Burning issue
Certified and safe
Although separate smoke extract fans have traditionally been installed alongside day-to-day air treatment fans and only used in the event of a fire, today, there is a growing trend for the installation of dual-purpose units. This is where fans are used with inverters and can be operated daily as well as in an emergency. Simon Plummer, national fire safety manager at FläktGroup, argues that safety and compliance should be top of mind when specifying these products
W
hen it comes to smoke control, extract fans are essential in keeping escape and access routes free from smoke and
assisting fire fighters with entry. However, they can represent a sizeable expenditure for building owners and a greater return on investment is achieved if fans can operate in more than just emergency mode. As a result, dual-purpose fan units with smoke
extract fans deployed with inverters (also known as variable speed drives – VSD) are increasingly being deployed to provide day-to-day ventilation as well as smoke extraction in emergencies. Operating a fan through a VSD offers reduced energy use, especially when there is fluctuating demand, as well as lower installation and maintenance costs.
With the use of these solutions on the rise, it is
more important than ever to ensure that they comply with the latest fire regulations. So what do specifiers need to consider?
Direct on-line operation
Legislation and design guidance of products and systems related to the control of fans used in mechanical extraction systems have undergone various updates over the past few years. In 2015 a key change was made to BS EN12101 part 3, which specifies the product characteristics of powered smoke and heat control ventilators (fans). An important part of this update is the ruling that the use of inverters in emergency mode is precluded unless they have been fire tested with the fans, or equipped with additional voltage waveform filters and the fan motors de-rated by 20 per cent. To avoid non-compliance with this standard, a system can be designed to bypass the inverter when an inverter driven fan switches to smoke mode. It is then operated directly on-line in the event of fire. However, by-passing the VSD during a fire can
force the fan to run at full speed regardless of the conditions. If the fan is used within a pressurisation
10 June 2019
system, doing this could cause system component damage by over-pressurising them. Worse still, over-pressurising stairwell
pressurisation systems may make it impossible for occupants to open escape route doors. Therefore, by not using a VSD, it becomes much more difficult for a system designer to achieve the desired level of smoke control and implement extraction strategies safely.
Overcoming heat
Where fans and VSDs have not been tested and certified as a packaged solution, the fan motor must be de-rated, i.e oversized by 20 per cent, and Sinusoidal or du/dt voltage waveform filters must be applied between the inverter and fan. This reduces the over-heating effect of the inverter on the motor and avoids early failure of the fan during fire mode. However, this is a costly option. Voltage
waveform filters are very expensive, often costing more than the VSD. A larger panel enclosure is also needed to house the filters inside a remotely mounted control panel along with the VSDs, and extra wiring would be needed.
Tested and trusted
To fully comply without losing the ability to implement the desired smoke extraction strategy, it is essential for a third-party to test the fan and VSD combination in fire mode. Strategies such as zonal control and reversing extraction flows become possible without the need for additional equipment such as dampers and contactors. Many existing solutions currently on the market
are not certified as a package. But Fläkt Woods, together with Danfoss Drives, now offers a fully certified EN12101-3: 2015 packaged solution in the UK and Europe. In order to pass the required tests, the fan and
VSD combination must meet certain standards for operation in high temperature environments. For
example, individual components, such as the motor impeller, must be made of materials that can withstand stress limits of up to 300˚C. It is also important that there is a sufficient gap between the impeller tip and the casing because under high temperatures, this component can expand. Bearings also need the correct gaps for heat expansion and the appropriate lubrication for fluctuating conditions.
Improved maintenance
In terms of maintenance, having a certified VSD dual fan package in place can improve reliability and reduce the costs associated with regular service checks. A dual-mode fan is effectively being “run tested” every day. In contrast, dedicated fire mode fans and associated equipment would only typically be tested at monthly, six-monthly or even yearly intervals. If fan or motor bearings are not turned regularly, they can degrade (flat spot) and then prematurely fail, especially when stressed. Dual purpose fans that are operated via a VSD
provide a wide range of benefits. But the most important consideration for specifiers must be safety and compliance. Opting for a package that is independently certified will ensure that it is fit for purpose should the worst happen and a fire occurs.
Simon Plummer, national fire safety manager at FläktGroup
www.heatingandventilating.net
www.heatingandventilating.net
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