Thread: HP and Impellers

before I comment on your problems let me ask a few questions

1) What are the sizes of the flexies labelled 1 and 2 in this photo?



RE: I have now got it parked outside my shed on a slab and a half built enclosure ala BobL's setup. Just waiting for the rain to ease off a bit so I can finish it.

2) What is the distance between the shed door and the slab on which the DC sits?

G'day Bob, Thanks for the interest.
The first one 5", the original fittings and No2 is 6" where I replaced the original fitting which was a 6" into a two by 4" outlet.
I realise the outlet to the filter and bag is smaller than the inlet and that it probably restricts airflow but no idea how much it is restricting the system. It was fairly good as a single machine unit but??? Cheers, Ian

Oops, the slab is just to the right outside the door and the inlet will be through the shed wall up high. Ian

Originally Posted by Nanigai

Oops, the slab is just to the right outside the door and the inlet will be through the shed wall up high. Ian

This is not good. When placing or venting to the outside the placement or vent should be on the opposite side of the shed from the door and downwind from the door. If the DC is too close to the door and upwind of the prevailing wind the low pressure inside the shed will immediately suck the fine dust back into the shed.

Originally Posted by Nanigai

G'day Bob, Thanks for the interest.
The first one 5", the original fittings and No2 is 6" where I replaced the original fitting which was a 6" into a two by 4" outlet.
I realise the outlet to the filter and bag is smaller than the inlet and that it probably restricts airflow but no idea how much it is restricting the system. It was fairly good as a single machine unit but??? Cheers, Ian

The 5" hose is THE critical weak link in your system and you can't change it for a 6" hose because the impeller port will need to be modified. The more I look at that particular 2HP unit the more of a dog it looks.

The 5" port means increasing the HP will make little difference not that it would anyway on that that impeller because the impeller looks to small to take advantage of the 6" ducting.

If you haven't seen it maybe take a look at this post.

The other thing that worries me about your system is the chip collector. Unless they are really well designed they turn into turbulence chambers and just slow things down

Thanks Bob, I have got no choice for the time being. I have a monster air filter inside the shed already (Visible behind the DE) and hopefully it will keep me a little bit safer until I can move it out the back. During use I may keep the roller door down and use the other doors, during the cooler months anyway.
Ian

Originally Posted by Nanigai

Thanks Bob, I have got no choice for the time being. I have a monster air filter inside the shed already (Visible behind the DE) and hopefully it will keep me a little bit safer until I can move it out the back. During use I may keep the roller door down and use the other doors, during the cooler months anyway.
Ian

Unfortunately Roller doors are notoriously leaky but keeping it down is better than not.

Originally Posted by BobL

The 5" hose is THE critical weak link in your system and you can't change it for a 6" hose because the impeller port will need to be modified. The more I look at that particular 2HP unit the more of a dog it looks.

The 5" port means increasing the HP will make little difference not that it would anyway on that tat impeller because it looks to small to take advantage of the 6" ducting.

Maybe take a look at this post.

Thanks Bob, I have been trolling the forum for months looking for info and haven't seen this one.
Looks like I might have to make/buy a new blower/impeller unit with a 5HP motor to get the result I want.
If anyone has info on where I might source one of these advice would be appreciated.
Ian

Originally Posted by Nanigai

Anybody out there know if an increase in HP on the same impeller will help improve the CFM or do I need a bigger impeller as well. I'm thinking 5HP would be good if I can find a suitable motor, if the same impeller would work.
Thanks, Ian

Hi Ian,

Fans are/should be designed to provide the max flow/pressure for the least amount of power consumed to do the work, this moreso applies for a speciffically designed system from start to finish and would be the ideal, however in general for mass production purposes a range of fans are made to fall within an acceptable range of flow/pressure requirements to meet the needs of whoever might want to buy a fan "off the shelf" Jet have most likely selected something to suit for their 2hp unit.

For the home user there are two tests that can be done to determine whether the motor is at a rough guess big enuff

1 disconnect everything from the blower unit, no piping or anything and run unit and see if motor gets hot, or trips any overloads/protection devices, you should be able to comfortably hold a palm on the motor housing, it will get hot but if you cant it's getting to hot and will be on the verge of letting out the blue smoke if given enough time ideally check the current draw (need a sparky) this is where the fan will move the max amount of air at the lowest pressure and draw the most current, this is one end of the fan curve

2 completly block off the outlet, this will give you the max pressure and the lowest air flow and should draw less power, this is the other end of the fan curve

When we add filters, piping, hose, machines, dust hoods, etc into the mix the fan will be operating somewhere between these two end points (ideally at the most efficient point but probably not)
So if your motor drives your system OK as it is then adding a larger motor will do little to assist and will be only draw more power due to it being a larger motor, adding things to the system all consume air flow so larger fan is what is needed to maintain airflow, larger can mean in diameter/width, it could also mean a different blade shape etc.

It is possible to increase airflow with an increase in rpm but a dowside is that power consumed is cubed with speed

Pete

Thanks for that Pete. just what I needed to help me decide where to go.
I will follow this up to see how it goes but it looks like my best bet may be to buy a cheap new dusty to get a new fan and motor that moves more air and bolt that to my system.
I have a definite bottleneck in my system between my impeller and the filter, thanks Jet. The outlet is only 5" but the inlet is 6" and modifying that is not something I can do easily.
Now if we can just get the rain to stop for a bit we may be able yo male some progress.
I'm not impressed with the prospect of a higher power bill for a small increase in volume either, cubed you say. From what I've seen on new (replacement) motors most wont give much more rpm anyway so probably I'm better off as above, new blower/motor unit.
Cheers, Ian

Hi Nanigai,

The centrifugal fan design you use is often built on a trolley, with a length of hose to be attached most likely to some stationary machine. The original hose length has probably been no more than 2 to 3 meters and that is about the vacuum that this particular fan design can cater for. The maximum vacuum is so poor that little of it will be left at the end of your 6 meter system. Motor overload, by the way, will only occur when the system is used “open”, without lengths of inlet and outlet tubing attached. The airflow will then be huge that the load rpm of the motor will drop too much, overheating it. With meters of tubing attached to it and with maximum airflow being limited, you needn’t worry about an overheating motor. The motor load will be least with either inlet of outlet blocked, minimising airflow and fan wheel load. Since the motor has its own cooling fan, the cooling airflow will be better when the motor will run at light-load rpm, as in your system with long bits of tubing attached.

You suggested cranking up the vacuum by using a stronger motor. Judging the fan design, the present motor is 4 pole unit, with a no-load rpm of approx. 1450. At full load its rpm will be somewhere between 1200 and 1380. If you would use a 5 hp 4 pole motor, this motor would experience the fan as a lighter load than the 2 hp motor would, so the load rpm would increase a bit, but would still not be more than 1450 rpm. With the 100 rpm extra you would gain a few tens of millibars extra, which would not be worth the trouble or the cost. The prospect of using a 2 pole 2800 rpm motor would be tempting, but I doubt that the fan design can cope with double the rpm. The yield of a fan is the square value of the rpm increase. So doubling the fan’s rpm would quadruple its yield and vacuum, but would also require four times the original motor power. You would need to build an 8 hp motor on top of this thing, with the risk of fan blades being ripped off or the wheel distorting into dangerous imbalance. So this is not recommended.

The reason for your fan design having such poor vacuum capabilities is, because it is a design meant for soiled input. The throughflow of debris has to be taken into account in such systems, meaning that vanes/blades are fewer in number and their shape needs to be coarser and more robust to make them more wear resistant and to prevent debris buildup and blockages. This coarser shape and fewer blade numbers compromises the efficiency and vacuum capability of such a pump system. A tight fit between pump wheel and its enclosing housing is also not possible, again because of blockage risk, and this loose fit further worsenes efficiency.
The story is much the same for air and water, so pump wheel designs for soiled air or water are both coarse and “open”. Sewage water pumps designed for reliable operation in worst case situations are known to have as little as two or even one pump wheel vane only, because that is the only array that in practice has proven not to get clogged by cleaning rags or garment fibres flushed down the toilet. In sewage pumps there is also much room between the pump wheel and the housing. Sometimes the pump wheel is recessed, protruding just enough to create a vortex that takes over the centrifugal slinging toward the pump's outlet. Needless to say that the vacuum and pump head figures of this design are very moderate.

Another well known “soiled input fan design” is that of vintage vacuum cleaners; the ones with the motor and suction nozzle mounted on wheels close to the ground, exhausting into a large dust bag which is hung on the push stick. Early vac motors could not yet reach speeds beyond 8000 rpm and their input was less than 150 Watts, so the vacuum was only fractional of the amount that modern vacs can provide, so there was good reason to position motor and fan and nozzle close to the ground to pick up any dirt at all. Amazingly, even the latest and notoriously overpriced Kirby models still use this outdated principle, but that’s mostly because the motor is also used to drive various accessories that are sold with the machine.

Pump systems meant for “clean input” are not bothered by the need for a coarse low-efficiency anti-clogging design, because their pump wheels process air or water with few or no impurities. They can run at much higher rpm and their vane design and tight pump wheel enclosure can all be optmised for maximum efficiency and pressure difference. E.g. multistage boiler feedwater pumps can manage pressures up to hundreds of psi, and a single stage filtered-air type vacuum motor can reach up to five times the vacuum levels of a soiled flow Kirby type fan system. Another example of an optimised centrifugal fan systems is the turbocharger on combustion engine, capable of pressures up to 3 bar (43 psi) with one centrifugal fan wheel only, spinning at 50,000 rpm.

For your needs you would need a larger diameter pump wheel with smaller/sleeker vanes, reducing the cfm-rate but increasing the amount of vacuum, to overcome the flow resistance in the 6 meters of tubing in your shed. In principle 2 hp would be more than enough, but the vacuum of your particular Jet fan is just too modest for the present setup. It's not the motor power, it's not the hose connections or the changing from 6'" to 5" or reverse; it's the fan design's vacuum rating, trust me. If your pre-filtering system ( in the blue barrel) can manage to reduce the debris diameter to approx 4 mms or less, you could choose from a range of better built centrifugal fan systems of a less coarse and “open” design, giving you more vacuum for the same motor hp. There may be second hand units around, saving you lots of money.

Interesting to read:
http://www.chiblo.com/bulletins/icf-105.pdf
and lots more on this topic on the web.

Good luck!

gerhard

Thanks for that Gerhard, I am amazed at the amount of information available on the subject and you response is very helpful and much appreciated.
I have seen a home made system on the net using a large home made fan with narrow blades which supposedly ran a 6" system well and only used a small HP motor. I now believe I understand why that was possible thanks to your info on the blade size and configuration etc.
I have decided to go the easy way out (read expensive) and buy a new motor fan combination with a higher airflow so I can get back to my woodworking quicker. All of this dust control stuff is important to me but the end objective is to get back to my shed and make some more stuff, safely.

Cheers, Ian

Gerhard makes some good points there and goes into a little more detail, in the pic there is a backward curved enclosed fan, this is what I made for my system as my fan/motor is downstream from my cylone and therefore should not normally encounter any chips/dust, it however does when I have let the collection box fill right up but it just spits chips outside until I notice and empty the box. The backward curved design is a refined design over the straight blade, a further design refinement is the airfoil blade shape. If you are going to look for a fan/motor combo it might be worth looking for a backward curved shrouded fan wheel now that you have got your separator inline.

In your Jet DC the fan had to deal with a steady stream of chips/dust and at a guess it is built a little more robust like the straight blade designs in Gerhard's pic, these designs don't move as much air as the one like mine for a physically similliar size fan wheel and therefore less hp motor needed, Gerhard makes a good point that with a larger motor there could be a little increase in speed simply because the larger motor is not loaded as much and running closer to synchronous speed, a correspondingly small increase in airflow might be there to be had but maybe not as well

A few other things of interest is the skinny large diameter wheel for where high pressures/low airflow are required and small diameter/wide fan wheel for low pressure/high air flow, if you go to a fan manufacturer they will want to know system pressure and airflow you require so they can select an "off the shelf" fan wheel that is the right one, there are charts for coming to a pressure value for your piping layout/cyclones and separators or directly measure it and from BP we want 1000cfm.
The more stuff (piping/elbows/hose/filters/etc) that is in a given system then pressure goes up, should really say that the low pressure (vacumm) increases and airflow drops off, so if a system has enough airflow but the piping is modified (adding a separator) then to get back the airflow a fan that will produce a higher vacumm (larger diameter) is needed and will then deliver the same airflow as before but at a higher vacumm, 10" instead of 8" as an e.g.

Hope this is of some help and interest...

Pete

Originally Posted by pjt

. . . Gerhard makes a good point that with a larger motor there could be a little increase in speed simply because the larger motor is not loaded as much and running closer to synchronous speed, a correspondingly small increase in airflow might be there to be had but maybe not as well

Fans are funny things in that they don't load motors up like more direct loads e.g. saw blades or belt sanders. Fans will run at more or less at synchronous speed irrespective of load. If they experience increase resistance they just cavitate, ie rotate the air they are currently holding and the current they draw is then reduced. A fully loaded fan is one that is already running at it's synchronous speed and this is where it draws it maximum current. Hence, provided the motor can start and run the fan using a motor with potentially more HP will not move any more air.