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# Static Equipment: Understanding Heat Exchangers

## Take a closer look at heat exchangers, including the various types and designs available, applications, and selection considerations. This article helps the project engineer, who is not an equipment specialist, to check that economical choices are made across all involved disciplines.

The majority of mechanical equipment found in oil and gas facilities belongs to the static equipment group, which comprises pressure vessels (drums, columns, reactors, filters) and heat exchangers (shell and tubes, plate and frame, air coolers).

A previous article presented guidelines for cost-effective recommendations for the equipment’s design, materials, and fabrication. Its goal was to enable the project engineer, who is not an equipment specialist, to check that economical choices are made across all involved disciplines.

This article has the same goal for heat exchangers: to make sure that across all disciplines involved—process, heat exchange, and mechanical—the most cost-effective choices are made.

Many a time one discipline is not fully aware of the cost impact of what it specifies. It is also common for a discipline to go beyond a functional specification, making a selection instead, and thus depriving the project of a more economical selection down the line.

The first part of this section is common to all types, followed by sections that are specific to each type of heat exchanger.

## Process Input Data

On top of the data required for pressure vessels, the following shall be specified by process:

• Names of flowing streams, their inlet/outlet temperatures, and physical properties (density, thermal conductivity, viscosity and specific heat) at inlet and outlet temperatures
• In a case of a fluid condensing or vaporizing: heating/cooling curves showing how the heat duty and vapor mass fraction vary with temperature, and the corresponding thermal properties of the liquid and vapor fractions

Note that the mechanical design of the heat exchanger shall consider all possible operating cases. There could be operating cases at low turndown where the thermal expansion is increased, compared to the design case, due to increased cooling on the process side. The process data sheet shall therefore include the extreme operating cases, including the turndown case.

## Overdesign

An overdesign margin, i.e., excess heat exchange surface area, is provided to account for possible inaccuracy of the process data as well as possible plugging of tubes. As described hereinafter, the process discipline usually performs the thermal design of shell and tube (S&T) heat exchangers. It therefore includes such overdesign, commonly 10%, in the design. For the other types of heat exchangers, the overdesign margin shall be specified by type of process. Please refer to the specific sections hereinafter for guidance.

## Types of Heat Exchangers

There is quite a large choice in types of heat exchangers: S&T air-cooled heat exchangers (ACHE), pipe-in-pipe/hairpin, and plate and frame (P&F) are the most common. Other compact design exchangers like semi/fully welded plate exchangers, printed circuit heat exchanger, shell and frame exchanger, etc., are getting popular due to their compact footprint and weight which makes them very popular offshore.

The type of heat exchanger is indicated by process on the process data sheet. Because the process engineer is not often familiar with the cost of each type, an uneconomical selection may be made.

When energy savings is required, an exchanger, typically a feed/effluent exchanger, a steam generator, or a boiler feedwater preheater is used.

When cooling only is required, water is used wherever available. In case cooling water is not available, e.g., in desert areas such as in the Middle East, or if it is so corrosive that its use would result in the requirement for expensive corrosion-resistant materials, only ACHE are used.

ACHE have a much lower heat-exchange coefficient and are much more expensive than water-cooled exchangers on an initial cost basis. However, they are very economical on a total cost basis, as they eliminate the high cost of handling cooling water.

When winterization is required, i.e., when the minimum air temperature could cause the process fluid to freeze, selecting ACHE adds to the cost due to the heating coils, recirculation, and louvers required for their winterization. Finally, an ACHE can usually cool down economically to the air temperature +10°C only (compared to +5°C for a water-cooled exchanger). In addition, air-supply temperature is always greater than water-supply temperature.

A combination of air-cooling followed by downstream water (called trim) cooling is standard. For instance, at a plant site where the air design temperature is 40°C, cooling down to 55°C will be by air cooler and further cooling by water cooler. The term process outlet temperature breakpoint is used for 55°C. It is usually 15°C above the ACHE air design temperature. The choice is usually made by the client and indicated in the design basis.

Air coolers have a significant impact on the layout of the plant. Indeed they cannot be located above any facility due to the air draft they induce. They are usually installed on top of pipe racks.

Plate and frames (P&F), also called plate heat exchangers (PHE), are far more compact than S&T. Their heat transfer coefficient is much greater so that the surface is much reduced. They  can be cost-effective, in particular in cases where expensive materials of construction (alloy) are required.

There are two types of P&F heat exchangers: gasketed and welded. The gasketed type is the cheapest. It is usually limited to a design pressure of 25 bar g and a design temperature of 185°C. Expanding the heat exchange duty of this type of heat exchanger at a later date is very easy by adding plates in the frame.

P&F heat exchangers are also considerably more compact than equivalent-duty S&T. A P&F exchanger occupies about 15% of the plot area of an S&T heat exchanger (and only 11% considering the area required for maintenance, i.e., removal of S&T heat exchanger bundle).

When low-duties are concerned, pipe-in-pipe or hairpin-type heat exchangers are sometimes specified. Nevertheless, due to a restricted number of suppliers, they are not competitive compared to S&T heat exchangers with U-tubes.

Spiral heat exchangers (SHE) should also be considered for severe fouling service due to the high turbulence created by the continuous change in flow direction minimizing fouling. This type of heat exchanger is also quite compact but not to the same extent as PHE. They can handle design pressures up to 25 bar g and design temperatures up to 400°C, as they have conventional gaskets. Thus, the applicability of SHE is much wider than that of PHE.

There are also some heat exchangers such as MCHE—main cryogenic heat exchangers (coil-wound), plate and fin, printed circuit heat exchangers (PCHE) for very compact, high-pressure, multiple-process stream into a single unit, variable temperature service, close temperature approach, etc., with proprietary licensor/vendor design, highly efficient for specific applications such as LNG, refining, etc.

## Reference

Practical Thermal Design of Shell-and-Tube Heat Exchangers, 1st edition, Rajiv Mukherjee, Begell House Publishers

Profiles of the authors can be found here: Navid TajikSagar GaikwadMansour HamzaMarco GarofanelloVU NathanRajiv Mukherjee, and Hervé Baron