Glossary of Heat Exchanger Terms

There's a litany of terms and definitions to keep straight when it comes to heat transfer and heat transfer equipment. Add in the fact that certain industries and applications may use the same term to define different things and it can get pretty confusing. For this post, we thought it'd be helpful to create a list of terms that relate to heat exchangers - their design, thermal performance, and some topics relating to how they're manufactured.

Airflow - The flow of air or gas that goes over the outside finned surface of the coil. The direction of the airflow in relationship to the supply and return headers is important on certain coil types (see Right and Left Hand).

Air Entrapment - Air that gets trapped inside a coil preventing the fluid from touching the tube wall thus lowering the coil performance. A coil properly designed for the mounted orientation and piping connections will vent the air automatically (also see Gravity Drain).

Air Friction - The amount or resistance that needs to be overcome to push the gas through the fin side of the coil at the specified air velocity. Usually measured in inches of water column.

Air Velocity - The speed at which the air moves over the finned surface of the coil. Usually measured in feet per minute FPM.

Airtight - A method of fabricating a coil casing so it does not leak minute amounts of air around the tubes that penetrate the coil end plates and at the casing corners.

ACFM - The cubic feet of gas or air per minute when measured at its actual temperature and pressure conditions. This will need to be converted to SCFM or lbs./hr. to properly size a coil for the ACFM condition.

ARI-410 - A certification regulated by ASHRAE that states the theoretical calculations used to establish a coils thermal performance are true and that a coil built following certain ASHRAE design criteria will indeed provide the calculated performance.

Bent Stubs - Special tube stubs that are fabricated and attached to the header when the centerline of the header pipe cannot be positioned over or near the circuit tubes.

Bridging - The capillary adhesion of moisture across the fin gap between fins that are close together (>12 FPI). If water bridging occurs it closes off the air passage through the fin raising the air velocity until carryover occurs (typically around 500 to 550 FPM).

Capillary Tubes - A small tube used to create the needed pressure drop for an evaporator. The tube length and OD are sized for the particular coil performance. These tubes may be connected to a common header. Capillary tubes are sometimes confused with feeder or lead tubes that are used between a distributor and an evaporator coil.

Carryover - Also known as moisture carryover. Condensation on the finned surfaces of a coil that can be stripped off due to high air velocity (>500 FPM) and carried away in the airstream.

Casing - The metal frame that goes around the coil to hold the fin and tubes together and confine the air or gas so that it passes over the finned surface. Also see endplate and pans.

Casing Flange - The additional material on the casing - usually bent 90° - to provide a mounting and sealing surface. The flange also increases the rigidity of the coil casing and minimizes casing warpage when brazing.

Center Support Plate - A part of the coil casing used to add extra strength and to support the tubes at a maximum span of about 60” between supports for a coil that has a long finned length. It is similar to an endplate and fits between the top and bottom pans.

Charged Coil - A protective option that leave a positive low pressure nitrogen or inert gas charge in a coil after leak testing. This is recommended if the coil may sit in a warehouse or jobsite for an extended time period before installation and/or use for validation that the coil is still leak tight.

Circuit - When straight tubes, or hairpins, are connected to each other in series by using return bends to create one continuous serpentine shaped tube. Also see passes.

Circuiting - The number of circuits (tubes) actually connected to a header. This allows getting the optimal flow rate of the fluid in the tubes which is critical to proper coil performance. This can be different to the number of tubes high. Also see full, half.

Coil Face - The fin side of the coil that the air enters or exits. Also see Face Area.

Condensation - Moisture in the air or gas that condenses out on the cool coil surfaces.

Condensing Temperature - The temperature of the refrigerant when it was condensed (not subcooled) in the condenser unit or coil. This needs to be known when sizing an evaporator coil. Also see suction temperature.

Connection - The point where external piping is connected to the coil. Also see hub, miter, sweat, FPT and MPT.

Corrugated Fin - A type of embossing done to a flat plate fin that ripples the fin surface in the airflow direction to increase the heat transfer capacity of the fin but at a higher air friction. Also see Sine.

Crimping - The process of folding the end of a tube to reduce the size of its opening. This is mainly used to allow the connection of a smaller OD capillary or feeder tube to the coil tubes.

Distributor - A part supplied with the coil that takes the incoming liquid refrigerant, creates a specified pressure drop and distributes the liquid evenly to the feeder tubes. The feeder tube OD, length, type refrigerant, heat load and suction temperature are evaluated and a specific nozzle size is inserted or built into the distributor to create the desired pressure drop.

Desuperheat - Is the process of cooling steam that is not at the saturation point. The heat value in desuperheating steam is much less than the heat value in saturated steam when it condenses.

Direct Expansion - (DX) This refers to the more common refrigerant evaporator coil. This coil usually has a direct expansion valve (supplied by others) connected to the coil distributor.

Endplates - The coil casing parts located at the ends, or sides, of a coil where the tubes penetrate through the sheet metal. The header endplate refers to the endplate located on the header, connection, end of the coil. The hairpin endplate refers to the endplate located at the opposite end where the hairpin tubes are inserted.

Face Area - Is the finned area of the coils’ face that air sees when it first enters the coil. This area is calculated by multiplying the fin height and finned length dimensions together.

Feeder Tube - A small OD tube that connects a distributor to the coil tube. Also called lead tube.

Fin - The strip of material that the tubes goes through and the air or gas travels over.

Fin Height - (FH) The length of the finned area of a coil (tangent to the fin depth edge) measured in the perpendicular direction of the tube and the airflow. Also see Face Area.

Finned Length - (FL) The length of the finned area in a coil measured between the coil endplates. It does not include the parts of the tube that goes through the endplates. Also see Face Area

Fin Depth - The width of the finned area of a coil (tangent to the fin height edge) measured in the perpendicular direction of the tube and parallel direction of the airflow. Also see Rows.

Fluid Velocity - The speed at which the fluid moves through the inside of a tube. Usually measured in feet per second (FPS).

Fins Per Inch - (FPI) The number of fin strips counted in an inch when the inch measurement is in the tube length direction.

Finned Tube Heat Exchanger - A type of gas-to-liquid or gas-to-gas heat exchanger that uses thin strips of material attached to tubes to provide extended surface area for heat to be exchanged. Also see “plate fin."

Finned Tube Length - (FL) The measured length of the tube that has fins. The additional tube length for return bends and connection to the header is not included in this measurement.

Fluid Flow - The flow of liquid or gas that goes through the inside of the tubes in the coil.

Fluid Pressure Drop - The amount of resistance that needs to be over come to push the fluid though the headers and tubes of the coil at the specified fluid flow rate. Usually measured in Feet of water head.

FPT - Connection pipe is threaded or fitted with female pipe threads. Pipe threads are different than straight or standard threads. They are tapered to create a leak tight joint though interference fit.

Full Circuit - All the tubes in the first row or the same number of circuits, as the number of tubes high in the coil, actually connected to the header.

Gravity Drain - A coil properly designed for the mounted orientation and piping connections will drain the fluid in the coil by gravity automatically (also see Air Entrapment).

Hairpin - When a straight length of tube is bent back over itself at the midpoint creating a hairpin shape. This replaces two straight tubes, a return bend and the brazing or welding of the return bend to one end of the tubes.

Hub - A style of attaching the connection to the header. A hole for the next smaller pipe size is cut in the header pipe at the desired connection location. A connection pipe is then attached. The connection pipe cannot be the same OD as the header pipe, it must be smaller. Also see MITER.

Header - A large vessel, usually a pipe, where tubes are connected, and is used to distribute or collect the fluid to / from the coil circuits. It also has one or two large connections that is connected to external piping. Sometimes it is also refereed to as a manifold.

Holding Charge - See Charged Coil.

Inline Tube Pattern - The tubes in each succeeding row is located directly behind the tubes in the preceding row in the airflow direction. This offers the lowest air friction and good cleanability. Also see Staggered.

Intermediate Drip Pan - A secondary pan to catch condensation on a coil over 59” fin height. This removes condensation flowing down the fins in the top half of the coil from flooding the fins in the lower half of the coil. Such flooding lowers coil performance and causes moisture carryover.

Inverted Pans - This is when the normal top and/or bottom casing pan(s) can be turned up side down to make the overall coil height a little more than the coil fin height. The casing flanges are kept to a minimum width and they point into the finned area of the coil. Depending on the type of coil, header size, etc. it may be difficult to also keep the header within the coil height dimension.

Latent Load - This is the heat released when water vapor in the saturated air condenses. It is added to the sensible load of the air to get a total heat load.

Left Hand - When looking at the coil face in the same direction as the airflow (air coming over your shoulder) a left hand coil has the header connections or the supply header on the left. This position is considered standard unless otherwise stated.

Miter - A style of attaching the connection to the header. One end of the header is cut at a 45° angle across the OD and the cut piece (connection) is rotated 180° and attached back to the header. The connection size has to be identical to the header and the connection location is dependent on the required header length and location of the last tube stub. Also see HUB.

MPT - Connection pipe is threaded or fitted with male pipe threads. Pipe threads are different than straight or standard threads. They are tapered to create a leak tight joint though interference fit.

Oil Logging - A condition that can occur in refrigerant coils where the circulating oil can block or “log” up a part of the coil causing lowered coil performance and cycling. This can be avoided if the coil is design for the mounting orientation of the coil and operating temperature.

Pans - The part of the coil casing that forms the top and the bottom of the frame. This describes the shallow pan shape that is created when fabricating these casing part.

Passes - The number of times fluid moves back and forth though the straight sections of one circuit before exiting the circuit. Also see circuit.

Plate Fin - A type of fin used on finned tube heat exchangers. A single flat strip of material (or plate) is stacked together and is attached to the tubes of the coil.

Raised Face Slip On - (RFSO) A type of flange that has a raised surface that mates with another flange and is attached to a pipe by slipping it on then braze or weld it to the pipe.  

Right Hand - When looking at the coil face in the same direction as the airflow (air coming over your shoulder) a right hand coil has the header connections or the supply header on the right. This position is considered not standard and needs to be specified.

Row - (R) The number of rows of tubes seen in the airflow direction through the coil. This in not to be confused with the number of tubes seen in the face area of the coil. Also see Tubes High.

SCFM - The cubic feet of air per minute measured at standard conditions (70 F and 14.7 PSIG - Sea Level). All coil ratings are based on SCFM to ensure proper coil performance (also see ACFM).

Saturated - This is when any gas is saturated with liquid vapor to the point the vapor is ready to condense.

Sensible Heat - The heat load when heating or cooling air. If any moisture condensation occurs during cooling the air then the water vapor portion is considered latent load. The sensible and latent loads are added together to get the total heat load

Sine Wave Fin - A type of embossing done to a flat plate fin that ripples the plate surface in the airflow direction in a sinusoidal wave pattern increasing the heat transfer capacity of the fin more than a flat or corrugated fin surface but at a higher air friction. Also see Plate and Corrugated.

Stacking Flanges - This is when the normal flanges on the coil casing is bent another 90 degrees. This provides extra casing strength and an additional support surface so that coils can be stacked on top of each other or slide smoother in a guide track.

Staggered Tube Pattern - The succeeding row of tubes is offset so as not to be directly behind the preceding row of tubes in the airflow direction. This increases thermal capacity over an inline pattern at the cost of a higher air friction.

Stub Connection - The customers system will connect directly to the actual coil circuit tube(s). The coil usually has no headers.

SST - Abbreviation for Stainless Steel or Saturated Suction Temperature of a refrigerant.

Sweat Connection - Connection is a plain pipe with no threads.

Swaged Connection - The ID of a tube or pipe is expanded to create a cup so that a mating tube or pipe of the same OD can fit inside the expanded cup.

Subcooling - This is cooling the liquid from the condensation of a gas or steam below the point where it condensed. A small part of a condenser coil will have a sub-cooling section to prevent the liquid refrigerant from boiling “flashing” back to a gas as it picks up heat on the way to the evaporator coil.

Suction Temperature - The temperature of the gaseous refrigerant that a refrigerant compressor must see entering on its suction side in order to do the required performance. This is needed when sizing an evaporator coil. Also see condensing temperature.

Superheat - Heating the vapor, that formed from boiling the liquid, well above the boiling point. Usually this refers to steam or extra heat applied to the leaving refrigerant gas off an evaporator.

Total Heat - The summation of sensible heat and latent heat. Usually used or assumed in cooling applications where some condensation will occur.

Total Heat of Rejection - (THR) The summation of the heat needed to condense refrigerant that was evaporated “boiled away” in an evaporator and the heat picked up by the gaseous refrigerant as it cooled the compressor and the heat of being compressed (Evaporator Coil + Heat of Compression + Heat In Compressor). This is the total heat that needs to be rejected by the refrigerant condenser unit or coil to convert the gaseous refrigerant back to a liquid. Also see Subcooling.

Tube Face - (TF) See Tubes High

Tubes High - The number of tubes counted in the fin height direction of only one row of the coil. Also refereed to as Tube Face (number of tubes across the face of the coil).

Tube Stub - The short piece of tube that is connected between the header and the tube.

Water Hammer - A condition that can occur in coils where a slug of liquid inside a tube is propelled at a high velocity or is subjected to a quick high pressure spike. This rapidly moving slug of liquid or pressure spike “hammers” liquid (a non-compressible) against the inside of the coil tube or header and over time can cause premature leaks in the coil. Water hammer can come from condensate or water being partially flashed to steam, air or gas entrapment in the liquid stream, cavitation, rapid closure of fluid flow control valves down stream of the coil, etc.

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