Need Help With Blower CFM Sizing 1

[dfowler]

I am building a pressure chamber that is 10'deep, 13'tall, and 22'wide. It is to be equally divided in depth so both sides are essentially 5'x13'x22' in volume. This will be used to test variable pressure loads on garage doors to be placed in between the two halves of the chamber. The variable pressure loads will then correspond to effective pressure from different wind speeds.
The volume of air on each side of the door is approximately 1400 cubic feet. I want to use a blower to suck from one half of the chamber and blow into the other half.
The test criteria is that a pre-load be placed on the door (a pressure delta of only about 2 psi or 30 pounds per sq ft.) for one minute. The door is unloaded and then the test load of twice the pre-load (4 psi) is enacted for one minute. The door is unloaded again and then the safety load of 1.5 times the test load (6 psi) is placed on the door for 10 seconds.
Minimizing the time to reach full load is crucial especially for the safety load so I want to size my blower fairly large even though the pressure differential and the volumes are relatively low.
I realize that control over the blower speed alone would result in slow response time for pressure adjustments in the system. Therefore I want to run the blower at full speed and divert most of its flow into a duct with a control valve looped right back into the blower intake. This could be pinched off slowly to deliver more pressure/ suction to the test chamber with better control.
With these volumes, time frames, and pressures (specifically the delta of 6 psi) could anyone suggest a proper blower size in cfm? Or better yet a good source or handbook that I can look up easy to use formulas for air flow and pressure differentials. We are plastic wrapping the door and assuming minimal air leaks between the halves of the test chamber.
I am a mechanical engineer with minimal HVAC exposure. Thanks in advance for any help!

David Fowler

 

[quark]

First of all, I think I would like some clarification. Do you have to test at 2 psi? or 30 psf?

2 psi = 2 x 144 psf = 288 psf, which is approximately 55.5 inches of water column.

Secondly, why you want to apply pressure on both sides of the door? Can you expose the door to pressure from one side and the other side to the atmosphere?(still you can manage the required dP)

The volume of one compartment is 5 x 13 x 22 = 1430 Cu.Ft.

In the absence of leakages, the volume of air inside the compartment to maintain a pressure of 6psi can be calculated as (14.7+6)x1430/14.7 = 2014 Cu.Ft

So extra volume to be pumped is 2014-1430 = 584 cu.ft.
Now if you want to raise the pressure in one minute, your blower capacity is 584cfm. If you require it in half minute then the blower capacity is 584 x 2 = 1168cfm and likewise.-(1)

If you directly connect the blower discharge to the test skid with short pipe/duct, you can neglect friction and the static pressure of the blower should be atleast 6psi or whatever it may be.

The pressure indicated here seems to be very high for centrifugal fans. However, more experienced souls in this forum may help you in this regard.

If at all you are going for a positive displacement blower, I caution you not to control it's discharge.

Finally, your assumption on minimum leaksis superficial. Don't take it for granted.

ASHRAE suggests the following equation to know the leakage through small openings.

Q(cfm) = 2610 x A(sq.ft) x (dP)^1/2(in.wc)

If there is an opening of 1" by 1' in your test chamber,
then Q = 2610 x (1/12) x 1 x 166^1/2 = 2802 cfm.-(2) So your total flow input should be (1) + (2)

If it is 90psf instead of 6 psi then the leak rate will be 897 cfm.-(2)

Good Luck,



Believe it or not : Had we trusted Archimedes and assigned him the work of lifting the earth(or any mass equivalent to that of earth on earth),with a lever of suitable length, it would have taken him 23 million million years to lift the earth by one centimeter, if he worked at the rate of 1 HP.

 

[dfowler]

Thanks quark! I am looking for 30psf and was attempting to translate it into psi which I felt everyone (and me) would be more familiar with. I goofed and slipped a digit initially and did not even look back at my own numbers (shameful!). I am after .2psi, .4psi, and .6psi overall. We have an equation for translating wind velocity to pressure that is psf=.00256(mph)^2. Testing for 90mph = 20.736 pound-force/square foot. Our test criteria calls for .5x, 1x, and 1.5x the test pressure for the entire test. These we call the pre-load, design load, and test load respectively.
I rounded to 20 psf to make it easier for everyone to make a simple judgment, give me a rule or thumb, or best yet point me to a good source. (I am woefully inexperienced with HVAC having only been exposed to it basically in Physics and Fluids classes about 8 years ago in college). This would then be a test for maximum 30 psf or about .2psi which is much more reasonable (I understand I am doing some wild roundoffs, but I am just trying to simplify).
The reason the test chamber was to be pressurized on both sides is that I was thinking of the wrong scale of numbers (2, 4, and 6 psi). I did not want to subject either side of my chamber to that sort of delta against the atmospheric pressure so I was attempting to split it up to +3psi on one side and -3psi on the other side. I see now that I will probably be easier to make this an open system not a closed one since this is a small dP.
Thanks for your quick equational analysis and pointing out my major mistake in my test criteria! Could you suggest a handbook or source for easy to look up and use equations? I have had some fluids background so I don't need a beginner's guide, but I don't want too technical of a source. I am familiar with compressible, uncompressible, laminar flow, turbulence, Reynold's numbers, psychrometric charts, etc.

Davey

 

[quark]

You need not bother about type of flow, Reynolds number and psychrometry for your problem(unless you have big leaks in the system). For preliminary estimation, the equations given by me are sufficient. You can prepare the test chamber either with a centrifugal fan or a positive displacement blower. I think PD Blower will be cost effective.

My advice will be to speak to some manufacturers(of both fans and blowers) based on this initial work out.

I came across plenty of references about Fan Engineering Handbook by Bufallo Forge Company, but I couldn't have my hands on it. (for me, the postage is as costly as the book itself)

Anyhow I will go through your system leisurely and let you know if anything comes up.

One more question.
Why you want to have 13' height?

Regards,


Believe it or not : Had we trusted Archimedes and assigned him the work of lifting the earth(or any mass equivalent to that of earth on earth),with a lever of suitable length, it would have taken him 23 million million years to lift the earth by one centimeter, if he worked at the rate of 1 HP.

 

[fredt]

The problem is fairly simple. As a previous correspondent suggested the volumetric flow required will be governed mainly by leakage. I would recommend a centrifugal machine which if correctly selected has a limiting maximum pressure characteristic. The volumetric flow required should be very small if you do a reasonable job on sealing and in practical terms you should be looking for a small centrifugal blower in the range 3-5HP. A single stage fan about 17" dia running around 3450rpm would do the job but there are probably more compact multistage units available. If reasonably leak tight you should still have too much volume and will need to regulate pressure by a damper controlled vent in the chamber. If you make the damper big enough you can almost instantly drop the pressure by venting to atmosphere. This approach means that you don't need to know exactly what the leakage is as the fan should have plenty in reserve.

A chamber of this size will have to be quite heavily stiffened (you may be surprised how much support you need even with these low pressures). Unless you really want to pressurise in either direction you don't need two chambers (one side can be atmosphere). Also the chambers could be very narrow unless you need to be in there when testing.

 

[dfowler]

Thanks for the tips guys! I feel more comfortable about this project now. I think stiffening the structure of my chamber will be my next challenge. And then onto the controls and data recording systems.
And quark, the 13' height is because we need to test door for commercial applications as well. These doors need to have an exposed surface area of 10'. We need at least 2' of free space above the 10' opening to mount the shaft and torsion spring assembly. The depth is for the installers and for entering and viewing a damaged or collapsed door after a harsh test.
I will need to regulate my dP across the door even as the door bows and allows more flow through its cracks. I intend to monitor pressure data from each side of the door and input it to a PLC which in turn would hopefully regulate a solenoid butterfly or damper valve to allow greater flow as needed to maintain pressure. I am just not sure if a solenoid valve is readily available in the size that I am looking for.
Is someone familiar with a better solution with rapid response for accurately controlling air flow? Fredt, I do like the idea of dampers on the chamber itself and not having the damper inline with my fan or blower like I was imagining. That would probably not be the best way to control my overall pressure and would unnecessarily load my fan or blower too I'd imagine. I do need a fast-response automated solution for regulating my pressure differential at a set point for a set period of time.

Davey

 

[fredt]

I do think controlling the pressure by venting the chamber to atmosphere is the best approach. If you use a centrifugal blower of the size suggested (it would have a flow rate in the range 500-1000 cfm) either continuosly running or with on-off logic controlled by your PLC. On when pressurising and holding pressure, off when depressurising. I reckon that the vent damper diameter should be around 9" and when fully open this would result in a very small positive pressure even with the fan running. I suggest that the damper be a simple butterfly type - it can be quite crude and you will need a modulatiing actuator and pressure transducer. My experience is with much bigger scale airflows and since I am metric I suspect I'm not in your region but I think you can get a simple damper/actuator pack from the HVAC industry locally although decent modulation control may be an issue on the low cost options. The damper can be a very simple sheet metal construction because it doesn't have to be completely leak tight and the pressure is very low.

 

[quark]

FredT is right. A damper cannot be a fool proof solution and it may create problems rather (as they are not leak proof). In this rocket age, simple solutions are as rare as a sales engineer who would give you an economic solution. Infact, a VFD controlled centrifugal fan (which is selected at the highest pressure required and flowrate at that pressure)which is controlled by a DP transmitter (or a pressure transmitter if one side is atmospheric) is a better option. Most VFDs(for example Danfoss) can take 4-20mA signal directly and control is inbuilt. You don't need a PLC here. But as I said earlier, this is not the simplest solution. (I call this 'Blindness of Perception)

I can't come up with a better solution until and unless I shrug off this technical advancement

Regards,


Believe it or not : Though he couldn't prove, Einstein never agreed with Heisenberg on his Uncertainty Principle.

 

[lilliput1]

Have you considered doing static test by using sandbag either laid on or hung from wires on the door. Use at least a safety factor of 2 x 1.6 = 3.2. The factor 2 is for impact factor and the 1.6 is typical safety factor for building structures. Wings of airplanes are tested using static test. I don't know what safety factor they use though.

 

[dfowler]

Thanks, but yes we have tried a static test by orienting sections horizontally. We built a wood frame around it and put a bag inside like a waterbed. We slowly filled it until the door buckled in the middle.
It was a good test to just give us a benchmark to know kind of where we are. The time period of the loading was unnaturally slow however and we think the specimen prematurely failed on account of that. It took 15 minutes to get enough water to simulate a strong wind gust that in nature will appear and disappear within 10 seconds often times.
We have a foam (eps) core in our doors that is glued to the metal outside shell. I believe this structure is highly suseptible to creep (I doubt that is what you call permanent deformation in foam) and the longer the load is on more deflection will occur.

Davey

 

[ChasBean1]

Stagnation pressure at Hurricane Category 5 force is 0.83 psig, or about 23 in. w.c. This equates to a force of about 19,000 pounds on a 20' x 8' garage door, perpendicular impact. I don't know of any commercial HVAC blowers that are capable of that ESP (nor TSP). These usually max out at 8-12 in w.c. TSP. As you first said, you should suck from one side and blow to the other. This might mean this is a job for an industrial hivac (high vol. vacuum) system, which could mean more amps than what you were planning...

 

[quark]

CB!

My idea of pressurising the door only from one side is to avoid double trouble. The cross flow between the two sides is difficult unless the door is properly sealed. 16" wc pressure is quite possible with centrifugal fans and beyond that a roots blower will do the work. I think, If we have to maintain the required PD from both sides, time may become a crucial factor which may unnecessarily load the door at some intermediate pressure levels for longer time. (PS: I don't have actual experience with these tests).

BTW, I have a good paper on pharmaceutical air locks which are in vogue now. This paper is actually appeared in Cleanrooms International magazine(free) and I modified it into a pdf format. If you are interested, give your email id.

Regards,


Believe it or not : Eratosthenes, a 3rd century BC true philologist, calculated circumference of earth with the help of a stick and it's shadow. The error was just 4% to the present day calcuated value.

 

[lilliput1]

The type failure evident is buckling of the unrestraint portion of the thin sheetmetal door panels. When I used to work for a sheet metal office equipment fabricator, I found out that using AISC design of light gauge sheetmetal panels (I can't remember the exct title, I will see if I still have the book somewhere, was useful in predicting failure. It had a factor of safety of 1.6 so after calculation I multiplied the allowable load by 1.6 to come up with the ultimate load for various configuration of steel shelving. The book gives criterias for wich the portions of thin waled elements are effective. If an element is too thin & far from reinforcing member (perpendicular edge elements part of or welded to the section) then the section modulus is calculated with this section rendered as ineffective and thus excluded. We will want to be conservative so consider that if it fails in the static test, it should also fail in the "live" test. Static test should have appropriate safety factor included to approximate live loading. If loads will be evenly distributed & suddent gust the minimum safety factor of 2 is required for inpact. Tack on another 1.6 factor to be safe. Total factor then is 2 x 1.6 = 3.2. That is the system must pass static test 3.2 times the live load to pass.

 

[dfowler]

Wow, that assumption seems incredible. Should a static test be more than 3x the anticipated live (wind) load? Safety factors are great and all but I'm just not sure about those numbers. The static test already failed at a lower weight (distributed force) than we wanted. By that reasoning we should only certify our door for 1/3 of the weight distributed by our horizontal static weight test.
I think that your are assuming that the initial force be counted as an impact. This will not be true as the force will be ramped up from zero over a short period of time (probably 2 seconds).
My assumption is that a real live test would be much less stressful than was our static loading test. This being largely due to the time sensitive nature of the necessary loading. The loads only need to be on the door for short periods of time (less than 1 minute) and this should makes a real difference.
Quark, I think I am leaning to your idea of pressurizing and depressurizing only one side of the door. It will be easier to control and I agree that 16"wc of pressure should not be too hard to maintain on just a single side. If nothing else I can install two entry blowers and have the control vents wide open at first.
Thanks ChasBean1 for your input on stagnation pressure for hurricane for winds. I did not know that and it is an interresting benchmark to know what we are testing against. We do not however have to test for full hurricane force winds and a dP of .5 to .6psi (appx. 16"wc) is the maximum that this system will ever see.

Davey

 

[lilliput1]

90 mph = 20.736 lb/SF = 0.144 psi

.5 x 0.144 = 0.072 psi = 2.0" wc
1.0 x 0.144 = 0.144 psi = 4.0" wc
1.5 x 0.144 = 0.216 psi = 6.0" wc

Using SMACNA Duct Construction Standard at 4"wc:
Width Min Ga. for no reinforcement for up to 10' span:
8" & dn = 24 ga
>8 to 10" = 22 ga
>10 to 12" = 20 ga
>12 to 14" = 18 ga
> 14 to 16" = 18 ga
> 16 to 18" = 16 ga

To use lighter ga metal you have to add reinforcements per SMACNA Table. Deflection is limited to width/200.

What gauge metal, panel width & panel length are you using for the door. At what static load did it fail?

 

[ChasBean1]

Okay, so it becomes a matter of selecting a blower with 16" ESP. My thought was that you might be able to do the job with a slightly smaller (lower HP) device if you use the inlet of the blower as well to draw from the low side chamber. What sort of leakage do you anticipate? Use the following to ballpark selection of a device:

Q = 2610 A dP^.5 (Q in cfm, A in ft2, dP in in. w.c.)

at the known dP of 16 in. w.c., this becomes:

Q = 10440 A

So if you expect a total of a half a square foot leakage area, you need at least 5220 cfm from your blower at a 16" ESP. Either way you decide to do this, could you re-post as to how things go? Thanks, -CB

 

[dfowler]

Ok a recap, here is the criteria so far:

Testing for up to 100mph = 25.6 pound-force/square foot
Our test procedure calls for .5x the load for pre-load which is 12.8psf for 10 seconds.
Then 1x the load for the design load which is 25.6psf for to 52 seconds.
And then 1.5x the load for the test load which is 38.4psf for 10 seconds.

12.8psf = .089psi - rounded up to .1psi
25.6psf = .178psi - rounded up to .2psi
38.4psf = .267psi - rounded up to .3psi

In a perfect leak-proof system all the pressure I would need to generate would be approximately .3 psi.

Now I am left with the time constant. This is a simple volumetric calculation. However, we have not ironed out the volume of the test chamber yet as we are still debating the ultimate width of door to test (16' - 20').

For no losses:
(14.7+(dP))x(volume)/14.7 = (desired volume)Cu.Ft

Let’s assume 1000 cubic feet of volume in the test chamber:

(14.7+.3) x 1000 Cu.Ft / 14.7 = 1020.41 Cu.Ft

1020.41 - 1000 = 20.41 Cu.Ft (this is volume of air needed to force into the 1000 Cu.Ft region to create a dP of .3psi).

Thus a 20.41 CFM blower would create this pressure in one minute. We need it in 2 seconds so we need a blower 30x that strength or 20.41 x 30 = 612.3 CFM for the blower.

With a small air gap (12 square inches) at that pressure I used the equation posted above by quark (and you ChasBean1):

Q(cfm) = 2610 x A(sq.ft.) x (dP)^1/2 (in.wc) = 2610 x (1/12) x (8.3)^.5 = 626 CFM.

I realized that this doubled the requirement of the blower under these conditions and again mislead everyone by doubling my pressure requirement instead of my volume requirement. Not having much experience with HVAC I doubled the wrong number. I should not have doubled the dP required from .3psi to .6psi. Sorry, I just can’t get past my southern upbringing I guess

Here’s the deal, I am thinking I need a 1500 CFM blower to accomplish the pressure that I need under these conditions (8.3” SP). I am sure that I will encounter many more leaks across the door that I am anticipating so we will probably need more like 3000 CFM at 8.3” SP. Does this all sound right so far? I've steered myself wrong several times already

Now from what I understand the CFM output of a typical blower will increase as the static pressure decreases. Is this true for all blowers?

Davey

 

[lilliput1]

I hope you realize that at 0.3 psi the force normal to the 22' x 13' side is 12,355 lb.

 

[dfowler]

I do and we are going to frame the entire unit out of 3" x 3/16" wall square tubing. Similar construction to the 2x4's in a residential wall with vertical tubes on 24" centers. We will face the outside of the chamber with 3/4" thick plywood that is both glued and bolted to the frame. This will weigh thousands of pounds, but we should not have to worry about deflection of the walls of our test chamber. Thanks for bringing up that point though, you never know with me

Our doors are made from .018 steel to .022 steel. We have 1-1/2" thick and 2" thick versions. We have open backed and closed backed doors. We have styrofoam core doors as well. We tested a 26 gauge (.018) section that was 16' long, 21" high, 1-1/2" thick, and with a styrofoam core. It failed at a static pressure around 5 psf. The door panel cross sections are the shape of a super wide channels open sides together. Upon failure the legs of the channel shape straighten out and then the door buckles and creases. We have not put several door sections together and tested yet a garage door as a whole. I think that the door sections being hinged together will prevent the channel leg from kicking out and buckling somewhat - hopefully!

Davey

 

[lilliput1]

I think you had better pose this in the structural engineers forum. A span of 16 ft with depth of 1 1/2" would result in way too large deflection. To prevent failure by deflection, limiting it to 1/240 of the span, usually the depth (thickness of the panel for this case) has to be at least 1/24 of the span. For 16 ft this will be 8".