Regarding the use of wedge-wire screens. They are only self cleaning if a significant portion of the flow continues past the screened intake because if the bulk of the flow is through the screen it must become blocked because there I nowhere for the debris to go.
Regarding drum screens. Most of the problem with screening is when river flows are high and there is plenty of water but as you rightly say, there is a problem when flows are low. This can be overcome by running the screener on demand (ie. It runs only when the screen needs cleaning) This is achieved by closing off the 'drive water' or by using a 'back flushing' design, that only operates when the head-drop across the screen in more than about 50mm. It is impossible to design one screener which will fit all heads, flows and debris, so one has to compromise. Many site suffer from a wide range of material from leaves and grass, right up to trees, plastic containers and boats! Usually a 'guard screen' or skimmer channel' will remove the big or floating objects, leaving the automated screener to cope with just the fine stuff. If you are screening less than about 70mm, it is often better to screen very fine (less than 25mm) if there are sticks that can get lodged in the bars ( even a 25mm stick will take a lot of power to shear it) you will have to make it all very strong or have a slip clutch (as soon as the screener stops the plant will probably have to shutdown, which is why I have always favoured 'back-washing'. The system will still function even it you have a number of sticks tangled in the actual screen. Sorry to go on a bit, but screening is one of the most overlooked areas and one of the most common reasons for hydro schemes being abandoned. Scrapers, in my experience only work if they are simple, and in line with the screen bars. Regards Rupert -----Original Message----- From: Marc de Piolenc [mailto:[EMAIL PROTECTED] Sent: 25 April 2004 08:14 To: [EMAIL PROTECTED] Subject: [microhydro] Re: New (???) trash rack design [EMAIL PROTECTED] wrote: > > Anyone who is thinking outside the box, so to speak, is to be congratulated. > It just might work. Have you ever considered using a tilted wire wedge wire > screen? There are no moving parts, minimal maintenance and for all practical > purposes are totally self cleaning. There is usually a way to either utilize an > existing structure or easily build something that will hold the screens. They > will remove all debris larger than .25mm and the material of construction is > 304 stainless steel. When the value of time and lost generation are considered > they are well worth the money. Dear Robert, I've visited your Web site (difficult, because Netscape 4.7 can't display it and I had to install and - gulp! - open Internet Exploiter to see it). As far as I can figure out from the pictures - there doesn't seem to be any verbal explanation of how your various products work - these screens need a continuous flow of water over them, bypassing the turbine, to be self-cleaning. The plant I am concerned with (and this will be true also of all future projects) cannot afford to divert water year-'round, as surplus flow only exists during peak flow periods, which are rather short. So IF I have understood your products' MO correctly, they are not applicable. Too bad, because I must say they are very elegant. Best, Marc 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 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/
