Choosing the right heat exchanger for wastewater applications
By Matt Hale, International Sales & Marketing Director, HRS Heat Exchangers
Tuesday, 06 October, 2020
Heat exchangers are crucial to many processes in the water industry, whether as part of an aerobic or anaerobic digestion plant, for cooling or evaporation, and for sludge concentration. There are several different types of heat exchanger available and choosing the right one can appear daunting. For both economic and environmental reasons, maximising productivity is essential for companies to remain competitive, and correct heat exchanger selection is key to this, as is maximising the use of all outputs, including power and digestate produced through anaerobic digestion (AD).
What is productivity?
Productivity is measured by the ratio of output per unit of input. Many water companies see it as a balancing act: how can they increase their output without increasing their costs? It is intricately linked to efficiency, ensuring that every piece of equipment and every part of the process is optimised, delivering greater value for money to the operator.
Due to the pressures on water availability, Australia already has some of the most effective water treatment infrastructure in the world, and many areas are following Perth in considering the ‘direct potable reuse’ of treated sewage in the water supply. There is real potential to increase the efficiency of existing AD plants, not least because some of the oldest wastewater AD facilities are now able to upgrade and take advantage of the latest technology.
The potential of such upgrades is illustrated by the 163 wastewater AD plants in the United Kingdom, which account for around 28% of the country’s total AD capacity. Thanks to project upgrades and efficiency improvements, the UK AD sector delivered a 25% increase in power production from just a 12% increase in capacity between 2010 and 2015.
Understanding the process
The most important criterion when specifying a heat exchanger is to ensure that it will be fit for purpose. To do this, it is important to understand the material being treated, the heating (or cooling) medium and the process involved. Is the material in question liquid (dirty or clean water) or semi-solid (sludge or digestate)? Is it viscous? What is the extent of the temperature difference required? Is there the option to use a source of hot or cold water from another part of the plant, and will heat regeneration technology help to improve overall energy efficiency?
Other information must also be provided for correct heat exchanger selection. For example, does the product contain solid particles? Consider also whether it is shear sensitive, if the process needs a scraped surface heat exchanger (SSHE) or whether corrugated tubes will be enough to prevent fouling. Also bear in mind the space available for the heat exchanger, and the connection pipework and fittings. The more information an operator can provide, the more likely they are to get the right heat exchanger for the job.
Avoiding a false economy
After thoroughly researching the options available it may be that more than one type or model of heat exchanger is suitable for the purpose. To ensure optimum productivity, it is important to consider the total cost of ownership (TCO). Although some operators can be swayed by the initial purchase price, this can be a false economy. The lifecycle cost of a heat exchanger — particularly one used to process difficult materials such as sewage sludges — heavily depends on the costs of cleaning and maintenance. Buying a cheaper design that is more prone to fouling can quickly become more expensive to operate. To keep productivity high, it is therefore essential to consider the TCO of a heat exchanger, not just the purchase price.
The running costs of a heat exchanger also have a large effect on its overall financial viability, so selecting a heat exchanger that is energy efficient is key to enhancing productivity. Some heat exchangers have features specifically designed to improve the operational energy efficiency.
Scraped-surface heat exchangers also minimise potential fouling by keeping the tube wall clean and create turbulence within the material. Both actions help to increase heat transfer rates, and together they create a highly efficient heat transfer process ideal for high-fouling viscous materials like sludges.
Recapturing heat is also one of the easiest ways to improve efficiency, and heat exchangers represent the best way of doing this. They are an established technology, but despite their widespread use in industries such as food manufacturing and the chemical sector, they are often under-used in wastewater AD plants.
Making the most of heat
Surplus heat can be recovered for use in the AD process itself, for example to pre-heat feedstock or digesters to improve gas production efficiency; for water treatment, pasteurisation and concentration processes; to heat offices or for space heating; or to provide hot water for cleaning. Using surplus heat is also free, without the need to buy additional fuel, and all of these applications can be carried out using a suitable heat exchanger — a well-designed system could recover and utilise 40% of the heat produced by the plant.
Furthermore, heat exchangers can be used to pasteurise digestate (the biofertiliser left over at the end of the AD process) and are more efficient than using tanks with heating jackets, as they have a much lower heat requirement. Using heat exchangers means that effective digestate pasteurisation is possible using surplus heat, rather than needing to install an additional heat source such as a biomass boiler, which could add hundreds of thousands of dollars to a project. The right system can also provide a continuous pasteurisation process, using less energy than alternative systems, while allowing additional thermal regeneration levels of up to 60%. This saved heat can then be used elsewhere, such as an evaporation plant.
Heat can also be used to separate water from digestate by concentration. This technique can reduce the overall quantity of digestate by as much as 80%, greatly lowering the transport costs associated with the removal of digestate. A well-designed concentration system will include measures to retain the valuable nutrients in the digestate, while the evaporated water can be condensed and reused. For example, the captured water can be added back to the feedstock as it enters the digester, making the entire process almost self-sufficient in terms of water use and eliminating liquid discharges from the plant. After concentration, the treated digestate dry solid content can be as much as 20% (often a fourfold improvement), making it much easier to transport and handle.
Using the wrong heat exchanger can soon prove costly in terms of maintenance and spare part costs. In fact, in some cases it becomes necessary to replace entire heat exchangers when they have become too expensive to clean and maintain, something commonly seen in the wastewater sector. It is therefore important to make sure that heat exchangers are designed to make cleaning and maintenance as easy and flexible as possible, while reducing cleaning and maintenance intervals.
Choosing equipment that can be remotely monitored also makes it easier to anticipate problems, enabling predictive maintenance with the help of the manufacturer. Being able to quickly identify issues helps to keep efficiency high and provides more time to plan for maintenance, resulting in less downtime for operators.
Beware hidden costs
Another consideration is the overall installation cost, and whether fitting the heat exchanger will require the purchase of ancillary equipment. Corrugated tube-in-tube heat exchangers result in improved efficiencies, providing comparable thermal performance to other designs, but in a smaller footprint. These can often be skid- or frame-mounted, reducing the need for significant support infrastructure.
A balancing act
While a low purchase price may appear attractive, much of the TCO comes from running costs, and keeping these lower over the lifetime of the heat exchanger will provide a greater return on investment in the long run. Whatever solution you choose, make sure to consider its energy efficiency, maintenance requirements and the costs of any ancillary equipment before signing on the dotted line. Put simply, specifying the wrong heat exchanger could decrease your productivity — choosing the right heat exchanger will almost certainly increase it.
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