Since the first spiral heat exchanger was proposed back in the 19th century, they have become synonymous with the treatment of slurries and wastewater; materials which have a high fouling risk, are viscous, or which contain fibres and solid materials.
However, in many situations the performance of a corrugated tube heat exchanger is equal to or better than that of a spiral design. Not only that, but corrugated tube designs provide a number of additional advantages in terms of day-to-day operation and maintenance.
The Advantages of Spiral Heat Exchangers
Proponents of spiral heat exchangers (sometimes known as shell and coil heat exchangers) cite the following advantages as making them suitable for handling challenging fluids: good thermal performance; the spiral design and use of a single channel is claimed to prevent fouling and be ‘self cleaning’; compact design makes them suitable for installations where space and/or access is restricted; and the counter-current flow provides an effective way to recover waste heat.
Based on such claims, it would seem that spiral heat exchangers are the ideal option for high-fouling wastewater and sludge situations. However, given how frequently clients choose to replace their existing spiral heat exchangers (SHEs) with corrugated tube units, some of these claims need closer inspection.
Thermal Performance & Efficiency
There is no doubt that in theory SHEs offer greater thermal efficiency than conventional smooth surface tubular designs due to their large surface area and true counter-current flow. However, this assumes that the barrier between the product and service fluids is kept clean and operates efficiently at all times. In practice, fouling frequently occurs, interfering with thermal transfer. Where the heat exchanger is used for sludge-to-sludge applications, this fouling layer can create a double barrier to efficient heat transfer.
Although these comparisons hold true for smooth tube heat exchangers, they do not always apply to corrugate tube heat exchangers. For example, like SHEs, the tube-in-tube HRS DTI Series is a true counter-current heat exchanger with the product flowing through the inner tube, and the service fluid flowing through the surrounding shell. The use of HRS corrugation technology increases heat transfer and operational efficiency, while also minimising fouling.
Non-Fouling and ‘Self Cleaning’ Designs
The design of SHEs can create turbulent flow inside the exchanger. In turn this is claimed to reduce the likelihood of fouling, and that where blockages do start to occur, product flow speeds up; creating a ‘scrubbing’ effect that dislodges the blockage (so-called ‘self cleaning’). This is fine in theory, but in our experience is unlikely in many real-world situations, particularly where SHEs are used with sludge.
In fact, when you read further into manufacturers’ brochures, ‘self cleaning’ often becomes ‘virtually no fouling and clogging’ – two very different claims. Newer designs do not have the same support framework to separate the coils as some older models, as such structures were ideal at picking up rags and fibres from the sludge, leading to further blockages and a consequent reduction in heat exchange efficiency. As a result, manufacturers of SHEs also claim that their units are easy to clean.
However, it is not uncommon to have to remove 40-50 bolts just to open the unit. Once opened, the construction means that the service fluid channel contains a large number of brackets which not only give the heat exchanger its rigidity, but also make cleaning difficult in sludge-to-sludge situations. There is also the issue of the expensive bespoke gaskets which are used to seal SHEs, and which need to be replaced each time the cover is refitted.
Manufacturers of SHEs know that users don’t want to remove 50-odd bolts before they can clean the unit, (and have to do them all up to the correct torque again). They also acknowledge that many materials, such as sewage sludge, require frequent cleaning. To overcome this they produce SHE models with hinged covers, which are held shut with C-clamps, so you don’t have to undo the bolts. Although this type of design improves access, it increases the amount of space required for the installation and contradicts claims of SHEs being self-cleaning.
Choosing The Best Heat Exchanger
In many situations, a corrugated tube heat exchanger is the best solution for challenging materials such as sludge. However, in the most extreme cases with a very high fouling risk, it will be necessary to use a scraped-surface heat exchanger such as either the HRS rotating R Series or the reciprocating Unicus Series.
To select the best solution it is necessary to accurately assess the physical properties of the product or sludge to be treated, as well as the service fluid, inlet and outlet temperatures and the amount of heat regeneration required (if any). When comparing specifications or quotations for different solutions – for example SHEs compared to corrugated tubes – it is important to make sure that you are comparing like-for-like and that the performance specifications are identical. You should also assess whether any gains in efficiency or lower capital cost will be offset by increased fouling or operational costs over the full life of the unit.
Matt Hale is with HRS Heat Exchangers