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needed as concentration levels reach set limits . This also allows that not all fans have to operate simultaneously . To note , smoke and fume extraction design is done and signed off by persons with the appropriate competence .
SELECTING THE CORRECT FAN Fans for HVACR applications come in a variety of sizes and configurations from very small units to large custom-made units measuring as much as two metres in diameter . They can be made from different materials including plastics , steel and aluminium , further be fitted with various blade quantities , and run at different revolution-per-minute ranges .
Depending on the application environment , bearings are the only real failure-susceptible or ‘ wear component ’ in a fan and indirectly determine the fan ’ s lifespan , measured in work hours which can run into tens of thousands of hours . Some fan-mechanisms can be maintained , thus prolonging life expectancy . Newer technology includes magnetic driven motors without any bearing .
“ One of the biggest misconceptions in the market is the understanding and interpretation of efficiency .”
As with most industries there are a number of cheap imports available in the country which is difficult to compete against the supply of good quality , more expensive products . The South African mentality remains very much aligned to the cheapest options with the hope of making a 12-month guarantee . Under the current economic environment , securing contracts has become tough , and looking to the bottom-line means looking for cheaper products . What is also happening more and more regularly , and is pulling the wool over people ’ s eyes , is that quality brands are being copied and sold cheaper when in fact they are poor quality .
“ Certainly , many engineers are applying principles to ensure efficiency and good practice , and this is often driven by the client or those who seek green building accreditation . But South Africa is lacking regulations in efficiency , essentially allowing the industry to use whatever products they choose . This is a particular challenge for suppliers as often products are not evaluated on an even playing field in an increasingly price-sensitive environment . Off-the-shelf extraction systems are generally not very energy efficient and often misleading information results in selecting old or poor technology ,” says Anderson .
Extract fan selection needs to meet application requirements and regulations , and should further be certified , for example if set to run at 300ͤ degrees C for two hours . Some suppliers build fans and then don ’ t have them certified . This is a potential risk for the engineers and contractors who wouldn ’ t necessarily be able to tell the difference . “ This is something that is often overlooked , but in an emergency situation a fan failure resulting in harm or death means someone is going to possibly find themselves in jail ,” Anderson stresses further .
When selecting a fan , two main elements need to be considered : air volume required , and pressure drop ( system resistance ) as Dube has mentioned . A particular error in these factors revealed by suppliers is that often the distances that air needs to travel are miscalculated . This can be due to factors such as additional ducting added that would stress the fans or ducting challenges on installation deviating from planned paths . Each fan is designed to produce a certain air volume against a certain pressure . If the parameters change , the output will change . Reducing pressure means volume decreases .
The pressure calculation would be determined by the size of the ventilation space as well required volume , as mentioned . The actual pressure is then determined by the ducting , whether there is any means of filtration or an air handling unit , how many bends are included , how many outlets are in the design , and so on . When all the parameters are known , suppliers are able to identify which model will work for the application , mostly suggesting one size larger to accommodate any inefficiencies .
When it comes to the bigger axial flow fans , these have adjustable-pitch propeller blades that can be set to exactly what the engineer specifies and thus the fan is then made according to specification . To note , changing the blade angles will increases or decreases the amount of power on the motor , so adjusting blades can result in drawing a whole lot more power from the motor resulting in premature failure . The addition of variable speed drives for these fan types is another method of electronically controlling the exact duty required .
“ When considering fans generally , there are two aspects that stands out specifically pertaining to the product ranges in the HVAC category that engineers and contractors should be aware of . One of the biggest misconceptions in the market is the understanding and interpretation of efficiency . South Africa has representation of products from around the world , however because the local market leans heavily towards American standards through various guidelines based on ASHRAE documents , the way the industry compares technology is very different . So , when a product has an EU standard , there is often a misunderstanding in whether the
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RACA Journal I June 2021 www . hvacronline . co . za