In my indoor lab I have a fume hood which I have been using successfully for years. I use it for some experiments where I need venting off stinky of even noxious gases. I use a fan which vents off 270m3 an hour and works nice. But sometimes I use it for melting small quantities (0.5kg / 1lb or less) of copper or silver alloys in a small furnace using a natural gas / air burner inside the fumehood. In the latter case the fan gets very hot and sometimes it shuts off itself because of its overheating protection. So now I want to make a dual exit in the ceiling of the fumehood. The exhaust is 100mm / 4" inner diameter. I want to attach a 45 degree Y shaped 'tee' (actually a 'wye') which is available at local shops here. The plan is cutting off a piece of the exhaust, attaching the Y and the straight leg into a new hole in the ceiling of the fumehood. That exit I'll use for the furnace and the other 45 degree leg I attach to the fan with another 45 degree knee to another hole in the ceiling. See the sketch. Now my question is: when the furnace is running and (very) hot air is exiting through the straight leg, it has actually an open connection to the (then not running) fan. Does the heat still reach the fan ? And when I am using the fan and perform 'cold' experiments, in what extent does the fan push back the (smelly) air back into the fumehood via the straight leg ? And here how the small furnace is inside the fume hood:
if your drawing is correct, the vast majority of the hot air will go through the fan we do something similar at work for our ventilation, but the fan sucks outside of the hood, the air blowing into the hoods exhaust creates a low pressure area increasing the suction in the hood, its a air injector basically , works very well for welding exhaust V/r HT1
HT1: Maybe I am not clear enough. There are two scenarios: 1. 'cold' use, i.e. with fan ON which uses the left exhaust in the drawing and forces the air, fumes and gases out of the fumehood (which is the scenario HT1 describes) 2. 'hot' use: fan OFF and furnace placed below the right exhaust in the drawing (possibly an extra hood to force the hot air into that hole) where the hot furnace exhaust is blown by the heat of the furnace Hence I need a second hole in the fumehood ceiling. Is this correct or does the dual hole have some adverse effects ?
If you use the fan drawing outside air to power a venturi junction, you could have a single hole in your fume hood and still be able to have passive or active operation and avoid any hot air through the fan. You could even have a spring loaded flap valve to prevent reverse flow to the fan when it's inactive. The flap valve could even be placed at the Y junction in your original drawing to block the fan when inactive and cover the passive tube when the fan is active as the fan flow would overcome a spring on the flap valve.
I understand now , and i cannot answer that , you are asking if a "chimney" effect will naturally draw hot fumes up through the exhaust pipe. it depends on a whole bunch of things that you still have not told us, naturally drafting always has the possibility of a weird wind gust forcing air back down the "chimney" and filling your shop full of fumes. V/r HT1
