In applications prone to high fouling, consider using a spiral heat exchanger. Its single-channel design minimizes fouling and erosion and helps ensure high flow velocities even with heavy process slurries.

Figure 1. A spiral heat exchanger consists of two long flat plates wrapped around a mandrel or center tube, creating two concentric sprial channels.
Heat exchanger fouling is a major source of maintenance costs and lost production time. It has been estimated that fouling costs U.S. industry more than $5 billion annually. Over time, as material builds up on the heat transfer surfaces of a typical heat exchanger, an insulating layer is formed that reduces the heat transfer rate and increases pressure drop through the exchanger. Eventually, the heat exchanger must be cleaned to restore the heat transfer rates and pressure drops required by the process. With many traditional heat exchanger designs, cleaning is time-consuming and costly, and it may need to be performed frequently. A spiral heat exchanger can help processors avoid these problems.

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.

Figure 2. In a spiral heat exchanger, the hot fluid flows into the center of the unit and spirals outward toward the outer plates while at the same time, the cold fluid enters the periphery and spiral inward, exiting at the center.

Construction and Operating Principles

A 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).

Figure 3. In a spiral heat exchanger, each fluid flows through a single channel. If suspended solids settle on the heat transfer surface, the increased fluid velocity creates a scrubbing effect that removes the deposits.
At the same time, the cold fluid enters at the periphery and spirals inward, exiting at the center. Channel spacing can be varied from approximately 0.25 to 1" to optimize channel velocities and heat transfer coefficients.

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.

Figure 4. A spiral heat exchanger can be cleaned by removing the cover and spraying the interior with a pressure washer or by using other mechanical means. Most spiral designs are only 6' deep, so it is easy to access the entire heat transfer surface area.

Fouling and Plugging

In 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 used in many processes prone to heavy fouling and erosion.
For example, if 0.25" particles settle in a spiral heat exchanger channel that is 0.5 high by 24" wide, the channel geometry is reduced to 0.25" by 24". Because the fluid has no alternative channels in which to flow, the channel velocity doubles and the shear rate increases by a factor of four, causing the solids to be resuspended and flushed out of the exchanger.

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.

Spiral heat exchangers can be used in processes where erosion is expected. Because a spiral heat exchanger is less prone to fouling, local fluid velocities are subject to less variation.
Spiral heat exchangers can be used in most applications in the chemical process industry. In difficult services where fouling is a concern, life cycle cost should be considered when a heat exchanger is designed. Calculating the installation, operation and maintenance costs up front will provide a perspective on the equipment's total cost.

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.