Varying CFM on Shaded Pole or PSC Direct Drive Blowers

First of all, thank you very much for the help ! I can follow what you posted even though I know I'm in over my head as to how to wire and set up a PID. I do have a power column drill set up with a VFD and a 3 phase motor, but with manually set speed. If I got you right, the best idea would be to get a 3 phase PSC blower and an Anacon 1 phase to 3 phase 200-240V Controller with a PI module (these seem to be add-on and not built in, at least for Anacon). I have some questions about the use of a transducer. It is a device that measures a pressure gradient of gas passing through it. Does that mean attaching a tube coming from the pre-filter area to the device and another from the post-filter area ? If this is a positive feed-back loop that will keep increasing the blower speed overtime, how will I know when the pre-filter pressure has reached the point that I must change the filter ? Do I need a manometer as well?

This will be a positive pressure hood, with laminizing air guides, usually made from a bed of soda straws packed tight and glued in place. All you need is 3-4" lengths to fill the upper plenum that feeds your filtered air down. This plenum is arranged as an inverted funnel(not circular, but the size of your straw bed at the bottom, and about 1/10 that area on top and it should be about 3 feet above it to allow the air to equalize in pressure acroos the top. You will then force air through the HEPA filter and you will have a variable throttle linked to a pressure controller that opens/closes a bypass duct to maintain a constant flow through the HEPA filter. At some point the pressure drop gets too great and the throttle limit is reached and the filter is changed. Some people have a dual system and when one gets plugged, it senses it and the parallel air flow starts, and a red light goes on the change the filter.

The laminizing bed can also be made from corrugated plastic board, much like corrugated cardboard, but from plastic. You can also use corrugated cardboard to make the laminizing bed, but you will then need to dip it into thinned gloss paint, let drain dry and then dry until fully dry. This will place a coat of pain on all surfaces of the cardboard, thus cementing any fluff down. You can also use a clear varnish. The corrugated plastic is better and once make it can be cleaned with a pressure washer after you remove it. The cardboard will still absorb water through the paint, so it can be washed, but must be warm air dried to avoid this. I have made laminar flow cabinets this way for budgetary rasons. You have no idea what the monkeys that sell these things want for them.

lots of plastic makers here

http://www.google.ca/#hl=en&source=hp&q=%22corrugated+plastic%22&btnG=Google+Search&meta=&aq=f&oq=%22corrugated+plastic%22&fp=7310e1b076ed5f89

Let me see if I got this: Above the hood I must have 2 chambers. One chamber between the admission pre-filter and the blower. A second chamber between the blower and the HEPA filter. What you are saying is that the second chamber should be built as a rectangular inverted funnel with a rectangular base that is 10 times larger in area then the top (where the blower outlet will be) ? Also that the height of that second chamber should be at least 3 ft and that at the bottom of the chamber I should have the straw bed (or corrugated plastic) with a thickness of about 3-4" laying directly on top of the HEPA filter ? I have seen polycarbonate cellular sheets. Could I use those stacked together? How do the variable throttle, pressure controller and duct bypass work ? What is the system configuration ? Is it similar to what we discussed above ? Thanks for the help!

best look here. You will see top to bottom laminar flow and side to side. Most are top to bottom with an open grid floor, some use fresh air, some recirculate, it depands on the application. Some want to filter the outflow into a stack to exhaust, etc.

As you read among these, you will find many explanations and good ideas.

http://www.google.ca/#hl=en&source=hp&q=%22laminar+flow+hood+construction%22&btnG=Google+Search&meta=&aq=f&oq=%22laminar+flow+hood+construction%22&fp=7310e1b076ed5f89

By now I've googled almost every aspect of hood construction and read at least a dozen pdfs on step by step hood construction. All of them are nice, but there seems to be a lot of guesswork involved. All of them use a fixed speed blower and most use plywood. I don't need anything fancy, as I will do mostly orchid seed sowing and flask replating, but I want to build something that will remain useful for years and is sort of comfy to use. I've decided on a vertical hood design, without any need for exhaust or recirculation. I'm concentrating on how to choose the best blower/blower speed control system and also how to best guide the laminar flow and monitor the flow output. Can you please explain to me again how to use the straw bed ?

any laminar flow system needs to have an air flow diffuser that spreads the air out and send it on a uniform path downwards or across. You do not want a tornado in the fume hood.

So the corrugated plastic/straws are one way to do it. You need to have a very high degree of transparency. Thin walled straws are probably about 97% through flow and 3% plastic. It depends on the amount of adhesive you use. The corrugated is probably about 90%.

As to guess work, yes, you need a slow flow of air in parallel with every straw at the same speed, and the rest is cut and try. No need for precision as long as you understand the basics.

Slow flow = not turbulent.

parallel flow at constant speed = no mixing that differential flow would cause.

Laminar cabinets are low velocity devices.