Exchange Your Exchanger
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.
When the application requires sludge in both sides of the heat exchanger (to have sludge-to-sludge heat recovery), then the tube-in-tube arrangement will cause the flow in the outer pipe to be slow, which will result in baking and plugging. Therefore, a design engineer would want to have at least 3 ft/sec fast flow to limit or avoid baking to the surface due to the difference in the temperatures of the two liquids. This technology cannot have sludge in both sides of the heat exchanger.
An alternative to these approaches incorporates these three components:
Utilize direct, sludge-to-sludge heat recovery. It requires less bio-gas, natural gas or electricity, which could be used for other heating purposes. Alternately, the processor can reduce the overall usage and lower utility costs.
Preheat the cold raw sludge with the hot water. This will preheat the cold raw sludge to a temperature in between the cold raw temperature of about 60°F (15°C) and the desired temperatures of 95 or 135°F (35 or 52°C). Inserting preheated sludge into the digester will improve the digester’s process and lower the liquid temperature inside the digester.
Boost efficiency by lowering the carbon emissions via direct sludge heat recovery from the wasted digested hot sludge. If this component is not included in the solution, it is simply wasting energy. A small heat recovery exchanger (500,000 to 1 million BTU/hr) can reduce the CO2 pollution by 29,215 tons in a year, or the equivalent of removing more than 19,000 cars from the roads.
When designing a system to satisfy the specified criteria, there are 10 requirements that the design engineer must comply with to lower or eliminate the risk of plugging, baking or ending up with holes in the channels.
There should be 3" gaps at a minimum so sludge blockage does not occur due to the height of the channel. The system must be designed for the optimum height for each liquid.
To avoid baking and achieve the fastest flow, the system must be designed for optimum width. This means a minimum of 3 ft/sec is required.
A non-block design - without spacers or obstacles - in the flow in both sides of the heat exchanger for both liquids also is necessary.
More turbulent flow than is possible with tube-in-tube or spiral technologies is required for better heat transfer due to the zigzag channels and side bends design.
A more efficient solution will have a smaller footprint than tube technologies so less area is required. Also, a modular system can be used for water-to-sludge or sludge-to-sludge heat recovery. Backup units of water-to-sludge, while not in use, could be used for sludge-to-sludge heat recovery.
Maximize Space Too
It will have a larger circumference than tube-in-tube technologies, which translates into a larger heat transfer surface with the same flow area.
Direct sludge-to-sludge systems do not require a macerator and blades to grind sludge, which can result in capital investment and ongoing maintenance savings. In addition, the heat recovery on a more efficient system can provide payback within one year.
Individual doors should be designed in to provide full access to the internals. Davit arms allow the doors to swing easily.
As safety measures the system should include:
A design calculation that uses realistic viscosities, usually in the range of 2 to 40 cPs.
Plate thickness of a minimum 0.25" and allowance for corrosion and abrasions.
Heat exchangers that are tested and proven.
Simplify Maintenance Demands
Little maintenance should be required. Many older technologies require weekly cleaning and service.
Technology that embraces these 10 components does exist in the form of rectangular channeled heat exchangers, which can utilize direct sludge-to-sludge heat recovery without plugging or baking and with low maintenance.