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HVAC Bending


Tubular parts are often used in the HVAC industry, most often as a part of a heat exchanger. Tubular heat exchangers come in several varieties depending on the industry served. Heat exchangers consist of a tube making multiple passes through a given area to maximize heat transfer. A series of return (180 degree) bends in a single tube is the simplest solution. This is referred to as a serpentine configuration.

Serpentine bending, (making multiple and alternate 180 degree bends) provides some challenges for the tube bender manufacturer.
  • Special tooling is often required to allow the clamp die to hold the part for the second or third bend without crushing the previous bends.
  • Special low profile clamp die holders are required for part clearance.
  • Custom pressure die holders are required to allow clearance for the bend radius as well as provide support of the bend at tangent point.
  • Pressure die opening must be limited so as not to unbend the previous bend.
  • Wiper dies must be extremely thin to allow for 180 degree bends (plus springback) and not interfere with the part shape.
  • A custom wiper die holder may be required to mount the specialized wiper die.

. . . . and all of these special machine and tooling requirements are determined by the bend radius and final part shape. Make no mistake. Serpentine bending is a highly engineered product and requires close co-operation between product design, production, tooling designers, and machine designers to achieve optimum results.


REFRIGERATION AND COOLING

Copper

Copper heat exchanger coils used in small refrigeration applications are often bent on NC benders. Usually these coils are not self-supporting and require a table or flat surface to support the part during bending. The table limits rotations to 180 degrees so any out of plane bends must be done manually after the flat coils are bent.

Standard CNC benders cannot be used due to the difficulty in rotating the part. The length and weight of the coils make the part shape flexible enough that automatic rotations are unreliable and problematic.

Customized CNC benders can be used for this type of application as long as any out of plane bends are done first (at one end of the part) and then the serpentine bends can be formed by bending alternately to the right and then the left. Very large coils also require an assist to help move the weight of the earlier bends during the final bends so the tube does not kink or bow in the middle.

Steel

Mandrel bending is seldom used in this application due to the cut length of the part.

Industrial heat exchangers use serpentine bent tubes (multiple 179 degree bends). These are usually bent on manual or NC benders. Extremely long tubes in excess of 60 feet long are placed into the machine and bent. After each bend the operator manually positions and rotates the tube prior to the next bend. Again a table or support mechanism is required to maintain the flatness of the finished part. Very long coils also require an assist to help move the weight of the earlier bends during the final bends so the tube does not kink or bow in the middle.

Mandrel bending is never used due to the extreme length of the part. Steel heat exchanger coils are typically a larger OD than copper heat exchanger so care must be taken to prevent collapse in the bend area. Without the use of a mandrel, shaped groove tooling is the next best solution to maintain bend quality.

CNC benders are not well suited to this application due to the difficulty in rotating this type of part. A custom right and left bending machine could fabricate this part with an adequate support structure. One disadvantage to this solution is that the part shape will now sweep out a larger area of the shop floor as it sweeps on both sides of the bending machine..


HEATING

Forced air furnace heat exchangers

Many residential forced air furnaces use a tubular heat exchanger. The tubes used are a larger OD typically 1.5 to 2 inch and are made from a variety of steel products including stainless steel and aluminized steel.

A typical heat exchanger has from 3 to 7 bent tubes in the final assembly. Each tube typically has 3 - 180 degree bends. Some tubes are designed to lay flat with 180 degree rotations between bends. Others are designed to occupy less vertical space (but more width) and have rotations varying between 110 and 140 degrees.

Air flow through these tubes is critical for the proper exchange of heat so bend quality requirements are more stringent than other applications. Many of these tubes are mandrel bent. Correct sizing of the mandrel and proper selection of mandrel lubrication is important. The mandrel lube must rinse clean and leave no residue in the tube.

Many manufacturer have changed their designs to eliminate mandrel bending. The solution is to use a custom bend die that has machined cavities to allow the material to pucker on the inside of the bend and reduce the flattening on the outside of the bend. This pre-wrinkled die eliminates the mandrel (usually) and associated mandrel lube issues. Most wrinkle designs effectively lock the tube onto the bend die when 180 degree bends are made. In order to remove the tube the bend die must be split and opened vertically after each bend. A split die actuator adds this function to the bending machine.

These parts can be bent easily on a NC or CNC tube bender. Production rates with a good operator are similar. The CNC advantage is change over time between parts. If the tooling is not changed a CNC bender can change to a similar part program in seconds. An NC machine operator will take many minutes to adjust rotation and distance stops before restarting production.

Steam Heat Boilers

Commercial and residential boilers often use bent tube heat exchangers to heat the water to steam. Since these tubes are subjected to high heat they are made from heavy gauge steel (or stainless steel) tubing or pipe. Multiple serpentine bends are made in a single piece of tube often with 90 degree bends at one or both ends of the part.

These parts can be manufactured on NC or CNC benders. A heavy duty CNC machine is required due to the extra weight of the part and the extra power required to rotate the heavy part. A table or support structure is usually used to help support the part shape and weight during bending. A mandrel may be required to maintain ovality in the bend area due to the tight bend radius requirements.


HEAT EXCHANGERS

Fin Type heat exchangers

Copper or Aluminum heat exchanger coils used in air conditioner or refrigeration evaporator and condensing coils as well as air handler systems and are often bent on NC benders. Single return bends with long legs are made and inserted into a fin package. Return bends are then brazed onto the free ends to create a single cooling loop throughout the fin unit.

Mandrel bending is seldom used due to the cut length of the part. Occasionally these are pulled directly off a spool and straightener and cut to length prior to the bend.

CNC bending machines offer no advantages for this application.

Finned Tube heat exchangers

In some applications stamped steel fins are impractical. In order to increase the heat transfer ability, fins are directly attached to the tubes themselves. It is then necessary to bend the tubes after the fins have been attached. In some cases the fins are only applied to the areas of tube that remain straight. In this case the parts can be bent with more conventional serpentine tools. A split die actuator that can open the bend die a large amount is required.

In other cases the entire length of the tube is covered with fins and the tooling must be designed to accommodate the fin diameter.


 

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