Introduction
I have noticed, after purchasing a ribbon burner and a crossfire for a new shop I'm setting up (my first, actually), that it would be very easy to spend a small fortune, if I had one, on equipment (old pros, don't laugh too hard.) And you don't always get what you pay for, either (the old pros are now laughing hysterically.)
Situation
I suckered for one of the "precision needle valves" specified for the ribbon burner in the vendor's catalog. Just one - for the gas valve. Thirty bucks, as I recall, of fairly hard-to-obtain cash. After using scrounged odd-ball and miscellaneous valves for the crossfire and hand torch, and a 1/2 inch water gate valve for the air on the ribbon burner, guess what? The junk valves work perfectly - easy to adjust. The thirty buck wonder? One half turn, from full off to fire on the ceiling. Some days are like that.
So when it came time for a proper air blower, the concept of "buy new" was rapidly dismissed. However, whatever was to be used has to work right - all the time. Nothing I've experienced this side of Classic glass is more frustrating than fires that intermittently blow out from being too lean or reduce the lead out of the glass from running too rich. Let's face it - the fires 'gotta be stable.
Insight
I had an instructor at UCSD Craft Center's School of Neon, Ceramics, and Other Fun Stuff (fish painting?) mention that sign shops have been known to use spa blowers for their air source. Why not, I think. Heck, I've even got one (this is California - a hot tub at every house is required, otherwise the Culture Committee will come and speak with you.) Blower even works, I think. After fast work with wire cutters and a hacksaw, and over minor whining and protests from the family (great sense of humor they have - even bought the line "it's only temporary - I'll put it back"), there we have it - cheap air.
[Sad note: the UCSD Crafts Center has been shut down and bulldozed into oblivion, for most likely political reasons.]
Only it's not quite that simple. Never is, it seems. You can't just plumb a couple of fires into this thing, turn it on, and bend away. You see, the air going through the blower is also used for cooling the motor. Not enough airflow and you get this baby imitation of Chernobyl as the hot glowing remains of the motor descend through the plastic blower case and start working on the floor. Problem number two: flip the lever on the ribbon burner from full military power back to idle, and the crossfire jets start blowing out. We don't even want to talk about the hand torch.
Solution
However, there is a solution, and it's a reasonably cheap solution (the best kind.) And better yet, it even works! What we did here is build what I'll call a "bypass regulator".
Here's how it works (see drawing, right): As the air pressure in the manifold builds up, it increases the force on the wooden piston. When it gets high enough (I set mine for slightly less than two pounds), the pressure acting on the piston overcomes the force of gravity pulling down the weight on the other end of the rod, and the piston starts moving up towards the open end of the pipe. As it moves up, it starts uncovering a slot cut into the side of the pipe, which allows excess air to escape, which reduces the air pressure, which reduces the force acting on the piston, which stops moving up the pipe. The trick here is that the air pressure acting on the piston must always balance the force of the weight, otherwise the thing will automatically reposition itself until they do balance. Piece of cake.
Details
The piston is a 2 inch long piece of wood. Use some kind of hardwood - it works better. Make the thing round and smooth such that it moves easily up and down inside the pipe, with minimal side clearance. Don't make it too tight a fit, however, or it will stick in the pipe. Make it smaller diameter in the middle than at either end - sort of spool shaped. I put some furniture oil on mine to seal it up from moisture attack.
The rod between the piston and the weight is made from a length of 1/8 inch gas welding rod. The copper coating on the rod keeps it from rusting. Drill a 1/8 inch hole through the center axis of the piston for the rod. While you're at it, drill a 5/64 inch hole in the bottom pipe end cap for the rod to pass through. Bend the end of the rod so that it can't depart company with the piston. Drive a small finishing nail into the piston, down along side of the rod to wedge it, keeping it from sliding down the rod towards the weight when you shut the blower off.
All plumbing is done with 2 inch schedule 40 PVC water pipe and fittings. The slot is cut with a 1/8 inch Dremel tool bit chucked up in a power drill. Don't use the Dremel motor - it turns too fast and just melts the PVC into a gooey mess. Make the slot about four inches long. Note that the slotted pipe is not glued into the tee, but just pressed into place, and thus, removable for service.
Note that PVC pipe isn't terribly round or smooth inside. I cut a slot in a short length of wood dowel to hold a strip of sandpaper, chucked the creature up in the power drill, and used it to spin sand the inside of the pipe smooth.
The frame to hold the whole beast was made from pine 2 x 8 lumber and plywood. Trick: cut the three holes in the top piece of 2 x 8 the size of the outside diameter of the pipes, then rip saw the wood in half lengthwise. Remove about 1/4 inch of wood. Use two 4 inch long 1/4 inch diameter lag bolts to tightly clamp the wood halves around the pipes.
The 1/4 inch plywood corner braces give the whole structure strength. A coat of your favorite wood finish gives it beauty.
The design of the weight I leave up to your imagination. I just used a rock that in a previous life was the shop doorstop. Just make sure that, when complete, the piston's lower edge completely covers the slot in the pipe when the weight is resting on the base, and completely uncovers the slot before the weight hits the bottom pipe cap. Adjust the weight of the weight to get the air pressure that you want.
Technical Info
Area of piston = pi x radius2
= 3.14 x 1 x 1
= 3.14 square inches
Force on piston = 2 PSI (air pressure) x 3.14 square inches
= 6.28 pounds
therefore, weight of the weight = 6.28 pounds
For a different air pressure, adjust the weight of the weight accordingly.
Caveat
While this setup works great, there is a potential problem, of the big and bad kind. However, it is easy for the thinking person to avoid.
The problem is this: the motor used in a spa air blower is what is called a "universal AC-DC" type of motor. It is a close relative of the common household vacuum cleaner. It has brushes and a commutator and such. All motors of this type spark at the brushes when running (look at the back of the motor in your power drill for a demonstration of this.)
WHATEVER YOU DO, NEVER, EVER let gas from the fires to get back through the air plumbing into the blower motor. Otherwise, you're six o'clock news.
To keep this from happening, I added a common electric gas valve, like the one used in a household forced air furnace, to the supply pipe for the fires. You can probably scrounge one from a deceased gas clothes dryer, although I haven't checked this out.
Add the electric valve after your main gas shutoff valve. Wire the primary input of a standard 11 volt doorbell transformer (the right voltage for my valve) in parallel with the blower motor. Use the secondary output of the transformer to energize the gas valve. If you inadvertently shut off the blower or loose electrical power while the gas is on and the fires are running, the gas will automatically shut off. End of problem.