Vented Extraction

Why use a vented to atmosphere LEV System?

There is no definitive answer to this question but there is a range of factors we need to consider when deciding on whether to use a Vented to Atmosphere LEV system. The first things we need to consider are:

• What type of materials are being used?
• What type of contaminate is being produced?
• How is the contaminate being produced?

• How much contaminant is being produced?
• How many points/ work stations is extraction needed from?
• What is the working area/ space like?

Once we have ascertained these questions, we can begin to build up a picture of what type of LEV system will be needed.

The working area and available space is also important. Some workshops/ work places won’t have the floor space required for a larger recirc unit. We also need to be aware of the amount of air being removed by a vented to atmosphere LEV system. Small areas could need make up air as the vented LEV systems basically suck it out of the working area and chuck the air outside (hence they are sometimes referred to as a “Suck it and Chuck it” system. Recirc systems pass the cleaned air back into the working area, thus removing the need for make-up air.

It may also be the case that it is only a small number of points that need extracting with low volumes of contaminates being produced. In this case a vented to atmosphere LEV system could be an expensive solution as opposed to compact individual recirc units.

Vented to Atmosphere LEV Systems really come into their own for multi-point systems that need to extract fumes, gases and vapours – such as reflow ovens, welding or chemical decanting.

These kinds of applications would produce a high level of fume and vapour that could quickly saturate the filters in a recirc system, meaning they may have to be frequently changed – driving up running costs.

Most dry, industrial dusts are not suitable for extracting via a Vented to Atmosphere Extraction System without further components such as cyclone filters, dust collector devices and others. Dust laden air-streams need to be cleaned before being vented outside. Most dry dust extraction systems terminate in a dust collection/ filter unit.

Components of the LEV system

Let’s go through an example of a small Vented to Atmosphere LEV system.

Company A has two reflow ovens and 6 hand soldering stations. Reflow ovens produce a lot of hot fumes, combined that with the 6 hand soldering stations and we have a lot of solder fume in need of extraction.

Company A’s workshop is producing a high amount of solder fume. To filter solder fumes, carbon filters and HEPA filters are needed, carbon filters with this high amount of fume could saturate quickly and need regular changing. This would not be cost-effective in the long term. The working area is large enough to not have to worry about adding make up air (this is needed when the LEV system can remove the air in the room faster than it can be replenished – see more below). As a result, we have advised Company A to use a Vented to Atmosphere LEV system.

The main LEV system will comprise of a header duct running along the walls/ from the ceiling around the workspace. Each reflow oven will connect via a downpipe – or “drop” as it is commonly referred to – this will then connect via a flexi-hose to the ovens exhaust port.

The hand soldering stations will each have an extraction arm that mounts on the table and connect to more drops via flexi-hose.

Solder fume has a high level of vaporised solid particulate – which is very small, less than 30 microns – in it so we will need an in-line filter to protect the fan and remove most of the microscopic dust particles from the air before it gets exhausted outside.

Next, we need to look at the fan. There are many different types of fans. We need to ensure we select a fan type to suit the application and contaminate that also has enough performance to provide enough airflow and pressure to capture the solder fume, pull it along the duct and push it out the exhaust.

Carbon Filters

The Exhaust Ducting

In order to get the exhaust stack outside the LEV system need to penetrate an exterior wall or penetrate through the roof.

This can be a problem, especially as more and more companies don’t own the buildings, they are in. Many companies lease their building and landlords may not permit them to cut holes into the building. There is also the factor of weathering.

When Vodex penetrate the wall or roof we always add exterior weathering protection to the hole, to ensure they rain doesn’t come in through the hole we cut. But this is not guaranteed forever and over time weather proofing can fail and need replacing.

The other factors in exhaust design is how to cap the exhaust – we can’t just have an open duct pointing skyward – and how long it needs to be. The regulations state that an exhaust stack must be at least a meter above roof height with the discharge point above the highest point of the building.

Extracted air, clean or otherwise that is vented outside must not be able to re-enter the building or enter other buildings unless it has been diluted enough in the air to a negligible concentration.

We need to consider the:

• The buildings surrounding “boundary layer” of air (we need to go over it)
• The recirculation eddy produced by the leading edge of the roof
• The downwind wake
• Wind Direction
• Discharge speed of extracted air

Exhaust stack height can be a problem. Let’s say Company A’s solder shop is on the ground floor of a 5-storey building. The LEV system runs around the work shop and out the ground floor side wall. The exhaust stack would now need to run 5 stories up the outside of the building to a point above the roof.

This is along exhaust system and can cause issues with the system if it is not allowed for and can increase the initial costs of the system.

For our example, we will say Company A is in a single storey building and need to run a short exhaust, around 3 mtrs up the side of the building.

We now need to terminate the ducting. There are several ways to do this. The cheapest way is to use a “bird beak and mesh”.

Make-up air

A component that is often over look with Vented to Atmosphere LEV’s is that of “Make-up” Air. Unlike Recirculating LEV’s which usually return cleaned air back into the work place, Vented to Atmosphere LEV systems take air out of the work place they are situated in and exhaust it outside. This means the LEV takes air away and doesn’t put any back in. Many rooms will have natural ways of letting air “back in” such as windows, doorways, false ceilings etc (most rooms are not “air tight”. But if an LEV extracts air quicker then it can be replaced then the room may become negatively pressurized – it can cause a vacuum in the room – this can be too such an extent that it may be hard or even impossible to open doors, especially in smaller rooms.

If an LEV system does remove air faster than it can naturally be replenished then Make-up air will need to be allowed for. This can be done by either adding more ways for the air to naturally refill the room (such as simple louvre grilles in a door or wall or by adding a fan to blow more air into the room. Fans need to be carefully specified so they don’t cause the opposite effect and put to much air back in.

Heat Loss

As detailed above, Vented to Atmosphere LEV’s remove air from an area with out putting any back in. This can lead to another issue that can be very apparent in winter months. The area in which the LEV is located maybe being heated for operator comfort. This means any Make-up air (either naturally drawn in or added via a fan) coming into the room could be cold – especially if it is from out side . This can mean the nice warm area the operators are working could become very cold and uncomfortable. It can also result in higher costs for heating the area – as you may find you are effectively trying to heat a constant stream of cold air.

Not all air being drawn back into the room maybe cold. It may be the make up air is from another part of the factory/ office etc that has already be warmed but where the make-up air is coming from needs to be taken into consideration.

Summary

A vented to atmosphere LEV system is ideal for many applications. They can incorporate a large number of components and cover a large area with multiple extraction points, can handle high volumes of contaminate and can be cost effective to use with energy saving options like automatic flow control. But as with most extraction solutions one size does not fit all and these kinds of LEV systems need to be custom designed and fitted to suit your environment and application and be an effective control of the hazardous contaminate that meets the legal requirements.