Types of Heat Exchangers

Based on the design characteristics indicated above, there are several different variants of heat exchangers available. Some of the more common variants employed throughout industry include:

• Shell and tube heat exchangers
• Double pipe heat exchangers
• Plate heat exchangers
• Condensers, evaporators, and boilers

Shell and Tube Heat Exchangers

The most common type of heat exchangers, shell and tube heat exchangers are constructed of a single tube or series of parallel tubes (i.e., tube bundle) enclosed within a sealed, cylindrical pressure vessel (i.e., shell). The design of these devices is such that one fluid flows through the smaller tube(s), and the other fluid flows around its/their outside(s) and between it/them within the sealed shell. Other design characteristics available for this type of heat exchanger include finned tubes, single- or two-phase heat transfer, countercurrent flow, cocurrent flow, or crossflow arrangements, and single, two, or multiple pass configurations.

Some of the types of shell and tube heat exchangers available include helical coil heat exchangers and double pipe heat exchangers, and some of the applications include preheating, oil cooling, and steam generation.

A close-up view of a heat exchanger tube bundle.

Image Credit: Anton Moskvitin/Shutterstock.com

Double Pipe Heat Exchangers

A form of shell and tube heat exchanger, double pipe heat exchangers employ the simplest heat exchanger design and configuration which consists of two or more concentric, cylindrical pipes or tubes (one larger tube and one or more smaller tubes). As per the design of all shell and tube heat exchangers, one fluid flows through the smaller tube(s), and the other fluid flows around the smaller tube(s) within the larger tube.

The design requirements of double pipe heat exchangers include characteristics from the recuperative and indirect contact types mentioned previously as the fluids remain separated and flow through their own channels throughout the heat transfer process. However, there is some flexibility in the design of double pipe heat exchangers, as they can be designed with cocurrent or countercurrent flow arrangements and to be used modularly in series, parallel, or series-parallel configurations within a system. For example, Figure 4, below, depicts the transfer of heat within an isolated double pipe heat exchanger with a cocurrent flow configuration.

Figure 4 – Heat Transfer in a Double Pipe Heat Exchanger

Plate Heat Exchangers

Also referred to as plate type heat exchangers, plate heat exchangers are constructed of several thin, corrugated plates bundled together. Each pair of plates creates a channel through which one fluid can flow, and the pairs are stacked and attached—via bolting, brazing, or welding—such that a second passage is created between pairs through which the other fluid can flow.

The standard plate design is also available with some variations, such as in plate fin or pillow plate heat exchangers. Plate fin exchangers employ fins or spacers between plates and allow for multiple flow configurations and more than two fluid streams to pass through the device. Pillow plate exchangers apply pressure to the plates to increase the heat transfer efficiency across the surface of the plate. Some of the other types available include plate and frame, plate and shell, and spiral plate heat exchangers.

A close-up view of a plate type heat exchanger.

Image Credit: withGod/Shutterstock.com

Condensers, Evaporators, and Boilers

Boilers, condensers, and evaporators are heat exchangers which employ a two-phase heat transfer mechanism. As mentioned previously, in two-phase heat exchangers one or more fluids undergo a phase change during the heat transfer process, either changing from a liquid to a gas or a gas to a liquid.

Condensers are heat exchanging devices which take heated gas or vapor and cool it to the point of condensation, changing the gas or vapor into a liquid. On the other hand, in evaporators and boilers, the heat transfer process changes the fluids from liquid form to gas or vapor form.

Other Heat Exchanger Variants

Heat exchangers are employed in a variety of applications across a wide range of industries. Consequently, there are several variants of heat exchangers available, each suitable for the requirements and specifications of a particular application. Beyond the variants mentioned above, other types available include air cooled heat exchangers, fan cooled heat exchangers, and adiabatic wheel heat exchangers.

Heat Exchanger Selection Considerations

While there are a wide variety of heat exchangers available, the suitability of each type (and its design) in transferring heat between fluids is dependent on the specifications and requirements of the application. Those factors largely determine the optimal design of the desired heat exchanger and influence the corresponding rating and sizing calculations.

Some of the factors that industry professionals should keep in mind when designing and choosing a heat exchanger include:

• The type of fluids, the fluid stream, and their properties
• The desired thermal outputs
• Size limitations
• Costs

Fluid Type, Stream, and Properties

The specific type of fluids—e.g., air, water, oil, etc.—involved and their physical, chemical, and thermal properties—e.g., phase, temperature, acidity or alkalinity, pressure and flow rate, etc.—help determine the flow configuration and construction best suited for that particular heat transfer application.

For example, if corrosive, high temperature, or high pressure fluids are involved, the heat exchanger design must be able to withstand the high stress conditions throughout the heating or cooling process. One method of fulfilling these requirements is by choosing construction materials which hold the desired properties: graphite heat exchangers exhibit high thermal conductivity and corrosion resistance, ceramic heat exchangers can handle temperatures higher than many commonly used metals’ melting points, and plastic heat exchangers offer a low-cost alternative which maintains a moderate degree of corrosion resistance and thermal conductivity. Another method is by choosing a design suited for the fluid properties: plate heat exchangers are capable of handling low to medium pressure fluids but at higher flow rates than other types of heat exchangers, and two-phase heat exchangers are necessary when handling fluids which require a phase change throughout the heat transfer process. Other fluid and fluid stream properties that industry professionals may keep in mind when choosing a heat exchanger include fluid viscosity, fouling characteristics, particulate matter content, and presence of water-soluble compounds.

Thermal Outputs

The thermal output of a heat exchanger refers to the amount of heat transferred between fluids and the corresponding temperature change at the end of the heat transfer process. The transference of heat within the heat exchanger leads to a change of temperature in both fluids, lowering the temperature of one fluid as heat is removed and raising the temperature of the other fluid as heat is added. The desired thermal output and rate of heat transfer help determine the optimal type and design of heat exchanger as some heat exchanger designs offer greater heater transfer rates and can handle higher temperatures than other designs, albeit at a higher cost.

Size Limitations

After choosing the optimal type and design of a heat exchanger, a common mistake is purchasing one that is too big for the given physical space. Oftentimes, it is more prudent to purchase a heat exchanging device in a size which leaves room for further expansion or addition, rather than choosing one which fully encompasses the space. For applications with limited space, such as in airplanes or automobiles, compact heat exchangers offer high heat transfer efficiencies in smaller, more lightweight solutions. Characterized by high heat transfer surface area to volume ratios, several variants of these heat exchanging devices are available, including compact plate heat exchangers. Typically, these devices feature ratios of ≥700 m2/m3 for gas-to-gas applications and ≥400 m2/m3 for liquid-to-gas applications.

Costs

The cost of a heat exchanger includes not only the initial price of the equipment, but the installation, operational, and maintenance costs over the device’s lifespan as well. While it is necessary to choose a heat exchanger which effectively fulfills the requirements of the applications, it is also important to keep in mind the overall costs of the chosen heat exchanger to better determine whether the device is worth the investment. For example, an initially expensive, but more durable heat exchanger may result in lower maintenance costs and, consequently, less overall spend over the courses of a few years, while a cheaper heat exchanger may be initially less expensive, but require several repairs and replacements within the same period of time.

Design Optimization

Designing the optimal heat exchanger for a given application (with particular specifications and requirements as indicated above) involves determining the temperature change of the fluids, the heat transfer coefficient, and the construction of the heat exchanger and relating them to the rate of heat transfer. The two main problems which arise in pursuing this objective are calculating the device’s rating and sizing.

The rating refers to the calculation of the thermal effectiveness (i.e., efficiency) of a heat exchanger of a given design and size, including the rate of heat transfer, the amount of heat transferred between fluids and their corresponding temperature change, and the total pressure drop across the device. The sizing refers to the calculation of the required total dimensions of the heat exchanger (i.e., the surface area available for use in the heat transfer process), including the length, width, height, thickness, number of components, component geometries and arrangements, etc., for an application with given process specifications and requirements. The design characteristics of a heat exchanger—e.g., flow configuration, material, construction components and geometry, etc.—affect both the rating and sizing calculations. Ideally, the optimal heat exchanger design for an application finds a balance (with factors optimized as specified by the designer) between the rating and sizing which satisfies the process specifications and requirements at the minimum necessary cost.

Applications of Heat Exchangers

Heat exchangers are devices used throughout industry for both heating and cooling processes. Several variants of heat exchangers are available and find application in a wide range of industries, including:

• ASME heat exchangers
• Automotive heat exchangers (typically as car radiators)
• Brewery heat exchangers
• Chemical heat exchangers
• Cryogenic heat exchangers
• Marine heat exchangers
• Power generation heat exchangers
• Refrigeration heat exchangers

Table 1, below, indicates some of the common industries and applications of the types of heat exchangers previously mentioned.

Table 1 – Industries and Applications of Heat Exchangers by Type

Type of Heat Exchanger	Common Industries and Applications
Shell and Tube	Oil refining Preheating Oil cooling Steam generation Boiler blowdown heat recovery Vapor recovery systems Industrial paint systems
Double Pipe	Industrial cooling processes Small heat transfer area requirements
Plate	Cryogenic Food processing Chemical processing Furnaces Closed loop to open loop water cooling
Condensers	Distillation and refinement processes Power plants Refrigeration HVAC Chemical processing
Evaporators/Boilers	Distillation and refinement processes Steam trains Refrigeration HVAC
Air Cooled/Fan Cooled	Limited access to cooling water Chemical plants and refineries Engines Power plants
Adiabatic Wheel	Chemical and petrochemical processing Petroleum refineries Food processing and pasteurization Power generation Cryogenics HVAC Aerospace
Compact	Limited space requirements (e.g., aircrafts and automobiles) Oil cooling Automotive Cryogenics Electronics cooling

Summary

This guide provides a basic understanding of heat exchangers, the designs and types available, their applications, and considerations for use. Additional information on purchasing heat exchangers is available in the Thomas Heat Exchangers Buying Guide.

For more information on related products, consult other Thomas guides and white papers or visit the Thomas Supplier Discovery Platform, where you will find information on over 500,000 commercial and industrial suppliers.

Other Heat Exchanger Articles

All About Air To Air Heat Exchangers - What You Need To Know

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