Dear Rupert, Many thanks for taking the time to comment. More below.
"I have been designing and making drum screeners for many years and would like to summarise the main design decisions for the benefit of those interested." That answers one question I had - it isn't a new idea! "Mechanically scraped screens have to be strong (especially if they are inward flow ie. The pressure is trying to collapse the screen) because even small twigs and debris can exert a considerable force on the screen it they get jammed in the scraper. Scrapers tend to shred the debris an send quite a lot through the turbine (which may or may not matter depending on the type of turbine, even small amounts of material going through a fixed geometry propeller turbine can reduce the output considerably, because it can build up on the leading edges of the runner blades, causing the flow to become turbulent. The plant in question runs two back-to-back coaxial Francis turbines with stainless steel runners. They are very strong indeed - an incident early in the plant's operation resulted in chunks of concrete passing through the turbines, with no damage resulting. They are impervious to small debris, silt and so on - the small debris only become a problem when held back by larger stuff caught on the trash rack during peak flow, causing serious head loss. The screen spacing of the trash rack is fairly coarse, but I didn't take measurements. The only requirement imposed on the trash rack is to prevent entry of branches, logs and other show-stoppers. The rest should preferably go right through. "Getting back to your vertical screen, it is a bit of a problem with a spiral scraper because it will have to cut any sticks that poke through the screen unless the pitch is very fine or the screen mesh is very fine. From my experience it is always a bit of a compromise between cost, maintenance and the frequency of jamming. One golden rule is that the screen must be able to withstand the hydrostatic head with total blockage or there must be a totally failsafe system to shutdown the plant(I have seen many plants with collapsed or damaged screens, caused by blockage)" I've been very concerned with the problem of shearing off intruding twigs, or maybe even larger stuff, thus defeating the purpose of the device. The design I'm working on is intended to lift the debris without cutting it. To accomplish that, the turning, cylindrical grate consists of coarsely spaced spiral members opposite in rotation to the sense of the fixed spiral scraper. The result (I hope) is that the branch or whatever is pushed up the spiral ramp to the trash conveyor without catching on anything. This does pose an interesting design problem for the moving grate, because it can only be supported by radial members, with no bracing between members like in the usual orthogonal grid. I wish there were some way I could test this at small scale, but where to you get scale debris? Best, Marc de Piolenc ------------------------ Yahoo! Groups Sponsor ---------------------~--> Buy Ink Cartridges or Refill Kits for your HP, Epson, Canon or Lexmark Printer at MyInks.com. Free s/h on orders $50 or more to the US & Canada. http://www.c1tracking.com/l.asp?cid=5511 http://us.click.yahoo.com/mOAaAA/3exGAA/qnsNAA/FGYolB/TM ---------------------------------------------------------------------~-> Does your company feature in the microhydro business directory at http://microhydropower.net/directory ? If not, please register free of charge and be exposed to the microhydro community world wide! NOTE: The advertisements in this email are added by Yahoogroups who provides us with free email group services. The microhydro-group does not endorse products or support the advertisements in any way. More information on micro hydropower at http://microhydropower.net To unsubscribe: send empty message to [EMAIL PROTECTED] Yahoo! Groups Links <*> To visit your group on the web, go to: http://groups.yahoo.com/group/microhydro/ <*> To unsubscribe from this group, send an email to: [EMAIL PROTECTED] <*> Your use of Yahoo! Groups is subject to: http://docs.yahoo.com/info/terms/
