In processing plants, a waste heat recovery system will usually include economiser/s and/or a waste heat boiler.
An economiser unit is typically installed on the exhaust system from a steam and hot water boiler to reduce the flue gas temperature and increase the thermal efficiency.
This will be formed of an appropriate tube configuration that will be determined by taking into account the type of fuel that is being burnt, the temperatures that are being handled, pressures and the space available.
A waste heat boiler recovers heat from hot gas streams, that still have a relatively high energy content which would otherwise go up the chimney and out into the atmosphere.
Capturing the exhaust energy from diesel or gas engines by reducing the flue gas temperature from, for example, 400°C to 180°C, allows processing plants to maximise energy usage and increase production efficiency.
For a heat recovery capital investment project, plant managers would need to consider the following factors for the return of investment (ROI):
- Available waste energy in the exhaust gas system
- Manufacturing and installation cost of the heat recovery unit
- Physical space available in the plant for the heat recovery unit
- Efficiency savings based on plant operational hours
- Fuel cost savings from increased plant efficiency
- Reducing carbon and harmful emissions
Common applications for reusing the waste heat that has been recovered:
- 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
- other heating or pre-heating for industrial processes
However, before a waste heat recovery system can be designed for a processing plant to obtain these benefits, managers need to provide the supplier with information that will help to inform an effective system.
This includes the type of fuel that will be used and data about the flue gas including flowrate, composition and inlet temperature. It is, of course, essential to know where the system will be located and the space available which will be used to determine the best materials and tube arrangements.
Using this information, the supplier will work on the optimum thermal design which will include outputs such as the best working fluid i.e. water, saturated steam, superheated steam or thermal oil, flowrate, thermal duty, waterside pressure drop, gas side pressure drop, working fluid outlet temperature and operating pressure.
All of this information will then be used to formulate pressure part calculations and develop the mechanical and structural design that specifies factors such as the system’s weight, width and length and, in some cases seismic calculations and wind loading calculations.
Use of systems in processing plants brings particular challenges for their waste heat recovery systems. For example, if the plant has a dusty atmosphere, this will influence the types of tubes that are used and which fins are employed.
Generally, the fins need to have relatively large gaps between them, which is known as a wide pitch specification. They also need to have as high a heat transfer surface as possible making H-finned tubes a frequently specified solution.
Processing plant managers should also ensure that their chosen supplier has the required experience, track record and industry accreditations to be able to design and manufacture a system that meets all regulatory and insurance needs.
This can only be achieved by companies that have stringent quality and technical assessment processes in place and is best checked by visiting their facilities to see what they offer first hand.
Having invested in a waste heat recovery system with the aim of making energy usage more efficient and reducing costs, it is highly recommended to initiate a planned maintenance programme to ensure that its lifespan is extended as much as possible and that unplanned outages are minimised.
The environment in which equipment operates, varying quality of feed water and rigorous operating patterns can result, over time, in reduction in efficiency or even total machine failure.
To avoid this, it is vital to commission a proactive inspection service to identify potential problems with waste heat recover equipment so that they can be addressed in order to maintain optimum efficiency and before they become unnecessarily costly or result in avoidable downtime.
This will usually include examination of the internal surfaces of the economiser and waste heat boiler during planned outages to ensure fouling and erosion issues are not occurring and online cleaning equipment such as soot blowers, are operating correctly.
Typically, a comprehensive report will be submitted to the client after the examination work, stating the condition of the examined surfaces and recording corrective measures needed so that a plan can be put into place that keeps these vital pieces of equipment working efficiently.