What are the key elements of WWHRS Heat Exchanger design?

WWHRS Heat Exchanger design

Waste Water Heat Recovery for Showers (WWHRS) heat exchanger design varies between different manufacturers and different product types. The different designs ultimately affect the system's overall efficiency; the measure of how well heat is exchanged from the wastewater to the cold mains supply. Heat exchanger material choice also plays a large part in this. Combined they provide one of the key comparison points between systems.

WWHRS Heat exchanger design

There are two main ways WWHRS heat exchangers operate;

Shower wastewater passes through the centre of the heat exchanger with cold mains water running around the outside of it. Typical for most vertical exchangers like the Recoup Pipe HEX.

The cold mains water is contained within the heat exchanger and the shower waste water will flow over and/or around it. As it does with most Horizontal exchangers like the Recoup Easyfit+ and Recoup Drain+ Range.

There are several design and layout types of these counterflow-type heat exchangers available on the market. This article mainly focuses on the vertical type, although many of the principles apply to both.

The ultimate aim is for the heat exchanger to be as efficient as possible. The best way to achieve this is by creating as much physical contact as possible between the heat-transferring part of the exchanger and the waste shower water. If there are interruptions in this contact due to poor direction of the wastewater or deformations in the heat exchanger's surface, this can impact efficiency. Shaping of the exchanger can increase the available surface area for contact, such as the corrugations or ridges seen on some exchanger designs on the market, But this can also have a detrimental impact on the amount of water that makes contact with the heat exchanger, which then, in turn, reduces the efficiency. Therefore, a design that maximises surface area while maintaining surface contact of the wastewater will produce the optimal heat exchanger efficiency

Likewise, the contact of the cold mains water with the heat exchanger is just as important. A larger volume of water will take longer to heat. However, restricting the volume and flow can have a detrimental effect on the pressure of the system. Therefore a balance needs to be found between maximising surface contact while keeping any restriction of flow to an acceptable level.

Unlike the Pipe HEX which is designed to locate next to an SVP there are some vertical wwhrs systems on the market that take the place of a section of the SVP. These designs can be much larger than the Pipe HEX and so potentially can provide a greater area for surface contact. However, a downside to this style of heat exchanger is that as they are effectively part of the SVP they carry "black water" rather than only "grey water", black water carries more contaminants and debris which long term could have a detrimental effect to the internal surface, and therefore efficiency of the heat exchanger.

The design of these systems is often 100% copper, with an external coil carrying cold mains water around the central "core", carrying the black water. While Copper is highly conductive and is an excellent material for heat exchanger construction, this amount of copper potentially makes this style of heat exchanger prohibitively expensive, and also more attractive to theft. The efficiency can also be affected detrimentally due to the amount of copper that will be heated, holding heat that is not transferred.

The manufacturing process to produce the coil and core is also one with more potential for lack of consistency, compared to the simple pipe within pipe style of the Recoup Pipe HEX.

Materials

Copper or Stainless steel is typically used as the heat-transferring material for the heat exchanger. This is due to their thermal conducting properties. Copper conducts heat more efficiently than Stainless Steel at a given thickness, heating up more rapidly than Stainless steel does. Stainless steel is a cheaper material choice, but a greater surface area is needed to perform to an equivalent level as copper. Copper is also anti-microbial, meaning that it is much more resistant to bio-film build-up on its surface than Stainless Steel is.

The Recoup Pipe HEX with it's smooth surface and highly conductive copper heat exchanger produces a large surface area for contact, and thanks to the proprietary Turbo Rotator the amount of water that is able to make thin film contact with the heat exchanger surface is optimised, resulting in the high-efficiency figures published on the PCDB.