Fitting an economiser on a boiler is a small addition that can provide big results in terms of helping companies reduce their operating costs and decrease their environmental impact, says heat and energy generation equipment provider Steam Generation Africa MD John Rundle.
Aware of the negative impact of rising fuel costs and the importance of energy efficiency for operating companies, Steam Generation Africa’s design and manufactur- ing partner Cochran Boilers UK first introduced economisers internationally about 25 years ago. Owing to the success of the units, Cochran now fits most of its newly manufactured boilers with economisers.
“However, these environmentally responsible units can also deliver impressive cost savings when retrofitted to almost any brand of industrial boiler,” notes Rundle.
Since 2015, Steam Generation has, thus, included economisers as a standard add-on to the range of Cochran boilers it supplies to the South African market.
Rundle explains that economisers – which usually sit unobtrusively on top of a steam or hot water boiler – recover previously wasted heat from the hot exhaust gases they generate. The econo- miser then uses this heat to preheat the boiler feedwater, significantly improving the system’s overall thermal efficiency.
“The harvesting of this previously wasted exhaust heat as it is expelled through the flue regularly delivers operational cost savings in excess of 5%, frequently pushing system efficiency to over 95%. The use of these heat recovery units are, thus, an important factor in maximising the efficiency of modern steam and hot water generation systems.”
A short heat recovery and economiser survey enables Cochran’s system efficiency experts to determine a plant’s heat recovery potential. While economisers are most often fitted to natural gas-fired systems, Cochran can also design and fit specialist systems to some liquid-fuelled boilers.
The standard economiser consists of banks of finned tubes, which the boiler exhaust gases pass over before entering the chimney stack. The finned tubes are used to increase the amount of heat transfer surface available. The boiler feedwater is pumped from a thermal deaerator or hotwell tank, through the economiser tubes, to increase its temperature – typically by about 45 ºC – before entering the boiler. The finned tube is usually made from carbon steel for gas firing, with a suitable bypass system included to prevent corrosion when using a standby liquid fuel.
If a customer is primarily using a liquid fuel, the economiser can also be manu- factured using stainless steel to ensure longevity.
“Economisers are perfect where there are low levels of condensate return and high levels of relatedly cold make up water,” highlights Rundle.
Proof is in the Project
China-based cigarette manufacturer Shanghai Tobacco is an example of a Cochran customer that has benefited significantly from the development and retrofitting of economisers to its industrial boilers.
Rundle notes that the company uses large volumes of steam during the drying and processing of tobacco and, thus, operates three Cochran Twin furnace boilers at its factory, producing 70 t/h of steam.
Having visited the factory to review Shanghai Tobacco’s specifications to add a further four 20 t/h boilers to their facility, the Cochran survey team advised that a two-stage economiser system should be fitted to each boiler.
Cochran designed this two-stage system, consisting of a conventional carbon steel first stage economiser mounted at the rear outlet of the boiler and a stainless steel second stage economiser mounted in the flue ductwork. The high volume of cold make-up water is passed through the second stage economiser, entering at ambient temperature and leaving at a raised temperature. This water then enters the deaerator where it is further heated to 103 ºC before being pumped through the first stage economiser and entering the boiler at 132 ºC.
“This has increased the boiler design efficiency by an impressive 9%. These results are typical of what our local customers are getting with their Cochran boiler 7 Economiser combination,” says Rundle.
He adds that the Cochran design is based on a standalone economiser unit rather than being integrated into the rear smoke box of the boiler. This has the benefit of flexibility to fit the boiler/economiser system within the confines of the boilerhouse, whether it be vertical or horizontal.
Whether the economiser is fitted to the rear of the boiler or is standalone, the removable side panels, moreover, allow easy access for cleaning and maintenance. Unlike other designs, there is, therefore, no need to isolate the boiler and cut into the rear smokebox to access the economiser for maintenance.
“Adding an economiser to your new boiler plant or requesting an investigation into retrofitting a unit to your existing boiler plant is an ideal way to optimise the efficiency of your plant and achieve paybacks that will please your accountant and improve your plant’s green credentials,” concludes Rundle.