[Speedslug]

Ok, First off you will only need one fan on the exhaust duct work.

The 'make up air' will come in to replace the exhaust air because the one fan will suck it in as it pushes the burnt gases out. You only have to make sure that there is a large enough opening for the make up air to come in through.

If you use the window on the other side of the room you will need to route the make up air to the torch bench through ducting.

Now for the details;
Fans push and pull air through ducting.
How much energy it takes to move the 1000 cubic feet of air per minute through the ducting depends on the size of the duct .
Think of the difference between a garden water hose and a 55 gallon drum. The hose would take an enormous amount of energy to move that much air that fast and the 55 gallon drum would hardly slow the air down at all.

So bigger duct work is preferred, to a point.
You do want the duct small enough that the air keeps moving at a pretty good speed so that particles don't settle out of the air and just collect on the inside of the duct work.

The other thing about duct work is length. Since there is resistance to moving air through a section (say three feet ) of ducting it makes sense that the longer the ductwork the more resistance the fan is going to 'feel' so keeping ducts short in length is more effective at moving the air with the least amount of energy.


One of the other things about duct work is the smoothness of the inside walls.
Just like water over a rapids slows the water in a river, a rough surface on the inside of the duct will slow down the air movement and, in effect, will reduce the size of the free flowing area in the duct.
So those handy looking crinkle foil 10 inch ducts at the home center will actually act like they are only 5 inches inside because the jagged inside surface interferes with the smooth flow of the air.

This is my round about way of saying you really want to use hard flat surfaced ducting material.

Another thing about flowing air is that it gets slowed down if it has to make lots of changes in direction.
I think the rule of thumb is to limit the number of 90 degree bends to 3 at the most and if you can help it two 45 degree bends with a little distance between them is better than one 90 degree bend.

Some times it works out that you just have to have more bends and or longer duct work and increasing the power of your fan can make up for these but you have to work through the details in those cases and you might be best off having one of the local heating and air conditioning companies come and give you an opinion.

Just be sure you hold your ground at the volume of air you want to move. Most of those folks are used to kitchen and bath room fans and all their furnace gases go up larger chimneys.

The gases from melting glass can have trace amounts of metals and some of them are not healthy for humans like copper and cadmium and we want the air to move fast enough to get out of the ductwork before the particles can settle out of the air stream. Once outside weathering and rain will dilute them enough so that you won't have a toxic lawn but if they settle on the inside of the duct work this 'dust' can collect enough to vibrate back down the ducting and settle on the bench. Then you wind up with it on our hands and everything that gets on our hands winds up in our food.


My advice to you for starting out in our crazy addiction of melting glass is to use the one window above your troch bench for both exhaust and make up air.

It is my opinion that you can go as low as 850 cfm for the fan on the exhaust duct and start it about 18 inches from your torch bench.
Run it up and out through the window and then run it another six or ten feet from the window, mount it pointing upward and put one of those pointed rain caps on the top of it.

Some of the hardware stores have round disks that fit inside the duct work with a handle outside that you turn to change how much air moves through the duct.
One of those would be good to block off the duct work when you are not using it.
( Well it will slow 98 % of the air flow anyway.)
Those should be much cheaper than the $40 I spent on the blast gate I use.

As for the make up air I suggest you bring that in through the same window above the torch bench.
You can use the same mounting outside as you did for the exhaust side with the pointed hat and mounting both duct work ends some 18 to 30 inches off the ground. Just make sure these ends are some 10 feet apart.

Bring the make up air duct down to the bench.

Due to the air flow you might find it best to mount the make air dump on the side of the bench. There is a real possibility of turbulence causing the troch flame to dance so you may have to play with how you introduce the make up air to the torch bench area.

Another one of those round disk duct blocking things in the make up air line will allow you to block it off when you are not using it.



Good luck and happy hunting.



ETA: You might want to skip the hot water out the window idea. You would find that it wont melt as much as you might think before it gets cool and then it will freeze to the cold metal. Better to mount it up a little higher. Of course if you get snow drifts on that side of the building then you will need to go higher still or find another spot to mount it.