required pressure

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Paul-K
Posts: 6
Joined: Wed Apr 22, 2020 12:37 pm

required pressure

Post by Paul-K »

Your web site mentioned 6 bar as the needed pressure for these systems. Until now we have been hearing about a value we haven't managed to find clearly defined, but most often referred to as "between 2 and 3" bar. That's a big difference and seems likely to redefine all our assumptions about what kinds of containers and valves and compressors might be required. Can you bring us up to date on what your experience has been?

kochju
Posts: 16
Joined: Fri Apr 24, 2020 3:58 pm

Re: required pressure

Post by kochju »

Hi it's Julian, nice to meet you Paul!

Yes indeed, we started out thinking the more pressure the better. With higher pressure we could cycle faster and thought we increase productivity. We suspect that with pressures such as 6 bar, we do not have a proper flushing of the tube, maybe due to turbulence. Very vague though.

But moving forward we realized that a pressure of 4 bar or slightly above is sufficient for our small setup and still allows for relatively short cycle times. And as you can see with our latest prototype Ox01c, with a pressure of 4.4 bar, we get decent production at 1.5 l/min and 90% oxygen.

In addition, we have less stringent requirements for our parts used in the setup the lower the pressure is so thats a plus too.

luurs
Posts: 1
Joined: Tue Apr 28, 2020 2:25 pm

Re: required pressure

Post by luurs »

Hi Paul, it's Urs

Standard PSA works with two pressures: lower and higer. In the simplest case (which we are dealing with) lower pressure P(low) is equal to ambient pressure. Still, the choice of higher pressure P(high) is not that simple. So far, I have not done any detailed calculations, but here are some thoughts about the choice of the higher pressure.

Capacity:
Each zeolite has a specific (temperature dependent) capacity for the adsorption of nitrogen and oxygen. For standard air-separation zeolites (such as 13X), nitrogen capacity is higher than oxygen capacity.

Amount of adsorbed nitrogen:
Given pure nitrogen, the amount of nitrogen that is adsorbed depends on pressure, and at very high pressure P(sat,N2) the amount of adsorbed nitrogen is equal to the specific nitrogen capacity (the zeolite is saturated with nitrogen). If we increase nitrogen pressure from P(low) to P(high) some nitrogen adsorbs on the zeolite. Above P(sat,N2) the zeolite can not adsorb more nitrogen, thus it does not make sense to incease pressure above P(sat, N2).

Amount of adsorbed oxygen:
Given pure oxygen, the amount of oxygen that is adsorbed depends on pressure, and at very high pressure P(sat,O2) the amount of adsorbed nitrogen is equal to the specific oxygen capacity (or the zeolite is saturated with oxygen). If we increase oxygen pressure from P(low) to P(high) some oxygen adsorbs on the zeolite. Above P(sat,O2) the zeolite can not adsorb more oxygen.

Selectivity:
If we use air (say 21% oxygen 79% nitrogen) instead of pure nitrogen or pure oxygen, both gases can adsorb on the zeolite and there is a competition. Because there is more nitrogen adsorbtion (higher affinity for nitrogen adsorption and higher nitrogen content of air) than oxygen adsorption (lower affinity to adsorb and lower oxygen pressure). I guess that there is an optimal amount of air per gramm of zeolite that can be filled to the zeolite bed. If we fill less (lower pressure), we get lower productivity per cycle. If we fill more (higher pressure) we get lower purity.

Productivity:
As already mentioned productivty per cycle depends on P(high), the zeolite, the apparatus you use, and how you are using it (cycle time). Factors that influence optimal cycle time depend on the size of your column, air flow per second in or out of the column (during pressurization and depressurization), and particle size. Here valves and tubing (size, length) can be limitting, in general bigger and shorter tubbing is better and your valves (at the air inlet and exhaust outlet) should have a high "smallest diameter".

Long story short: it depends! Maybe moost of this does not help, since you might not know the saturation pressure of your zeolite.

When we started the project, we likely used too high pressures. Right now I think that 2-4 bar is enough. You can start with a small pressure (e.g. 1 bar overpressure) and increase it until the pressure specs of your system are reached. If you need to know more about valves maybe this homepage helps you: https://tameson.com/selection-guide.html.

I hope, this answer helps you more than it confuses you. I also hope that experts would consider my explanations as correct (enough).

GM5
Posts: 3
Joined: Mon May 25, 2020 5:39 pm

Re: required pressure

Post by GM5 »

https://www.cedengineering.com/userfile ... ystems.pdf is primer on compressors for research on DIY and inexpensive alternatives. It shows The required "Quality" of air has three main considerations:
1. Maximum acceptable moisture content or pressure dew point;
2. Maximum acceptable oil content;
3. Maximum acceptable particulate concentration.
Different end-uses require different levels of air quality. The quality of the compressed air produced by a system can range from normal plant air to high quality breathing air.

Air Quality (in descending quality) Applications:

Breathing Air - Hospital air systems, refill diving tanks, respirators for cleaning and/or grit blasting and spray painting
Process Air - Food and pharmaceutical process air, electronics
Instrument Air - Laboratories, paint spraying, powder coating, climate control
Plant Air - Air tools, general plant air

GM5
Posts: 3
Joined: Mon May 25, 2020 5:39 pm

Re: required pressure

Post by GM5 »

https://hongchuan.en.alibaba.com/produc ... 3dcbKIhamz samples $130 a piece but don't know shipping, most likely price of item or $130. This is most likely gold standard to beat for DIY because these are most likely used in manufactured Oxygen Concentrators
280 L/min@0 bar
219 L/min@2 bar
180 L/min@4 bar
160 L/min@6 bar
120 L/min@8 bar

fawazahmed0
Posts: 8
Joined: Fri Jul 10, 2020 6:05 pm

Re: required pressure

Post by fawazahmed0 »

If someone is going to buy air compressor for 130$, I think it will be much better to just buy oxygen concentrator unit at that price , for example:
https://www.alibaba.com/product-detail/ ... I8XCrv&s=p

kochju
Posts: 16
Joined: Fri Apr 24, 2020 3:58 pm

Re: required pressure

Post by kochju »

fawazahmed0 wrote:
Wed Jul 15, 2020 12:11 pm
If someone is going to buy air compressor for 130$, I think it will be much better to just buy oxygen concentrator unit at that price , for example:
https://www.alibaba.com/product-detail/ ... I8XCrv&s=p
Yes we have also ordered some from Aliexpress to understand the mechanics. However, our project is also more focused on building oxygen concentrators from easily accessible parts when common supply chains break down (as was the case in some parts of the world during lockdowns). Therefore we will continue to look for simpler solutions such as your idea with the candle burning!

fawazahmed0
Posts: 8
Joined: Fri Jul 10, 2020 6:05 pm

Re: required pressure

Post by fawazahmed0 »

What I was trying say was, we don't have to buy expensive compressors for this DIY project.
we could also use a cheap chinese made 12v car compressors, which is around 3-7$.
I am not sure if this would work, but they can give pressure of upto 150psi(or 9bars)

we just have to open the case of the compressor or attach a fan to it, so that it doesn't heatup.
oiling (very very slight oil, so oil doesn't go to into the system) in the piston will also help.

For example:
https://www.ebay.co.uk/b/12V-Vehicle-Ai ... =nc&_sop=2

fawazahmed0
Posts: 8
Joined: Fri Jul 10, 2020 6:05 pm

Re: required pressure

Post by fawazahmed0 »


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