Project TEH – Bagging Plant Ducting

AT A GLANCE:

• Design / Consultancy to improve ducting design and dust levels
• Improved extraction
• Re-routed ducting to allow easier maintenance and access

CONSULTANT’S REPORT:

We were contacted via the Engineering manager as he had been told about some previous projects we had completed for some other sites.

The site had been having some issues with ducting getting blocked after a short while when the LEV system was running. We visited site to have a look at the existing ducting which was connected to a Donaldson filter and take some volumetric measurements and pressure readings.

Unfortunately, we could take any viable readings due to the ducting being blocked at the time of the visit. After looking at the ducting arrangement it became apparent that the design of the ducting was incorrect and not sized correctly which due to the low carrying velocities was causing drop out in the ducting.

As we didn’t know the overall volume and filtration area of the filter, we needed to contact Donaldson to get the details of the filter and fan specification.

The unit on site was a C-20R with a filtration area of 20m² and the fan is a K5 which is a 2.2kW fan. Looking at the fan curve with the K5 fan arrangement the maximum volume the fan can pull is around 2,900m³/hr @ 125mm pressure indicate by the red lines on the fan curve.

A quick calculation showed the filtration velocity of the filter to be 2.41m/min which is extremely high.

We worked out the pressure drop of the existing ducting which was 175mmwg which reduces the extraction volume to 2,200m³/hr.

Donaldson told us that the maximum filtration velocity of the filter on site is 1.5m/min or a maximum volume of 1,800m³/hr.

Therefore, we needed to design the ducting extraction based on the design point of the filter to avoid causing issues to the filtration efficiency and life expectancy of the filter media.

To do this we installed correctly sized ducting and blast gate dampers to balance the system accordingly.

Even with the volume set at 1,800m³/hr we will be able to get 1.08m/sec capture velocity.

According to HSG258 the capture velocities are as below:

The existing ducting was difficult to get access to for maintenance and the main extraction point was in the incorrect position. We spoke with the plant manager and discussed re-routing the ducting to enable the operatives to get better access for future maintenance.

We took measurements from site which allowed us to design the site layout in 3d to give our client a visual representation of what we were going to install prior to order being placed.

Once the ducting had been agreed and quoted the client then placed the order.

The ducting was split over two levels, the first-floor level where the extraction points were and the second floor where the filter was situated.

We used the 1,800m³/hr to design the ducting to maintain a carrying velocity of above 15m/s as they only wanted to extract the fine silicates in the air rather than the product itself.

Originally, we had planned to install the ducting during the week, but our client then wanted the installation to be over the weekend to avoid any downtime on production.

This subsequently formed a change order.

The ducting was designed to the maximum operating volume of the filter, which is 1,800m³/hr or 1.5m/min filtration velocity:

The ducting was fabricated out of 3mm galvanised sheet with 5mm flanges.

We installed access hatches within the ducting and blast gate dampers to allow us to balance the system once it had been installed.

To allow flexibly on site some of the flanges we slotted, and we included slip joints in each plane.

The existing ducting was removed, and the new ducting was installed within 2 days to ensure minimal downtime for site.

Once the install was completed, we balanced the system to allow ample extraction at both points.

We had three test points where we took volumetric, pressure and velocity readings.

Results pre-balancing

Test Point	Volume	Pressure	Velocity
1*	1,650m3/hr	115mm	19m/s
2	440m3/hr (turbulent)	125mm	33m/s
3	1,350m3/hr	120mm	45m/s

*Overall volume was taken with the flexible connections off thus reducing the pressure and increasing the volume.

Results after-balancing

Test Point	Volume	Pressure	Velocity
1	1,350m3/hr	175mm	15m/s
2	300m3/hr	120mm	8.64m/s
3	1,100m3/hr	125mm	32m/s

Due to the increase in pressure meant a slight reduction in the overall extraction volume as per the fan curve:

We now feel that the extraction points have been balanced are all above the HSG258 guidelines as stated in the COSHH regulation 9 especially as all the extraction points are enclosed.

Client testimonial:

Filter Designs attended site to review the current dust filtration system that had never worked effectively. Time spent investigating, enabled a detailed design proposal and scope of works giving us confidence in their ability return the LEV system into service.

Despite numerous issues with the existing filter, the team installed and commissioned the new ducting and balance valves, providing a detailed and clear commissioning report.

Job well done