Industry demand sets the pace for prefabrication
This prefabricated boiler rig for the refurbishment of the boiler room of Mount Grace School in Brentwood comprises two condensing boilers, an unvented DHW cylinder, expansion vessels and system pump. It has a SEDBUK A rating for energy efficiency. There are also extensive controls, remote-monitoring capability and a header arrangement. This rig was designed to the requirements of Dunlop Haywards Building Consultancy to replace a gas-fired heating boiler and electric immersion heater for DHW.
Prefabrication for one boiler manufacturer was driven by its customers, leading to a ‘can-do’ response to customer requests.Far from prefabrication being a concept that is foisted on a less-than-receptive market, Nick Stevenson, managing director of Keston Boilers, tells us that his company’s prefabrication services have developed steadily in response to customer demands. One simple statistic he cites is that a quarter of Keston boilers rated at 55 kW are installed into rigs, and that proportion is growing quickly. ‘Boiler sales are growing, but rig sales are growing more quickly,’ he says. Hint
The hint of the need for a prefabrication service came 10 years ago when some contractors began installing several 50 kW boilers to serve a single heating system and enquired how it should be done. That led to Keston building a frame on which were assembled groups of three or four boilers. The key to the successful operation of such multiple-boiler rigs is control. Control features include sequence control of boilers and rotation of running order. Another option can be to run the fewest number of boilers at full output to meet the load or all the boilers at the minimum output possible. Nick Stevenson explains that boilers with pre-mix burners — all of Keston’s product range — are more efficient at part load than at full load, so running all boilers in a rig is more energy efficient. More-imaginative customers then started asking how the benefits of prefabricated rigs could be applied to multiple-zoned heating systems. That led to the incorporation of low-loss headers with an appropriate number of tappings. ‘Every enquiry is different, so every rig has to be different,’ explains Nick Stevenson. ‘Our prefabrication service has evolved into a bespoke facility with a general lead time for delivery of three to four weeks. Indeed, shorter lead times can, and have, been achieved.’ Planned work
Such lead times could imply that prefabrication is best applied to a planned programme of work. If a project has to be executed quickly, such as an urgent boiler replacement, buying individual components and installing them on site may see the job completed sooner. However, site time saved through prefabrication should also be considered. In contrast, an installer could, for example, complete many more education projects during the school summer holidays using prefabricated rigs due to the site time savings provided through this prefabrication service. Because much work is carried out off site, prefabrication lends itself to minimising downtime for planned replacement projects. One such project that Nick Stevenson recalls is the replacement of two cast iron boilers with a total output of 400 kW serving a hotel. The first stage was to build a rig with two 110 kW boilers and a header already sized for the 400 kW load with isolating valves on the spare tappings. One boiler was replaced by this rig, and the project was completed with another rig of the same size connected to the header of the first rig. Complement capabilities
The expertise and experience of companies like Keston can complement the capabilities of installers. ‘We can take care of boiler module design and control issues, so that the only electrical work required can be the provision and connection of mains supply,’ says Nick Stevenson. Consultants, too, can benefit from the prefabrication approach. ‘Not only are drawings prepared of exactly what will be delivered to site, but solutions can be developed for particular challenges,’ he explains. One such project involved two 55 kW boilers, a 200 litre mains-pressure cylinder, 80 litre expansion vessel, electronic modulating system pumps and all controls and control valves to deliver weather compensation and optimisation — all within a rig 2 m wide. Another challenging project demanded an L-shaped rig because the plant room was not wide enough for any other shape. Challenges
‘Although it was the initiative of contractors that prompted us to develop a prefabrication service, the more interesting challenges are undoubtedly posed by consulting engineers,’ says Nick Stevenson. ‘Our philosophy has evolved accordingly. Whereas we used to ask what components were required in a rig, we now asked what the rig is required to do and design it accordingly. ‘Whatever the question, the answer has to be “yes”. A “can-do” approach is essential.’ Keston Boilers is at 34 West Common Road, Hayes, Bromley BR2 7BX.