Baffle Plate. Also called support plate. Tubes pass through this plate for support. Provides a blocked path for the shell-side medium, forcing the medium across the tubes for better heat exchanger performance.

Baffle Spacing. The space between baffle plates on a tube bundle. Baffle spacing is adjusted to achieve maximum heat exchanger performance.

End Plate. A cover that has been welded to the heat exchanger. Most end plates are used on bonnet assemblies.

Fixed Tube Sheet. A nonremovable tube sheet; the tube sheet on a core assembly; or any tube sheet that is an integral part of the shell assembly.

Floating Tube Sheet. The tube sheet at one end of a removable-tube bundle. The floating tube sheet always has a smaller diameter than does the stationary version. The floating tube sheet moves freely with the expansion and contraction of the tube bundle due to temperature changes in operation.

Gasket. A sealing device used between two parts to prevent leakage. Types include inside-the-bolt circle (no bolt holes) and full face (with bolt holes and the same diameter as the flange).

Impingement Plate. A small perforated- -plate or bar assembly placed inside the shell-side nozzle, usually a dome-type nozzle. The plate also can be attached directly to the bundle by being tack-welded to the tie rods. The impingement plate protects and prolongs tube life by breaking up and slowing down the shell side fluid, which otherwise would erode the tubing.

Nipple. A short piece of pipe threaded on both ends.

Design Pressure. Also called maximum allowable operating pressure. Used by engineers to calculate part thickness and heat exchanger design. Generally, it is slightly higher than the most severe condition or highest operating pressures seen by the heat exchanger.

One, Two, Four Pass. The number of times the fluid passes through the tube bundle. In a one-pass unit, the tube-side medium passes through all the tubes once. In a two-pass unit, it passes through one-half of the tubes and returns through the other half. A four-pass unit goes through approximately one-quarter of the tubes, down and back four times. Greater-than-one pass heat exchangers are referred to as multipass units.

Operating Pressure. The pressure a heat exchanger is actually operating at while in use.

Outer-Tube Limit (OTL). The diameter created by encircling the outermost tubes in a tube layout. Engineers use the design OTL to calculate clearances between bundle parts. The actual OTL is usually a few-thousandths less than the design OTL.

Test Pressure. Generally 1.5 times the design pressure. It is used during a hydrostatic test that detects leaks at any joint on the heat exchanger.

Removable Bundle. A type of heat exchanger in which the tube bundle can be removed from the shell pipe. This provides easy cleaning of the shell side and a less expensive way of replacing worn out tubes.

Shell Assembly. The assembly into which the tube bundle fits. The shell also contains the shell-side connections.

Shell Head. A formed plate welded to the shell, or bonnet, pipe. It can be many styles or shapes, including flanged and dished, elliptical, ellipsoidal and hemispherical. Generally, as a head gets flatter, it gets weaker; therefore, designers can use a flat-end plate or a thinner formed head to do the same job.

Shell Side. The side of a heat exchanger where the fluid circulates around the outside of the tubes.

Stationary Tube Sheet. The tube sheet at one end of a removable bundle. It has a larger diameter than the floating tube sheet and is held together in a permanent position between the bonnet and shell flanges.

Tube Sheet. A plate that secures both ends of the tube in a heat exchanger. Both the shell-side and tube-side mediums come in contact with it.