Advice on waste heat recovery in industrial applications

A waste heat recovery unit/heat exchanger recovers heat from hot streams, water or gasses, that still have a relatively high energy content which would otherwise go unused into the atmosphere. The most common examples are from steam from cooling towers or flue gases from a heat source such as a diesel generator. However, now utilising waste heat from smaller industrial processes is becoming ever more popular.

Given the need to reduce carbon emissions, protect our environment and reduce fossil fuel consumption, waste heat recovery is increasingly important. We have completed recent waste heat recovery projects that have required us to design, manufacture and install an economiser or flue gas cooler into applications in all corners of the world. These include sugar cane processing plants in the warmest parts of Queensland, Australia, where the sugar cane waste by-products are used to heat and heat and reheat water. We have also been involved in projects at pulp and paper mills operating in the coldest parts of Finland where flue gas coolers are used to reheat water for other plant applications. Industrial waste heat recovery is also prevalent in refineries, chemical plants, oil and gas pipelines and general manufacturing.

Anywhere you can find an industrial process that involves transforming raw materials into useful products – heat is likely to be generated as a result. If not captured and used, this heat is released to the atmosphere and wasted so using this heat source presents a winning solution to a really wide range of applications.
How the process works

Waste heat found in exhaust gas, water and/or hot air is simply extracted by some form of ancillary equipment.
This equipment is invariably one or more of the following:
1. Economiser – in the case of package boilers for example, waste heat in the exhaust gas is passed across the outside of metal tubes thus heating the feed water that passes through the inside if them before it enters the boiler. This results in less fuel being used by the burner to generate steam. Waste heat from other sources such as diesel engines or gas turbines can also be used to pre-heat boiler feed water.
2. Waste heat boiler (WHB) or exhaust gas boiler (EGB) – a stand-alone unit is used to create steam without the need for a separate boiler. Steam from this unit might then be used in another process or, if superheated, used to drive a steam turbine to generate electricity.
3. Recuperator – a form of heat exchanger in which hot waste gases from a furnace are conducted continuously along a system of flues where they impart heat to incoming air or gaseous fuel.
4. Regenerator – a type of heat exchanger where heat from the hot fluid is intermittently stored in a thermal storage medium before it is transferred to the cold fluid. To accomplish this, hot fluid is brought into contact with the heat storage medium and the fluid is displaced with the cold fluid which absorbs the heat.
5. Rotary heat exchanger or wheel – consists of a circular honeycomb matrix of heat absorbing material, which is slowly rotated within the supply and exhaust air streams of an air handling system.
6. Heat pump – a device that provides heat energy from a source of heat to a destination called a “heat sink”. Heat pumps are designed to move thermal energy opposite to the direction of spontaneous heat flow by absorbing heat from a cold space and releasing it to a warmer one. A heat pump uses some amount of external power to accomplish the work of transferring energy from the heat source to the heat sink.

In all cases, the heat that would ordinarily be lost from the industrial process is used. Common applications are:
• pre-heating combustion air for boilers, ovens and furnaces
• pre-heating fresh air used to ventilate the building
• hot water generation including pre-heating boiler feed water
• direct steam generation for process or power generation
• space heating
• drying
• other heating or pre-heating for industrial processes

What are the advantages of an industrial waste heat recovery application?
• Adds to the overall efficiency of the industrial process.
• Reduces fuel consumption and so decreases both the cost of fuel and energy consumption
• Reduces carbon and harmful emissions
• Reduction in the equipment size – as fuel consumption reduces, so does the requirement for handling the fuel, pumps, filters, fan etc.

Obviously waste heat recovery comes with a capital cost. Projects need to be assessed on an individual basis, taking into account current fuel costs, running hours, future factory requirements, condition of existing equipment and so on. But whatever the application taking steps to determine whether a valuable heat source can be used rather than wasted is always worth the effort.

This information was published in the October issue of Plant and Works Engineering magazine