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In process heating, economisers are essentially heat-exchange devices that extract the residual heat from the exhaust gas generated by the combustion process. Often, that recovered heat is fed back into the process.
Economisers for steam generators have been around for many years but few manufacturers offer economisers for thermal fluid heating systems and even fewer still provide integral economisers.
With thermal fluid systems becoming an increasingly popular process heating method because of their very high overall efficiency, ease of use and compact nature, it’s a wonder why more manufacturers don’t provide economisers and why more customers don’t demand them.
If you use a thermal fluid heating system because of its energy-saving factors, then buying one with an integral economiser would seem to be a no-brainer. After all, they provide added savings from recovering what would otherwise be waste heat.
The main reason they’re not provided seems to be that they are more difficult to integrate into the process, and return fluid economisers – as used with steam generators – are not really practical owing to the high fluid temperatures.
Systems exist that work to recover heat to an external heat sink, but there is the challenge of matching the heat load of the slave system to that recovered from the thermal fluid heater. This is not always an easy task and it often results in having to dump valuable recovered energy when the needs of the two systems do not match up.
The other choice is to opt for a thermal fluid heater that uses an integral, double, combustion air preheater. The standard preheated jacket is bolstered with an additional shell and tube-type heat recovery device built into the case of the heater. It enables even more heat to be recovered from the flue gases by heating the combustion air, which reduces the amount of energy required to take it from ambient to normal combustion temperature. This can mean a flue gas temperature of 200°C, while still heating the thermal fluid to 300°C.
This design allows 5-6% energy recovery with no operator involvement or further heat sink required; it’s a self-contained solution. Extra floor space is needed – up to 50% more than thermal fluid heaters without economisers – but, as the systems are so compact in the first place, this is rarely an issue.
Take a thermal fluid system with a 1,000kW load as an example. To reach the heat load absorbed by the process using a standard heater you would need 1,300kW gross heat input, which allows for energy loss from chimneys and so on. Having a high surface area integral economiser fitted in that heater will reduce the energy input demand by 65kWh. Over a single year, assuming the heater is working 60-70% of the time, this could equate to anywhere between 340,000 and 400,000kWh.
If we assume a gas cost of 0.02p/kWh gross, we can easily see a saving of £7,000 a year, and that’s just on the smaller heaters. Average payback is in one to two years, although for larger heaters the payback is even quicker – just a single year on a 2,000-3,000kW heater.
With an integral economiser there is no water or other intermediary fluid required, so no water treatment and no risk of corrosion. The inbuilt preheater works on clean combustion air and high-velocity flue gas, so is virtually maintenance-free and will provide very high performance for the life of the heater.
So, with the efficiencies and energy savings achievable by using thermal fluid heating systems with integral economisers, the question remains: why aren’t more companies reaping the benefits of this proven technology?
Content published by Professional Engineering does not necessarily represent the views of the Institution of Mechanical Engineers.