Vent Size For Bulk Material Receiving Bin 1

[BruceTheEngineer]

Hello everyone,
Long-time reader, first-time poster.
I have a question regarding how to size an open air (gooseneck) vent on a bulk solids receiving bin.
First some background. I have a receiving bin that gravity feeds a polypropylene feeder. A dilute phase blower blows air at about 925 ICFM and carries these polypropylene pellets at 22,000 lbs/hr into this receiving bin. The receiving bin itself has 240 cubic feet of volume, a 5" inlet, and a 6" outlet. The polypropylene coming out of the receiving bin can come out between 275-2500 lbs/hr.
The problem we have right now is that pellets are shooting out of the 6" vent that's on top of this receiving bin. I think it's because the vent is undersized and there's a bird cage on this vent further reducing the surface area. I'm just thinking that because some online forums gave a general rule of thumb that the surface area of your vent outlet face has to be more than the surface area of the inlet and outlet pipe face combined. This receiving bin has a spare 8" flange on top of it, so I'm thinking about either:
a) Taking off the 6" gooseneck and putting an open air gooseneck on the 8" flange (with birdcage)
b) Putting an 8" gooseneck on the 8" flange and leave the 6" gooseneck where it's at so that now I'll have two vents.

Any help would be much appreciated. I tried looking at API 2000 & API 650 but I couldn't make heads or tails out of them.

 

[mk3223]

For a better reference, a sketch of the system is really helpful.

 

[BruceTheEngineer]

Here is a dimensional drawing for reference

 

[mk3223]

IMO, the mass of the pellets is too less to separate it from the air and escape to the vent on the top of the cyclone vent. As the larger vent may help the condition, the after filter system may be more efficient to collect the pellets from the vent.

 

[dhengr]

BruceTheEngineer:
What about a internal deflector which directed the inlet flow down and along the tank wall? The pellets probably get sucked up by the vent air flow in the region/arc length (340̊ to 30̊) where they lose enough momentum to start falling out of the inlet flow stream. Baffle the vent air flow around the 6" center vent in this same general arc length. An 8" vent at 180̊ would likely see less of this action.

 

[BruceTheEngineer]

MK3223:
Would you happen to know of any equations or standard formulas I could use to mathematically show the lift experienced by the pellets suspended in the air? I understand that lift is generated when faster air is on top and slower air is on bottom (like the wings of an airplane). Is that why the pellets are being suspended in air?

dhengr:
Sorry if these questions seem stupid because I don't have a whole lot of experience, but just for clarity, are you saying that the pellets are losing the momentum when they are at the 30 degree to 340 degree range going around the bin in a circle because of the way the pellets are entering the bin? I'd just like to know where those numbers came from (did they come from a manufacturer or did you do some calculations?). I see what you're saying about directing the flow down but what I'm worried about is if we fill this receiver bin too full, then that deflector shooting pellets down could back up with pellets because now instead of filling pellets straight and perpendicular to the receiver bin, shooting them down reduces the amount of pellets we can have in this receiver bin.

I'm not familiar with baffles and how they would work for vent air flow. I tried searching online for what one would look like for this but I had no luck. Do you happen to know of a vendor that sells such air baffles or can you please provide a picture of what one looks like?

 

[lilliput1]

Looks like the velocity of air coming out the vent is too high such that it blows the pellets out. The falling pellets will block the 6" outlet. Assuming all the air comes out the 6" vent its velocity will be 4711 FPM. With 6" vent + new 8" vent, the velocity will be 1696 FPM. Still high. You should reduce the airflow, flare out the 6" and new 8" vent to the terminal velocity of the pellets, screen and weather hood at each vent. If you need 22,000 lb/hr but can only get 2500 lb/hr from each bin then either you need 8 more bins or consider another delivery method like screw conveyors or dust collector like arrangement with filters.

 

[LittleInch]

Are you sure about that air flowrate? I get a velocity of about 75 ft/sec out of that vent!!

I don't know what velocity you need to get down to to not entrain your pellets but I would guess you're looking at a reduction of a factor of 10. That's a big nozzle.

I think you need a set off baffles and grills inside the vessel if you're going to keep using these vents. Either that or vent it into a large box and let the entrained pellets drop out and pick them up on a regular basis.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[BruceTheEngineer]

Lilliput1 & LittleInch:
Can you please tell me what equation you used to get to those results just to verify. I used this online calculator at

http://www.tlv.com/global/TI/calculator/air-velocity-through-piping.html?advanced=off

to get near the same results as you guys and I just want to make sure. I'd like to be able to show my boss the calculations as to why we need to get baffles instead of getting a larger vent and show him to what degree I'd need to flare out to on those vents to keep pellets entrained in the receiving bin. Using that online calculator (and assuming a 6" + 8" vent is the same as having a 14" vent), I see my velocity coming out of the vents is 16 feet/second. Is that a good assumption I made about the vents? Then again, I do have 2 spare 8" flanges on this receiving bins so I could have 3 vents if I want to haha.

Sorry for returning back to the gooseneck idea, but I'm still struggling with the baffles because I don't know what they'd look like. I've never seen a baffle inside a receiving bin before so I don't know if pellets would get stuck in it, how would that affect our receiving bin's capacity, etc.

LittleInch:
What's not shown in that receiving bin diagram I sent is that there's a valve just below the receiving bin so what we do is blow pellets to the receiving bin from a silo when needed and fill up the receiving bin to a high level. Then the valve will open releasing pellets to our feeder. Once the level in the receiving bin gets low enough, the blower sends more pellets over and the process repeats itself from there.

 

[LittleInch]

I wasn't actually sure what you meant by ICFM, but assumed it was as close to standard volume as makes no difference.

So 925cfm = 15.4 ft^3 per second = 26640 in3/sec
Sq area of a 6" pipe is (approx.) 3 x 3 x 3/142 = 28.3 in2
So velocity = vol / area = 941 inches =78.5 ft = 24 m per second.

All fairly rough and ready, but that is absolutely whistling....

I can see that internal mods might be difficult, which is why I suggest you take the vent into another atmospheric pressure sealed box where you can let the velocity drop and recover the beads blown out.

a 6" and a 8" nozzle doesn't equal 14. You need to take the square root of the squares of the diameter.

So 8 + 6 is more like a 10" nozzle. That would get you dow to around 30 ft/sec on average, but two nozzles of different sizes wouldn't split flow equally.

Your two 8" nozzles look a better bet but I would then expand to 12" or 16" and let the pellets sit there until you stop the filling operation when they should then fall in. However as the 8" nozzles are closer to the edge you might get more pellets in the air vent.

Remember - More details = better answers
Also: If you get a response it's polite to respond to it.

 

[MortenA]

Bruce,
If this has not always happened - then something must have changed? DO yu know what? This may help you determining if the solution will work.

You can always give it a go with the "Terminal velocity" - wkipedia will give you the basics. Maybe also look for STokes law. You can most likely find diagram for settling velocity in air for grains with different wights and then check against your expected air velocity?

Best regards, Morten

 

[BruceTheEngineer]

MortenA:
According to our technicians, this has always been a problem but we just never had someone to really push this project.
So then with terminal velocity, I'm assuming I'm looking to see if the force of air carrying the pellet upward is less than the force of gravity, right? Or am I trying to see if the terminal velocity of my particle will be less or more than the velocity of air carrying it? Sorry for all the silly questions.

 

[miningman]

My immediate thoughts here are that the calculated velocities are indeed very high, and more than adequate to carry solid matter with them . However, it strikes me than any of the remedial actions already suggested have some significant costs associated with them. It also strikes me that with an inflow rate of 22,000 lbs / hr and a withdrawal rate of only a fraction of this, the dynamic conditions within the vessel change quite dramatically between the fillings to hi level and then draw down. As the filling cycle approaches the high level cut off condition there must be a very small volume of open bin remaining to act as surge capacity. Perhaps the OP might suggest to management that they lower the high level switch so as to maintain a larger volume of air within the vessel at full condition and perhaps simultaneously reduce the inflow rate of 22,000 lbs per hour. Would probably have minimal initial costs and if succesfull , would gain brownie points for the engineer.

 

[BruceTheEngineer]

miningman:
Yeah you're right about that, if you look at the drawing of the vessel the pellets are always around where the high level alarm is at. Basically, the PLC logic is that as long as that high level alarm is activated, the bin isn't refilled. As soon as that level drops below the high level alarm level, the blower turns on and sends pellets to the receiving bin until the high level is activated again.

Right now the worry is about the cost. I agree that it would be great to not fill the receiving bin up so high, but to do work on the tank, go through the inspections on the weld, pay for the electrical work to move the level switches, etc. we might be spending four times as much as it would cost to just make a gooseneck for all three receiving bins that we have.

And just as a note of clarification, when the receiving bin fills up it only fills up for about two to five minutes. The reason why the rate is so high is that this blower also blows pellets to other feeders via a system of diverter valves and as such needs a lot of oomph to move those pellets to where they need to go.

 

[BruceTheEngineer]

As a note to all, sorry about me leaving out details, this is my first post so I'm still kind of new to this.

 

[lilliput1]

Replace fan motor with inverter duty type. Provide variable speed drive on fan. Provide indicator switch at each diverter valve. Set up schedule of motor speed required to correspond to number of diverter valve open. However since it is not recommended to operate below about 20% speed provide bypass-recirculation valve to modulate and divert excess airflow around pump if operation below 20% CFM is required.

 

[lilliput1]

Also provide shaft grounding ring for VFD motor.