In 2008 APMG’s Plastics division designed, fabricated, installed, and commissioned three venturi slot plate and multistage Davidson Clark impingement plate wet scrubbers for Watson-Marlow Fluid Technology Solutions (WMFTS) in Cornwall, UK.
The carefully considered design was created by APMG Plastics for the specialised application. For over fifteen years, these tough, efficient, and reliable units have removed pollutants from industrial exhaust gases, improved air quality, and met regulatory emissions control requirements.
APMG Plastics has worked with WMFTS over the intervening years upgrading the system and fitting an additional wet scrubber. The partnership has proven so successful that as the company expands its US operations it has turned to APMG Plastics to supply similar fume scrubbers to its new facility in Devens, Massachusetts, USA.
End User – Watson-Marlow Fluid Technology Solutions
Backed by a global network of technical and support teams, WMFTS specialises in high-quality fluid path technologies for the life science and process industry sectors. It offers a comprehensive portfolio of scalable, robust solutions, including peristaltic pumps developed to optimise process performance.
A peristaltic pump is a type of positive displacement pump. It uses rotating rollers or shoes to create a series of “peristaltic” waves along a flexible tube or hose, which moves fluid through the pump. The squeezing action of the rollers or shoes on the tube pushes the liquid in an unidirectional manner, preventing backflow.
Peristaltic pumps are commonly used in applications where precise and gentle fluid handling is required, moving fluids that are sterile, or aggressive chemicals for example. The tube material can be selected to withstand the specific requirements of the fluid being pumped.
Hodess Cleanroom Construction provides turnkey cleanroom design and build services across the US. It works with architects and cleanroom engineers to provide documentation, code analysis, structural design and all services required for the complete installation of a cleanroom of any size or type and related structures and systems.
Following the design, fabrication, and installation of the original three wet scrubbers in 2008, APMG Plastics has worked closely with WMFTS to ensure the units continue to offer the highest levels of efficiency.
The discharge stacks have benefited from some redesign and refurbishment since they were initially installed, with extra droplet separators added to prevent any liquid carryover.
in 2020 the Cornwall facility expanded its manufacturing plant and introduced a fourth oven into its clean room. APMG again fabricated and installed a fourth scrubber system to manage this extra capacity.
The level of professionalism and service offered by APMG Plastics, together with the longevity and successful operation of the Cornwall-based fume scrubbers, led WMFTS to engage the company to deliver three more virtually identical systems for its new plant in Devens, Massachusetts.
WMFTS takes its responsibility to effectively neutralise any fumes, particulates, and gases extremely seriously, and a reliable wet scrubbing system to assure the protection of its operators, the wider community, and the environment is absolutely imperative.
It manufactures silicone tubing for its range of pumps. This tubing is cured in ovens within its cleanrooms, and a by-product of the process is silicone oil in the air stream.
This oil consists of submicron particles of silicon which must be amalgamated to be of a size where they can be captured and neutralised before the airstream is exhausted safely to atmosphere.
The experienced engineers at APMG Plastics designed a hybrid venturi and impingement plate wet scrubber to deal with WMFTS’ very specific application. Building on its success in the UK, this design has been replicated, with some slight amendments, for the new US plant.
The combination of the venturi slot plate and multistage Davidson Clark impingement plates provides efficient contact between the gas and the scrubbing liquid, maximising the removal of contaminants. Combining these two different types of wet scrubber delivers an effective solution to the challenge.
The fume scrubber begins with a venturi slot plate, which is a specially designed throat section that creates a high-velocity gas flow. It operates on the principle of a venturi effect, where the narrowing of the throat causes an increase in gas velocity. This acceleration of the gas stream enhances the scrubbing efficiency by promoting thorough mixing and contacting of the contaminants with the scrubbing liquid.
The WMFTS fume scrubbers extract the contaminated gas from the curing ovens. From here the venturi pressure drop accelerates the sub-micron silicone oil particulates and the scrubbing liquor through the venturi slot plate.
This causes the silicon particulates to amalgamate into droplets of a sufficiently significant size to be scrubbed.
Now the silicon particulates are of a suitable size, they can be separated through the two-stage Davidson Clark impingement plates.
The gas stream enters the series of Davidson Clark impingement plates located downstream from the venturi slot plate. This network of perforated plates creates a labyrinthine path for the gas flow. As the gas passes through the impingement plates, the particulate matter and liquid droplets in the gas collide with the plates, causing impaction and separation. The liquid droplets coalesce and drain down the plates due to gravity, while the cleaned gas continues to move through the plates.
The scrubbing liquid – in this case water – is stored in the recirculation tank at the base of the scrubbing tower. The water serves two purposes: first, it acts as a carrier medium for the removal of contaminants, and second, it helps to cool the gas stream if necessary.
In the WMFTS fume scrubber, the contaminated air enters the gas inlet. Here it is cooled to its adiabatic saturation temperature by humidifying sprays in the lower part of the scrubber. (When water evaporates into air, it has a cooling effect on the air. Adiabatic saturation temperature is the maximum temperature that can be achieved when water evaporates into air, and it represents the point at which the cooling effect of evaporation balances with the heating effect of the air.)
The humidifying sprays ensure saturation, remove larger particulate matter, and start the absorption of any gaseous compounds present.
The liquor flows into the recirculation pump and is fed to the upper impingement plate and the flushing and humidifying sprays. It flows horizontally across the plates, down the weirs and back to the recirculation tank by gravity. The spray liquor also drains back into the tank.
This bespoke scrubber system has a freshwater make-up to allow for evaporation losses and continuous or intermittent liquor drain-off from the system.
As the gas stream passes through the venturi slot plate and impingement plates, the high-velocity gas flow and impingement action cause the scrubbing liquid to capture particulate matter, liquid droplets, and gaseous contaminants.
The impaction, diffusion, and condensation mechanisms effectively remove the pollutants from the gas stream. The liquid carrying the contaminants is collected at the bottom of the scrubber, and the silicone oil floats to the top of the tank sump. It is withdrawn from here for further treatment, recycling, or disposal.
Manufactured from PVC, the discharge stacks have been meticulously sized for a velocity of 9m/sec as required for wet emissions. This velocity is key. Any faster and there is the risk of side wall creep of any condensate – droplets escaping from the top of the stack. 9m/s or lower will allow the droplets to fall back down into the scrubber.
The APMG stacks have been designed to provide an overall height of around 9 metres from ground level. It was agreed that WMFTS would use a local US steel ductwork company to extend the stacks above roof level by introducing steel attenuators and further ductwork to achieve the recommended discharge velocity. The combination of height, velocity, and distance factors allows discharged air to be dispersed far enough from the building to avoid downdrafts created by wind passing over the roof. This will also protect neighbouring properties from any potential noise pollution.
To create the high-pressure drop across the venturi, a fan capable of 60” wg is necessary. APMG Plastics designs, fabricates and installs a wide range of plastic industrial fans. However, to deliver the required volume and velocity of air movement for this application requires steel fans which were sourced from a reputable manufacturing partner.
The fume scrubber system, the stacks, the body of the scrubber, and its internal chambers, plates, and accesses, have been fabricated in Manchester, UK by a committed team of time-served specialist craftsmen.The scrubber has been manufactured throughout from polypropylene materials with the recirculation tank reinforced with glass fibre.All the connections for gas and liquid are flanged, and clear access doors have been provided at each stage allowing quick and easy access for maintenance and troubleshooting.
The completed scrubbers were shipped to the US by APMG Plastics, which worked with Hodess, the main contractor for WMFTS’ project, to agree timescales and schedules for the arrival, unloading, and installation of the three large wet scrubbers.
Two of the company’s highly experienced installation engineers flew out to oversee delivery and unloading and to install the scrubbing system. This was successfully achieved in less than 3 weeks.
‘The US project was a logistical challenge however APMG’s proactive approach to problem solving really helped deliver the project on time. A lot of detailed technical information was needed at the start of the project, but APMG were more than happy to provide as much information as required to satisfy the local permitting requirement. The product is manufactured from high quality materials and due to the unique design to solve our issues we could not find a similar product in the US that delivered the same quality & performance. The can-do attitude of the APMG team is exemplary. ‘
Darren Etherington Head of Engineering – Manufacturing Technology