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The following is a glossary of various heat exchanger and design terms. You can search for a word below or scroll down the list.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
- A -
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 trapped inside a coil preventing the fluid from touching the tube wall thus lowering 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 of resistance to overcome so as 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 FPM (feet per minute).
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.
- B -
Bent Stubs - Special hand bent 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 unobstructed air passage through the fins is decreased, raising the air velocity until carryover occurs (around 500 to 550 FPM).
- C -
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. Accumulated condensation on the finned coil surface that can be stripped off by a high air velocity (>500 FPM) and carried away in the air stream. Also see Bridging. Wet surface coils are designed to have a low air velocity.
Cascade Freeze-Up - A condition where ice develops at the tube-liquid film layer even if the bulk liquid temperature is several degrees above freezing. This ice film can build rapidly and unexpectedly, slowing the liquid flow in the tube and causing it to freeze faster in a cascading effect.
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 90o, to provide a mounting and sealing surface. The flange also increases the rigidity of the coil casing and minimize warping of the casing when brazing.
Cavitation - A condition where entrapped air or gas bubbles in a liquid stream rapidly expands in areas of low pressure (pump impellers, sharp edge transitions, bends). These growing, then collapsing bubbles can cause surface pitting, corrosion and/or erosion. They can also coalesce, creating large gas pockets that effect proper system operation.
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 coil pans.
Charged Coil - A protective option that leaves a positive low pressure nitrogen or inert gas charge in a coil after leak testing. This is recommended for coils sitting in a warehouse or at a jobsite for an extended time period before installation, and/or used to validate that a coil is still leak tight.
Circuit - When two or more straight tubes, or hairpins, are connected to each other in series by return bends, creating one continuous serpentine shaped tube. Also see passes.
Circuiting - The number of circuits (tubes) actually connected to a header. This sets the optimal fluid flow rate in the tubes, critical for proper coil performance. The number of coil circuits can be different to the number of tubes high. Also see double, full, half.
Circuiting Pattern - How the tubes of a circuit are positioned and interconnected within a coil so the fluid in the tubes moves in a certain direction relative to the airflow direction through the coil. This is important to maximizing thermal performance, liquid venting/draining, steam loading/condensate removal, or the proper condensing/evaporating of refrigerants.
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 where refrigerant vapor condenses to a liquid inside the condenser unit or coil. Not to be confused with liquid temperature. This needs to be known when sizing an evaporator coil. Also see liquid temperature and 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 capability 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 capillary or feeder tube to the coil tubes.
- D -
Distributor - A part supplied with the coil that takes the incoming liquid refrigerant and creates a specified pressure drop, then distributes the liquid evenly to the feeder tubes. The feeder tube OD, length, type refrigerant, heat load and suction temperature are evaluated so that a properly sized nozzle is inserted, or built into the distributor, to create the desired pressure drop.
Desuperheat - Is the process of cooling steam or gaseous refrigerant that is not at the saturation point or not ready to condense. The heat value in desuperheating steam is much less than the heat value in saturated steam when it condenses. Refrigerant desuperheating occurs in condenser coils.
Direct Expansion - (DX) Refers to the more common refrigerant evaporator coil. This coil usually has a direct expansion valve (supplied by others) connected to the coil distributor.
Double Circuit - Every tube in the first and second coil rows, or twice the number of circuits as the number of tubes high in the coil, are actually connected to the header.
Dry Surface - Infers that the air contains little or no moisture, because when cooled to the desired temperature, no condensation occurs on the finned surfaces of the coil (see Wet Surface). Thus a sensible air heat load.
- E -
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.
- F -
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 - Small OD tube that connects a distributor to the coil tube. Also called lead tube.
Fin - The strip of material that the tubes go through, and the air or gas travels over.
Fin Height - (FH) The length of a fin strip (perpendicular to the fin depth edge) measured in the perpendicular direction of the tube and the airflow. Also see Face Area.
Finned Length / Finned Tube Length - (FL) The length of the tube that has fins, 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 a fin strip (perpendicular to the fin height edge) measured in the perpendicular direction of the tube and parallel direction of the airflow. Also see Rows
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" and "helical fin".
Fluid - Any substance that cannot retain the shape of its container once removed from the container. So gases and liquids are considered fluids.
Fluid Flow - Usually refers to the liquid or gas that flows inside the tubes of a coil.
Fluid Flow Rate - The amount, over time, of liquid or gas being supplied to the coil. Usually measured in GPM (Gallons per Minute) or Lbs. /Hour.
Fluid Pressure Drop - The amount of resistance to over come so as to push the fluid though the headers and tubes of the coil at the required fluid velocity. Usually measured in Feet of water head.
Fluid Velocity - The speed at which the fluid moves through the inside of a tube. Usually measured in FPS (feet per second).
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, are actually connected to the header.
- G -
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).
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- H -
Hairpin - 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.
Half Circuit - Every other tube in the first row, or half the number of circuits as the number of tubes high in the coil, are actually connected to the header.
Helical Fin - (Also referred to as "Spiral Wrapped"), where a narrow strip of material is tension wound onto individual coil tubes, like threads on a screw.
Hub Connection - A style of attaching the connection to the header. Ideally a hole for the next smaller pipe size than the header, is cut in the header pipe at the desired connection location. A smaller connection pipe is then attached. Also see MITER.
Header - A large vessel, usually a pipe, where tubes are connected, and used to distribute or collect the fluid to / from the coil circuits. It also has one, sometimes more, large connections that are connected to external piping. Headers are sometimes refereed to as a manifold.
Holding Charge - See Charged Coil.
- I -
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 cleanablity. Also see Staggered.
Intermediate Drip Pan - Additional pan to catch condensation on coils over 59" in 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.
- J -
- K -
- L -
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.
"L" Footed Fin - A type of fin-to-tube contact for helical fins where a portion of the fin is laid down parallel with the tube. Then the edge of the next fin overlaps slightly onto the previous fin's foot, keeping the fin-to-tube contact tight.
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, or the default hand, unless otherwise stated.
Liquid Refrigerant Temperature - This is the temperature of the condensed gas refrigerant + the sub-cooling of the liquid.
- M -
Manifold - See Header.
Miter Connection - 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) rotated 180° and attached back to the header. The connection size is 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.
- N -
Nucleoid Boiling - A condition where a liquid in contact with hot coil tubes develops gas bubbles (boils), even if the bulk liquid temperature is well below the boiling point. Areas where bubbles develop can get overheated, build-up deposits, and become pitted when bubbles rapidly collapses after contacting the cooler bulk liquid, setting up localized shock waves. Over time tube failure can occur.
- O -
Oil Logging - A condition in refrigerant coils where the circulating oil can block or "log" up a part of the coil causing lowered coil performance and system cycling. This can be avoided if the coil is design for the required mounting orientation and operating temperature.
- P -
Pans - The part of the coil casing that forms the top and the bottom of the frame. This describes the shallow pan shape that's created when these casing parts are fabricated.
Passes - The number of times fluid moves back and forth though the straight tube sections of one circuit before exiting that 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.
- Q -
- R -
Raised Face Slip On - (RFSO) A type of flange that has a raised surface that mates with another flange that slips onto a pipe then is attached by brazing or welding.
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 not considered as 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.
- S -
SCFM - The cubic feet of per minute of air 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 while cooling the air, then the water vapor portion is considered a 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 capability of the fin more than a flat or corrugated fin surface, but at a higher air friction. Also see Plate and Corrugated.
Spiral Wrapped Fins - See Helical Fins
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 or fitting.
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 - Cooling the liquid, resulting from the condensation of steam or vapor/refrigerant, below the temperature point where it condensed. Most condenser coils have a small sub-cooling section. This extra cooling prevents the liquid refrigerant from "flashing" back to a gas as it picks up heat on the way to the evaporator coil. Also see "Liquid Refrigerant Temperature".
Suction Temperature - The temperature of the gaseous refrigerant that a refrigeration 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 its boiling point. Usually this refers to steam, or the extra heat applied a refrigerant gas before it leaves an evaporator coil. This helps the thermal expansion valve to properly modulate refrigerant flow for the cooling load, so no or minimal liquid is sent to the compressor.
- T -
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 required to "boil away" refrigerant in an evaporator, plus the heat picked up by the gaseous refrigerant as it cooled the compressor, plus 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 in the first row across the face of the coil).
Tube Stub - The short piece of pipe that is connected between the header and the tube.
- U -
- V -
- W -
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. Over time this can cause premature leaks in the coil. Water hammer can come from condensate or water being partially flashed to steam, entrapped air or gas in the liquid rapidly expanding at low pressure points, or shock waves from rapid closure of fluid flow control valves down stream of the coil, etc.
Wet Surface - Infers that the air contains significant moisture, because when cooled to the desired temperature, condensation occurs on the finned surfaces of the coil (see Dry Surface). Thus a sensible + latent, or total air heat load.
- X -
- Y -
- Z -
For more definitions and unit conversions visit:
http://www.engineeringtoolbox.com/hvac-terms-d_246.html