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EXCLUSIVE Why steam has no sell-by date

02 February 2012

Incredibly, steam accounts for around 35% of energy consumption within UK industry. In the context of potential cost and carbon savings this figure is significant - however steam is often relegated to the back burner when it comes to plant upgrades

Part of the reason why may be the perception of steam as old-fashioned - after all, it’s been an industry staple for over three centuries now - but this couldn’t be farther from the truth.

Today steam is essential to almost every food process, and has a seemingly endless number of applications. For example, steam-heated brat pans and jacketed kettles process everything from soups, stews, sauces, and syrups, to ready meals, dips and desserts. Pasteurisation – which uses pressurised steam to control micro-organisms such as E. Coli, Salmonella, and Listeria – is used in the treatment of meat carcasses, processed meat, dried fruits, spices and tea and extensively within the milk and dairy industry.

The process of sterilisation takes this a stage further, using steam to remove micro-organisms entirely. But microbes aren’t the only ‘unsavoury’ element of food which steam is used to eliminate. Odour neutralisation is another common application, such fat deodorisation where steam is blown through heated fat to remove volatile components.

Steam is applied directly for moisture addition. In the manufacture of air-dried instant noodles, for instance, the noodles are steamed before being deep fried. This process allows starch to swell and gelatinise, and increases the rehydration rate of the finished product. It also denatures protein and helps to fix noodle waves. A similar process of steam conditioning is used in the transformation of soft, dusty wheatmeal – a by-product of flour production - into hard pellets of animal feed.

Steaming success
The reason why steam has universal appeal is clear, especially when compared to common alternatives such as thermal fluids and hot water. To begin with, the water required to produce steam is cheap, accessible and environmentally friendly. Steam itself is relatively easy and inexpensive to transport thanks to the natural flow of steam from high to low pressure (so no need for costly circulating pumps) and the high heat content of steam, which requires much smaller bore pipework. This latter benefit not only reduces heat loss by radiation and convection, but makes installation more simple and cost-effective.

Once it reaches the plant, steam can then be transferred to the process, product or space to be heated with precision control over the delivery, quantity and temperature, and it can instantly respond to fluctuations in demand. What’s more because steam has the advantage of a constant temperature, gradients on the primary side of the heat transfer surface are removed, avoiding many of the quality and health and safety problems - such as product deformity and contamination - which are associated with other heat transfer media.

Increasing efficiency
Steam has a high latent heat of evaporation so it can hold six times more energy than the equivalent mass of water, and even more than thermal oils. With industrial steam boilers commonly exceeding the Energy Technology List’s (ETL) 89% efficiency threshold, the current technology is clearly designed to take advantage of this.

Take modern shell boilers, for example. Most feature three passes, which increases the efficiency of heat transfer and reduces exit flue temperatures and emissions. Certain vertical boilers take this a step further with four passes, a development which reduces maintenance by eliminating metal stress and refractory problems and also improves productivity by enabling the boiler to reach full capacity quickly.

Burner technology has also made significant strides in efficiency. Modulating burners match heat input to demand and, according to the Carbon Trust, can reduce fuel costs by up to 5% compared with on/off control. Even the position of the burner has changed. On our high-efficiency UL-S horizontal boilers, the burner is located in an off-centre position rather than centrally. This promotes good water circulation within the boiler and prevents steam bubbles formed by the lower smoke-tubes fouling the heating surface of the higher tube bundles. The result is dry steam and high heat transfer efficiency from a compact boiler.

Other improvements, which are usually incorporated as standard or available as add-ons, include: automatic operation and touch screen PLC controls systems (reducing costs associated with running, maintenance and human-error); better turndown; reduced excess air; and combustion trim controls. Furthermore, heat recovery on the blowdown system minimises the heat lost through flash steam to atmosphere, and economisers (which use the residual heat in the flue gases to pre-heat the incoming feedwater before it enters the boiler) can increase total efficiency by around 5%.

Full steam ahead
Generally speaking, steam is still the most practical and cost-effective choice for many applications within the food processing industry. The benefits of steam directly translate to cost savings - it’s been estimated that installing a new ETL approved steam boiler can result in savings of 15%. In our experience, this figure can be much higher with savings of 30% not uncommon.

The answer to why steam technology receives so little attention might be simply that it’s already accepted as the de facto choice. Steam delivers the performance and cost savings that all manufacturers need to stay competitive and meet targets - now and in the future.

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