Heat Exchanger Types and Selection

aprile 09, 2019

Heat Exchanger Types and Selection

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:

purchase cost
installation cost
operating cost (pumping, fan…)
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
Advantages	Disadvantages
Widely known and understood since it is the most common type	Less thermally efficient than other types of heat transfer equipment
Most versatile in terms of types of service	Subject to flow induced vibration which Can lead to equipment failure
Widest range of allowable design pressures and temperatures	Not well suited for temperature cross conditions (multiple units in series must be used)
Rugged mechanical construction - can withstand more abuse (physical and process)	Contains stagnant zones (dead zones) on the shell side which can lead to corrosion problems
	Subject to flow mal-distribution especially with two phase inlet streams

Compact Heat Exchangers
Advantages	Disadvantages
Low initial purchase cost (plate type)	Narrower rage of allowable pressures and temperatures
Many different configurations are available (gasketed, semi-welded, welded, spiral)	Subject to plugging/fouling due to very narrow flow path
High heat transfer coefficients (3 or more times greater than for shell & tube heat exchangers, due to much higher wall shear stress)	Gasketed units require specialized opening and closing procedures
Tend to exhibit lower fouling characteristics due to the high turbulence within the exchanger	Material of construction selection is critical since wall thickness very thin (typically less than 10 mm)
True countercurrent designs allow significant temperature crosses to be achieved
Require small footprint for installation and have small volume hold-up

Air Cooled Heat Exchangers
Advantages	Disadvantages
Attractive option for locations where cooling water is scarce or expensive to treat	High initial purchase cost
Well suited for cooling high temperature process streams (above 80^oC when using cooling water should be avoided)	Require relatively large footprint
Low maintenance and operating costs (typically 30-50% less than cooling water)	Higher process outlet temperature (10-20 ^oF above the ambient dry bulb temperature)

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 -250^oC to 800^oC. 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.

Typical applications include condensers, reboilers and process heaters and coolers.

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Heat Exchanger pictures courtesy of Festival City Fabricators http://www.steelcraft.ca/fcf/index.php

Design tips http://www.hcheattransfer.com/index.html