The primary loop is where the lake or river water circulates. It usually includes the intake pipes, one or several filters and the first heat exchanger(s). The water contains impurities, nutrients and small aquatic organisms, so that the primary loop is exposed to biochemical problems such as colonization by algae.
The heat exchangers in the first circulation loop are directly in contact with lake or river water (filtered or not). Tubular (generally shell and tube) heat exchangers and plate heat exchangers are the two main construction types. Tubular heat exchangers are easy to clean physically (especially in the case of straight tubes) (Kazi, 2012), while plate heat exchangers are popular because of their high effectiveness, small size, and low cost. Heat exchangers are a vital part of thermal use infrastructure, and especially two of their properties:
For most heat exchangers operated with lake or river water, the occurrence of biological growth must be considered. In particular, it is common to observe the growth of bacteria, algae and mussels, which is known as biofouling (Kazi, 2012). Biofouling is likely to negatively impact both of the above-mentioned properties (Kazi, 2012). Indeed, the buildup of organisms on the exchanger walls increases friction and turbulence, therefore reducing the flow (or increasing the pressure drop if a constant flow is maintained). In addition, this layer lowers the heat exchange coefficient. Both effects add up to increase the energy needs of the system, as more fluid has to be pumped to exchange the same amount of heat. Sometimes, biological growth promotes other types of fouling (e.g., chemical precipitation) by changing the conditions locally (Langford, 1990). Scaling and corrosion are rare in thermal use of freshwater.
Biofilm growth is particularly critical. Biofilm is a layer of bacteria which attach to a surface and develop progressively, protected from external perturbations. By covering the inner surfaces, the biofilm ultimately increases friction and reduces heat transfer efficiency. The factors favoring biofilm growth are: low shear velocity, warm water, abundance of nutrients and rough attachment surface (Rajagopal et al., 2012; Bott, 1995; Kazi, 2012; Jenner et al., 1998). Under harsh conditions, especially rapid flow, biofilm will expectedly grow slower and be thinner and denser. It is worth to note that suspended solids can have an erosive effect and help preventing biofilm formation (Bott, 1995).
It has been observed that colonization by macroorganisms also depends strongly on water velocity (Rajagopal et al., 2012). Zebra mussels were found abundant in areas where velocity is in the range 0.1-0.5 m s-1 and absent where it exceeds 1 m s-1. Even though colonization sometimes also occurs at higher velocities, a velocity of ~1.4 m s-1 (at 1 mm from the walls) has been recommended to prevent fouling by macroorganisms (Jenner et al., 1998). Ideally, the surface should be as smooth as possible.
Many mitigation and cleaning techniques can be applied against biofouling (Jenner et al., 1998):
In the case that biofouling occurs sufficiently slowly, on-line techniques are generally not necessary. Regular physical cleaning (e.g., brushing or jet washing) is generally the optimal option, at a frequency ranging from every few months to every few years. For installations requiring constant operation, several heat exchangers are generally mounted in parallel, allowing for cleaning of one exchanger with no system interruption. Chemical cleaning should only be considered in case physical cleaning is not feasible, as it requires cautious flushing and additional safety measures to protect both workers and the environment (Bott, 1995).
In the end, the choice of the governing parameters to be included in the operational scheme (flow rates and working temperatures) will be a trade-off to maximize heat exchange and minimize biofouling problems, so as to reduce overall costs. Basic design concepts include: simplicity, maximization of flow velocity, minimization of the temperature of the exchange surface, use of materials unfavorable to bacterial attachment and easiness of maintenance (Bott, 1995). For instance, heat exchangers are often made of copper as this material has a high thermal conductivity and resists well to biofouling. Operating heat exchangers for thermal use of lakes and rivers implies irregular conditions, which makes biofouling control more difficult.