Maintaining higher air pressure in an "open" leaky container

[cashflow]

TL;DR Summary

I need to buy an air pump for a project

Hey all,
I've got a container that needs to maintain a certain amount of air pressure for the contents inside to develop correctly. However, it is an open container (like a big box with a hole in it), so without a pump constantly pushing air into it, it will rapidly drop pressure to equalize with the atmospheric pressure.

Now, I need to maintain a certain pressure, say N pascals, and I know the container is L x W x H centimeters and has a rectangular W2 x H2 cm hole in it. I need to figure out how many V liters per minute I need to pump inside given that I need to achieve N pascals and outside environment is in standard room temperature and pressure. I would be pumping normal "air" -- so whatever the normal air composition is.

Any ideas or direction as to the relevant formulas would be greatly appreciated. Thanks!

 

[russ_watters]

Any ideas or direction as to the relevant formulas would be greatly appreciated. Thanks!

The dynamics of this will be different depending on the size of the container, hole and pressure. Can you be more specific?

 

[cashflow]

The dynamics of this will be different depending on the size of the container, hole and pressure. Can you be more specific?

Hi Russ, thanks for your reply. The size of the container is 10 ft (~3 meters), in all directions, cubic. The hole is in one spot at the top of the container, centered. The air pump will be in one location at the bottom of the container. It's ok to have a pressure gradient within the container, so long as the mean pressure near the bottom is around N pascals. I need to figure out how much air I need to pump inside (rate, L/min) to maintain the average pressure that I want.

Basically, I am going to have some materials inside the container which need to be kept at a higher average pressure than atmospheric pressure, but the container is part of a larger system so I can't exactly seal it. After some time at higher pressure, there's a barometric sensor connected to a computer that will wait for the container to leak and reach atmospheric (room) pressure, at which point the container opens fully and new material goes in. Then the container mostly closes and my task is to build up the needed pressure. Originally I was going to use the barometric sensor to turn on and off the pump to maintain the minimum pressure but unfortunately the application requires the pressure to be relatively stable so I have to figure out what L/min to set the pump to, as its liters/min setting can be programmatically configured.

I also need to figure out what max rating L/min the pump needs to be for some arbitrary dimensions and pressures, as each time we may be dealing with different values here, unfortunately.

 

[russ_watters]

Hi Russ, thanks for your reply. The size of the container is 10 ft (~3 meters), in all directions, cubic.

...
I also need to figure out what max rating L/min the pump needs to be for some arbitrary dimensions and pressures, as each time we may be dealing with different values here, unfortunately.

The dynamics of this will be different depending on the size of the container, hole and pressure. Can you be more specific?

...we need the scenario specified exactly. Obviously a single pump/fan can't handle every scenario between "computer case" and "open full-scale wind tunnel" and the calculations are entirely different for 5 pa vs 500,000. You need to specify a maximum hole size and differential pressure to get a pump/fan size.

 

[Baluncore]

If the gas pumped into the container is heavier than air, such as CO2, then the hydrostatic pressure at the bottom, on the floor of the container will be higher than atmospheric pressure. The hole at the top may then have zero differential pressure, so it will remain open, but will not leak.

 

[cashflow]

Understood, it is 10 ft x 10 ft x 10 ft and needs to maintain 106,000 Pa. Maximum hole size surface area is 1 sq ft. What blower / air pump do I need to buy is the question. It won’t have heavier air — just plain air, whatever is in the environment. Thanks!

 

[davenn]

After some time at higher pressure, there's a barometric sensor connected to a computer that will wait for the container to leak and reach atmospheric (room) pressure,

Maximum hole size surface area is 1 sq ft.

Whith a 1 sqr ft hole ... that is going to be very fast, not instantaneous but very close to it
It's going to be a major air pump that would be able to hold the internal container pressure at 106kPa (15 psi)
whilst air is constantly escaping from such a big hole

 

[cashflow]

Whith a 1 sqr ft hole ... that is going to be very fast, not instantaneous but very close to it
It's going to be a major air pump that would be able to hold the internal container pressure at 106kPa (15 psi)
whilst air is constantly escaping from such a big hole

Hi Dave, how did you calculate that? I may have jotted the size down wrong. I will confirm tomorrow.

 

[davenn]

how did you calculate that?

calculate what ?

 

[cashflow]

calculate what ?

When you said it is going to need a major pump for a hole that big? Was it intuition? Looking over my notes again, it is specified as the max opening surface area but the size can be configured programmatically. Most users will never need an opening that big, perhaps. Say the size was 4 sq in. on a 10 x 10 x 10 ft container, or 1 sq in, what about then? Thanks

 

[Baluncore]

... and needs to maintain 106,000 Pa

The standard atmosphere is 101.325 kPa.
Do you need to hold pressure at 4.675 kPa above the local air pressure?

 

[cashflow]

The standard atmosphere is 101.325 kPa.
Do you need to hold pressure at 4.675 kPa above the local air pressure?

Yes sir, I have to design a controller for variable pressure control, where the differential pressure is just a few kPa (abs pressure a few kPa over local air pressure).

 

[Baluncore]

This may seem like reverse logic, but the potential energy due to pressure, will be converted to kinetic energy as air exits the hole.

Ram air pressure in Pa = ½ · density · velocity²
To maintain an air pressure difference of 5 kPa would require an airflow of about 90 m/sec.

As a quick estimate that ignores viscosity, you can expect the air to come out at that velocity.
With a 1 foot square hole, ( 0.093 m² ), you will have a flow rate of about;
0.093 * 90 = 8.37 m³/sec.

 

[256bits]

When you said it is going to need a major pump for a hole that big? Was it intuition? Looking over my notes again, it is specified as the max opening surface area but the size can be configured programmatically. Most users will never need an opening that big, perhaps. Say the size was 4 sq in. on a 10 x 10 x 10 ft container, or 1 sq in, what about then? Thanks

You say a hole of variable size now.
Question - ANY ductwork leading from the hole to the outside atmosphere?
That will change the pump sizing.

 

[Rive]

- I think it'll be a 'fan' and not exactly a 'pump'
- with the (pressure dependent) output flow I think the solution will not be about constant (?) inflow, but rather some kind of variable inflow/control system.

 

[256bits]

Pump, fan, blower, compressed air tank,...
Problem is not well defined as to what is being done.
There is a simple solution, but..., need more info regarding...

 

[Dullard]

Your pressure requirement is approx 20 inH2O (gauge). That rules out fans and centrifugal blowers. A regenerative blower is probably what you'll wind up with. The blower size is strictly a function of the 'leak' size.

 

[256bits]

Your pressure requirement is approx 20 inH2O (gauge). That rules out fans and centrifugal blowers. A regenerative blower is probably what you'll wind up with. The blower size is strictly a function of the 'leak' size.

And if he uses his "exit" as his entrance when he is pressurizing ( ie one hole, not two ), should simplify his selection. Unless of course, he needs ventilation of the space.

 

[cashflow]

Here's a drawing of what I'm looking at:

When the machine is stopped, air will leak and equalize with the surrounding atmosphere (at normal temp / pressure). It can leak in any direction: the tubing is setup to take outside air in, and the hole on the top releases air back outside too, but it doesn't matter where the air leaks. What matters is maintaining constant pressure while the machine is on. So I have to buy a pump or blower or fan or whatever, then get the pressure up to the desired pressure (within some time t), then configure the pump so the leak rate is the same as the input rate such that the overall pressure is maintained.

I've gotten the rest of the software in place and integrated with the other logic, now this pump / opening part is the one I'm stuck on.

 

[jrmichler]

Yes sir, I have to design a controller for variable pressure control, where the differential pressure is just a few kPa (abs pressure a few kPa over local air pressure).

Before designing a controller you need to know:
1) The maximum and minimum pressure over ambient. Fans and blowers, at least in the US, have the pressure specified in inches of water.

2) The maximum and minimum air flow rate.

3) Will the maximum pressure and maximum flow rate happen at the same time?

Then look for an air mover that will deliver the maximum pressure, the maximum flow, and the maximum combination of pressure and flow. There are several good manufacturers. I have had good experience with New York Blower, plus they have a good web site. Here is an excerpt showing their range of high capacity blowers:

Look for a blower that meets your needs, then read the power required. That will get you an approximate size, which you can use to decide if you really want to do this or if you should go back and design for lower pressure, lower flow, or both. Remember to calculate the duct loss from the blower to the box.

If you decide to proceed, call the company and have one of their sales engineers size and quote a blower and motor for you. You will then need a variable frequency drive (VFD) for the blower motor. And a pressure sensor. And a controller to connect the pressure sensor to the VFD.

The pressure you are talking about (20 in w.c. was mentioned) will result in a high noise blower. There will be noise from both the inlet and outlet of the blower. The manufacturer can tell you the noise levels, and possibly provide silencers if needed. There will also be noise from the high velocity air exiting the box. If any of those noise sources require silencer(s), you will need to add in the duct loss of those silencers to the total blower pressure needed.

 

[cashflow]

Thanks jrmichler! I will reach out to them!