02.12.2013

Silos and Ducting

Ducting and Silos are necessary to enable prefoam to be transported from the Expander, into Intermediate ageing in the silos, prior to transfer to the shape or block moulding area for moulding. The equipment that needs to be designed will include:

  • Properly sized ducting to the silos
  • Ducting design to minimise damage to prefoam and for easy access should a blockage occur
  • Diverter valves or other flow distributor, for feeding of silos on a one at a time basis
  • Silo size, number, mesh type and whether it is supplied as bag mesh only or the silo bag with the supporting metal frames
  • Safe access to top of silo area
  • Level sensors
  • Silo outlet distributors for shape moulding
  • Ducting from silos to moulding(block or shape) - requirements are different
  • The possible need to design the system with options to include blending for granulate use
  • The need to manage fire risk
    • Through segregation of the silo storage area
    • Adequate ventilation in the silo area

Please contact Polymac to discuss your needs.

The design process to optimise the silo processing step of EPS moulding should consider the follow:

Design for process optimisation and safety risk management to avoid static induced fires is critical. Read the following in regards to Pentane safety risk management http://www.hse.gov.uk/pubns/ppis1.pdf

When prefoam exits the expander, or drying bed, it is in a fragile state. Having cooled from the expander temperatures, the cell structure is left with a vacuum within the cells and prefoam is still soft enough to be easily deformed. During the silo ageing period the prefoam will cool to ambient, it will absorb air into the cell structure to dissipate the vacuum, it will lose moisture from the expansion process and prefoam will lose pentane.

The silo size and layout must enable this process to happen as optimally as possbile

  • Optimum ageing conditions will assist in fastest possible moulding cycle with minimal number of part rejects caused by variations in the condition of prefoam
  • Variability in prefoam condition can often be exaggerated when switching from the end of one silo into the start of the next silo

Ducting requirements from the expander to the silos:

  • Should be of adequate diameter to enable the transfer of prefoam from the expander to the silos in the blower air stream in laminar flow
  • Ducting should be smooth internal bore, and should be made from metal duct rather than plastic pipe
    • Generally a 200mm diameter duct might be required, but size can be as small as 150mm and upto 250mm subject to expander throughput rates
    • Where plastic duct pipe needs to be used it should be with metal reinforcing where the metal can secured to metal duct at either end of the plastic pipe to assist with electrical continuity.
    • Ducting should be designed to avoid static build up
    • Ducting should employ large radius bends to avoid the prefoam from being buffeted during transfer
    • Ducting should have electrical continuity at each joint, linking the expander through to the ageing silo frame
    • Ducting on vertical runs should have quick release at joints to enable easy access to clear blockages should they occur
  • Diverter valves
    • Should not restrict flow diameter of duct
    • Operation will either be manual or automatic
    • Various design options are possible, even to avoid the use of diverter valves

Silo design and layout:

  • Silo location
  • Design should maximise surface area relative to volume of bead being held, typically 3m x 3m square
  • Mesh type should be considered
    • Maximise open area without hole size being large enough to allow prefoam to escape
    • Anti-static mesh is available to reduce static build up
  • Design layout to enable access to clear blockages in ducting
  • Silos should ideally be segregated in a room away from factory moulding area, for safety reasons
    • Silos and silo filling system can be fully manual
    • Operated manually but with high and low level sensors to alarm when silo is full or empty
    • Be part of a fully automatic silo management system
    • Where level alarms are to be included they should be specified upfront and positioned appropriately
  • The volume of silo space needs to be sized adequately to enable optimum ageing of prefoam prior to moulding. Silo space will depend upon:
    • Down stream moulding throughput rates
    • The type of raw material being used
    • The ambient temperatures
    • The density of prefoam to be moulded
  • The silo room should be sized for the number of silos required, anticipating future growth, rather than silo size and number designed to fit the room
  • The silo room is a source fo pentane accumulation and consequently it is a fire risk area. Pentane is heavier than air
    • The silo room should have good natural ventilation at ground level
    • Air circulation around the silos is important to speed up the ageing process and to keep pentane concentration well below LEL

Ducting from silos to moulding machinery:

  • All ducting should be metal duct/pipe with electrial continuity

Block moulding

  • Generally 150mm duct is adequate and prefoam can be sucked from the silos and taken through the suction/blower fan as the prefoam now has adequate strength to withstand mechanical impact without damage
  • Consideration needs to be given at silo design and layout as to whether recycled granulate will be mixed with virgin prefoam prior to moulding
  • Ducting should be installed to have electrical continuity from the ageing silos through to the block mould fill hopper

Shape moulding

  • Generally 1.5" to 2" light weight pipe or ducting is adequate
  • Avoid using plastic hose, as plastic will generate static on the prefoam and make filling harder to achieve
  • Where plastic hose has to be used for short lengths, use plastic hose with metal flex to ensure electrial continuity by baring the metal flex at either end to fix to the metal duct/pipe

Polymac has the experience to assist you in proper design to optimise processing, whilst better managing potential fire risks.

 

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