Harbor Freight Blower

[Ralph Tesson]

The ducting and fan specification is all based on conveyance velocity (the speed at which the air is moving so that a given particle size will be conveyed by the air instead of resting in the duct). In a 6" duct 400 cfm is equal to a conveyance velocity of approximately 2100 feet per minute. Change the duct to 4" and to maintain a conveyance velocity of 2100 fpm you only need to move 180 cfm of air. A 5" duct would be 280 cfm. This is perhaps important to note for any of you that happen to be reducing or enlarging your ducts with a collar and some duct tape thinking that it has little to no effect!
For the rest of you who wanted an alternative for a Zing you can rest assured that you have an appropriate solution. Feel free to PM me if you have any questions about ducting and exhausting your engraver or laser system.
[QUOTE=Richard Rumancik;1323680]Dan, I looked at the Zing and it still asks for 400 cfm at 6". I don't know what cools the laser on the Zing or other Epilogs.

[Richard Rumancik]

Yes, I agree that Mike raised a valid point pertaining to the heat loss resulting from a high cfm blower.

Mike suggested just lowering the CFM exhausted to the outside by using a smaller blower.

My suggestion to members, if heat loss is an issue, was to try to use makeup air from the outside. If the laser could be set up so that it is drawing a significant amount of its makeup air from outside (rather than 100% from the heated room) the cost could be reduced considerably.

Your suggestion to reduce heat loss is to recirculate the air through an air purification system. Although your solution will reduce heating costs, it is not inexpensive either, in terms of capital cost and maintenance of filters.

. . . . With some quick math I calculate that on a 6" duct 650cfm would be reasonable; it would equate to an air velocity of about 3200 fpm which is the conveyance velocity for average industrial dust. Change that to a 4" duct at 650cfm and you accelerate up to 7500fpm, enough to fire lead shot. I would call that overkill. . .

I don't quite follow the approach you are taking on this. Generally we start with what the manufacturer specifies for a particular laser system. Now some people may disagree with this (as Mike does) but let's follow this for a moment. If the manufacturer asks for a blower capable of 400 cfm at 6" water column, my inclination would be to try to follow the recommendation. Also, most of the small lasers are using 4" ducts although there are a few that use 6". Also, most of the lasers call for 250-400 cfm at 6" water. (There are some exceptions.)

You seem to be starting with a 3200 fpm number which is apparently a typical "conveyance velocity" for an "average industrial duct". If I was sizing the system, I would first look at what the manufacturer is asking for, and size my blower appropriately. The fpm would not be a starting point, it would be a consequence of other decisions.

I would estimate that for a blower of 400 cfm a user might be paying $0.50 to $1.50 per hour operation for energy lost up the pipe during the heating season. (Depends on climate and local energy costs.) I think that by using outdoor makeup air this cost could be halved or better. For the rest of the year, energy savings would be negligible. I am running a 250 cfm exhaust and it probably costs about $0.50 per hour of operation with no makeup air, at -30C outside temp. (This is in Canada, it's cold but electricity is inexpensive.) Sometime I will take my own advice and install a makeup air duct and get the cost down to $0.25.

I have not considered air conditioning. I have a suspicion that few shops are air conditioned but I could be wrong.

Assuming around $2K for the fume extraction unit and a few hundred dollars of filters/year some users might be able to pay for it in a few years. I my case it would take quite a number of years to save $2000 in energy costs.

[Richard Rumancik]

The ducting and fan specification is all based on conveyance velocity (the speed at which the air is moving so that a given particle size will be conveyed by the air instead of resting in the duct). In a 6" duct 400 cfm is equal to a conveyance velocity of approximately 2100 feet per minute. Change the duct to 4" and to maintain a conveyance velocity of 2100 fpm you only need to move 180 cfm of air. A 5" duct would be 280 cfm. . . . .

Ralph, I read this a couple of times trying to figure out how to apply this info and understand what you are getting at.

Seems like you are saying that if a manufacturer asked for 400 cfm through a 6" duct, they are really asking for a velocity of 2100 ft/min. And if what they really want is 2100 ft/min, then by using a SMALLER duct, you can get the same velocity with lower cfm (smaller blower). Is this what you are saying?

If so, how small of a duct can I use? It seems contrary to what we have been told before - which is keep system impedance low by using large diameter ducting (even if larger than the laser exhaust port itself.) None of the manufacturers specify air velocity requirements; they specify cfm @ inches water column.

It would seem to me that lower flow would mean that less contaminated air is captured and exhausted.

The other point is that for laser engravers, I don't think there is a substantial amount of particulates. The main reason for the exhaust vent is for smoke and vapor. Now I realize that smoke is potentially particulates, but it is not on the same scale as conveying sawdust. A smoke particle is a few magnitudes smaller than sawdust. I really don't expect smoke particles or acrylic vapors to settle in the duct to any significant amount.

Some laser processes can be dusty, but in those cases a lot of the heavy particulates remain in the laser itself and need to be vacuumed up.