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Flue Gas Economizers:
Helping the Food Processing Industry Re-Process Heat
by Giulia Sicarrdo & Brandon Conard
At the heart of every food processing facility is a relentless
boiler, which is often an unknown source of waste. Running full-time
and emitting pounds upon pounds of scorching hot flue gas, the
boiler is a prime candidate for efficiency-inducing retrofits,
such as installing a flue gas economizer. BlueMap Inc., a sustainability
research firm, has explored flue gas economizer technology in
order to assess the benefits that installing economizers can
provide to the food processing industry. The best part is that
the installation of this device has a payback time of three
years or less.
The Technology
A flue gas economizer saves energy by recycling waste heat from the combustion exhaust gasses back to the boiler. Using the heat from the exhaust gasses, or flue gasses, to warm the boiler's feed water minimizes the energy needed to heat the boiler in the first place. Therefore, this process reduces the company's required energy input.
An indirect contact economizer removes heat from flue gasses by passing them through one or more shell-and-tube, or tubular, heat exchangers. Shell-and-tube heat exchangers, sometimes used in other sectors of the food processing industry, consist of a shell that can be subjected to high pressure containing a series of tubes running through. The water that will eventually be used to preheat the boiler runs through the tubes, while the hot flue gasses flow over them, transferring heat to the water without ever coming into direct contact. The now-warm water from the heat exchanger is cycled back to the boiler, where it lowers the needed energy input and reduces the firing rates required to achieve necessary boiler output.
Economizers come in a wide range of models and sizes, from small coil units to large waste heat recovery systems, and are engineered to work with existing systems while still fitting their customers' unique needs.
A Source of Savings
In a food processing plant, the boiler operates throughout the day. Increasing the efficiency of such a machine-a machine that runs constantly-has the potential for huge savings. The exact savings from installing an economizer depend largely on the stack temperature, the feed water volume, and the hours of operation. However, an industry rule of thumb is that at least 5% of boiler input capacity can be recovered through the implementation of an economizer. In fact, industry expert Alain Descoins of GreenHeat Solutions Inc. emphasizes that since economizers are always able to recover heat from flue gases, they provide great efficiency solutions.
It is especially important for food processing companies to note that for each 40F, or approximately 22C, that the flue gas is cooled by an economizer in a conventional boiler, overall boiler efficiency increases by 1%. Since industrial economizers typically decrease flue gas temperature by anywhere from 200F to 500F, the efficiency potential can certainly exceed the industry rule of thumb. In some cases, boiler efficiency can be improved by over 10%.
Addressing Technical Concerns
Although flue gas economizer technology has been in widespread use since the mid 1800s, its implementation can sometimes have detrimental effects if certain simple precautions are not taken.
One concern is corrosion to the exhaust stack. When the temperature of flue gasses is lowered, condensation of these gasses occurs. As a result, acidic condensate made up of sulphurous acid (H2SO3) and sulphuric acid (H2SO4) is deposited onto the piping which can cause corrosion. For example, condensate can accumulate in the economizer penetration site, or the juncture in the piping from the furnace to the economizer. However, chances of corrosion damage can be significantly decreased through careful design and material selection. In fact, Descoins of GreenHeat cites the implementation of a stainless steel economizer and exhaust stack to be an effective preventative solution.
Another concern in limited cases, such as with an indoor boiler, is that installing an economizer can contribute to carbon monoxide accumulation. In a boiler, adequate venting is essential for complete combustion. Incomplete combustion is dangerous as it produces carbon monoxide. When a flue gas economizer is installed, especially on a retrofit basis, additional pressure is lost through the venting system. If this added pressure loss is significant enough, it may detract from the performance of the combustion air fan. This could allow carbon monoxide to be produced and released into the plant. In cases like this, installing an economizer constitutes an over-load of the existing infrastructure. To prevent such consequences, a basic review of the processing plant facilities must take place to make sure an economizer is a viable option. This review involves an assessment of atmospheric pressure using a barometer. Companies such as GreenHeat, which specialize in economizers, regularly include such an inspection as part of the installation process.
An Attractive Green Solution
Saving the environment is a great cause, but in the industrial sector, conservation is most attractive when it saves money. In fact, flue gas economizers can bring substantial cost cuts to a food processing plant. By recycling heat that would normally have been wasted and expelled into the ambient air, an economizer will reduce the amount of heat necessary to operate the boiler-a savings of both energy and money. As a result, for a plant that operates a boiler regularly, installing a flue gas economizer can be a great business decision. Economizers are not just about going green; they're about saving green.
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