Spiral tube heat exchangers are suitable for anumber of traditional shell-and-tube applications.

Figure 1. A spiral tube heat exchanger uses multiple parallel tubes connected to pipe-like manifolds to create a tube-side flow path.
Spiral tube heat exchangers have been used for years in a number of applications. However, many OEMs and end users are still not acquainted with this type of heat exchanger. Spiral tube heat exchangers can provide a good alternative to shell-and-tube and other types of heat exchangers.

A spiral tube heat exchanger's design is based on the use of multiple parallel tubes connected to a pipe to create a tube-side flow path. If two flow paths are required for heat exchange, the multiple tube bundle is spirally wound to create a spring-like coil. This coil assembly is fixed by connection to a round, flat plate that also serves as the head of the shell assembly. The spaces or gaps between the coils of the spiral tube bundle assembly become the shell-side flow path when this coiled assembly is bolted or welded to a shell. Tube manifolds protrude through the head to provide tube-side connections; shell-side connections are made directly through the head. Threaded, flanged or weld connections also are possible as well as placing connections on the bottom of the shell assembly for different flow path configurations. The number, size and length of tubes as well as the diameter of the bundle and shell can vary, providing a variety of parameters for heat transfer optimization. A typical cross section is shown in figure 1. Spiral tube heat exchangers differ from shell-and-tube designs in a number of ways.

Figure 2. The square dimensional profile of the spiral tube heat exchanger allows the unit to have a smaller footprint than the long, rectangular shell-and-tube unit.
Efficiency.Because of the spiral flow paths imparted to the tube- and shell-side fluids, the effects of centrifugal force and secondary circulating flow enhance heat transfer on both sides in a counterflow arrangement. Tube-side enhancement is provided without the associated potential for plugging on both the shell- and tube-side of the heat exchanger. Because there are multiple parallel tube configurations, efficiency is not compromised by limited shell diameter sizes.

Construction. A number of features set a spiral tube design apart from a shell- and tube-design. A difference in profile allows the spiral unit to be more compact and and offer a smaller footprint than a shell-and-tube unit (figure 2). Space requirements for tube bundle removal are virtually eliminated.

When exotic material is required for the tube side, the spiral tube heat exchanger minimizes the material used. Heat transfer coefficients related to spiral tube design can reduce the total required heat transfer area resulting in less tubing for the same service.

Typical Heating Applications

Spiral tube heat exchangers can be used for many different functions in a number of heating applications.

Vaporizers/Steam Generators. The compact footprint and thermal shock resistance of a spiral tube heat exchanger make it suitable for use as a miniature boiler. The shell side is enlarged and acts as a disengagement space. Demister pads or baffles also can be included. Hot fluid or gas is circulated through the tubes as the heat source. Because of its compact size, it is ideally suited for skid mounting as part of an equipment package. A spiral tube design also may be a good choice for as a low flow, high purity steam generator because of the lower cost of construction for stainless and other exotic alloy materials.

Process Fluid Sample Heating. When a sample must be heated to get the optimum analytical results and pressure drop is critical, the multiple tube capability of a spiral tube heat exchanger makes it a solid choice for this application. The unit remains compact and can fit inside or outside an analyzer enclosure or on small sample analysis panels.

Acid/Caustic Heaters. Much like a shell and tube unit, a spiral tube heat exchanger can be manufactured with heavier wall tubes and other pressure parts to provide for added corrosion allowance. This is a significant advantage when compared to a plate type heat exchanger. When compared to a shell and tube heat exchanger, the spiral requires less tubing and other materials.

High Pressure/High Temperature Water Heater. Because of its high tube side pressure rating range, the spiral tube heat exchanger generates high pressure hot water for washdown and other cleaning purposes. Steam pressures up to 150 psig for the shell side are standard and higher pressure ratings are available.

Preheat and Regenerative Heat Exchangers. Spiral tube heat exchangers also are well suited for use as trim heaters/preheaters in a number of processes. Because the shell side can be removed without disconnecting piping, heat recovery from a fouled stream to a clean stream is possible when higher pressures preclude the use of a plate heat exchanger.

Immersion Heater. A spiral tube coil can be used as an immersion heater in tanks, vats or other vessels. Its flanged type head can be connected to a flanged connection on the vessel or a coil can be fully immersed in an open tank.