Efficient heat exchange for major energy savings

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Related tags: Heat exchanger

Industrial boilers, which are used in a number of food processing
applications, have become the focus of a decisive trend towards
greater energy-efficiency. Food processors are more aware of the
potential savings to be made from reducing energy consumption, and
the new legislative regulations that have to be met, writes
Anthony Fletcher.

But the problem for food processors is that energy consumption is still necessary right along the production line. A chip manufacturer such as McCain for example needs steam power for peeling, chipping and cooking. Abiding by stringent emission regulations and achieving production cost cuts must therefore be weighed up with the ongoing need for energy.

Efficient processing is the answer, and Loos Europe​ sales manager Franz Doerr contends that flue gas loss is a key value in assessing the efficiency of a boiler system. The quantity of energy contained in the flue gases that are blown out unused through the chimney can translate into significant energy loss.

Because of the laws of physics, the flue gas temperatures of high-pressure steam boilers or high-pressure hot water boilers are relatively high. The water temperature of a high-pressure boiler with an average working pressure of 10 bar, for example, is around 180 °C.

The flue gases taken through the boiler can thus never be cooled to lower temperatures, however modern the boiler may be. This represents a massive loss of energy, which can be avoided by the use of convection passes known as flue gas heat exchangers.

"The boiler produces steam for energy, and the flue gas heat exchanger reduces the temperature of heat gases to reduce gas losses,"​ explained Doerr. "This reduces the amount of energy waste that goes out the chimney."

The temperature of flue gas can reach 250 degrees, but with a heat exchanger, this can be reduced to 130 degrees. This, said Doerr, translates into major savings.

"If you reduce the temperature of flue gas by, say, 20 per cent, this represents a one per cent saving in fuel. Reducing the temperature of flue gas from 350 degrees to 150 degrees would result in a five per cent saving. If you then work out that a typical factory operates over 8,000 hours a year, the cost savings in fuel are significant."

With cost and regulatory obligations taken into account, Doerr contends that all boiler operators should already, for economic reasons, be concentrating on fitting their boiler systems with a flue gas heat exchanger as soon as possible. Loos claims that flame-tube smoke-tube boilers of all sizes can generally be easily retrofitted with flue gas heat exchangers. The efficiency of a boiler system can be increased by up to 7 per cent in dry operation and up to 15 per cent in condensation operation.

"We have now achieved standardisation in this field,"​ said Doerr. "This means that the cost of a flue gas heat exchanger is coming down. We can also integrate the exchanger into a boiler system, with saves on installation costs. I'd estimate that with the energy savings made, the pay back time of installing a heat exchanger is about two years."

Tighter legislation on CO2 emissions and energy consumption is evident right across Europe. Germany for example recognised the need for action back in 1998, and since then, German law on the prevention of emissions has set out clear limit values. The flue gas losses from oil and gas fired systems with higher outputs (over 50 kW) must not now exceed 9 per cent.

"On the one side, you have cost considerations,"​ said Doerr. "And on the other you have the government. There is great pressure in countries such as the UK to reduce emissions of CO2."


Related topics: Processing & Packaging

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