It is an ill wind that blows no good

[ian maybury]

It'd be worth measuring the impeller diameter and blade height you have Bruce, also the diameter of the fan inlet and exhaust. That would at least verify whether or not impeller size is an issue, or if it's something else. For what it's worth a bigger motor doesn't deliver higher CFM unless the impeller is big enough to load it/move the air, and the system is free flowing enough that the flow comes at a pressure drop the impeller is capable of at the motor RPM.

ian

[Michael W. Clark]

Michael, a clean -out plug is a good idea, were you born in KY ?

Currently reside in NKY, originally from northern AL.

[Michael W. Clark]

Bruce,
I think I follow you, but I may have missed something without a diagram. Here is what I assumed, feel free to correct me and I can change the calc pretty easily. I assumed the TS would be the worst case.

Branch A - 6" connection at the cabinet, 1000 CFM, 5100 FPM - 10' of straight and (2) 90s
Branch B - 4" connection at blade guard, 400 CFM, 4580 FPM - 6' of straight, (2) 90s, and 45 degree branch entry
8" main to fan inlet - 1400 CFM, 4,000 FPM - 17' of straight, (2) 90s
8" main from fan outlet - 1400 CFM, 4,000 FPM - 20' of straight, (1) 90

I assumed you may want to branch in the 12' of straight to the blade guard (8x6x4). This gives an external static pressure requirement for the fan of 5.5"wg. The connection at the cabinet of the saw has the most loss due to the high velocity. If you make the 4" out of flex for the guard, it should still have less loss than the 6" cabinet connection but is very close to the same loss.

If I have grossly missed some lengths or number of elbows, let me know and I can revise. If we have estimated everything correctly, you should have some good DC at the TS. The BHP calc would give you about 3 HP required at 40% efficiency. The fan table at the bottom of your curve, shows 3.13 BHP and 1373 CFM at 6" SP (41% fan efficiency).

I think it will work if the duct length and quantity of elbows listed above is correct.

Mike

[Bruce Seidner]

It'd be worth measuring the impeller diameter and blade height you have Bruce, also the diameter of the fan inlet and exhaust. That would at least verify whether or not impeller size is an issue, or if it's something else. For what it's worth a bigger motor doesn't deliver higher CFM unless the impeller is big enough to load it/move the air, and the system is free flowing enough that the flow comes at a pressure drop the impeller is capable of at the motor RPM.

ian

I think I would need more than a shoe horn for this Size 126 blower. It looks like it will only take a 12 impeller.

[Bruce Seidner]

First off, you are very generous to take this time to calculate this.

I always forget to account for the duct to the outlet, which you have included. It works better if you ignore the 20' of outlet duct!

But facts are facts. It is like my overlooking the undersized impeller for this blower. I never thought to measure it and as in the post above it is definitely 12" and not 15".

It looks like you have branch A ducting 6" to the cabinet and 4" to the blade guard and if I understand I would be using 8" duct all the way off the 8"main down the wall and out to the machine. Close to the machine I would use an 8x6x4 wye. That makes sense. I have seen metal duct wyes in this configuration and worst comes to worse I know for sure I can find an 8x6x6 and just use a reducer on the 6" outlet that branches up.

I was getting nervous about the blower capacity and may in the next year or so buy a larger impeller from ClearVue and build a blower box for it. In the meantime think I see a green light to keep moving.

I have been practicing plastic welding and want to get a speed tip for my Harbor Freight plastic welder. It did not come with one. Actually the cost of the tip is almost as much as the HF welder. Ha! But it has the outlet for the hot air and another inlet for the plastic welding rod which is preheated and blown into the seam making the weld faster and more consistent with material added to the seam. In the meantime it will be a combination of inner tubes short PVC stubs and metal fittings. Less than optimal but this whole thing has been an exercise in bootstrapping. It is hard to rebuild a shop when you have dismantled it first. Truly like painting oneself into a corner. But the worst is over.

Thanks again.

Bruce

[David Giles]

In high pressure systems, Ty is correct. It is more efficient to "push" the air than to "pull" it. The reason is that the air density changes as the pressure changes. In low pressure systems, like shop dust collectors, the pressure change is only about 6" WG (0.2 psi). The air density is within 2% (14.7/14.9 psia) from one end of system to the other. It doesn't make any practical difference where the blower is located in a low pressure system.

[ian maybury]

That more or less settles the reason for the lowish fan performance anyway. I've only got Bill P's blower to go by, but it's got 3in plus clearance at the impeller tips all round, 1 1/2in above and 1/2 in underneath - so as you say it looks like the 126 fan casing is of too small a diameter to use a bigger imepeller.

It's got unusually high vanes (what they list as the 5HP version), but the problem with that is that while it'll add some CFM it doesn't help high pressure performance in the way going for a larger diameter does.

The fan could do quite a decent job if let breathe, but as before running it with 8in ducting would worry me. The problem is that as before it'll drop CFM very quickly if faced with more restriction than expected - and 1,400cfm is only just enough to make 4,000fpm in that duct size.

Don't forget there's entry and exit losses to the duct, the possibility of restrictive machine hoods, flex hose, losses at blast gates, need for unforeseen bends, and leaks etc in the mix too.

It might well be OK (given no filter and cyclone), but if I was staying with that blower i'd think of 6in ducting. If i wanted to run 8in ducting i'd think of running your reconditioned motor with a bigger 15 or 16in dia fan from the start.

ian

[Anthony Whitesell]

That's the direction I was heading earlier. In conjunction with the question, do the blowers for our cyclones have more ability to suck or to blow? In the water analogy, pumps have much more ability to push than to suck (with physics playing a large part in the latter).

[Michael W. Clark]

No problem Bruce. I took into account the hood losses like Ian suggests and our calcs typically have a little conservatism built in. I didn't account for any strange fittings at the fan inlet, so that may hurt you a little if thats the case. Your fan is marginal, but I think I would give it a try based on the catalog information. I think if you use 6" duct, it will take too much SP for this fan. If you have problems, replace the outlet pipe with 10" and make a place to clean it out or inspect it for build-up. Or, get a larger fan and mount your 5HP motor to it.

Mike

[Michael W. Clark]

Anthony,
I don't think it matters in a dust collection system where the fan is located from an airflow/pressure loss standpoint. The duct doesn't care which way the air is flowing. A certain amount of loss will be created by the airflow regardless of flow direction. The fan has to overcome these losses whether it is on the inlet or outlet of the fan.

My preference would be to locate the fan after the cylcone just to knock out all the big chips and maybe protect the fan wheel. In large industrial systems, you will often see the fan located downstream of the cylone and filter (baghouse) then discharging into some type of stack. This is done to allow the use of a more efficient fan wheel that can only be used in "clean" air applications. It also protects the fan from abrasion if the dust is abrasive.

Let's say we have 20,000 CFM, 10"wg SP and a radial wheel at 50% efficiency, you need about 65 BHP. On this same system, if you use a backward incline wheel at 80% efficiency, you only need 40 BHP. On very large systems, this energy savings and installation cost adds up quickly. In addition, the fan cost can be much less because of wear on the fan and having to replace parts over a period of time if located on the dirty side of the collection equipment. However, fan wear is probably not an issue in a hobby shop. We would likely upgrade our system and replace it before wearing out the fan.

Ian has alluded to the fact that the fans that come with most of our home shop collectors are radial tip design and have an efficiency of about 40% +/-. You could probably use a more efficient wheel if locating the fan on the outlet of a cyclone, but I wouldn't recommend it on single stage collectors where the fan sees all the dust coming in.

One more reason not to locate the fan befor the dust collection equipment is that it would put all of your DC equipment under positive pressure. If you have any leaks whatsoever (think of a pin hole in a plastic dust bag on a single stage system and how much of a mess it can make) you will blow dust all over your shop. Keeping everything under negative pressure, keeps the dust inside the system.

Obviously, there may be circumstances where it makes sense to put the blower before the DC equipment. But I think downstream of it is the way to go if you have the choice. Just my opinion.

Mike

[Anthony Whitesell]

I agree system losses don't care where the fan is located. The question to the group is: Do we know if the fans in our DC system fans are better at creating suction or creating pressure?

As to you're other point, if a cyclone system is not sealed, you're sunk either way. On a push through design the dust will spew from the system under pressure as you mentioned. Under a suction or vacuum design, it will pull and push more of the fines and small chips into the filter. Sunk either way. Gotta have the system leak proof or you'll have trouble.

[Michael W. Clark]

Sunk either way. Gotta have the system leak proof or you'll have trouble.

Yes, especially on leaks that are a large percentage of your total volume. However, I think you will notice a leak sooner if blowing into your collection equipment. For example, if your leakage is 10 CFM on a 1000 CFM system, you probably won't notice it on the suction side, nor would you be able to measure it accurately and repeatably. However, if you have a pin hole leaking 10 CFM in your plastic bag on a single stage system, you'll notice the dust spewing out. It is very difficult, if not impossible, to get all of the leaks out of the system. Small leaks are less of a pain on the negative side of the system.

Can you elaborate on your question about creating suction or creating pressure? Are you referring to the fan design?

[Anthony Whitesell]

We always talk that the fans we use in the DC systems are designed to create CFM not SP. Unlike shop vacs, that are designed to create SP instead of CFM. So I posing a twist on the question. Are the fans we use better at pulling or better at pushing? I have a gut feeling on this, but am looking for some collaboratative insight and perhaps some techinical backing in these systems that I lack.

[David Giles]

No practical difference in pushing or pulling. The fan boosts the air flow pressure from the inlet to the outlet. It can go from 0 psig inlet to 0.2 psig positive if the fan is at the front of the system. Or it can go from negative 0.2 psig inlet to 0 psig at the discharge with the fan at the end of the system. Putting the fan at the end of the DC system has numerous advantages and no disadvantages.

[ian maybury]

See mid post no 6. According to manufacturer tables on operating at altitude (reduced pressure) it's probably the equivalent of about an extra 2% on the total pressure drop....

ian