The heat is on01 October 2007

Two of the key concerns for anyone involved in process heating are to save energy and reduce running costs. But how many organisations really know how well – or badly – they are doing? Brian Wall finds out

At least 40% of industrial energy in the UK is used in process heating, according to The Carbon Trust. The range of activities is vast, embracing everything from drying, to evaporation, separation, curing, heat treatment, melting and chemical processing – just as it's been for years. The good news is that there's nothing very revolutionary about most of the technology used – a tubular heat exchanger from the era of steam is not very different to a similar unit today – but the devil is in the detail.
So first some basics: while steam remains the most widespread process heating medium, direct combustion also provides an efficient means of heat transfer – accepting the risk of contamination and its obvious limits with flammable products. Clearly, indirect combustion gets around the problems, but is less efficient, while direct electrical heating is efficient and clean – but still expensive.
Whatever heat transfer approach is used, given that two key concerns are to save energy and reduce running costs, there's a range of precautionary measures – some well known, others less so. For example, in energy-hungry drying processes, online moisture content monitoring can help eliminate over- or under-cooking, but it's also worth investigating removing as much moisture as possible in the processes prior to drying. Then, self evidently, equipment should not be left running when not required, but also that your process is not operating at a higher temperature than necessary – which means accurate measurement.
Beyond these, energy losses are reduced by insulating the outer surfaces of process equipment and, where possible, covering the surfaces of liquids. Certainly, all steam pipework should be insulated: even a small leak results in considerable steam waste so it's worth investing in reporting and maintenance. Also, on older steam distribution systems, where some pipework may be redundant, it should be isolated. And, since a major source of steam loss is sticking steam traps, these must be checked regularly.
Much of this is about good maintenance: for example, regular checks on the combustion air/fuel ratio on your boiler or furnace will avoid excessive fuel consumption and emissions. But it's also about sometimes thinking outside the box – looking at the possibility of recovering waste heat from the process to power the process itself or to provide space heating. Returning hot condensate to the boiler also saves energy and reduces maintenance.
The Carbon Trust suggests a number of ways to increase efficiency and cut costs:

Check the time the process is held at the processing temperature is no longer than needed
Check if the heating rate can be increased to reduce the overall time spent at a higher temperature
Check your process is not running for longer than necessary and the product being over-treated
Check utilities are not left on when the process is not running
Consider changes to scheduling, where possible, to allow more efficient use of the process equipment
Think about installing variable speed fans. Air-flow control in dryers and furnaces is sometimes achieved by dampers interrupting the flow from a fan, which is usually working at full load. Variable speed fans can be much more energy efficient.

Clever stuff
But what about efficiencies from prolonging the life of the heat transfer fluid and the system components themselves? As Petro-Canada America Gaston technical advisor Gaston Arseneault advises, the best approach is to prevent fluid degradation and solids formation from the start. "Solids can be formed in the fluid through thermal breakdown, oxidative degradation, process leaks or when improperly transitioning between incompatible fluids. Thermal degradation can be greatly reduced by not operating the fluid beyond the temperature range and heat flux recommended by the manufacturer. It's not normally a problem in electrical immersion heaters, as long as a sufficient flow rate and adequate turbulence near the heating elements are maintained."
A common form of fluid degradation in an electrical immersion system, however, is oxidative degradation. Some manufacturers offer an optional nitrogen blanket in the expansion reservoir to eliminate this threat. "Oxidative degradation results from the reaction of oxygen in the expansion tank vapour space with the fluid," explains Arseneault. "In addition to odours and increased acidity, these reactions form polymers or solids, resulting in an increase in fluid viscosity. A more viscous fluid is more difficult to pump, has poorer heat transfer characteristics and increases the chance of coking and sludging." He advises keeping the heat transfer fluid in the expansion tank below 65oC.
Gerry Christensen, technical chief at Arla Plast, one of Europe's leading suppliers of extruded plastic sheet, recalls how his company struggled with this problem. "Oil degraded very quickly and got very black. After six months, we had to put new bearings in the pumps … which cost a lot of money." Christensen experimented with 10 different heat transfer fluids before settling on Petro-Canada's Calflo HTF, a premium high temperature fluid recommended for systems operating with bulk temperatures up to 326°C. "We have now been running without problems for three years," he concludes.
More fundamentally, though, achieving good value, low energy is first and foremost about choosing the right kit. Yes, little has changed with the fundamentals of shell and tubular heat exchangers, but the materials, construction and measurement and control technologies have certainly moved on. Consider plate heat exchangers (PHEs) and other compact devices, for example. Reductions in size have gone hand in hand with improvements in performance and efficiency, so that whereas once they were limited to lower temperature and pressure duties, now they are far more capable.
A typical example is provided by the AlfaNova, a PHE made of 100% stainless steel. It is manufactured using a fusion-bonding technique that enables it to withstand pressures up to 20bar and temperatures from –50oC to +550oC – ma\king it ideal for process cooling, but also hydraulic oil cooling, laser cooling, hygienic and sanitary duties and water cooling and heating.
It's an example of process intensification at its best. Another is the ART (Alfa Laval plate reactor). Essentially a reactor in a box, ART is a heat exchanger that not only heats or cools a process, but provides the reactor vessel itself. The company claims that processes are accelerated and that mixing rates are higher, while dwell times are shorter. So product quality is improved, plant costs less to operate, in terms of energy and manpower, and downtime for servicing and cleaning, is minimised.

In the zone

Process heating and drying applications vary, with individual industry sectors requiring different temperatures and conditions:
Uses of low temperature process heating (30oC-100oC) include drying, curing, fermentation and laundering, using warm air or water to provide process heat – although, in the case of drying, higher temperatures may be needed for some applications.
Uses of medium temperature process heating (100°C-250°C) include dyeing, sterilisation, cooking and baking, generally using warm air or water as the means for heating.
Uses of high temperature process heating (250°C-1,200°C) include kilns, glass manufacture and metal foundries. These processes generally use heat provided by gas and oil burners, and electrical heating.
Unlike hot water, warm air has a limited capacity to hold heat. It is generally used for the lower range of process heating, and one of its many uses is for drying, where it supplies heat and removes water. Hot oil is used where a higher temperature range is needed that can't be achieved with water.

Brian Wall

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