Cut out the fouling

Heat exchanger fouling is universally perceived as a very real and expensive problem, particularly for the chemical, petrochemical and food processing industries.

Fouling occurs across a broad spectrum of applications ranging from the heating of raw juice in the food processing industry - which leads to protein precipitate or crystallisation fouling - to chemical processes where polymerisation fouling can occur.

Since fouling increases the resistance to heat transfer, thereby decreasing the heat transfer coefficient, it makes the heat exchanger much less efficient, and in some cases may even cause it to fail completely. Once the heat transfer coefficient falls below a critical value then the heat exchanger has to be shut down for cleaning so that the deposit build-up can be removed.

Fouling can prove to be very costly. It has been estimated that as much as 50 per cent of the maintenance costs for heat exchangers and boilers is probably caused by fouling.

The traditional method of compensating for fouling and the accompanying decrease in efficiency is to specify a unit that is much larger than needed. Typically a heat exchanger can be up to five to six times larger than necessary, so that as deposits form on the heat transfer surfaces, and the unit becomes less and less efficient, it is still able to perform the duty necessary to carry out the processing application.

Specifying larger heat exchangers means that capital costs are higher, more space is needed, and often flooring has to be reinforced due to the weight of the units. Since increasing the heat transfer area does not actually prevent fouling, the best that can be achieved is to allow a longer time to elapse between cleaning and maintenance shutdowns. At worst it can lead to increased deposits, since the larger the heat transfer surface area the lower the velocity within the tube, resulting in an increased rate of fouling.

There are heat exchangers now available, however, that have been designed with non-fouling properties to eliminate, rather than simply compensate for, the fouling problems.

Exchangers that fall into this non-fouling category tend to be mechanical scraped surface or corrugated tube models. With the patented HRS Dynamic scraped surface heat exchanger, for example, the product is pumped down the inside of a tube containing a shaft on which scraper blades are placed at regular intervals. The shaft inside the tube is moved backwards and forwards giving a reciprocating longitudinal motion that moves the scraper blades along the tube wall, thus continuously scraping the wall. The result is that the product channels are kept clean at all times so the equipment works at optimum heat transfer efficiency. This type of self-cleaning element is also much less damaging than the more traditional cleaning methods and helps to ensure lower maintenance costs.

Mechanical scraped surface heat exchangers, since they are continuously self-cleaning, are ideal for treating the viscous fluids and products containing particulates often found in the food processing industry. They are equally suitable for evaporating water and solvents from industrial effluent. While a mechanical scraped surface heat exchanger is more forgiving than a static model, with regard to dealing with varying viscosity and particulate size, static units manufactured using corrugated tubes also have significant non-fouling properties.

Corrugated tube heat exchangers, because of their heat transfer enhancing properties, create turbulence in the media without using any mechanical shaking. Shallow indentations in the corrugated tube cause the boundary layer to be disrupted without significantly reducing the flow area. The enhanced turbulence created by the corrugations results in uniform temperature distribution and a higher partial heat transfer coefficient by increasing the Nusselt Number.

In the water industry corrugated tube heat exchangers are often used to maintain a constant temperature in the sludge digestion tanks of 35 degrees C ±2degC using hot water as the heating medium. The hot water is usually maintained at a temperature of between 60 degrees -70 degrees C to ensure that the sludge does not foul the heat transfer walls of the tubes. There are examples of corrugated tube heat exchangers undertaking these applications that, after six years in operation, have not yet needed cleaning.

By increasing the tubeside heat transfer coefficient, the temperature of the tube wall is closer to the temperature of the bulk fluid in the tubeside. This minimises any tendency to cause fouling due to 'burn-on', freezing or depositions. In addition, with a corrugated tube unit the higher turbulence created in lower viscosity fluids will minimise any tendency for deposition fouling to occur, even at low flow velocities.

The vegetable processing industry typically experiences polymerisation fouling, which means that a heat exchanger usually has to be cleaned every seven to ten days. If a corrugated tube model is used then cleaning needs to be carried out only every three to four months and, even then, purely for hygiene reasons.

Adeel Khan is Technical Sales Manager at HRS Heat Exchangers Ltd.