Portable Ventilation Fans – A guide to choosing and using fume extractor fans
Portable Ventilation Fans – Background
Site work often entails tasks that either create a hazardous environment or is work within a hazardous environment. It is in these situations that portable ventilation fans have their application. Use of tools on masonry or timber materials gives rise to airborne dust, making the working environment both difficult and hazardous to respiratory health.
Other sites such as wells, pumping stations, sewers, tanks can be intrinsically hazardous because of dust, specific gases, explosive atmosphere or depleted levels of oxygen. Portable ventilation fans as very useful in these situations and can be used either to extract hazardous air and fumes or supply fresh air. Portable ventilation fans also find use in wider industry where contaminants from work processes such as welding can be easily ventilated to outside.
Portable Ventilation fans – Airflow and Pressure
On their own portable ventilation fans are of little use but with the addition of flexible ducting they have a huge variety of applications. All portable ventilation fans are limited by their flow and pressure capability, and like all fans they have a pressure/flow curve, which relates how much air flow can be expected given an amount of pressure drop on the fan. The general rule is the more resistance to air flow (pressure drop) then the less airflow will result. In practical terms this means that for any portable ventilation fan the more flexible ducting that you connect to it the less the air will flow. Generally manufacturers of portable ventilator fans will not specify a pressure capability, but will specify a maximum flexible duct length that can be attached to the fan without a significant drop in its stated airflow.
For the majority of portable ventilation fans applications the outside is close enough to the ventilated area to get foul air to outside or conversely, clean air to inside by using a single fan and a length of duct within the manufacturers recommendations, however, where the distance is greater there are a some options;
a. To use multiple portable ventilator fans and flexible ducting lengths to connect the units. If the maximum recommended ducting length for a particular fume extractor fan is 10m then by using multiple units and multiple lengths of ducting the capability of the overall extract system increases to say 20m for 2 fans, 30m for 3 fans etc.
b. To use a higher pressure ventilation fan. Some portable ventilation fans have much more pressure capability than others and can more air along much longer lengths of ducting. Centrifugal ventilation fans are usually more costly than an axial extractor but cheaper than using multiple axial portable ventilation fans.
c. To use a portable ventilation fan with a higher flow capability than you need. If say you need to move 500m3/hr 20m, and the 500m3/hr fan is only rated to ducting of 10m, then try using a fan that moves 1000m3/hr rated to 10m of duct but connected to 20m of ducting the resulting airflow will not be 1000m3/hr, but may be close to the 500m3/hr that you actually require. In effect, the larger fan will be operating at a less efficient point on its pressure/airflow curve.
ventilation fan – Sizes
Portable ventilation fans are generally referred to by their diameter, and the bulk of the market is served by standard diameter sizes, of which 8″ Ø and 12″ Ø are the most common.
|Imperial Ø||Metric Equivalent Ø||Example Flow Capabilities|
|8″||200mm||1350m3/hr, 1450m3/hr, 1500m3/hr, 2100m3/hr|
|12″||300mm||3600m3/hr, 4200m3/hr, 6808m3/hr|
Generally greater size of fan equates to more airflow, however because for a given size the manufacturers use different impellers and motors, there can be oddities such that the capabilities of a particularly capable fan of one size might move as much air than a less capable fan of a size larger. Ducting also follows the sizes of the fans, which means that 8″Ø flexible ducting will fit on 8″ ventilator fans from a number of manufacturers.
Centrifugal or Axial Portable Ventilation Fans
Beyond the main market for ventilator fans there is also a number of ventilators that are centrifugal type.
Generally these units have duct connection spigots that follow the industry standards (e.g. 200, 300, 400mm etc.) but the fans can handle much greater pressure resistance in the ducting and are therefore capable of moving air along a greater distance, typically up to 30 or 40m. The largest units can move 20000m3/hr through 100m of 630mmØ ducting. The choice of centrifugal or axial is almost always driven by considerations or airflow and distance. If your requirements are small look at axial ventilators, and if they are large airflow and over longer distances look at centrifugal ventilators.
Plastic or Steel Portable Ventilation Fans ?
Cheaper portable ventilation fans tend to be made from steel and are perfectly capable machines, so why spend more on a plastic cased ventilator?
The simple answer is durability. The plastic cased portable ventilation fans tend to
be far more resistant to knocks and bumps, and if they take a knock they tend to absorb it far better than a metal cased ventilator which is more likely to dent.
Although a metal cased ventilation fan can sometimes take a few dents without affecting its operation, in worst cases a knock can deform the casing to the extent that the impeller is damaged and either breaks or strikes the casing when used. Some plastic cased portable ventilation fans also have the capability of being stacked one on the other which is a bonus if storage space or site space is limited.
Safe Ventilators and ATEX Rated Ventilators
Some applications are hazardous and there may be a risk of an explosive atmosphere. This means that occasionally explosion proof or ATEX rated ventilator fans may be required. There are a couple of ways round this problem. Firstly, you could use an explosion proof portable ventilation fan for the application such as the VF250EX ATEX rated portable ventilation fan,
The other solution is to use a Clustajet ventilator which does not use electricity
and has no moving parts (therefore no ATEX rated motors and controls to worry about) but instead uses a compressed air line to the ventilator. It works by having special nozzles in the unit which discharge the compressed air in the direction of airflow, which in turn educt the surrounding air in the duct to be conveyed along with the discharged compressed air. Whatever safe or ATEX rated solution you choose don’t forget that there is more to preventing explosions that an ATEX rated fan – the ducting is also important. Choose antistatic earthed flexiducting.
Ducting for Portable Ventilation Fans
The flexible ducting that attaches to portable ventilation fans is similar whatever the manufacturer. For a particular size the differences are typically little more than the colour and the means of fixing and joining (some use Velcro circumference straps whilst others use a corded strap).
It is worth understanding that compared to rigid smooth more ducting, the
flexible ducting has a much higher pressure resistance, particularly where the ducting has to go round bends. As mentioned above pressure resistance comes at a cost to airflow and some customers whose applications have torturous ducting routes use a combination of flexible ducting and smooth bore duct pieces particularly for corners and splitters.
Flexible ducting usually comes in fixed lengths; 5m, 7.5m and 10m are common lengths and as well as the ducting some manufacturers supply the ducting in a case or compression sack.
Power supply for Portable Ventilation fans
Generally the axial type portable ventilation fans are offered as either 110v or 230v versions. Where the intended purpose for the unit is in a workshop say for a welding bay, then 230v is normally preferred, but if the unit is to be used on site then the safer 110v is preferable, particularly if 110v supply is available. For the centrifugal ventilation fans there is not usually a choice of voltage for any model, but depending of the size they are either 230v for the smaller units and 415v, 3 phase for the larger fans.
Air Filtering for Portable Ventilation Fans
Some of the ventilation fans that we supply can be connected to a filter unit. Where the fan is connected to 12″ or 300mm Ø flexible ducting the filter unit can be connected to it. The unit can be used for filtering air being supplied into a confined space or for air being extracted from one space to another. The filter canister can be operated under positive pressure or negative pressure i.e., suction, however the filter is ‘handed’ so there is only one correct direction of airflow through the unit.
Filter socks are another solution available for some ventilation fans. The filter socks are inflated by exhausted air
from the fan and the dust is retained in the sock.
As with all mechanical filters they have a pressure resistance to the air so it is inevitable that the airflow from the fan will be less when a filter is attached. Also as the filter collects dust the contaminants will increase the pressure resistance of the filter and the airflow will drop further. Ultimately the filter will require changing when the airflow being extracted is no longer sufficient to meet the requirements of the application.
Other Tricks with Portable Ventilation Fans
Although ventilation fans have a typical design and purpose, several of the fans that we supply have functionality that straddle the capability boundaries between ventilation fans and other fan types. Some of the larger, shorter cased ventilator fans are equipped a stand that allows the fan to be pivoted to direct the airflow in any direction, which can be used without ducting as man coolers i.e., the same way as a drum fan, or for general air circulation.
The other benefit of an open based swivel frame for the ventilation fan is that when the fan is orientated to vertical it can be used without bends to move air through a vertical duct, for
instance in a shaft or chamber. Vertical airflow also finds use at shows and exhibitions where inflatable figures or streamers can dance to catch your attention.
Some of the ventilation fans that we supply can have multiple spigots attached so that air can be supplied to, or extracted from several rooms or areas simultaneously using a single fan.
Basic ventilation fans are not expensive to buy, and indeed at the entry end of the market there are numerous options available which are perfectly capable fans. But if your requirements are less straightforward or are for larger airflows, longer duct runs, and for more functionality then inevitably the price increases. Our advice for choosing a suitable ventilation is to consider;
- your requirement carefully, not just your immediate application for the fan but also potential future requirements
- length of duct runs
- other uses for a ventilator fan eg. general cooling, exhibition attraction
- ease of storage for more than one fan
- that larger fans cover a wider range of applications.
If you need any help to choose a fan or accessories, give us a call on 0845 6880112