Shell-and-tube heat exchanger

Class of heat exchanger designs

A shell and tube heat exchanger is a class of heat exchanger designs. It is the most common type of heat exchanger in oil refineries and other large chemical processes, and is suited for higher pressure applications. As its name implies, this type of heat exchanger consists of a shell (a large pressure vessel) with a bundle of tubes inside it. One fluid runs through the tubes, and another fluid flows over the tubes (through the shell) to transfer heat between the two fluids. The set of tubes is called a tube bundle, and may be composed of several types of tubes: plain, longitudinally finned, etc.

Two fluids, of different starting temperatures, flow through the heat exchanger. One flows through the tubes (the tube side) and the other flows outside the tubes but inside the shell (the shell side). Heat is transferred from one fluid to the other through the tube walls, either from tube side to shell side or vice versa. The fluids can be either liquids or gases on either the shell or the tube side. In order to transfer heat efficiently, a large heat transfer area should be used, leading to the use of many tubes. In this way, waste heat can be put to use. This is an efficient way to conserve energy. Heat exchangers with only one phase (liquid or gas) on each side can be called one phase or single phase heat exchangers. Two phase heat exchangers can be used to heat a liquid to boil it into a gas (vapor), sometimes called boilers, or to cool the vapors and condense it into a liquid (called condensers), with the phase change usually occurring on the shell side. Boilers in steam engine locomotives are typically large, usually cylindrically shaped shell and tube heat exchangers. In large power plants with steam driven turbines, shell and tube surface condensers are used to condense the exhaust steam exiting the turbine into condensate water which is recycled back to be turned into steam in the steam generator. They are also used in liquid cooled chillers for transferring heat between the refrigerant and the water in both the evaporator and condenser, and in air cooled chillers for only the evaporator.

There can be many variations on the shell and tube design. Typically, the ends of each tube are connected to plenums (sometimes called water boxes) through holes in tubesheets. The tubes may be straight or bent in the shape of a U, called U tubes. In nuclear power plants called pressurized water reactors, large heat exchangers called steam generators are two phase, shell and tube heat exchangers which typically have U tubes. They are used to boil water recycled from a surface condenser into steam to drive a turbine to produce power. Most shell and tube heat exchangers are either 1, 2, or 4 pass designs on the tube side. This refers to the number of times the fluid in the tubes passes through the fluid in the shell. In a single pass heat exchanger, the fluid goes in one end of each tube and out the other. Surface condensers in power plants are often 1 pass straight tube heat exchangers (see surface condenser for diagram). Two and four pass designs are common because the fluid can enter and exit on the same side. This makes construction much simpler. There are often baffles directing flow through the shell side so the fluid does not take a short cut through the shell side leaving ineffective low flow volumes. These are generally attached to the tube bundle rather than the shell in order that the bundle is still removable for maintenance. Countercurrent heat exchangers are most efficient because they allow the highest log mean temperature difference between the hot and cold streams. Many companies however do not use two pass heat exchangers with a u tube because they can break easily in addition to being more expensive to build. Often multiple heat exchangers can be used to simulate the countercurrent flow of a single large exchanger.

To be able to transfer heat well, the tube material should have good thermal conductivity. Because heat is transferred from a hot to a cold side through the tubes, there is a temperature difference through the width of the tubes. Because of the tendency of the tube material to thermally expand differently at various temperatures, thermal stresses occur during operation. This is in addition to any stress from high pressures from the fluids themselves. The tube material also should be compatible with both the shell and tube side fluids for long periods under the operating conditions (temperatures, pressures, pH, etc.) to minimize deterioration such as corrosion. All of these requirements call for careful selection of strong, thermally conductive, corrosion resistant, high quality tube materials, typically metals, including aluminium, copper alloy, stainless steel, carbon steel, non ferrous copper alloy, Inconel, nickel, Hastelloy and titanium. Fluoropolymers such as Perfluoroalkoxy alkane (PFA) and Fluorinated ethylene propylene (FEP) are also used to produce the tubing material due to their high resistance to extreme temperatures. Poor choice of tube material could result in a leak through a tube between the shell and tube sides causing fluid cross contamination and possibly loss of pressure.

In shell and tube heat exchangers there is a potential for a tube to rupture and for high pressure (HP) fluid to enter and over pressurise the low pressure (LP) side of the heat exchanger. The usual configuration of exchangers is for the HP fluid to be in the tubes and for LP water, cooling or heating media to be on the shell side. There is a risk that a tube rupture could compromise the integrity of the shell and the release flammable gas or liquid, with a risk to people and financial loss. The shell of an exchanger must be protected against over pressure by rupture discs or relief valves. The opening time of protection devices has been found to be critical for exchanger protection. Such devices are fitted directly on the shell of the exchanger and discharge into a relief system.

Shell and tube heat exchangers are integral components in thermal engineering, primarily used for efficient heat transfer. The design and arrangement of the tubes within these exchangers are fundamental to their operation and effectiveness. The precise design and specification of tubes in shell and tube heat exchangers underscore the complexities of thermal engineering. Each design aspect, from material selection to tube arrangement and fluid flow, plays a vital role in the exchanger's performance, showcasing the intricacies and precision required in this field.

Tubes in these exchangers, often termed as condenser tubes, are distinct from typical water tubing. They adhere to the Birmingham Wire Gage (BWG) standard, which dictates specific dimensions such as the outside diameter. For example, a 1 inch tube according to BWG will have an exact outside diameter of 1 inch. Detailed specifications are available in specialized references.

The tubes are made from a variety of materials, each chosen based on specific system requirements including thermal conductivity, strength, and corrosion resistance.

The arrangement of tubes is a crucial design aspect. They are positioned in holes drilled in tube sheets, with the spacing between holes known as tube pitch being a key factor for both structural integrity and efficiency. Tubes are typically organized in square or triangular patterns, and specific layouts are detailed in engineering references.

Tube count refers to the maximum number of tubes that can fit within a shell of a specific diameter without weakening the tube sheet. This aspect is crucial for ensuring the structural integrity and efficiency of the heat exchanger. Information on tube counts for various shell sizes can be found in specialized literature.

In shell and tube heat exchangers, there are two distinct fluid streams for heat transfer. The tube fluid circulates inside the tubes, while the shell fluid flows around them, guided by baffles. The movement of the shell fluid, whether it is side to side or up and down, and the number of passes it makes over the tubes, are controlled by segmental baffles, essential for maximizing heat transfer efficiency. These aspects are elaborated in dedicated references.