Rectangular channeled heat exchangers can replace older technologies and relieve associated problems.
Heat exchangers have been used in wastewater treatment plants for many years, usually in applications with hot water on one side - from boilers that use bio-gas recovered energy, if possible - and mixed digested recirculating sludge from the digester on the other side. The basic principal in this application is to heat the sludge to the desired 95°F (35°C) for mesophilic process (Class B bio-solids) or to the preferred 135°F (57°C) for thermophilic process (safer Class A bio-solids, according to new regulations).
In the past, older equipment and technology were used to flow the cold raw sludge, which enters the wastewater treatment plant directly into the digester, where it was mixed with treated and already digested sludge. However, this process of digestion actually requires long retention times inside the digester for the mixed sludge. Plus, it lowers the temperature inside the digester, which is not a desired outcome.
In addition, there were other disadvantages associated with technologies such as spiral and tube-in-tube. For instance, spiral technology, which is well known and commonly used in wastewater treatment plants, cannot have more than a 1" gap. Yet normally in this application, a 3" gap is required to avoid or limit plugging. The 1" gap limit creates the need to send the cold raw sludge, which is at about 5 to 7 percent solids and very viscous (40 cPs), into the digester to mix it with the already digested sludge to reduce its viscosity and allow the mixed, recirculated sludge to flow in the spiral heat exchangers. The spiral heat exchanger also has pins on one side. That limits the use of that technology even further by allowing only water to flow in the side with the pins. Spiral exchangers cannot have cold raw sludge in them and definitely cannot have sludge in both sides.
Tube-in-tube technology can have sludge flowing only in the inner central tube, where a 3" gap could be achieved. While this is promising, if sludge were to flow into the outer tube, the outer pipe would require a 9" dia.