Sources of central gas supply Flashcards
Sources of central hospital gas supply (3)
- Cylinder manifolds (supply nitrous oxide, entonox, and oxygen at a constant pressure)
- Vacuum insulated evaporators (store liquid oxygen - which is usually used to supply piped oxygen in hospitals)
- Large oxygen concentrators
Cylinder manifolds: structure, cylinder size, pressure, function
Structure:
* Two equal banks of large cylinders (e.g. size J), known as duty banks and standby banks
* Number of cylinders in each bank is determined by the expected demand
* Cylinders in each bank are connected through non-return valves to a common pipe.
* All cylinders in each bank are turned on and interconnected
* Each bank of cylinders has separate pressure regulator valves
* ** Nitrous oxygen, entonox and oxygen are supplied at a constant pressure** via a control panel
* Pipelines are fed from pressure regulators and work at about 400kPa
Manifold is housed in a well-ventilated room built of fireproof material, away from the main buildings of the hospital. Not used as a general cylinder store.
Duty and standby banks alternate in supplying the pipelines. (see picture of automatic changeover manifold with a central monitoring unit). Changeover from one to the other is an automatic process
* Should occur at a cylinder pressure that will ensure maximum usage of the contents of each bank
How many days supply is the total storage capacity of a manifold
One weeks supply
Each bank of cylinders should contain no less than 2 days supply, with three days supply of spare cylinders kept in the manifold room
Relative sizes of nitrous oxide and oxygen manifolds
The nitrous oxide manifold is often larger than the oxygen manifold. This is because nitrous oxide is present in cylinders only, whereas liquid oxygen is normally used to supply piped oxygen in hospitals, so the oxygen manifold is a back-up.
Cylinder manifold changeover unit: power supply, function, structure
- Normal operation dependes on an electrical supply but design ensures that in the event of an electrical supply failure, there is no disruption of gas flow into the piped medical gas and vacuum system
- Cylinders can be changed, or pressure-regulating valves removed for overhaul without a loss of continuity in the gas supply
- Changeover unit is housed in a lockable, steel enclosure with a glass-fronted door. Has monitoring regarding function of the two banks and pipeline pressure.
Picture: automatic changeover manifold with a central monitoring unit
Vacuum insulated evaporators: structure, advantages, how is temperature maintained, delivery capability
- Made of steel**: inner wall stainless steel, outer wall carbon steel **
- Advantages: Most economical way to store and supply oxygen, allows for easy maintenance and access
- Should be capable of delivering up to max of 3000 L/min oxygen. Can be supplied in up to 50 different sizes depending on oxygen use in hospital
Temperature maintenance: liquid oxygen is stored at temperature of -150 to -170 degrees C (below its critical temp of -118 degrees C)
* High-vacuum shell (essentially a vacuum flask)
* Outside surface is painted white to reduce absorption of ambient heat
* Evapouration of oxygen: for liquid oxygen to evaporate, it requires heat (the latent heat of vaporization). This heat is taken from the liquid oxygen -> helps to maintain low temp
What temperature and pressure is liquid oxygen stored at in the VIE?
How does the volume change at 15 degrees and atmospheric pressure?
Stored at:
* -150 to -170 degrees C (i.e. below critical temp of -118 degrees C)
* 5-10 atmospheres
At a temperature of 15 ° C and atmospheric pressure, liquid oxygen in a VIE can give 842 times its volume as gas.
Liquid oxygen supply system: structure, pressures throughout system, adaptations to changes in demand
- Cold oxygen vapour (NB -150 to -170 degrees, 5-10 atmospheres in the VIE) taken from top of VIE via copper tubing
- Heated via a super heater -> causes an increase in its pressure
- Passes through a pressure regulator that allows it to enter the pipelines and maintains the pressure through the pipelines at about 400kPa (4 bar)
Excessive demand on system ->
* Control valve opens, allowing liquid oxygen to evaporate by passing through super-heaters made of non-insulated coils of copper tubing
Under-demand for oxygen ->
* Pressure in VIE starts to build up
* Safety valve opens at 1700kPa (17 bar), allowing gas to escape.+ reducing the pressure
1kPa = 0.1 bar = ~0.0987 atmospheres
How do you measure the mass of liquid oxygen in a VIE? How is the VIE refilled?
Meaure the mass of liquid oxygen in a VIE by:
* Differential pressure gauge that measures the pressure difference between the bottom and the top of the liquid oxygen -> calculate the contents of the VIE As liquid oxygen evaporates, it’s mass decreases, reducing the pressure at the bottom.
* Rest the storage vessel on a weighing balance in order to measure mass of liquid
Refilling
* Fresh supplies of liquid oxygen are pumped from an insulated tanker into the vessel via cooled hose.
* Hose is cooled to below the critical temperature by allowing the tanker’s liquid oxygen to flow down the hose
* NB considerable wastage of oxygen during the filling process
VIE safety measures
- Any liquid oxygen storage vessel should be housed away from main hospital buildings (fire hazard)
- Storage vessel protected by a caged enclosure which also houses 2 banks of reserve cylinders
- Reserve banks take over automatically if the VIE output fails
NB: liquid spillage -> risk of fire, cold burns, frost bite, hypothermia
Oxygen concentrators: describe the mechanism of action
Extract oxygen from the air by differential adsorption.
Over one cycle: (around 20 seconds)
* **Ambient compressed air is filtered and pressurized to ~137kPa **
* Enters one of the two parallel, alternating adsorber towers located on either side of the central ‘mix tank’
* Air in the tower is forced through a molecular sieve composed of zeolites (columns of micoporous crystals)
* When under pressure, the zeolites strongly attract nitrogen molecules, allowing oxygen molecules to pass through
* By time air reaches top of the tower: all nitrogen, most other impurities have been removed, leaving oxygen (max concentration 95%) and trace amounts of inert argon
* Oxygen passes into mix tank
Adsorption vs absorption
Adsorption = occurs when the host material does not change its characteristics when the added substance adheres to it. E.g. zeolite material is a rigid crystalline structure. Nitrogen molecules chemically attach to it but do not change its physical structure
Absorption = occurs when a substance combines with another substance to change the physical characteristics of the host material (e.g. paper towel)
Zeolite towers of oxygen concentrator: structure, lifespan
- Rigid framework of silica and aluminium, with an extra cation of calcium or sodium to make up the missing positive charge in the structure
- Life of zerolite crystal is approx 20,000 hours (about 10 years of use)
Oxygen concentrators: how is a continuous supply of oxygen produced?
Through pressure swing adsorption
* While one adsorber tower is pressurizing, the pressure in the second tower is reduced to zero by applying a vacuum -> causes release of its nitrogen into the atmosphere, thus regenerating the zeolite molecular sieve
* When this is complete, the cycle reverses and the newly regenerated tower pressurizes and produces oxygen, while the other is purged and regenerated
Typical oxygen concentrators have cycles of around 20 seconds
Oxygen concentrators: what is the maximum oxygen concentration achieved? What is the max flow rate achievable
- Max oxygen concentration = 93-95% by volume. Argon is the main remaining constituent
- Flows of up to 10L/minute can be delivered
Portable oxygen concentrators: power supply, flow rates
- Produce <1L/minute of oxygen (c.f. max 10L/minute of larger oxygen concentrators)
- Use a demand valve to deliver oxygen only when the patient is inhaling
- Typically plug into a wall outlet
Label the areas of the gas supply
