It’s a given that industrial machinery produces heat as a byproduct of the heavy-duty work it does. Ovens, furnaces, incinerators, kilns, dryers and thermal oxidizers (used for pollution control) all give off residual heat.
Industries that use large amounts of fuel and electricity to produce heat for specific processes also generate large amounts of waste heat. The question in this era of recycling and increased efficiency is if it makes good financial sense to recover and reuse waste heat to reduce energy costs. As industry looks to tighten its belt, creative ways to save money and increase efficiency become more important — and establishing a system of waste heat recovery is one proven way to do just that.
Waste heat recovery is simple: capture the waste heat and, using a heat exchanger, transfer it to another medium to be put back into the process. In general, recovery reduces fuel bills by recycling heat that would have otherwise had to be recreated. It also reduces plant emissions and improves productivity.
The product of waste heat recovery can be preheated combustion air, hot water or steam. Hot water and steam can be used for plant services, as part of the original process heating or to run steam turbines for mechanical work or electricity production, run absorption chillers or regenerate desiccant dehumidifiers.
Waste heat recovery is usually a good idea when:
- its temperature is hotter than what is required for the process; and
- fuel savings are greater than operating and maintaining heat recovery equipment.
Determine Waste Value
When it comes to air or liquid waste heat stream value, the higher the temperature, the better. Beginning at about 500ºF (260ºC), waste heat is considered a viable source for recovery.
Other waste value considerations include pressure drop and the chemical makeup of the waste gases. Waste heat recovery devices can produce pressure drops that can negatively impact waste heat source operation. Also, corrosive components and the dewpoint of the gas stream may necessitate the use of exotic metals, and the presence of materials that could foul the heat exchanger’s surfaces may affect its design.
Basically there are three types of waste heat recovery equipment.
• A recuperator is a gas-to-gas heat exchanger placed on the stack of the oven or exhaust of a prime mover in a combined heat and power (CHP) installation. Recuperators transfer heat from the outgoing gas to incoming combustion air without allowing streams to mix. All recuperator designs rely on tubes or plates to transfer heat. They are the most widely used waste heat recovery devices.
• A regenerator is basically a rechargeable storage device for heat. They can work with gas-to-gas, gas-to-liquid or liquid-to-liquid waste heat sources and can be installed on ovens, prime movers and chemical reactors and with steam condensate. It is an insulated container filled with material capable of absorbing and storing large amounts of thermal energy. The waste stream flows through the regenerator, heating the storage medium and then un-heated stream flows through the regenerator, absorbing heat from the medium before it enters the process. The cycle then repeats itself. In continuous processes, two regenerators are required. There are many designs for regenerators such as heat wheels, passive, fin-tube and shell-and-tube.
• Waste heat and exhaust gas boilers/steam generators are similar to conventional boilers except they are heated by the waste heat stream, not their own burner. They are of most value to process industries that require large amounts of steam. They generally will not replace existing boilers but will supplement the steam that they produce, reducing the energy cost to operate the direct-fired boilers. Steam is available only when the process is running, so waste heat boilers are generally designed to operate with existing boilers or with steam generators in a combination system.
Other Recovery Prospects
Waste heat recovery systems are a good choice in distributed generation (DG) locations, where the power generation facility is near or at the end user’s location.
By recovering the waste heat from the prime mover and generating steam and/or hot water, the user reduces the amount of energy that would otherwise be purchased. This DG project then becomes a CHP project. The importance of heat recovery in a CHP project is highlighted by the impact it has on the overall efficiency of the project; for example, for a total efficiency for CHP of 85 per cent, electrical production accounts for 35 per cent and steam production accounts for 50 per cent.
There are other ways waste heat recovery saves energy costs. Versatile types of heat exchangers — for example, simple boiler economizers to preheat feedwater or more sophisticated systems recovering heat from thermal oxidizers for high pressure process steam — exist for nearly all applications. Partner with your heat recovery equipment supplier to reap big savings in short order. p
Glenn Adgey is the general manager of Clayton Sales & Service Ltd. For more information, visit www.claytonindustries.com.