One large hood or multiple small hoods?

Here’s the dilemma:

A glassworker wants to set up 3 work stations…

I suggest:

3 hoods, 3 fans, 3 exhaust duct systems. This way if one or two stations are not being used, the fan is off and no air is flowing through that duct work.

I’m not a big supporter of one hood/one fan for multiple torches — the reason being that the hood coverage area can result in a very large capacity fan, sometimes in excess of 2000 CFM, and these typically need professional installation — not saying you can’t do it yourself, but large capacity fans are *HEAVY* and usually require 220 volts and a separate electrical run, and then there is the makeup fresh air to be considered — 2000 CFM can have a huge impact on the room temperature/humidity.

And as I’ve posted elsewhere, I don’t particularly like in-line fans for several reasons:

1) Heat issues — in-line fans, even the so-called duct blowers, still have a maximum temperature rating of about 140 degrees F. The fan motor relies on what is called “air-over” cooling, which means it relies on the air moving over the motor to keep it cool. The fan motors usually have an operating temperature of about 120-130 degrees F, and if the airstream temperature exceeds 140 degrees, the fan motor will naturally heat up to match the air temperature. This will result in the insulation in the motor beginning to break down and will fairly quickly burn out the motor. Now, a small torch, like a Lynx or Minor won’t raise the airstream temperature to 140 degrees, but a large torch, like a Mirage or CC/CC+ certainly will. I’ve seen airstream temps coming off a Mirage running full out exceeding 180 degrees.

2) Many times, these in-line fans have plastic blades. See #1.

3) Service issues — if the fan ever needs cleaning, and they do, the entire duct system will need to be torn apart to get at the fan. Messy. Dusty. Nasty.

4) Performance issues. Many times in-line fans rely on already moving air and act only to “push” more air into a duct section that has already low flow due to poor design. They really are not designed as the sole method of air movement in the duct, and don’t have the necessary horsepower to handle a higher than normal static or total pressure in the duct system. This will cause them to over heat, and see #1.

Separate hoods with separate fans and duct work (I don’t recommend ducting 3 hoods into one duct system and using one fan) may cost more money, but in the long run you are going to be much happier with the performance of the system.

Another thought for you to ponder: You COULD do one hood, but it would need to have baffles in it to separate the hood into sections.

My key point was that by separating the hood into sections with separate fans (and ducting), you will avoid issues of having to run one huge fan continuously even though you may be running only one or two torches.

It’s far more efficient (electrical and ventilation-wise) to use only the fan(s) you need as you need them.

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2 thoughts on “One large hood or multiple small hoods?

  1. I’m helping my daughter-in-law set up an exhaust system for her bead making. The system will be 3ft. wideX 2 ft. deep with one 90 and one 45 degree bend and a total run of 6 feet from hood to outside. Velocity should be around 3,000 FPM. We’re looking at Grainger fans 1TDT 5-975 CFM and 1 TDU1-990 CFM. Unfortunately the charts you reference in your article were not available to me so I cannot calculate losses but in the information I have available I think 7″‘ ductwork is best for this system. What do you recommend? Thanks much.

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