Shell and Tube Heat Exchanger

The most basic and the most common type of heat exchanger construction is the tube and shell, as shown in Figure 1. This type of heat exchanger consists of a set of tubes in a container called a shell. The fluid flowing inside the tubes is called the tube side fluid and the fluid flowing on the outside of the tubes is the shell side fluid. At the ends of the tubes, the tube side fluid is separated from the shell side fluid by the tube sheets. The tubes are rolled and press-fitted or welded into the tube sheet to provide a leak tight seal.

In systems where the two fluids are at vastly different pressures, the higher pressure fluid is typically directed through the tubes and the lower pressure fluid is circulated on the shell side. This is due to economy, because the heat exchanger tubes can be made to withstand higher pressures than the shell of the heat exchanger for a much lower cost. The support plates shown on Figure 1 also act as baffles to direct the flow of fluid within the shell back and forth across the stainless steel tube.

Shell and tube heat exchangers are commonly used for the transfer of heat in industrial process applications. They are available in a variety of diameters and lengths. The U-tube heat exchangers are designed for a wide range of liquid to liquid and steam to liquid applications.

The shell and tube heat exchanger is constructed using copper tubes and a carbon steel shell. It is manufactured using the most advanced technology available and is specifically engineered for a wide range of applications. Every unit is designed, manufactured and tested to meet the quality requirements of the ASME code: Section VIII Division #1 and ISO 9002.

Shell and tube heat exchangers consist of a series of stainless steel tube. One set of these tubes contains the fluid that must be either heated or cooled. The second fluid runs over the tubes that are being heated or cooled so that it can either provide the heat or absorb the heat required. A set of tubes is called the tube bundle and can be made up of several types of tubes: plain, longitudinally finned, etc. Shell and Tube heat exchangers are typically used for high pressure applications (with pressures greater than 30 bar and temperature greater than 260°C. This is because the shell and tube heat exchangers are robust due to their shape.

There are several thermal design features that are to be taken into account when designing the tubes in the shell and tube heat exchangers. These include:

Tube length: heat exchangers are usually cheaper when they have a smaller shell diameter and a long tube length. Thus, typically there is an aim to make the heat exchanger as long as physically possible whilst not exceeding production capabilities. However, there are many limitations for this, including the space available at the site where it is going to be used and the need to ensure that there are tubes available in lengths that are twice the required length (so that the tubes can be withdrawn and replaced). Also, it has to be remembered that long, thin tubes are difficult to take out and replace.

Tube pitch: when designing the tubes, it is practical to ensure that the tube pitch (i.e., the centre-centre distance of adjoining tubes) is not less than 1.25 times the tubes' outside diameter. A larger tube pitch leads to a larger overall shell diameter which leads to a more expensive heat exchanger.

Tube corrugation: this type of tubes, mainly used for the inner tubes, increases the turbulence of the fluids and the effect is very important in the heat transfer giving a better performance.

Tube Layout: refers to how tubes are positioned within the shell. There are four main types of tube layout, which are, triangular (30°), rotated triangular (60°), square (90°) and rotated square (45°). The triangular patterns are employed to give greater heat transfer as they force the fluid to flow in a more turbulent fashion around the piping. Square patterns are employed where high fouling is experienced and cleaning is more regular.

Baffle Design: baffles are used in shell and tube heat exchangers to direct fluid across the tube bundle. They run perpendicularly to the shell and hold the bundle, preventing the tubes from sagging over a long length. They can also prevent the tubes from vibrating. The most common type of baffle is the segmental baffle. The semicircular segmental baffles are oriented at 180 degrees to the adjacent baffles forcing the fluid to flow upward and downwards between the tube bundle. Baffle spacing is of large thermodynamic concern when designing shell and tube heat exchangers. Baffles must be spaced with consideration for the conversion of pressure drop and heat transfer. For thermo economic optimization it is suggested that the baffles be spaced no closer than 20% of the shell’s inner diameter. Having baffles spaced too closely causes a greater pressure drop because of flow redirection. Consequently having the baffles spaced too far apart means that there may be cooler spots in the corners between baffles. It is also important to ensure the baffles are spaced close enough that the tubes do not sag. The other main type of baffle is the disc and donut baffle which consists of two concentric baffles, the outer wider baffle looks like a donut, whilst the inner baffle is shaped as a disk. This type of baffle forces the fluid to pass around each side of the disk then through the donut baffle generating a different type of fluid flow.

Features

U-tube construction
Units designed for liquid to liquid and steam to liquid applications.
Available in 2 or 4 pass construction.
Removable tube bundles for convenience in cleaning and inspection.
Strong, durable construction with copper steel tubes and rugged cast-iron or steel head in standard units. (Optional materials are available).
Coil and baffle arrangements designed to maximize heat transfer performance.
Tubes expanded into stationary tubesheet allow for tube expansions and contractions, due to thermal fluctuations, without causing stresses on the joints. Welding tubes to the tubesheet is optional.

Advantages

The advantage of the shell and tube design cannot be ignored. Each unit comes with:

connections that come in standardized sizes for easy assembly and feature additional thread and surface protection for clean installation
U-bend tubes expanded into a tubesheet which allow for tube expansions and contractions due to thermal fluctuations.
gaskets that are made of high quality compressed fibres which lends to reusability.
a standard cast-iron or steel head for heavy duty services (also available as a spare part).
saddle attaches which make for quick and easy mounting.
punched baffles with minimal clearances between tubes guaranteeing correct fluid flow and minimized bypass
a welded shell protected with high quality paint for corrosion resistance.
copper steel tubes which allow for strong, durable performance over a wide range of applications. Unique tube bundle layout (chevron corrugated pattern) minimizes deposit buildup at the edges and optimises media flow for high velocity flow turbulence.

Applications

Shell and tube heat exchangers are frequently selected for such applications as:

Process liquid or gas cooling
Process or refrigerant vapor or steam condensing
Process liquid, steam or refrigerant evaporation
Process heat removal and preheating of feed water
Thermal energy conservation efforts, heat recovery
Compressor, turbine and engine cooling, oil and jacket water
Hydraulic and lube oil cooling

Shell and tube heat exchangers have been around for over 150 years. Their thermal technology and manufacturing methods are well defined and applied by the modern manufacturer. Tube surfaces range from standard to exotic metals with plain or enhanced surface characteristics. They can help provide the least costly mechanical design for the flows, liquids and temperatures involved.