By Matt Hale
Since the first spiral heat exchanger (SHE) was introduced 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.
Proponents of SHEs (sometimes known as shell and coil heat exchangers) cite the following advantages as making them suitable for handling challenging fluids:
- They have good thermal performance, even where the temperature difference between the two materials is small.
- The spiral design and use of a single channel is claimed to prevent fouling and be “self-cleaning”.
- Their compact design makes them suitable for installations where space and/or access are restricted.
- The counter-current flow provides an effective way to recover waste heat.
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.
While these comparisons hold true for smooth tube heat exchangers, they do not always apply to corrugated 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 minimizing fouling.
Non-fouling and self‑cleaning designs
The design of SHEs can create turbulent flow inside the exchanger. 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 it is unlikely in many real world situations, particularly where SHEs are used with sludge.
The corrugated tubular design of the HRS DTI Series reduces fouling in a number of ways. The tube-in-tube design offers a much larger channel than spirals, thereby reducing blockages. The corrugated inner tube also encourages turbulence, which increases heat transfer and reduces fouling. Tubes within the DTI Series are easy to clean and maintain. Removable bends make them easy to access, and there are no expensive gaskets to replace.
Unlike SHEs, the tubes in a corrugated tube design heat exchanger can be easily removed for inspection, cleaning and maintenance and a variety of flange connections are available. The unit is also made in easy-to-replace sections. Should the heat exchanger suffer internal damage, the failed section can be easily bypassed by moving the interconnecting bends. A replacement can be manufactured and fitted when it is ready.
In contrast, if a spiral is damaged, the complete heat exchanger must be sent to a workshop where many layers have to be cut out to access the point of failure. This is then re-welded and all the cut-away sections must be re-welded into place. The cost is significant and downtime can be many weeks.
Choosing the best solution
In many situations, we believe that a corrugated tube heat exchanger, like the HRS DTI Series, 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 patented reciprocating Unicus Series.
In order 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 examining specifications or quotations for SHEs compared to corrugated tubes, it is important to make sure to compare like-for-like and that the performance specifications are identical. Specifiers 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. This article appears in ES&E Magazine’s October 2019 issue.