32 Fans Staying safe with twin fans
Use of contra-rotating twin fans in commercial kitchens can reduce risk while enabling the kitchen to continue operating in the event of a single fan failure, says Alasdair Howie of S&P.
IN THE LIGHT of investigations by the Health & Safety Executive into carbon monoxide (CO) poisoning from indoor charcoal grills there is now a renewed focus on the importance of effective ventilation in commercial kitchens.
With reference to potential CO poisoning, Michael Eyre, product director at Jestic Foodservice Equipment, advises users to leave extraction on low overnight and adds: “We also recommend the use of a Soler & Palau fan which has two separate motors controlled by Hall-effect bearings that talk to the controller. If one fan motor fails the other will adjust its speed to compensate and the controller will let you know that a fan motor has failed.” Contrafoil Plus is a contra-rotating fan, using two fans that both operate like normal fans in standard operation. The contra-rotation of the impellers increases the fan pressure (up to 500Pa), enabling a straight-through fan to be used with a ducted kitchen ventilation system, providing easier installation and requiring less space than a centrifugal fan. As Mr Eyre notes above, a key feature of the Contrafoil Plus is that if one fan fails the other will switch to full speed to maintain ventilation at a minimum of 70% of full capacity.
This arrangement has two clear benefits. Firstly, it provides a safeguard against carbon monoxide poisoning due to fan failure. Secondly it addresses the requirements of BS6173 and the design criteria of DW/172 Specification for Kitchen Ventilation Systems, which includes a requirement of a safe gas interlocked ventilation system. This is a key
benefit for commercial kitchens that might otherwise be forced to close down in the event of fan failure.
The purpose of the gas interlock system is to switch off the gas supply if the ventilation system is not operating correctly, thereby preventing a dangerous build-up of carbon dioxide and carbon monoxide levels. The interlock system uses sensors to determine that the ventilation system is operating within acceptable parameters. As long as this is the case the interlock allows the gas isolation valve to open. If a problem is detected with the ventilation
system the isolation valve is closed. However, traditional gas/ventilation interlocks are plagued by a number of disadvantages.
They are either current monitoring systems, which are very unreliable as the current draw of the fan changes constantly in relation to factors such as operating temperature, fan loading due to grease or the filters’ resistance.
Or, even worse, they are based on pressure differential or airflow switches which, given the aggressive nature of the in-duct environment, rapidly become clogged with grease and therefore ineffective.
Therefore the ability of the Contrafoil Plus fan to maintain airflows above the required threshold for extended periods allows the kitchen to continue cooking food for customers.
In parallel, the failure of one of the fans in the Contrafoil Plus is indicated by visual and audible alarms to ensure a service engineer is contacted. However, as the system continues to run the service call can be dealt with in normal working hours. A key innovation within the Contrafoil Plus is its ability to sense and detect many different kinds of fan failure by profiling the effect that they had on the other working fan.
This is the result of integrating Hall- effect sensors into the bearings. These convert the rotation of each impeller to an electrical signal that is proportional to the
ACR News January 2014
Contrafoil
speed of the fan. The development of the controller took over a year to refine and combine all the essential functions – gas interlock, fan speed control, fan speed programming, BMS, fan fail detection, for example.
The key challenge was to be able to sense and detect many different kinds of fan failure by profiling the effect that they had on the other working fan – this involved months of test work to look at upstream and downstream fan windmilling speeds at all the possible fan duty points and at all the possible settings on a fan speed controller.
During commissioning, a routine is manually triggered that runs both fans at all the possible speed settings and stores the speeds of each fan independently for future reference. In this way the fan ‘learns’ the particulars of the fan and installation conditions so that any deviation from the normal pattern of operation is quickly detected.
This close monitoring ensures that conditions such as clogged filters and under-performing fans are detected and indicated more quickly than in other systems, allowing timely servicing and extending the life of the fan.
From the point of view of the building operator there are also potential cost savings, as the use of three-wire speed control consumes 30% less current than two-wire systems for the same motor operating speed.
Visit ACR News online at
www.acr-news.com
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44