Archive for the ‘Oxygen concentrators’ Category

Oxygen concentrators.

An Oxygen concentrator is a simple and convenient means of providing oxygen without having to buy and recharge cylinders.
Some people will tell you that it is free or at least very cheap oxygen, this is not quite so, they need electricity to run and even if you have been given the machine you will still have to maintain it from time to time, and you must check the concentration at the output as often as you can, some firms will say do it once a month, this will not be practical if you are looking after a number of concentrators spread over a large area and where you perhaps have to fly or drive a long way to look at them.
You will need an oxygen analyser to do the checking and do it perhaps a couple of times a year.
The new W.H.O approved concentrators have an analyser built-in to them.
This works on the principle of ultra sonics.

In some cases using cylinders is a cheaper option providing transport is reliable and leaks are kept to a minimum. Concentrators need a reliable and cheap supply of electricity.
There is one main type of concentrator that you are likely to come across.
These rely upon what is called a molecular sieve.

It works like this.
Air is compressed and passed through one of a pair of vessels containing a chemical in a granular form called artificial Zeolite.
The Nitrogen in the air is withheld by the Zeolite, adsorbed (Note the word has a ‘d’ in it not a ‘b’) which means it sticks to it) and the oxygen allowed to pass through the vessel into a small reservoir and eventually to the patient.
After a pre-set time an automatic control system diverts the compressed air, through a series of control valves, into a second vessel also containing Zeolite at the same time shutting of the first.
At this point you will find two different systems, the older vacuum type type employs a vacuum pump to regenerate the Zeolite, it sucks off the nitrogen and expels it into the room. The modern type is called a blow down or purge system system, with this a small amount of the oxygen is used to blow off the nitrogen and into the room.
I will explain this system at the end of the passage.
The reason for the two systems is as follows, one of the properties of Zeolite is that it has an attraction for moisture almost stronger than its attraction for Nitrogen, in the early days the purge system was not very reliable and an interruption in the purge could result in the chemical becoming saturated with moisture.
This would make it almost useless, it was found that the output concentrations were much better if a vacuum was applied to the Zeolite to remove the nitrogen.
These days with components and electronics being much more reliable the purge system has gained in favour, firstly there is less to go wrong without a vacuum pump and a less complicated electronic control sequencing and secondly there is less noise.


The sequence of the above purge system explained.

Air is drawn in by a compressor through an inlet filter. V2 opens, V1, V3, V4 and V5 closed and the air goes into No 1 tank containing zeolite, out of the other end and into the reservoir tank. V5 and V3 opens and a small amount of oxygen is blown down through tank 2 and out in to the room via V3. (V1 and V4 being closed). End of first cycle.

New cycle. V1, V2, V3 and V5 close and V4 opens to allow room air from the compressor into tank 2 and on in to the reservoir tank. V5 and V1 opens and a little oxygen from tank 2 blows down through tank 1 and out into the room. End of the second cycle.

And so it goes on cycling between one tank and the other.

The collected oxygen in the reservoir tank it sent to the patient via the flow meter.

The chemical Zeolite should last for at least 5 years and probably longer depending on how often it is used.
It should be positioned in a well-ventilated room, not near a source of heat, such as a sunny window, where the temperature may cause it to alarm.
Do not install it in a place where fumes (petrol, chemical or others) may be drawn in and passed to the patient.
Up to 200 feet of 1/4 outside diameter and 3/16 inside diameter tube may be put on to the outlet.

If humidifying equipment is put into the line between the patient and the machine it is most important that water cannot get back along the pipe work to the machine.
Do fit a water trap into the line if this is a possibility.
The tubing for the patient gas supply should be either non-toxic and odour free polythene or copper tube.
The oxygen concentrator is a very neat piece of equipment to have, it saves all the trouble of buying supplies of liquid oxygen or buying it in cylinders, with all the troubles of supply and possible delays.
On the whole they don’t seem to give a lot of trouble.

One of the most important things to remember.

With a little imagination they can be very useful, for example in the theatres you can do away with bottles of oxygen except as an emergency backup and use your concentrator like this:

I once saw a system in Kathmandu a little like the one described below.
First you have to get two large gas tight cylinders about 6 or 8 feet high and about 3 feet in diameter, the size doesn’t matter a great deal, within reason the larger the better but there is no point in making them too big.
Into one of these cylinders you will have to put a tubing connection to take the tube from the concentrator, then a pipe joining the two cylinders, then an outlet pipe at the bottom, this last pipe will lead off through a tap and then to the anaesthetic machine. You will also need a drain tap at the bottom of each cylinder to drain away any moisture that might accumulate and a pressure gauge on each tank.
At the start of the day you start the concentrator running and slowly fill these two cylinders, then when they are full you can start your operations with the oxygen inside.
The concentrator can now be turned off, now as the operations progress and the oxygen is used up slowly, after a while turn the concentrator back on to top up the system.
You should have some idea how much is in the cylinder as you will know how much the concentrator gives out and how long it has been on for, for example, suppose your concentrator gives 4 lpm and you had it turned on for 1 hour, you would expect to have about 240 litres in the tank.
If you are using 4 lpm during the operating then it’s going to last you for an hour. In this case you will need to turn the concentrator back on before your hour is up.
There is a neater way of doing this which will turn the concentrator back on by itself, that is to fit a pressure switch into the system that will turn on the concentrator when the pressure falls to a set level.
The only slight problem with this system is that the oxygen is only at a low pressure of around 7-10 psi which for most uses is fine, but may not be enough to run a gas-driven ventilator, though it should be plenty for most of the common ones that run on electricity and have oxygen added to them.
You can use more than one concentrator to supply your cylinders and this will obviously increase the flow rate available to you, you can also increase the pressure a little by adjustment inside the machine, and by adding further equipment increase the pressure a lot.
The extra equipment needed to increase the pressure is called a booster, it consists of a diaphragm compressor and a small reservoir, this will take the pressure from the concentrator and compress it to about 60 p.s.i. this is useful as you will now be able to run ventilators, which you can’t always do without the higher pressure.

The British company of Diamedica sell a system, click on the link, consisting of a concentrator and a small aluminium storage cylinder. The best of their three concentrators will give you up to 9 lpm at 94% and a pressure of 20 psi. The storage cylinders come in two sizes, 20 or 100 litres. They will hold a pressure of up to 75psi. This extra pressure comes from a compressor pump they also sell as part of the setup. You can also use this set up on a ward with their flow splitter that will feed up to eight patients.

These do need electricity to run, the ones that I have had dealings with have been made to run off 240 volts A/C, I mention this because I once had the embarrassment of taking one of these to a small provincial hospital in Nepal, everyone had gathered in a tiny room to see this marvel produce oxygen, I switched it on and nothing happened, a quick look inside, check the fuse in the plug all seemed to be OK, dead silence from all those gathered, I suspect that some patients had also come in to see what was going on, it was very hot and the sweat was beginning to run.
I then decided to check the mains voltage having noticed that one of the relays was arcing and having difficulty switching over, the voltage was only 115 volts, it should have been 220, so the hospital generator was switched on and all was well, the trouble was that the cost of running the generator, cancelled out the cheapness of the oxygen from the concentrator.
Anyway the point I am coming to is that they need a reasonable and reliable voltage on which to run.
The second point worth a mention at this point is that when you have a problem it is important to check for the simple reasons first, otherwise you could find yourself getting very involved and stripping it all down, only to find that there was no mains voltage or something else simple.

It could make you look a little stupid if there were people around to see.

Newer concentrators use a pulse system that only give oxygen when the patient makes an inspiratory effort, however sometimes the pulse system is dormant when the patient is asleep as they are not always good at detecting a low inspiratory effort. The older versions were wasteful giving a continuous flow even if the patient did not need it all.

Thanks to WHO for the chart below.

Cylinder and concentrator comparison chart

Click here for another excellent article from Frank’s hospital workshop.


Thanks to Frank for allowing me to link to it.

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