medical gases Flashcards
how is 02 manufactured?
fractional distilation - industrial O2
oxygen concentrators - field medicine, home O2
describe the process of fractional distilation..
liquidifies air
heats slowly, allowing different components with different boiling points to evapourate at diff temps
collects O2 at temp for 02 boiling point -183 degrees
99% pure
how do oxygen concentrators work
by pressure swing absorption
air is passed over a filter and then through 1 of 2 adsorbers made of zeolite (silica and aluminium)
as pressurised air passes over the zeolite, under pressure the zeolite adsorbs nitrogen and allows O2 to pass through.
this concentrates O2. there is also some argon present
the zeolite will eventually become saturated with nitrogen and to recycle this, it can be depressurised allowing nitrogen to escape.
there is 2 towers allowing one to pressurise while other depressurises allowing constant process of concentrating O2
95% O2 produced, some impurities and argon
what are the pros and cons of O2 concentrators?
pros
* allow portable method of concentrating O2. can be used as a back up on anaesthetic machine, for home O2 or field medicine
* relatively cheap
* reliable
* flows up to 10L/min
cons
* 95% and still has impurities
* needs power
* fire risk
* life span of zeolite is 10yrs
* requires routine maintainance - change of filters
define adsorption
the process of molecules adhering to a surface, without change in chemical structure / properties
define absorption
material bind materials and react with them to change their structure.
how is O2 stored?
cylinders e.g. cylinder manifolds, also as extra portable back up on wards and anaesthetic machine
vacuum insulated evaporator (VIE) - more practical i.e. stored as a liquid so takes up a lot less room
how much can a VIE hold? what are the pressures and temp
1500L of liquid O2
at -150 to -170 degrees C
at 5-10bar pressure
in a large hospital what is the main O2 supply?
VIE
cylinder mannifolds are only a backup
what is the recommended O2 storage capacity for a hospital ?
2 weeks
what are cylinder mannifolds?
group of size J cylinders and piping system that supplys N20, O2 and entanox for the hospital. O2 is mostly as a back up for the VIE.
these cylinders contain gases under pressure 137bar except N20 which is liquid + equilibrium with vapour.
they supply the pipelines at 4 bar
they are connected to a changeover control unit which measures pressure and allows a valve to switch between cyclinders when pressure is low. this will also alarm
each bank carries no less than 2 days supply
what are the pros and cons of cylinder manufolds?
Pros
* provide back up to VIE
* may be primary O2 supply in smaller hospitals
* cheap and simple
* have alarm system for low pressure
* automatic control unit to switch between them
Cons
* less storage capacity than VIE , so less practical as only O2 supply
* fire / explosion risk
how does a VIE work?
concentrated O2 is supplied to a VIE and cooled to -150 to -170 degrees and pressurised to 5-10bar.
this is below O2 critical temp and leads to liquidification of O2.
The liquid O2 is stored in a steel container with a vacuum insulation layer.
The O2 vapourisers and leaves the VIE via pipelines that go through a series of pressure regulating valves to reduce it to a pressure of 4bar. it also passes through a heat exchanger to warm the oxygen.
as the O2 vapourises, it cools the container via latent heat of vapourisation. this helps to maintain a low temp (as well as the vacuum shell)
there are 2 routes for vapourisation incase demand increases
how is the temperature of a VIE maintained?
vacuum insulated
latent heat of vapourisation
white reflective outer shell - reduces absorption of ambient heat.
what happens if no O2 is used in a VIE?
the temp rises (no latent heat) and more gas will vapourise and pressure will rise
if exceeds 15bar it will be released via pressure relief valve.
how is the content of VIE measured?
the VIE is placed on a scale so the mass can be weighed.
using moles = mass / mr
can work out the moles present and hence the volume of gas this could supply
new ones use a pressure transducer which compares pressure at bottom and top to determine how much liquid left.
pros and cons of VIE?
pros - large amount of O2 can be stored efficiently - most practical/ economical. no power
cons - explosion risk, expensive set up, if not used, O2 is wasted, needs thorough testing and checking for pressures/ contamination
how much O2 per L of liquid O2 in VIE?
840L of gaseous O2 supplied per 1L of liquid
what are the safety features of a VIE?
pressure regulators - reduce pressure
heat exchanger - increases temp
pressure relief valve - 15bar - O2 can escape
stored away from hospital incase of explosion
back up cylinder manifolds.
what is the boiling point and critical temp of O2?
-185 degrees = BP
-118 = critical temp
outline the features of piped O2 supplies in hospitals
supplied from central supplies - either VIE or cylinder manifolds
gases flow through copper alloy pipelines - 4bar
delivered to outlet valves located at different points in the hospital - some connected via shrader valves and some via wall rotameters.
what other piped lines are there?
piped lines for O2, N20, air, suction, scavenging
O2 and N20 and medical air = 4 bar
air for driving tools - 7 bar
suctioning and scavenging - negative pressures
why is copper used in piped lines?
bacteriostatic
prevents degradation of gases
how should cylinders be stored?
size F, G and J - upright to prevent damage of valve
C, D ,E either horizontal or vertical
entanox - horizontal
empty cylinders stored away from full ones.
in fire proof, dry, well ventilated room, normal temp
what is this ..
CD oxygen cylinder
modern alluminium alloy cylinder
commonly used in transport
what is this..
old type molibdenum steel cylinder
looks like a size D
can you describe a cylinder valve block?
larger cylinder dont have a pin index and instead have a bull nose valve
draw a graph to show the emptying of O2 and N20 cylinders..
O2 - linear, boyles
N20 initially constant P as liquid vapourises, then boyles when all liquid has gone.
- This is actually a simplification; in reality the initial pressure is not perfectly constant because as the n2o vaporises from the liquid phase it cools slightly (latent heat of vaporisation), and therefore gauge pressure does fall slightly.
