What if I can't afford dust collection??

[ian maybury]

Air flow requirements can vary so widely. Chris' question about dust collection for carving on the other thread (very small shop) kind of underlines it.

You can get quite decent dust collection with low air flows on a tightly hooded machine - where a well designed hood can be positioned to intercept flying dust, and where the fairly high air speed require to collect it only has to be maintained over a very small open area. The Festool example mentioned above.

The problem is that this is not possible on so many machines. A wide jointer or planer, or a decent sized table saw with a leaky bottom chute and wide top guard making angled cuts is another. Even quite small band saws can be another - if as is often the case you can't place the pickups so that they create the above high speed air flow through a small area, then the CFM/air flow requirement needed to do a decent job can be quite large.

Worst of all is perhaps the downdraft sanding or carving table, or fume hood type set up. To get these working properly you don't need much airspeed (maybe 0.8-1.5ft/min - don't know what the official recommendation is), but you need to maintain it over the entire open area of the table or the hood. Which for even a smallish 3ftx4ft = 12ft2 runs up to 12x0.8x60 = 576cfm. 1,080cfm for the higher end of the airspeed range.

What I'm driving at is that the above range of airflow/CFM numbers takes you (in a small shop scenario) from maybe vacuum levels of flow (on small well hooded hand power tools), through the 450CFM or so that's the minimum for low pressure systems on most floor machines, to the 1000CFM plus of a large Pentz or Oneida type dust system.

Pressure capability is the other variable - tightly shrouded and small ducted hand tools and the like need much higher (vacuum like - say 100in WG) pressures than smaller floor machines at say 12in WG/450CFM if well hooded. Larger floor machines/less effective hooding may require 15in WG and 1,000CFM plus. Against that a downdraft table or cabinet can be power by a low pressure axial flow fan capable of only maybe 0.5in WG but shifting lots of CFM - if that is you don't mind exhausting outside to avoid the need to filter and/or cyclone, and pulling in fresh air to compensate. Recirculation probably means with the extra pressure drops etc going back to a large Pentz/Oneida type system at the above 15in WG/1000CFM plus.

There's no easy answer to all of this, but it very much underlines the need to be clear about the nature of the machine/dust collection application before (as we often do) rushing to the 'I've got an XYZ' discussion. When requirements get lost then all the human bias and selective perception kicks in. 'You can't possibly need that much CFM' etc ..

ian

[Jeff Morton]

I've used a shop vac with HEPA filter and collection bag for years with no issue. I added one of the Rockler dust collectors when they came out. It captures most of the debris with the vac getting the rest. You might have to build more efficient collector box for your TS and other tools that don't have efficient ports, but it is worth the effort. I also use box fans with filters on both sides when applying finishes.

[Ole Anderson]

I have a full dust collection system, but still use my shop vac with an upgraded filter on my 6" belt sander, works fine as it was built for a 2.5" hose, but building a hood so I can use the 4" hose is on my list. One reason I haven't upgraded it yet is that I have one button to push that starts the sander and the vac.

[mark marriott]

In my basement shop I have limited space and low ceilings, just 7 feet up to exposed floor joists above. I also have somewhat limited funds (that I'd rather put into tools honestly). Can I get by just using a good strong shop vac that I connect to the tools prior to starting them up?? Right now I have to move tools outside to use them when they make a lot of dust, like routing or table sawing. I don't want dust all over the house above, or the shop.

Consider looking at craigslist for people upgrading or getting rid of dust collectors. In my area (DC) there have been a few good deals w/canister filters available over the last few months.

[Michael W. Clark]

Air flow requirements can vary so widely...

This is why it is common practice to design the hoods first or, in this case, consider the equipment being exhausted when picking out a dust collection system.

Ian, you are correct on the fume and canopy hoods. This type can require many times more CFM to function properly than a close-capture hood. I've done some hoods that require 400-500 FPM face velocity and some small hoods in the chemical/food industry that require 50-100 FPM. It adds up very quickly over a large face. It is usually your last resort for a hood design. It is not just about the fan sizing and horsepower required resulting from this extra volume, but it mainly affects the size of the collection equipment. Depending on the process, the airstream may have to go through several pieces of pollution control equipment, each sized based on the ACFM from the hoods. This affects energy consumption, real estate, and capital costs of the project.

Imagine if you needed 5,000 CFM instead of 500 CFM. The electrical service would be an issue in a home shop and the real estate required for the cyclone, fan, and filters would be much larger.

Mike

Mike

[ian maybury]

The bit I haven't figured out Michael is if there's any convenient solution beyond a typical cyclone + filter type ducted dust system for high CFM table/hood type installations in situations where recirculation is required. You can switch to an axial flow fan if the need is to shift a lot of air against very little resistance (e.g. outside through a hole in the wall), but presumably not if you need to recirculate - to duct, filter the air and separate out dust in a cyclone.

The numbers as you say quickly go crazy when it's necessary to move the air over a large open area like a table of a hood...

[Michael W. Clark]

An axial fan can be used with a prefilter and good after filter (hepa like). However, filter pressure drop can affect exhaust volume if the filters are not cleaned regularly.