In order to achieve optimum process operations, it is essential to use the right type of process equipment in any given process. Heat exchangers, commonly used to transfer energy from one fluid to another, are no exception.
The selection of the proper type of heat exchangers is of critical importance. Selecting the wrong type can lead to sub-optimum plant performance, operability issues and equipment failure.
The following criteria can help in selecting the type of heat exchanger best suited for a given process:
- Application (i.e. sensible vapor or liquid, condensing or boiling)
- Operating pressures & temperatures (including startup, shutdown, normal & process upset conditions)
- Fouling characteristics of the fluids (i.e. tendency to foul due to temperature, suspended solids ...)
- Available utilities (cooling tower water, once through cooling water, chilled water, steam, hot oil...)
- Temperature driving force (i.e. temperature of approach or cross and available LMTD)
- Plot plan & layout constraints
- Accessibility for cleaning and maintenance
- Considerations for future expansions
- Mechanical considerations such as: 1) material of construction; 2) thermal stresses (during startup, shutdown; process upset and clean out conditions); 3) impingement protection
Shell-and-tube heat exchangers accounts for more than 50% of all heat exchangers installed. However, in many cases, there are more attractive alternatives in terms of cost and energy recovery. Any time a heat exchanger is being replaced, the opportunity should be taken to re-assess if the type used is best for the given process. Operating changes since initial installation as well as advancements in the field of heat transfer may point towards a different type as being optimal.
Heat Exchangers Types
Shell & tube heat exchangers
Baffle types
Segmental baffles
Double segmental baffles
No-tube-in-window (NTIW) baffles
Rod baffles
EM baffles
Helical baffles
Tube Enhancements
Twisted tubes
Low finned tubes
Tubes inserts (twisted tapes, Cal Gavin)
Compact type heat exchangers
Plate & frame heat exchangers (gasketed, semi-welded, welded)
Spiral
Blazed plate & frame
Plate-fin heat exchanger
Printed circuits
Air-cooled heat exchangers
Heat Exchangers Selection
Past experience, is always the best place to start to guide the selection of heat exchanger types. Understanding the reasons behind both successes and failures will lead to better equipment selection.
When comparing different types of heat transfer equipment, one must take into consideration the total cost of the equipment which includes:
1. purchase cost
2. installation cost
3. operating cost (pumping, fan…)
4. maintenance cost
In order to make the best selection, it is important to have some knowledge of the different types of heat exchangers and how they operate. The tables below offer the advantages and disadvantages of common types of heat exchangers. They can be used to arrive at a type that is best suited for a given process.
Shell & tube heat exchangers
Shell & tube heat exchangers
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Advantages
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Disadvantages
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Widely known and understood since it is the most common type
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Less thermally efficient than other types of heat transfer equipment
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Most versatile in terms of types of service
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Subject to flow induced vibration which Can lead to equipment failure
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Widest range of allowable design pressures and temperatures
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Not well suited for temperature cross conditions (multiple units in series must be used)
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Rugged mechanical construction - can withstand more abuse (physical and process)
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Contains stagnant zones (dead zones) on the shell side which can lead to corrosion problems
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Subject to flow mal-distribution especially with two phase inlet streams
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Compact Heat Exchangers
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Advantages
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Disadvantages
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Low initial purchase cost (plate type)
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Narrower rage of allowable pressures and temperatures
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Many different configurations are available (gasketed, semi-welded, welded, spiral)
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Subject to plugging/fouling due to very narrow flow path
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High heat transfer coefficients (3 or more times greater than for shell & tube heat exchangers, due to much higher wall shear stress)
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Gasketed units require specialized opening and closing procedures
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Tend to exhibit lower fouling characteristics due to the high turbulence within the exchanger
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Material of construction selection is critical since wall thickness very thin (typically less than 10 mm)
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True countercurrent designs allow significant temperature crosses to be achieved
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Require small footprint for installation and have small volume hold-up
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Air Cooled Heat Exchangers
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Advantages
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Disadvantages
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Attractive option for locations where cooling water is scarce or expensive to treat
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High initial purchase cost
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Well suited for cooling high temperature process streams (above 80oC when using cooling water should be avoided)
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Require relatively large footprint
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Low maintenance and operating costs (typically 30-50% less than cooling water)
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Higher process outlet temperature (10-20 oF above the ambient dry bulb temperature)
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Shell-and-Tube Heat Exchangers Construction Details:-
The shell-and-tube heat exchanger is named for its two major components – round tubes mounted inside a cylindrical shell.
The shell cylinder can be fabricated from rolled plate or from piping (up to 24 inch diameters). The tubes are thin-walled tubing produced specifically for use in heat exchangers.
Other components include: the channels (heads), tubesheets, baffles, tie rods & spacers, pass partition plates and expansion joint (when required). Shell & tube heat exchanger designs and constructions are governed by the TEMA and ASME codes.
Tubes :-
Tubing may be seamless or welded. Seamless tubing is produced in an extrusion process; welded tubing is produced by rolling a strip into a cylinder and welding the seam. Welded tubing is usually more economical.
Normal tube diameters are 5/8 inch, 3/4 inch and 1 inch. Tubes of smaller diameter can be used but they are more difficult to clean mechanically. Tubes of larger diameter are sometimes used either to facilitate mechanical cleaning or to achieve lower pressure drop.
The normal tube wall thickness ranges from 12 to 16 BWG (from 0.109 inches to 0.065 inches thick). Tubes with thinner walls (18 to 20 BWG) are used when the tubing material is relatively expensive such as titanium.
Tubing may be finned to provide more heat transfer surface; finning is more common on the outside of the tubes, but is also available on the inside of the tubes. High flux tubes are tubing with special surface to enhance heat transfer on either or both sides of the tube wall. Inserts such as twisted tapes can be installed inside tubes to improve heat transfer especially when handling viscous fluids in laminar flow conditions. Twisted tubes are also available. These tubes can provide enhanced heat transfer in certain applications.
Tubesheets:-
Tubesheets are plates or forgings drilled to provide holes through which tubes are inserted. Tubes are appropriately secured to the tubesheet so that the fluid on the shell side is prevented from mixing with the fluid on the tube side. Holes are drilled in the tubesheet normally in either of two patterns, triangular or square.
The distance between the centers of the tube hole is called the tube pitch; normally the tube pitch is 1.25 times the outside diameter of the tubes. Other tube pitches are frequently used to reduce the shell side pressure drop and to control the velocity of the shell side fluid as it flows across the tube bundle. Triangular pitch is most often applied because of higher heat transfer and compactness it provides. Square pitch facilitates mechanical cleaning of the outside of the tubes.
Two tubesheets are required except for U-tube bundles. The tubes are inserted through the holes in the tubesheets and are held firmly in place either by welding or by mechanical or hydraulic expansion. A rolled joint is the common term for a tube-to-tube sheet joint resulting from a mechanical expansion of the tube against the tubesheet. This joint is most often achieved using roller expanders; hence the term rolled joint. Less frequently, tubes are expanded by hydraulic processes to affect a mechanical bond. Tubes can also be welded to the front or inboard face of the tubesheet. Strength welding designates that the mechanical strength of the joint is provided primarily by the welding procedure and the tubes are only lightly expanded against the tubesheet to eliminate the crevice that would otherwise exist. Seal welding designate that the mechanical strength of the joint is provided primarily by the tube expansion with the tubes welded to the tubesheet for better leak protection. The cost of seal-welded joints is commonly justified by increased reliability, reduced maintenance costs, and fewer process leaks. Seal-welded joints are required when clad tubesheets are used, when tubes with wall thickness less than 16 BWG (0.065 inch) are used, and for some metals that cannot be adequately expanded to achieve an acceptable mechanical bond (titanium and Alloy 2205 for instance).
Baffles:-
Baffles serve three functions: 1) support the tube; 2) maintain the tube spacing; and 3) direct the flow of fluid in the desired pattern through the shell side.
A segment, called the baffle cut, is cut away to permit the fluid to flow parallel to the tube axis as it flows from one baffle space to another. Segmental cuts with the height of the segment approximately 25 percent of the shell diameter are normally the optimum. Baffle cuts larger or smaller than the optimum typically result in poorly distributed shell side flow with large eddies, dead zones behind the baffles and pressure drops higher than expected.
The spacing between segmental baffles is called the baffle pitch. The baffle pitch and the baffle cut determine the cross flow velocity and hence the rate of heat transfer and the pressure drop. The baffle pitch and baffle cut are selected during the heat exchanger design to yield the highest fluid velocity and heat transfer rate while respecting the allowable pressure drop.
The orientation of the baffle cut is important for heat exchanger installed horizontally. When the shell side heat transfer is sensible heating or cooling with no phase change, the baffle cut should be horizontal. This causes the fluid to follow an up-and-down path and prevents stratification with warmer fluid at the top of the shell and cooler fluid at the bottom of the shell. For shell side condensation, the baffle cut for segmental baffles is vertical to allow the condensate to flow towards the outlet without significant liquid holdup by the baffle. For shell side boiling, the baffle cut may be either vertical or horizontal depending on the service.
Other types of baffles are sometimes used such as: double segmental, triple segmental, helical baffle, EM baffle and ROD baffle. Most of these types of baffles are designed to provide fluid flow paths other than cross flow. These baffle types are typically used for unusual design conditions. Longitudinal baffles are sometimes provided to divide the shell creating multiple passes on the shell side. This type of heat exchangers is sometimes useful in heat recovery applications when several shell side passes allow to achieve a severe temperature cross.
Tie Rods and Spacers:-
Tie rods and spacers are used for two reasons: 1) hold the baffle assembly together; and 2) maintain the selected baffle spacing. The tie rods are secured at one end to the tubesheet and at the other end to the last baffle. They hold the baffle assembly together. The spacers are placed over the tie rods between each baffle to maintain the selected baffle pitch. The minimum number of tie rod and spacers depends on the diameter of the shell and the size of the tie rod and spacers.
Channels (Heads) :-
Channels or heads are required for shell-and-tube heat exchangers to contain the tube side fluid and to provide the desired flow path.
Many types of channels are available. The three (3) letters TEMA designation is the standard method for identifying the type of channels and the type of shell of shell-and-tube heat exchangers. The first letter of the TEMA designation represents the front channel type (where the tube side fluid enters the heat exchanger), the second letter represents the shell type and the last letter represents the rear channel type. The TEMA channel types are shown below.
The channel type is selected based on the application. Most channels can be removed for access to the tubes. The most commonly used channel type is the bonnet. It is used for services which do not require frequent removal of the channel for inspection or cleaning. The removable cover channel can be either flanged or welded to the tubesheet. Flanges are usually not provided for units with larger shell diameters. The removable cover permits access to the channel and tubes for inspection or cleaning without the need to remove the tube side piping. Removable cover channels are provided when frequent access is required.
The rear channel is often selected to match the front channel. For example a heat exchanger with a bonnet at the front head (B channel) will often have a bonnet at the rear head (M channel) and will be designated as BEM. However, there can be circumstances where they are different such as when removable bundles are used.
Pass partitions are required in channels of heat exchangers with multiple tube passes. The pass partition plates direct the tube side fluid through multiple passes. The number of tube side passes is normally less than eight, although more than eight passes can in some cases be required. Multiple tube passes allow to maximize the tube side heat transfer within the pressure drop constraint. Typically, heat exchangers with liquid as the tube side fluid have multiple tube passes. Most heat exchangers with large tube side volumetric gas flow rates have a single tube pass.
Typical Applications:-
The shell-and-tube heat exchanger is by far the most common type of heat exchanger used in industry. It can be fabricated from a wide range of materials both metallic and non-metallic. Design pressures range from full vacuum to 6,000 psi. Design temperatures range from -250oC to 800oC. Shell-and-tube heat exchangers can be used in almost all process heat transfer applications.
The shell-and-tube design is more rugged than other types of heat exchangers. It can stand more (physical and process) abuse. However, it may not be the most economical or most efficient selection especially for heat recovery applications or for highly viscous fluids. The shell-and-tube heat exchanger will perform poorly with any temperature crosses unless multiple units in series are employed.
Common Operating Problems for Air-coolers:-
Air-cooled heat exchangers (ACHE) are commonly used in industry. They offer definite advantages in certain types of applications. However, due to their use of atmospheric air, air-cooled heat exchangers experience operating problems not encountered in other types of heat exchangers. We present here some of the more common operating problems with air-coolers.
Reduced Air Flow Rate:-
Air flow is the single most important variable in the operation of air-cooled heat exchangers. In continuous processes, the heat load on an ACHE generally remains fairly constant while the air flow is increased or decreased based on the ambient air temperature. There are a variety of reasons why air-coolers may experience reduced air flow (see below). When an air-cooler experiences reduced air flow, its cooling capacity is reduced and it is during warm summer days when the impact on production is most often seen. Below is a list of several causes of reduced air flow and possible solutions.
Dirty Tube Bundles:-
Air flow is directly related to pressure drop. When the pressure drop through the tube bundle of an air-cooler increases, the air flow decreases. The most likely cause of increased pressure drop is a dirty tube bundle. As the tube bundle gets fouled over time, the pressure drop gradually increases leading to reduced air flow around the tubes. Tube bundles can become plugged with leaves, paper or poplar fluff pollen.
The most efficient way to determine if an air-cooler is dirty and experiencing reduced air flow is to develop an air flow profile using an anemometer. The anemometer is used to measure the air velocity at multiple locations and the data can be analyzed to display air volumetric profiles and overall air flow. Figure 1 below shows data generated using an anemometer. The most useful way to use the data measured from the anemometer is to compare the current air profile to an existing baseline profile taken for the same fan when first installed or after cleaning. The two profiles are then compared to determine if the overall air flow has dropped significantly over time. Reverse air flow at the tip of the fan blades (shown by negative air flow values on Figure 1) would be an indication of a fouled air-cooled heat exchanger. Reverse air flow is caused by excessive pressure drop through the tube bundles which leads the air to flow back around to the suction side of the blade.
Figure 1
Once the cleaning is complete, a second set of air flow measurements should be taken for two reasons: 1) the new air flow profile can serve as the clean baseline to evaluate future performance; 2) if the air profile still shows reverse air flow at the fan tips or fan hub, it could be an indication that other problems still exist (see Reverse Flow below).
The required cleaning frequency for an air-cooler will depend greatly on its location. Some ACHE will require frequent tube bundle cleaning while others may never need to be cleaned. Air flow measurements are a non-intrusive way to determine when the tube bundle of an air-cooler needs to be cleaned. Figure 2 shows air flow profiles before and after cleaning. As can be seen, cleaning can significantly increase the air flow through and air-cooler and lead to improved performance.
Figure 2
Reverse Flow:-
Reverse flow is a common problem in older air-coolers. This misdirected flow causes two problems for the heat exchanger: 1) the obvious one is a net loss in the amount of air that travels through the tube bundle to provide cooling; 2) the secondary effect is that when the air flow returns to the suction side of the fan, it is again sucked up and creates artificially high inlet air temperatures which ultimately lead to less heat transfer capacity.
There are two areas where reverse flow is most prevalent. The most common one is at the tip of the fan blade where it meets the plenum housing. Over time or with incorrect installation, a gap can be found which will increase the amount of air flow that loops around the blade and travels back to the fan suction side. This gap should be approximately 3/8” but not greater than 3/4” as per API-660. Reverse flow can be detected by measuring the fan tip gap but the recommended way to determine if reverse flow is present is to conduct an air velocity profile (shown above in Figure 2) and look for a negative air flow number. Again, it is better to look at the air profile from an exchanger immediately after it was cleaned. If the exchanger is not clean, the high pressure drop caused by the dirt could lead to reverse flow which could be eliminated with a simple cleaning. If reverse flow exists after cleaning, the fix is to install tip a seal on the plenum which will eliminate the gap.
A less common form of reverse flow in ACHE occurs if there is a gap in the area above the motor or hub. Air will loop back through the center of the fan blade and be caught in a recycle. This problem will again be obvious if an air flow profile is taken. The fix to this problem is more complicated and costly but in most cases installing a hub seal will eliminate this problem.
Blade Pitch:-
ACHE fans can have fixed or adjustable pitch blades. Adjustable pitch blades are most often used and the adjustment can be either manual or automatic. The blade angle on manually adjusted pitch fans can only be changed when the air cooler shutdown. Automatically adjusted fan blades can be rotated to various angles while the air cooler is in operation. Newer air-cooled heat exchangers are usually provided with manually adjusted fan blades and use variable speed motors to provide the required air flow variability. Blades require an initial angle setting to achieve optimum performance. Quite often, automatically adjusted fan blades get stuck after some time and the air flow variability from the variable blade pitch angle is not longer available for process control.
A common problem with air-coolers is improper blade pitch angle. This problem may result from efforts to decrease energy usage by reducing the fan motor load. If the blade pitch is set low to reduce the motor load, the air flow may be too low to provide the desired cooling. On the other hand, if the blade pitch is set too high the load on the motor may be too high and the motor may stall or burn out. Typically, the optimum blade pitch angle is in the range of 12 and 17 degrees. It is always best to refer to the manufactures specifications to set the optimum blade pitch angle. Generating air flow profiles can help narrow in on the optimum blade pitch angle.
Motor Amps:-
A related problem that is often encountered is motor not running near their full load amps (FLA). To optimize peak air flow and heat transfer, fan motors should operate near their full load amp (FLA) set point. If a motor is running below 70% of FLA, adjusting either the motor or the blade pitch angle to increase the air flow will lead to better performance. It is preferred to have the %FLA at or above 85.
Mechanical Integrity
The overall condition of an ACHE can greatly reduce its ability to transfer heat. The following list contains common areas where air-coolers can experience minor problems that are relatively easy to fix.
Louvers
Air-coolers sometimes use louvers to control the outlet temperature by throttling the air flow. Missing or inoperable louvers are a common problem. Louvers should be inspected periodically to validate that the actuators are working properly. At full open, the louvers should be at least 50% to 60% open to allow unimpeded air to travel through the tube bundle.
Plenum
Another structural component of every air cooled heat exchangers is the plenum housing. The plenum should be inspected periodically to confirm that no panels are missing or that no large holes exist. If there are gaps in the plenum, the air will have a path through which to escape without going through the tube bundle. This reduces the overall heat transfer capability of the heat exchanger.
Tube Bundles:-
Although a fairly common practice, it is not recommended to spray water on tube bundles to provide temporary additional heat transfer capacity during hot summer days. Operating plants which adopt this practice see a deterioration of the aluminum tube fins overtime from corrosion and fouling due to chloride formation in the heat transfer surface. This practice leads to a reduction in performance over time and in time the tube bundle needs to be replaced. Replacing tube bundles is expensive and time consuming and should be used as a last resort. If the fins are corroded or have become detached, there may not be any other option than replacing the tube bundle.
A common problem is bent or crushed tube fins. In this case, a comb type device can be used to rake through and lift the fins back into a position perpendicular to the tubes. This will help increase the heat transfer performance of the air-cooler.
Control Philosophy:-
This section covers a few of the potential control problems with air-coolers.
Inlet Process Conditions:-
Over time or periodically, the inlet process conditions can change. The process flow rate, composition and inlet temperature may vary from design conditions. Often air-coolers are thought of as under-performing when actually the total required heat duty has changed over time or suddenly increased.
Control Set Point:-
Another less common cause for air-cooler problems is improper control set points. Often the use of heat exchangers changes over time but the set points tend to be “fixed and forgotten”. A quick check of the current heat exchanger design documentation can provide insight into the expected process conditions and set points. The plant data should be compared to the design data and any discrepancies investigated.
Non-Condensable Purges:-
Many air-coolers are used as overhead condensers on distillation columns. Similar to all other heat exchangers, air-coolers performance can suffer if non-condensable vapor gets trapped in the tubes reducing the effective heat transfer area. The usual design technique to eliminate this effect is to provide a non-condensable purge line that will provide a way out of the system. If an air-cooler is not performing well in an overhead condensing service, one cause could be that the non-condensable purge line has been closed or plugged. This line should be checked periodically to assure a clear path and prevent gas build-up.
Process Side Issues:-
The easiest and most cost effective fixes to air-cooler problems usually occur on the air side of the exchanger. Unfortunately this does not fix all problems and we also find that heat transfer limitations can occur because of poor conditions on the process side.
If the process side of the tubes gets fouled or scale builds up, the performance of the air-cooler will be reduced. Many exchangers that do foul on the process side are well known and should be put on a routine maintenance schedule to keep them clean. If a heat exchanger is suspected to be fouled, pressure drop readings and tracking can be used to confirm fouling. As the tube inside diameter gets smaller when fouling is present, the velocity increases and the overall pressure drop starts to increase. If this is encountered, the exchanger should be taken off line and cleaned.
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