside, on the other hand, it’d be nearly impossible to take out. You may wind up needing to invest in an entirely new exchanger.

    If your freezing fouling consists of waxy deposits, you can generally remove these by melting, using steam, hot water or hot oil, or using chemicals to dissolve the wax.

    Freezing fouling may be a constant risk in your operation, or it may be an outlier event. If your equipment has sustained damage due to freezing and need a replacement part or a new custom shell and tube heat exchanger, reach out to the helpful engineers at Enerquip.

  5. What’s the difference between parallel flow, counter flow and crossflow heat exchangers?

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    A tube in pipe exchanger can have either a parallel or counter flow pattern.
    A tube in pipe exchanger can have either a parallel or counter flow pattern.

    One of the most critical factors in the efficiency and effectiveness of a shell and tube heat exchanger is flow pattern. Flow pattern refers to the direction that the tubeside fluid runs in relation to the shellside fluid. There are several distinct patterns engineers can choose from:

    • Parallel flow.
    • Counter flow.
    • Cross flow.

    But while each is unique and has its own pros and cons, there are many times when a combination of multiple flow patterns is beneficial, or even necessary. Here’s what you need to know about these three flow patterns for shell and tube heat exchangers:

    A parallel flow pattern, also referred to as a cocurrent flow, is one in which the shellside and tubeside fluids flow in the same direction. This is widely seen in double-pipe heat exchangers and can be replicated in shell and tube heat exchangers as well, according to Bright Hub Engineering.

    “The dramatic temperature difference at the inlet can cause thermal stress in parallel flow heat exchangers.”

    The inlet temperatures of the two fluids may be significantly different, but by the conclusion of the process they’re relatively the same temperature. With parallel flow, the wall temperatures throughout the exchanger will be more uniform than with other flow patterns, Thermopedia noted. When the goal is to wind up with two fluids that have a relatively insignificant temperature gap, a parallel flow shell and tube heat exchanger may be the ideal solution, Engineers Edge explained. When there is a notable temperature gap, the cold-fluid temperature will always be colder than the hot-fluid temperature, Marine Notes pointed out.

    It’s important to note that because the heat exchange rate isn’t as high as other flow patterns, parallel flow heat exchangers need greater heat transfer surface area. As such, it’s critical that facility executives ensure they have the available floor space for their parallel flow shell and tube heat exchangers, as they may be larger than another exchanger with a different flow rate.

    Additionally, the dramatic temperature difference at the inlet can cause thermal stress, which may result in vibrations that lead to equipment damage.

    Counter flow shell and tube heat exchangers

    A counter flow or countercurrent shell and tube heat exchanger’s construction is in many ways identical to that of a parallel flow shell and tube heat exchanger. The main difference is that the tubeside fluid enters the exchanger at the opposite end of the shellside fluid. This results in the two fluids running against each other rather than in the same direction.

    “The counter flow pattern in shell and tube heat exchangers is the most efficient.”

    The counter flow pattern is the most common in shell and tube heat exchangers, primarily because it’s the most efficient. This flow pattern allows for the greatest temperature change between fluids. Additionally, unlike in parallel flow exchangers, the cold-fluid can reach the hottest temperature of the hot-fluid since it exits at the end where the hot-fluid enters.

    The temperature difference in counter flow configurations is more uniform throughout the entire exchanger, which reduces thermal stress that can lead to shaking or motions that can become damaging to the equipment. Further, since the temperature difference is more consistent, the heat exchange rate is also more consistent throughout the exchanger.

    It’s easiest to achieve “true” counter flow, in which both fluids travel perfectly parallel to one another in opposite directions, in double-pipe heat exchangers, although shell and tube heat exchangers can get remarkably close. The baffles typically included in construction causes the shellside fluid to travel up and down within the shell as well as from the front to back (or vice versa), which lends to a somewhat cross flow pattern. The bigger the length-to-diameter ratio of the exchanger, the closer to “true” counter flow the exchanger will experience, Bright Hub Engineering pointed out.

    Crossflow shell and tube heat exchangers

    A crossflow heat exchanger is designed so that the two fluids flow perpendicular to one another. This is typically utilized when one fluid is a liquid and the other is a gas, as in a car radiator in which hot water flowing left and right is cooled by air moving up or down, Bright Hub Engineering explained. Crossflow exchangers are also common in steam condensers, in which a liquid transforms into a gas by the end of the process.

    Combining flow patterns in shell and tube heat exchangers

    In practice, shell and tube heat exchangers are highly complex pieces of equipment, which are integrated into complicated processes. As such, many heat exchangers utilize a combination of multiple flow patterns to accommodate limits related to:

    • Available space for the equipment.
    • Budget.
    • Fluid types to be used.
    • Pressure.
    • Temperature.
    • Weight of the equipment.

    A common combination is counter flow and cross flow, as seen in many multipass shell and tube heat exchangers. As the tubeside fluid flows back and forth between the two bonnets, the shellside fluid runs up and down, guided by baffles installed at regular intervals throughout the equipment. One advantage of this is maximizing the heat transfer rate while also minimizing the floorspace taken up by the exchanger.

    When you’re looking to either add or replace a shell and tube heat exchanger in your operation, it’s important to understand the significance of different flow patterns. The engineers at Enerquip are experts at determining the most beneficial flow pattern for various processes. Reach out to them to get advice, discuss a project or request a quote.

  6. How a pinch analysis can lead to energy cost savings

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    In today’s eco-conscious world, companies everywhere are taking a close look at their processes to determine just how green they are. Many find there are at least a few areas where they can improve.

    Companies may take a number of routes on their journey to greater energy efficiency. Sometimes, this includes incorporating new technology to cut costs, such as integrating a shell and tube heat exchanger to recover wasted heat. Another strategy is to re-evaluate fuel types used and adopt more renewable energy sources.

    While these are worthwhile efforts in many cases, there’s one method of increasing power efficiency and decreasing energy costs that’s often found to be the most effective, according to Oil and Gas Business. That method is ensuring your current equipment is working and being utilized to the best of its potential. A pinch analysis is a highly effective way to study the efficiency of current equipment and to design a new heat exchange system if the results show that improvement is needed.

    “A pinch analysis helps to determine the best locations for each piece of equipment.”

    Thermodynamics and the pinch point

    A pinch analysis uses the principles of thermodynamics; specifically, the first and second laws of thermodynamics, according to Mukesh Sahdev, writing for ChE Resources:

    The first law of thermodynamics

    The first law of thermodynamics states that heat is a form of energy, and energy cannot be created nor destroyed. Therefore, in all cases, including in a heat exchanger, heat cannot be created nor destroyed; only transferred.

    Sahdev writes that, in a pinch analysis, the first law relates to enthalpy changes in a heat exchanger system.

    The second law of thermodynamics

    The second law of thermodynamics is also known as the law of increased entropy. It states that even though the amount of heat (or energy) will always stay the same, the quality will change. Entropy relates to unusable energy inside a closed system.

    Sahdev explains that the second law relates to the direction of heat flow in an exchanger – it can only go from hot to cold.

    In a heat exchanger, the cold fluid will never be able to reach a temperature higher than the warmest point of the hot fluid stream. Likewise, a hot fluid stream can never be cooled to a colder temperature than the lowest point of the cold stream.

    Further, the hot stream can only be cooled to the “temperature approach,” or the minimum allowable temperature difference. This figure – the minimum allowable temperature difference – is also called the “pinch point.”

    What is a pinch analysis?

    A pinch analysis identifies the pinch point, which helps in determining the optimal size of a heat exchanger for a particular use. An analysis can also help companies recognize reasonable energy and capital cost targets. The whole idea is to determine the best locations for each piece of equipment; it’s kind of like solving a puzzle in your operation.

    There are a number of resources and technologies that companies can use to conduct a pinch analysis or to hire a professional to perform one in their facilities. Companies can also purchase pinch analysis software that will collect the necessary information. The company can then send that information to a third party to analyze it.

    Writing for Chemical Processing, Gary Faagau describes a do-it-yourself version of a pinch analysis. This is a good first step, as it avoids the time, labor and money required to conduct a formal pinch analysis.

    To begin, open a spreadsheet:

    1. In column A, list all streams in your plant that need to be heated, ordered from the lowest final target temperature to the highest.
    2. In column B, list all streams that need to be cooled, ordered from the lowest initial temperature to the highest.
    3. In column C, list all available utility heat (include steam pressures, fired heating and cost).

    Next, calculate your average heat capacity over the range of temperatures needed (as shown in columns A and B). You can use a heat exchanger simulation program to do this step.

    Then, you can begin analyzing which areas make the most sense for heat recovery. When doing this, prioritize heating or cooling capacity for your most important streams. If you don’t have a defined pinch point, you can assume an approach of 20 degrees Fahrenheit for a standard shell and tube heat exchanger, or a 10 or 15 degree approach for a more complex set of exchangers, Faagau noted.

    Benefits of a pinch analysis

    When formulating a process without a pinch analysis, engineers need to determine the design of the core process first, then the heat recovery system and finally the utility system. Each aspect of the overall design is created independently of each other.

    The pinch method, on the other hand, works with defined targets for each of the three components of the design. By taking into account the unique features of each part of the design as well as reasonable targets for each, engineers can identify opportunities for greater heat integration. Not only can this reduce energy costs, but can also contribute to a more useful heat exchanger design.

    “A pinch analysis can also contribute to a more useful heat exchanger design.”

    The pinch method was developed in the 1980s for the petrochemical industry. Its use has shown to be extremely helpful both in the planning of new facilities and the retrofitting of existing operations in various chemical engineering applications.

    Sahdev pointed out that companies have reported that pinch analyses have led to a:

    • 15 to 40 percent decrease in energy costs.
    • 5 to 15 percent reduction in capacity bottlenecks for retrofits.
    • 5 to 10 percent decrease in capital costs for new designs.

    Pinch analyses may be particularly useful for existing facilities, especially if they’ve been in operation for many years or have never been evaluated for efficiency. Considering that many petrochemical plants have been in operation since the 1960s or 1970s, as noted by Oil and Gas Business, a pinch analysis could be worth the time.

    Despite the many benefits of a pinch analysis, there are a few obstacles companies will need to overcome to make the most of them. First, they’re time-consuming and involve complex calculations. If a company decides that it would be best to conduct a formal pinch evaluation, it can be a high upfront expense. However, the savings and reduced energy expenditure may justify the cost.

    If you find that there are areas where a new or redesigned custom shell and tube heat exchanger would benefit your process, reach out to the helpful engineers at Enerquip. They’re experts at solving problems and making the best use of their exchangers.

  7. Preventing Cross Contamination in Your shell and tube heat exchangers

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    Cross contamination is a shared concern at all stages of the food industry. Chefs need to make sure their fresh veggies are kept away from their raw meat; storage facilities strive to keep common allergenic foods like nuts separate from other ingredients; and food production plants must ensure products sent through their process equipment isn’t affected by harmful bacteria, chemicals or other items.

    Shell and tube heat exchangers are popular in food production plants as a method to pasteurize fruit, vegetable or dairy products, or to achieve a desirable consistency, as in honey or maple syrup production. Cross contamination is also a risk factor in industries like pharmaceuticals and personal care.

    It’s important that these important pieces of equipment don’t contribute to any form of cross contamination. If this were to occur, it could reduce plant efficiency, lead to a ruined batch of product or necessitate a recall. There are many different ways to reduce the chances of cross contamination in your food or pharmaceutical production facility. Here are a few:

    Work with quality equipment fabricators

    The materials used in the construction of your shell and tube heat exchangers play an important role in the quality, sanitation, cleaning requirements and lifespan of your equipment. Many food industry companies turn to stainless steel for its fouling resistance.

    Choosing a stainless steel shell and tube heat exchanger is therefore a good step toward preventing cross contamination in your facility. However, you can take this one step further by finding out what sort of environment in which your shell and tube heat exchanger is fabricated.

    Cross contamination isn’t just limited to food items; you can also cross-contaminate metals. As such, it’s worthwhile to find out if your stainless steel shell and tube heat exchanger is being made in a facility that also utilizes carbon steel. If it is, there’s always a chance that this metal, which is more prone to fouling, can contaminate your equipment.

    At Enerquip, we value the integrity of stainless steel, which is why we don’t work with carbon steel. When you receive one of our heat exchangers, you can feel confident that it hasn’t been affected by this metal.

    Strategically choose your tubes

    When cross contamination does occur in a shell and tube heat exchanger, it may be caused by the shell-side fluid mixing with the tube-side fluid. To prevent this from happening, added barriers or an adjusted tube design can help.

    Enerquip’s high purity shell and tube heat exchangers are fitted with double tubesheets, which reduces the risk of cross contamination of this type. These custom and standard pharma-grade exchangers are particularly useful for pharmaceutical, nutraceutical, animal health and personal care industries.

    Double tube sheet configurations typically have a form of leak detection installed in the exchanger. If a leak were to occur in these models, the fluid should drain away from the exchanger and into a safety container rather than mixing with the other fluid, and alerts the operator that there is an issue to repair.

    Understand pressure differentials

    The engineers who create shell and tube heat exchangers must understand many complex formulas to know how the equipment will behave once it’s put to use. The pressure differential, or the difference between the pressures inside the exchanger, is an important one that relates to the likelihood of cross contamination. Typically, the pressure on the shellside would be less than inside the tubes. That way, if a leak springs, the product will flow into the heat transfer medium, rather than the medium mixing into the product and entering the tubes. This helps to keep the negative effects of a cross contamination incident as low as possible.

    Regular inspections and cleaning

    If there’s a chance of cross contamination in your equipment, it’s best to know sooner than later. Periodic visual inspections is the first step in identifying weak points and emerging problems that could lead to contamination, Business Standard pointed out. In your inspections, you might see early signs of leaks in your tubesheet or gaskets. If you catch this early, you may be able to replace or repair the damage before it leads to mixing fluids.

    You may also see early signs of fouling. If fouling is allowed to continue for too long, it can lead to spoiled product. If you do, you’ll want to clean the exchanger and determine whether you can make any changes to your process to prevent fouling. This might mean exploring new options for heat transfer fluids, cleaning more frequently or changing your sanitation methods.

    Whether you’ve experienced cross contamination at your facility or simply want to ensure you’re doing everything you can to prevent it, strategically choosing your shell and tube heat exchanger and making sure it’s kept in good condition can go a long way to help your efforts.

    To learn about Enerquip’s stainless steel shell and tube heat exchangers, reach out to our knowledgeable engineers.

  8. What you need to know about cleaning different tube configurations

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    When considering your options for a new shell and tube heat exchanger, one important factor is the tube configuration.

    Various options benefit different types of processes. For example, a floating head configuration is better suited to processes prone to significant thermal expansion because the tubes aren’t constrained by the tube sheet or the shell, and can therefore expand or vibrate without risking damage to the rest of the equipment.

    Beyond taking into account the intended use for the exchanger, and other elements like location of the exchanger or the product that will be introduced to it, it’s also a good idea to think about cleaning methods. Not all cleaning strategies are appropriate for all configurations, but all exchangers will need to be properly and thoroughly cleaned sooner or later. It’s best to know what cleaning capabilities you’ll have with a particular configuration beforehand so you can factor it into your decision, or at least prepare for new sanitation needs.

    How to Clean a Fixed Tubesheet Shell and Tube Heat Exchanger

    A fixed tubesheet is a popular shell and tube heat exchanger design for several reasons, including cost effectiveness and ease of cleaning. Since the tubes are straight and the tubesheet is welded straight to the shell, construction is relatively simple.

    To clean a fixed tubesheet shell and tube heat exchanger, the bonnet first needs to be removed. This is a relatively simple task with this configuration. The insides of the tubes can be cleaned mechanically, and the straight configuration makes it easy for brushes, hoses or other cleaning supplies to be fed into the bores. The tubes can also be cleaned chemically, and running the cleaning solution through the tubes is fairly easy, again, because of the straight design.

    While cleaning the tubeside is pretty straightforward, shellside cleaning is a bit more complicated with fixed tubesheet designs. Because the tubesheet is welded to the shell itself, it’s nearly impossible to mechanically clean the outsides of the tubes. Chemical cleaning must be done instead. However, it’s critical that operators are confident that the chemical cleaning agent can be thoroughly rinsed from the shellside before operation reconvenes. Leftover residue can damage the material of construction or contaminate the product.

    The bonnet type plays a role in how easy it is to reach the tubes. L-type and N-type bonnets, which have removable covers, grant easy access to the inside of the tubes without removing any piping. The M-type bonnet does not have this removable cover, which means the entire head needs to be taken off to access the tubes.

    The difficulty in shellside cleaning isn’t always a problem. If the shellside of this heat exchanger is only used for clean fluids rather than fouling services, there’s virtually no need for future cleaning.

    How to Clean a U-Tube Shell and Tube Heat Exchanger

    As the name suggests, the tube bundle of a U-tube exchanger is curved at the end and returns the fluid back to the same side it entered, rather than providing a point of exit on the opposite end of the exchanger. Thus, only one tubesheet is required, leaving the other end free to expand or vibrate without risking damage to the rest of the construction.

    While the U-shaped bend provides benefits in some ways, it becomes cumbersome when it comes time to clean the equipment. The curve at the end of the tube makes it challenging for mechanical cleaning, unless a flexible-end drill shaft is utilized. Chemical cleaning is possible, but certain types of fouling, make it challenging – particularly scaling that hardens to the sides of the tubes and is difficult to remove without physical force. Additionally, with scales forming at the point of the bend, it may be difficult to assess whether all fouling has been completely removed. The solution to this dilemma is to use clean fluids on the tubeside with this configuration, Thermopedia pointed out.

    An articulating brush is advantageous for cleaning U-tubes.

    While cleaning the interior of the tubes on U-tube exhchangers is a challenge, the shellside is very easy. Since there’s only one tubesheet, deconstruction is simple. Once removed, the shell and the outside of the tubes can be cleaned easily.

    How to Clean a Floating Head Shell and Tube Heat Exchanger

    The floating head tube bundle configuration is the best of both worlds. Only one end of the two tubesheets is welded to the shell, allowing the other to expand as needed according to the process it’s engaging in, similar to the U-tube configuration. Meanwhile, the straight tube design makes cleaning easier, comparable to the fixed tubesheet configuration.

    These advantages make floating head shell and tube heat exchangers a favorite among operators who are concerned both about thermal expansion as well as fouling on both sides, such as petroleum refineries or kettle reboilers, for example.

    A number of methods can be employed to sanitize floating head shell and tube heat exchangers and remove fouling. Mechanical cleaning is a practical solution, as the straight tubes make it easy for brushes, bits and sprayers to reach all areas of the bores. The floating head configuration makes it easier to remove the tube bundle than with the fixed tubesheet design, so it’s easy to reach the outsides of the tubes and the interior of the shell.

    Chemical cleaning is also a possibility, especially because it’s easy to spot inconsistencies in the cleaning job. When insufficiently cleaned areas are identified, they can be mechanically or chemically cleaned again before the equipment is put back into operation.

    The bonnet type associated with a particular exchanger’s construction plays a role in how easy this configuration can be cleaned. A P-type rear header, which is an outside packed header, gives convenient access to the tubeside but does not allow the tube bundle to be removed so the shellside can be difficult to clean.

    The S-type header also allows the tube bundle to be removed, but it is hard to take apart for bundle pulling, which can cause some complications when it’s being cleaned, inspected or repaired. The T-type header is easier to dismantle and remove than the S-type, often making it a more desirable configuration, though it also tends to be a bit pricier. The W-type header is also easy to remove and is often the least expensive of the options for a floating head heat exchanger.

    Proper Cleaning Prevents Fouling

    No matter what type of shell and tube heat exchanger you have, it’s important to know how to properly clean it to prevent fouling and ensure deposits left behind won’t cause corrosion.

    To learn about the right configuration for your operation, reach out to the helpful engineers at Enerquip

    More from the Enerquip Blog

    Preventing Cross Contamination in Your shell and tube heat exchangers

    Five Important Qualities to Look for in Pharmaceutical Process Equipment

    Pharmaceutical manufacturers must meet ASME-BPE standards

    Closed-loop process cooling can help reduce water, energy use in pharmaceutical manufacturing

    Enerquip Pharma Exchangers meet cGMP’s

  9. 3 possible causes for flange leaks and how to fix them

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    Shell and tube heat exchangers are integral pieces in a processing operation and must be fully operational at all times. However, there are a number of things that can go wrong when operating such highly pressurized equipment – such as leaks.

    Leaks are never OK in process equipment. Depending on the exchanger, where it is and what it’s used for, leaks can decrease productivity; impact a company’s bottom line; contaminate product or heat transfer fluid; create a safety or environmental hazard; or increase the risk of fire. As such, any leakage from a shell and tube heat exchanger must be addressed soon after it’s detected.

    One common area equipment operators observe leakage is around the flanges of a shell and tube heat exchanger. Flanges have three main components: a ring, a gasket and bolts to hold the flange together and secure it to the exchanger. When a flange leak occurs, it’s typically associated with the gasket – perhaps it’s not installed correctly or the wrong size is used – but that’s not always the case.

    When leaks happen, it’s important to identify them quickly, then determine the cause. Here are several possible causes of flange leaks in shell and tube heat exchangers:

    Thermal stress causing distortion

    The temperatures of product inside a shell and tube heat exchanger can vary dramatically; that was, after all, what the exchanger was designed for. However, distortion caused by the rapid change from hot to cool solutions is inevitable in some cases. If this is the case, it’s important to ensure all parts of the exchanger are designed to account for temperature distortion.

    “Take temperature fluctuations into account during the design phase to prevent future leaks.”

    Writing for Chemical Engineering Magazine, mechanical design engineer Pankaj Kumar Singla pointed out that American Petroleum Institute standard 660 outlines temperature limits and when they become a major factor to consider when designing shell and tube heat exchanger flanges; however, these standards are only a guideline. There are cases when an operation should take temperature fluctuations into account during the design phase, even when they have not met API 660.

    To determine if special consideration should be made regarding the flanges, Singla suggested first determining the temperature inlet and outlet for the tube side. Next, determine the temperature for a random area within the tube as well as a random area on the shell side. Then, calculate the difference between the tubeside inlet and outlet as well as the difference between the random area on the tubeside and that of the shellside. If either equation results in a temperature differential of more than 110 degrees Celsius, be sure to take this into account.

    Singla noted several strategies and design adaptations that may be beneficial in these cases:

    • Increasing the thickness of the flange and tubesheet.
    • Reduce the allowable flange rigidity index.
    • Reduce the allowable stresses for flanges and tubesheets.
    • Make the required bolt ratio 120 percent of design and perform full bolt-load calculations.

    The bolts aren’t secure anymore

    When your exchanger is in operation, there may be vibrations, temperature increases and high pressure, all of which can affect the security of your bolts. Over time, the bolts may loosen and cause the gasket to leak. Of course, the obvious answer is to tighten the bolts again and proceed with your operations. However, it’s important to consider the method by which you tighten the bolts, as well as how often you need to tighten them.

    Torquing is the typical method of tightening bolts when they become loose. Dennis Martens and Michael Porter wrote for Penn State that hot torquing and hydraulic tensioning are two methods that can be more effective in tightening bolts and keeping them secure.

    If you find that you’re tightening your bolts continuously with seemingly no effect on leakage, there could be another factor at play. The American Society of Mechanical Engineers pointed out that, when equipment has excessive paint over or around the bolts, this could reduce the bolt integrity. As the paint degrades, it can reduce the bolt load and cause leaks to occur, for example.

    Other times, the design of the shell and tube heat exchanger simply isn’t adequate for the stress within, and an addition to the equipment may be required to mitigate the problem.

    Additional hardware is needed

    Martens and Porter discussed in their report “Investigation and Repair of Heat Exchanger Flange Leak” a recurring problem in a shell and tube heat exchanger, whose bolts never seemed to stay secure, despite efforts to tighten them using proven techniques. Even after hot torquing and hydraulic tensioning, the bolts would loosen and cause problematic leaking in the plant.

    The researchers identified two problems: excessive bolt load and gasket scuffing. The excessive bolt load was caused by the temperature differential. To combat this issue, disc spring washers were installed before replacing the bolts. They found that these were more capable in handling the load to the bolts without allowing flange deformation.

    The second issue of gasket scuffing was likely due to movement during operation and caused enough damage to the gasket that, even with adequate bolt tightness, it would continue to leak. The gasket needed to be replaced, but the problem also needed to be addressed so the new gasket would not also fall into disrepair. To avoid this, a weld ring, which consists of two separate halves of the gasket ring being welded together to contain the gasket material and prevent deformation. This creates a sealed gasket.

    If you’re experiencing gasket leaks in your shell and tube heat exchangers, it could be an indication that something is amiss in your equipment. The engineers at Enerquip are happy to help you get to the bottom of the problem and help devise a solution for your process equipment.

    Contact us today.