Fume Hood Blower Installation

Quote: Originally posted by Abromination

After much thought, I have decided on the fume-hood centrifugal blower I wish to use. The only problem with it is that it does not come with a power cord and is meant to be hard wired directly into a furnace or wood stove. Does anyone have any suggestions on how to do the wiring? I have some experience in this but not enough to be confident. https://www.amazon.com/dp/B07DWBC1GY/ref=sspa_dk_detail_1?ps...

Quote: Originally posted by wg48temp9

Many tumble dryer blowers have the motor outside of the impeller housing (not in the air flow) and most are thrown away because the drum bearing is faulty, the drum belt is broken or have a door problem. Meaning the blower still works. The owners are usually happy if you offer to haul it away. As bonus you get the sheet steel of the housing and perhaps the plastic trunking used for ducting the damp air outside. Obviously metal trunking is safer if you do have a fire.

Below is a pic of a typical dryer blower.



Please note that the motor frame tends be be insulated from its mounting bracket by rubber bushes and some times not grounded. It should be grounded (earthed) to reduce static pickup and for electrical safety reasons.

Unfortunately they are induction motors so the speed can not be controlled easily. They are frequently arranged to run in alternative directions by reversing the polarity of one of the wingdings. That does mean that the impeller is bi directional and probably less efficient than ones with curved blades. If you must have simple speed control you could replace the motor with a DC brushed motor from a washing machine with a spin cycle but fitting the impeller could be tricky.

Quote: Originally posted by Abromination

Ohh by dryer you actually do mean a clothes dryer, I thought you meant one of the fans they use to dry out water from flood damage. Surely that cant produce enough cfms, but I will look out for one.

Quote: Originally posted by wg48temp9

The best we can do is compare it with the $100 blower you previously selected. Both use a four pole motor and the dryer motor is much larger but not smaller so that suggests a similar power rating (the dryer motor has to rotate the drum in a addition to pushing the air). The Impellers are a similar size and rotating at similar velocities that suggests they will have similar flow rates. So I guesstimate the flow rates will be within a factor of two or perhaps 3 the same. It would be nice to know the flow rate so I may try to measure it. It can be done by dropping a small piece of paper into the inlet of the blower and seeing how long it takes to be blown out of a length of ducting. Perhaps with different lengths of ducting would be useful too.

I got round to measuring the flow rate using the method above.

With 9ft (+ 1ft for the blower path) of 4in dia plastic ducting attached to the blow a small piece a paper sucked in to the blower inlet appears very quickly at the end of the ducting. Its so quick its hard to estimate the time. I estimate it is no slower than 0.25s but perhaps as fast as 0.125s. That gives a flow rate of bewteen 210 to 420 CFM that comperes well to the 463 CFM of the $100 blower. However we do not know under what condition the 463 CFM is specified. The blower replaces the Dayton 1TDR9 which specifies the a flow rate of 463 CFM with no back pressure (no ducting).
I checked two other fume hood threads and both were using about 400CFM blowers

At almost no flow of 50 CFM the pressure is specified as 0.6 in (presumably water head). With the dryer blower blocked off by my hand using a plastic tube part filled with water I measure the difference in water height between the end of the tube held in the blocked output of the dryer blower and the
other end of the tube as 0.75 in. Again similar to $100 blower.

The dryer blower I tested was from a discarded dryer that was noisy probably because in had a small plastic toy figure stuck in the blower blades and was rubbing on part of the housing.

I was thinking I could video the paper going in and then out and count the frames. I should also measure the back pressure of 9ft of 4in dia ducting. If I repeat the experiment I will use dye and detergent in the water. I would also like to try a Venturi tube to measure the flow but I guess a smooth transition is required to maintain laminar flow.


See https://www.grainger.com/product/DAYTON-OEM-Blower-1TDR9

Calculation:

PS: I looked up the flow resistance of ducting at https://www.engineeringtoolbox.com/duct-friction-pressure-lo...

Apparently at a flow of 400CFM 10ft of 4in dia ducting would have a pressure drop of 0.9in which exceeds the maximum pressure of the blower. This suggests the flow probably closer to 210 CFM than 400CFM.

[Edited on 1/10/2020 by wg48temp9]

[Edited on 1/10/2020 by wg48temp9]

My blower came in today, I ended up buying one for cheaper, with better reviews and higher cfms.
The motor is totally isolated from the air flow and the blade piece should be removable for cleaning. If I can figure out a good, temporary window mount, the blower will vent directly outside instead of pushing air through the duct first, insuring minimal leakage for imperfect seals in the duct. The duct itself is vinyl, which will have to be replaced after a few years. I do not foresee myself pumping a huge amount of corrosives or flammables from the fumehood, but its a nice security when I have to. Better to be able to work inside then in the weather we have here (-15°F here right now)
For the reference of others, the idea of using a dryer blower is not at all a bad one.

I was surprised how much resistance to air flow 10ft of 4in dia ducting produces.

Below is a graph plotting the back pressure versus air flow of a blower Dayton 1TDR9 (triangular data points). I also plotted the back pressure of 10ft of 4in dia ducting, 4ft of that ducting and the back pressure of 4ft of 6in dia ducting, respectively the top middle and bottom graphs on the right. The intersection of the blower graph with each of the ducting graphs indicates the air flow for that ducting using the blower.

As can be seen from the graphs the air flow of the blower is approximately halved for 10ft of 4in ducting. While 4ft of 6in ducting has almost no reduction.

Enginerringtoolbox.com gives the air flow resistance of round ducting as approximately inversely proportional to the fifth power of the duct diameter.
Putting it differently about 7.6ft of 6in ducting has the same back pressure
as 1ft of 4in ducting at the same air flow.

Quote: Originally posted by Abromination

... better reviews and higher cfms. ... the weather we have here (-15°F here right now)

Quote: Originally posted by Sulaiman

Quote: Originally posted by Abromination   ... better reviews and higher cfms. ... the weather we have here (-15°F here right now) One problem ... every cubic foot of air expelled through the fan/blower draws into the room a similar volume of external atmospheric air, which requires a significant cost in either heating (in this case) or cooling (in hotter climates)

Quote: Originally posted by Abromination

The problem I have with metal is the corrosion caused by acid vapors.

I'm using a 4" inline rule brand diesel exhaust fan for a boat in my so far unused hood.
Is there anything that i should know about like the difference between the one pictured above and the inline type i have?

The blower is ready to be installed, and I have high hopes for a very efficient hood. Sitting loose and unattached on the top of the hood, the lowest setting is capable of pulling a tissue from the sash into the fan (I intend not to let go again). I am going down a slightly different rout then I initially planned. Instead of permanently sealing the blower to the top of the hood or on the output (board in window) with flexible ducting, the blower will have a few inches of rectangular, wooden duct venting directly outside with the blower on top of the hood. To make this removable, the blower will sit on top of a rubber gasket with the heavy weight to make a seal. This way I can remove the board fro the window when I have to leave for a while or when its cold in the winter.
I have found that flexible ducting severely limits the fans ability to move air, so this will be a potential solution.

[Edited on 1-26-20 by Abromination]

Here are some quantitative values for the effects of bends in ducts in terms of equivalent length of ducting. For example a single right angle bend in 10ft of ducting restricts the air flow the same as 30ft of straight ducting. Two right angle bends and the equivalent is 50ft. Wow thats a significant, so yes right angle bends can restrict the flow very significantly. However the effect can be reduced by increasing the duct diameter. If you have bends at least double the duct diameter.



From: https://flexibleduct.org/images/ADC~IR5E.pdf

Quick update:
The gasket seal works very well. The plastic foam like material was tested with various solvents and corrosives and is very resistant. The weight of the fan on top of the motor makes a very good seal, and will make the fan easily removable and serviceable. It has also made the hood much more quiet and seems to have raised the cfms by quite a bit. I also have noticed very little turbulence inside of the hood body itself, even above the sash opening and around the baffles. When I get a chance, I will construct the outlet ducting and the window piece.