No More Fouling: The Spiral Heat Exchanger
A spiral heat exchanger is a useful alternative to shell and tube designs for many applications prone to fouling and plugging problems. For more than 60 years, it has been used in difficult services ranging from PVC slurry coolers to asphalt heaters. Its flow-channel geometry and single-channel design induce highly turbulent flow, so the compact exchanger can operate reliably with low fouling rates even in heavy fouling, fibrous or slurry duties. It can be opened quickly and easily for inspection, cleaning and maintenance.
Construction and Operating PrinciplesA spiral heat exchanger is composed of two long, flat plates wrapped around a mandrel or center tube, creating two concentric spiral channels (figure 1). The channels are seal-welded on alternate sides to provide a sturdy barrier between the fluids. A cover is fitted on each side, and a full-faced gasket is positioned between each cover and spiral element to prevent fluid bypassing and leakage to the atmosphere. Access to the hot and cold heat transfer surfaces is gained by removing the respective covers. Connections are installed in the center of each cover and on the peripheral pockets.
Countercurrent flow maximizes heat transfer. The hot fluid flows into the center of the unit and spirals outward in the long, flat, rectangular channel toward the periphery (figure 2).
The spiral heat exchanger can handle heat loads up to 20,000 kBTU/hr and flow rates of 3,000 gal/min, with heat transfer surface areas up to 6,000 ft2 in a single unit. Many applications with higher heat loads and flow rates -- ammonia liquor cooling in coke plants or foul condensate interchangers in pulp mills, for example -- are handled with multiple spiral units installed in parallel. Capable of handling pressures up to 500 psi and design temperatures up to 1,500oF (816oC), a spiral heat exchanger can be constructed of carbon steel, stainless steel, titanium or any other metal that can be cold formed, rolled and welded.
Fouling and PluggingIn a spiral heat exchanger, the single, curving channel and presence of spacer studs create a rigorous flow path that ensures turbulent flow regimens even at low velocities. There are no dead spots, so velocity is uniform throughout the channel. Its curved design causes the flow to continuously impinge upon the heat transfer surface, creating high shear rates and preventing solids from clinging to the wall.
Suspended solids present a major problem in most heat exchangers in applications such as pellet water coolers as well as catalyst slurry heaters and coolers. If the solids begin to settle on the heat transfer surface, the channel's cross-sectional area is reduced. In multiple-channel heat exchangers such as the shell and tube design, this creates flow distribution problems as flow is diverted away from partially fouled channels to nonfouled channels. As a result, flow velocity in the fouled channel is reduced. With lower velocity, the tube continues to collect solids until it eventually plugs, eliminating all flow through the tube. Once many tubes are plugged, the unit must be cleaned before it will operate effectively.
A spiral heat exchanger has a single channel for each fluid. If solids settle onto the heat transfer surface, the cross-sectional area of the channel is decreased, yet the fluid has no alternative channel in which to flow. In areas with solid deposits, the local velocity and shear rates are increased, creating a scrubbing effect that removes the solids from the wall (figure 3).
Spiral heat exchangers are designed with fouling factors between one-quarter to one-eighth of standard TEMA fouling factors. Even with these lower fouling factors, the spiral heat exchanger generally can be expected to operate effectively three to four times longer than a shell and tube exchanger before cleaning is required.
Another problem associated with heavy fouling applications is erosion. Erosion occurs when the local velocity in a heat exchanger becomes excessive and begins to wear away its walls. With many process slurries, a fouling problem can lead to erosion if the local velocity cannot be controlled effectively. Some examples of erosive services are TiCl4 slurry cooling in titanium dioxide plants and bauxite slurry heating in alumina plants. If the flow is not evenly distributed or if fouling diverts a large portion of the flow, local velocities may vary significantly within the heat exchanger. This can increase fouling in low velocity areas and accelerate erosion in higher velocity areas. With a spiral heat exchanger, fouling and variations in local flow velocities are reduced and erosion can be minimized.
Eventually, every heat exchanger must be cleaned. A spiral heat exchanger's heat transfer surfaces can be fully accessed for inspection and cleaning by removing a cover and cleaning with high-pressure water or other mechanical means (figure 4). Held in place with hookbolts, the covers can be removed and replaced easily. And, because the spiral channel is a maximum of 6' deep, all heat transfer surfaces can be easily reached. The covers are often fitted with hinges or davits to facilitate opening and closing the units. In general, a spiral heat exchanger can be opened, cleaned and resealed within 4 to 6 hours.
In many difficult applications where fouling and plugging are problems, a standard shell and tube design may not be effective. While a spiral heat exchanger often has a higher initial cost, it may provide a lower life cycle cost due to lower fouling rates and ease of maintenance.