Recent energy cost increases and the introduction of anaerobic thermophilic processes as part of wasted industrial process and municipal-sludge digestion has prompted the use of heat recovery exchangers. Attempts to modify existing spiral and tube-and-tube heat exchangers to prevent exchanger plugging or baking in heat recovery applications for wasted industrial process and municipal-sludge digestion have largely failed. This has led to the development of large-gap, rectangular-channeled heat exchangers. They are designed to provide non-plugging operation in the presence of stringy and sticky materials found in some industrial processes as well as municipal sludge.

Large-gap, rectangular-channeled exchangers are finding use in industrial pasteurization applications such as dairy, beer or other liquids. They also are being employed in wastewater treatment plants (WWTPs) as well as in dairy cow manure farms. They can be used to heat sludge in a mesophilic digester (95°F [35°C]) or a thermophilic digester (134°F [55°C]). They can help processors conform to the current Class A solids-disposal law via heating the waste to about 160°F (71°C) for pasteurization.

Many large-gap heat exchangers have been installed successfully and operated without any maintenance for many years in industrial applications, in the production of molasses and, of course, in WWTPs. In some of these applications, it may be possible to recover the cost for this type of direct sludge-to-sludge heat recovery equipment within six to 18 months. This allows for ongoing savings in heat recovery operations.

Large-gap, rectangular-channeled heat exchangers are composed of a number of horizontal passes positioned atop each other, forming separate and enclosed cold- and hot-media passages in between which the transfer of heat occurs. Each pass is composed of number of parallel channels. This configuration of channel passes allows for unobstructed liquid flow, making the construction of the unit void of internal structural supports such as studs.

Rectangular-Channeled Heat Exchangers

• The heat exchanger design allows liquids with a high percentage of solids (up to 26 percent solids) to flow without plugging or baking. The inherent design of the rectangular-channeled heat exchanger imparts specific benefits:
• It includes channels with 3” or more height gaps for each of the liquids. This helps reduce the risk of plugging.
• The design allows the width of the channel to have a fast flow rate of 3 ft/sec minimum. This also helps reduce the risk of plugging.
• Using rectangular-channeled heat exchangers allows the use of liquids with high percentage of solids and high fluid viscosity in both sides of the heat exchanger.
• This type of technology provides more heat transfer for the same flow. (By contrast, round tube has the lowest surface heat transfer area compared to any shape.) Having a larger heat transfer area provides positive benefits.
• Special systems enable users to clean both liquid sides quickly.
• Doors or gaskets are not required.
• The rectangular-channel technology offers the double-wall design as required by law for gray or black water recovery.
• Having outside bends increases the turbulence and lowers the risk of struvite occurrence.
• The counterflow design helps improve heat transfer.
• Insulating the units can improve the heat transfer and safety.
• Rectangular-channeled heat exchangers are smaller in size than tube-in-tube exchangers.
• The design provides the ability to have two heat exchangers in one frame. This allows for direct sludge-to-sludge heat recovery and then a trim heating via hot water of the preheated sludge. Both subunits are independent.
• It is possible to stack one heat exchanger on top of the other to save space.
• The units are designed to ASME high-pressure vessel requirements as well local requirements.

Large-gap, rectangular-channeled heat exchangers are designed to provide non-plugging operation even in the presence of stringy and sticky materials. Common uses include some industrial processes as well as municipal sludge.

Typically units are manufactured from welded A570 or 44W mild steel plates or 516-70 or 304L, 316L stainless steel plates. The units are composed of a number of horizontal passes positioned atop each other. This forms separate, enclosed cold- and hot-media passages in between which the transfer of heat occurs.

Each pass is composed of number of parallel channels. This configuration of channel passes allows for unobstructed liquid flow, making the construction of the unit void of any internal structural supports such as studs. Stacking one unit on top of the other is possible to save space for the plant and piping. This type of heat exchanger is designed to provide a fast flow even for liquids with high viscosity.

The design incorporates a plug for every two channels, which allows cleaning from both sides if required.

An available cleaning ports system for the sludge sides of the unit are designed to ease access for manual cleaning of hot- and cold-media internals if required. The design incorporates a plug for every two channels, which allows cleaning from both sides if required. The compact unit requires a small footprint while providing reliable operation. A pig cleaning option allows self-cleaning of the inside channels if performance is reduced due settling of the liquids on the surface.

All of these features combined can provide benefits for sticky and stringy media, or for liquids with a high percentage of solids.