3 or 4 blade blower wheel on snowblower

[Jerry/MT]

Since I cant change the diameter of the fan, and I need to rebuild the impeller anyway.
I would like to come up with somthing more efficient. I agree with you that a smaller fan should take less hp to turn, I just wonder how much the actual blade design plays a role in power consumption, and performance

My impeller is nothing more than a 20" round piece of plate with three pieces of c-channel welded to it.
I have seen some impellers that are contoured, some that are made from c-channel but have material cut away towards the center of the impeller, some with 3,4,5,and 6 blades........many different designs, and ones got to bet better than what I have now

Im just hoping somone smarter than myself could share some of their engineering wisdom.

thanks,
Brian

The bigger the fan diameter the higher the tip speed for a given rpm and that's what throws the snow. Most of these blowers are designed around a 540 pto rpm.

There is probably an optimum blade number as a function of fan diameter but I don't know what it is. There is also probably an optimum chord to span ratio for the blade, too. Increasing the blade chord can increase the throughput but also puts the horsepower requirements up. Too many blades and the impeller "wetted area" creates more drag on the snow (and wastes input horsepower) and more chance for snow blockage between impeller blades because they get closer together for a fixed diameter fan. With larger diameters, there maybe the ability to add more blades and then you'll also need to increase the auger speed since the fan capacity increases with diameter, and the exit area must get bigger.That also comes with an increase in power requirements.

Since you have an existing diameter, I 'd go with the OEM configuration. Pay attention to the tip clearance and don't let it get too big (decreased capacity) but don't let it get too small either or you may a problem with rubs or damage from gravel getting between the tip and the shroud.
A good way to reduce tip clearance is with a wearable material riveted or bolted with countersunk bolts to the shroud.

In my opinion, the blades are designed the way they are to "ruggedize" them because of all the crap that can go trough them in normal use. A fancy looking design would probly lose any benefit in the first several hours of use and would cost more to make.

That's my two cents worth.

 

[PILOON]

Hey, the impeller spins at 550 RPM. NOT 100 MPH.
Alignment and balance are not a problem here.
Just cut any parts to close copies and weld them as closely as possible to equal spacings.

On my Bervac 3 bladed 60" blower the impellor supports were so rotted that I had no choice but to replace them.
Instead of the thin formed struts I went with 3/16" x 2" flats with crazy angles to make a good fit.
I hammered the blades back to shape (they were bent back from rock tossing) and simply stick welded everything.
Even added some metal where edges were worn back some.
Assembled the whole thing and went to work.
That was 4 years ago.
AND: I'll add that I always run at 750 rpm (3 speed PTO).
No shakes and no issues. Still on original bearings.
About the only preventative thing I do is squirt oil on the bearings in the spring and rattle can spray a thin coat on any bare metal just to deter serious rust from forming.
The rattle spray lasts 'till the first snow event but then rust will not form during the cold winter, besides the blown snow keeps the metal nicely polished.

This is one area that we don't need to over engineer things.

 

[schmism]

Hey, the impeller spins at 550 RPM. NOT 100 MPH.
Alignment and balance are not a problem here.

ya but a 1.5' impeller does have a tip speed of about 30mph @550rpm

Ballance should always be something to check at least roughly, and alignment will have a huge impact on the how efficient the blower is when looking at the clearance of the impeller to the housing.

 

[bkc]

Hello Brian,


If you add an additional impeller you wil be reducing the volume of snow thrown out of the impeller drum per revolution * 540 revolutions per minute.

If you visit the Pronovost home page and look at page six of the snowblower PDF you will see three different types of impellers with 4,5 and 6 impeller paddles that are plat or curved.

I think if you have bad impeller you should order a replacement and add some steel to create a scoop on each of the three impellers taking care to balance the impeller after the welding is done at a machine shop as the impeller is rotating at 540 R.P.M.


They built the impeller inexpensively using channel iron and unfortunately your stuck with the design.

Is the impeller partially blocked with a small plate covering a portion of the entrance to the impeller? If not you are dealing with a lot of snow being thrown out of the housing when the snow caster is working and that may be the entire issue.

I have spent alot of time looking at provonost website, I have seen the page with the impellers and noticed that... as impeller DIA goes up, so does the number of blades.
I am gunna take a guess that more blades are needed for the large volume of snow the large DIA fan puts out.

You mention blocking a portion of the impeller housing, my blower does not have any plates blocking the entrance of the impeller. Again after looking at many blower pics and websites, I have noticed those "plates". They are always either on the top portion of the impeller housing right where the snow is would be exiting the housing and into the cute, or I see them covering a portion of the impeller housing on the discharge side.

What is the function of these plates?

Pics are worth how many words...

Plate twards top
Redirect Notice

Plate on the side

Redirect Notice

 

[bkc]

The bigger the fan diameter the higher the tip speed for a given rpm and that's what throws the snow. Most of these blowers are designed around a 540 pto rpm.

There is probably an optimum blade number as a function of fan diameter but I don't know what it is. There is also probably an optimum chord to span ratio for the blade, too. Increasing the blade chord can increase the throughput but also puts the horsepower requirements up. Too many blades and the impeller "wetted area" creates more drag on the snow (and wastes input horsepower) and more chance for snow blockage between impeller blades because they get closer together for a fixed diameter fan. With larger diameters, there maybe the ability to add more blades and then you'll also need to increase the auger speed since the fan capacity increases with diameter, and the exit area must get bigger.That also comes with an increase in power requirements.

A good way to reduce tip clearance is with a wearable material riveted or bolted with countersunk bolts to the shroud.

I didnt really think about more blades = more drag and "wetted area" but I makes sense. Like I said in my last post, I think the number of blades is a function of larger DIA impellers.
My impeller is 27-28" So I dont think any thing more than 4 blades would be wise.

I also think that the more blades you have the smaller bite the impeller wil have to take for a givin amount of snow.
Imagine if you had an impeller with one blade. That blade would try to compress the snow as it made its way around the blower housing. If you had multiple blades you would still move the same amount of snow, but in smaller "bites" per revolution.

I am just guessing at all of this, so all opinions are welcome

Im not trying to reinvent the wheel, (ha ha... get it?)
But why use an old design when a newer design is available.

I am just thinking that blade design can have a significant factor in blower performance with out too much of a compromise in durability

Brian

 

[Jerry/MT]

I didnt really think about more blades = more drag and "wetted area" but I makes sense. Like I said in my last post, I think the number of blades is a function of larger DIA impellers.
My impeller is 27-28" So I dont think any thing more than 4 blades would be wise.

I also think that the more blades you have the smaller bite the impeller wil have to take for a givin amount of snow.
Imagine if you had an impeller with one blade. That blade would try to compress the snow as it made its way around the blower housing. If you had multiple blades you would still move the same amount of snow, but in smaller "bites" per revolution.

I am just guessing at all of this, so all opinions are welcome

Im not trying to reinvent the wheel, (ha ha... get it?)
But why use an old design when a newer design is available.

I am just thinking that blade design can have a significant factor in blower performance with out too much of a compromise in durability

Brian

Blade design, whether on snow blower or a turbofan engine, is ALWAYS a compromise between performance and structural strength.

Your constraints are power, tip speed (rpm and diameter) and structural capability of the impeller blades. Chamfer the the blade edges so you don't have a sharp right angle leading edges. The sharp leading edge will eventually get rounded from snow and debris anyway. make the surface finish slick to help reduce drag, and keep it slick to maintain performance.

If this were a fan moving air there are some more tricks available but you are trying to move a two phase material (liquid water film and solid water) plus some dirt, rocks , etc.

As far as blade count, you are trying to accelerate the snow to the tip speed of the bade. This is the way I think these blowers work. The snow comes in across the blade axially from the auger and has to turn outward and then slides along the blade toward the tip, driven by centrifugal force. Remember 1/2 the flow is out board of the area swept by 0.707 x the blade diameter. If you had a single blade, only part of the total incoming snow on that blade would go out the chute. The rest of the snow would "pile up" until the blade could come by again to pick it up. The capacity would be very low Not all the snow coming into the impeller would not be able to be accellerated outward. If you went to two blades, then more snow could be accelerated and the capacity would increase but some power would be wasted (relative to a single blade) due to increased drag of the addiional blade. Adding a third blade, would increase the capacity some more but maybe not as much incrementally as goig from one to two. Similarly with going to four, some mprovement, but incrementally a bit less and so on. You'd eventually get to dimishing returns for adding blades so you' have to increase the diameter to get a meaningful increase in capacity. Then there is the power limitation. At some point, as you increase blade count, the power requirement gets greater than the power available so you are power limited.

This is why I recommend you stick with the OEM blade count.

While I haven't spent a tremendous ammount of time thinking about this problem, these are my thoughts on the subject.

 

[leonz]

The Plate that partially covers the left or right exit point of the chute prevents any snow entering the chute and impeller drum while the impeller paddle is rotating and throwing snow out through the chute and spout of the snow caster.

 

[bkc]

Thanks for the replies... at work now.
I gunna have to think about this some more.

Brian

 

[slowzuki]

Except on the smallest impellers, more paddles will move more snow but also requires more power. Lucknow used to offer an upgrade from the 3 blade to 4 blade to increase capacity but eventually made it standard.

 

[leonz]

Thanks for the replies... at work now.
I gunna have to think about this some more.

Brian

Hello Brian,

I think that you wil be sanity and money ahead if:

1. you purchase one of clarences impeller kits
2. a new replacement impeller
3. you purchase slick sheet remnants from Horn Plastics to line the snow caster auger housing and the impeller drum using either elevator bolts or small bolts and fender washers. The grain elevator bolts have a flat head and mount flush with the plastic using a a carriage bolt ind with the flat sides under the flat head. Check with fasten all as I think Farm Teks grain elevator bolt prices are too high in my opinion.