Confused about heat exchanger terms, or just looking for a good primer on the topic? Consult this glossary of terms.

To help users of heat exchangers get a handle on heat exchanger construction, parts and function, here is an edited version of a glossary of heat exchanger terms and definitions compiled ITT Standard, Buffalo, N.Y. You can find the complete glossary at

Plate and frame heat exchangers are designed to provide close temperature control of fluids where space is a prime consideration.
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.

Bolt Circle
The circular dimension line on which bolt holes are drilled.

Bonnet Assembly
The name of a tube sheet exchanger part that directs the tube-side medium through the tubes. It may also contain the tube-side inlet and outlet connections and/or pass ribs. It differs from a channel in that it does not have a removable cover. See Channel Assembly and Water Box.

Bundle Assembly
The name of the tubing assembly in removable-bundle heat exchangers. It usually consists of tubes, tube sheets, baffles, spacers and tie rods. See Core Assembly.

The threaded bolt used to hold the bonnet onto the core of BCF-type heat exchangers. Different head types are available. Capscrews always have a head on one end with threads at the other.

Channel Assembly
Performs same function as a bonnet assembly. However, a removable cover permits access to the ends of the tubes. It also is the name of a structural construction product shaped like the letter "C."

Core Assembly
The name of the shell assembly and tube assembly in fixed-tube sheet heat exchangers.

The part that is used to connect a user's piping to the heat exchanger. Couplings come in many styles.

Cover/Cover Assembly
The part used to cover an opening on a heat exchanger that is used in conjunction with a channel to seal off the tube side of the exchanger. Nozzle covers can be used to cover nozzle openings to keep heat exchanger internals clean during shipping and storage. Covers are different from end plates in that they can be removed from the heat exchanger to clean the interior of the tube side without disturbing any piping.

Cradle Assembly
The part used to support the entire heat exchanger. A cradle may be fixed or moveable. It also is used to secure the heat exchanger to the customer's mounting surface when welded or strapped to the shell.

Design Pressure
Also called Max-imum 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.

A type of nozzle connection. It provides a larger nozzle opening between the customer's pipe size and heat exchanger tube bundle, usually to prevent tube erosion from high inlet velocities.

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

End Zone
The first baffle space on a tube bundle. It is the space between the tube sheet and the first baffle plate. The end zone is adjusted to keep the baffle plates within the two shell-side nozzles.

A small piece of tubing approximately 1" long made of copper or stainless steel. The ferrule is crimped or squeezed onto the tie tube, up against the last baffle, thus locking all the baffles into position.

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.

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).

The cast or forged part of a BCF/SSCF core assembly. It is made up of a tube sheet, shell-side nozzle connection and vent/drain connections. The hub eliminates additional welding or brazing operations, making the exchanger less expensive.

Heat exchangers can be constructed with a packed floating tube sheet, removable bundle design.
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.

In-and-Out End
The end of the heat exchanger that contains the tube side inlet and outlet connections in a multi-pass unit.

Lantern Ring
A nylon or metal ring on some packed-joint heat exchangers. It fits over the outside diameter of the floating tube sheet between the packing rings. When tightened, the joint holds the packing rings in place. The lantern ring also has small holes that act as leak detectors, which let the customer know if one of the fluids is leaking around the packing rings. The fluids never mix.

Lap-Joint Flange
Also called Van Stone. These flanges are used to reduce the amount of stainless steel required to make a flange, thus lowering the cost. A less expensive steel ring is used in tandem with the more exotic material that is placed at the fluid-contact surfaces or where the fluid touches the flanges. The steel ring contains the bolt holes. Lap-joint flanges also are used on stainless steel C-200-type heat exchangers for tube sheets.

A short piece of pipe threaded on both ends.

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.

Packed End
The end of a heat exchanger containing the packed joint. This joint contains the packing rings. It also is called the floating end of the unit, where the floating tube sheet is located.

Packing Ring
A fairly soft nonmetallic ring used to seal the floating tube sheet or packed end of the heat exchanger. The packing ring slips over the floating tube sheet on either side of the lantern ring. It also fits into a groove in the shell or bonnet flanges, or is held in place by the retaining-ring assembly.

Pass Lane
A lane in a tube layout where there are no tubes. It is the surface on the tube sheets where the pass ribs mate.

Pass Rib
A separator plate inside a bonnet or channel. The rib, which mates with the pass-lane surface, creates a multipass heat exchanger. By arranging the ribs, a designer can control the flow of the tube side medium.

Protector Rod
Also called a Zinc. A sacrificial anode usually placed in the tube side of a heat exchanger to protect the tubes, tube sheets and bonnets against corrosion. When water flows through the tube side, the zinc is consumed instead of the other heat exchanger parts.

Heat exchange coils can be constructed with a drawer-type casing to allow removal of finned tubes and headers.
Quad Layout
A quadrant-type tube layout. Four separate quarters of a tube layout with pass lanes between them.

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.

Retaining-Ring Assembly
A metal assembly used on ACA, GC and/or OC packed-joint heat exchangers. The ring assembly performs the same job as a lantern ring, eliminating the need for a stuffing-box flange.

Reversing End
The end of a heat exchanger where the tube side fluid reverses its flow in a multipass unit. It usually contains only small vent and drain connections.

Ribbon Flow
A type of tube layout or pass-rib pattern that creates an end-to-end flow that looks like a piece of ribbon candy.

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.

Slip-On Flange
A type of nozzle flange that slips over the nozzle pipe and is welded in place.

A piece of tubing that slides over the tie rod or tie tube between the baffle plates. The spacer holds the baffle plate in a permanent position.

Two or more heat exchangers that have been stacked together either side by side or one on top of the other. These units have interconnected piping hooking them together.

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.

Stub End
The fluid-contact surface used with a lap-joint flange.

Stud/Stud Bolt
Used to hold two or more pieces together. It is threaded the full length and fits through bolt holes that are not tapped. A hex nut is used on both ends. A stud is similar to a stud bolt except it threads into a tapped hole and a hex nut is used on one end only.

Stuffing-Box Flange
Used at a packed-end joint. When a packed joint is tightened, the packing ring is forced into this flange by the lantern ring or lantern gland.

Support Foot
A support foot usually is bolted to the heat exchanger using the bonnet-to-shell-flange bolting. See Cradle Assembly.

Tubular Exchange Manufacturer's Association. TEMA standards cover the design, manufacture, installation, operation and maintenance of shell and tube heat exchangers. TEMA C is for generally moderate requirements of commercial and general process requirements. TEMA B is for chemical process services. TEMA R generally serves requirements of petroleum and related processing applications.

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.

Tie Rod
A small diameter rod that threads into the stationary end-tube sheet. The rod ties the baffles and spacers together.

Tie Tube
Takes the place of the tie rod in a small diameter heat exchanger. It serves the same purpose as a tie rod.

Tube Layout
The tube-layout drawing shows the positioning of the tubes inside the heat exchanger as well as the locations of the tie rods.

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.

Tube Side
The side of a heat exchanger where the fluid circulates through the inside of the tubes.

Water Box
Another name for bonnet or channel. See Bonnet Assembly and Channel Assembly.

Weld-Neck Flange
A type of nozzle flange used when a butt weld is required. This means the end of the flange butts up against a piece of pipe. Both the flange and the pipe are beveled. The joint provides a sound weld with no overlapping parts.

Zinc. See Protector Rod.