Valves and flow control
A liquified propellant such as butane would boil if it was at the normal atmospheric pressure of 1 bar. The liquid cannot boil inside the can because at high pressure the boiling point is increased. When the valve is opened this liquid and the product held in it suddenly move from inside the can,where the pressure is anything up to 10 bar (10 times atmospheric pressure) to outside the can, where the pressure is 1 bar (normal atmospheric pressure).
This high speed flow is controlled by a simple plastic sliding hollow stem with a tiny hole in the side. The hole is blocked by a rubber ring called a gasket. When you press the actuator the stem slides past the gasket and the product flows into the hollow stem and out through the nozzle. A spring pushes the stem up again when you let go.
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When the valve is closed the hole in the side of the stem is blocked by the rubber gasket.
Outside the can the pressure is 1 bar and propane is suddenly 60 degrees
above its boiling point and vaporises instantly. This causes a mix of
propellant (now a gas), droplets of liquid propellant still vaporising
and the product to accelerate through the nozzle. As the mixture is
blasted out of the nozzle it forms tiny droplets that eventually
evaporate, leaving just the product.
Droplet size for an aerosol is very important. Large droplets make a wet spray and smaller droplets make a dryer spray because the propellant vaporises more quickly.
The nozzle is often a precision made plastic part that looks like a simple hole, but behind the hole are specially shaped channels. The escaping propellant and product mix runs into the channels and as they flow towards the hole they force the flow to swirl. This breaks up the flow into smaller droplets.
This is a front and rear view of the nozzle insert with swirling channels.
The cutaway diagram shows the swirling channels in more detail.
Safe minimum droplet size
If the droplets are too small they could be breathed in and reach the alveoli in the lungs. The droplets must not be less than 10 microns (10 millionths of a metre).
Valves are designed to minimise the amount of small droplets. An exception is in asthma inhalers where the spray is designed to penetrate as far into the lungs as possible.
Metered Dose Valve
This type of spray used a metered dose valve
. When the stem is depressed the dosage chamber (green) fills up and then the dose flows into the upper stem. The chamber cannot refill until the stem is pressed again. Two gaskets are needed to achieve this.
As the propellant leaves the dosage chamber it vaporises rapidly, causing a high speed blast of propellant and fine drug particles. These drugs can reach deep into the lungs where they relive constriction in the bronchioles.
Image: ABPI www.abpischools.org.uk
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The formation of droplets is extremely fast. The propellant is vaporising, forming microscopic bubbles that burst, leaving only the active ingredient behind.
1. What are the three phases that water can have?
2. What are the phases of the propellant in the can?
3 What is your estimate for the diameter of the hole in the stem (stem orifice or IMO) using the scale in the picture?
4. What is your estimate for the diameter of the nozzle hole?
5. If a rain drop is 1mm in diameter, how many times smaller is a 10 micron diameter droplet?
6. Why do asthma inhaler sprays use a metered dose valve?
Find out how the boiling point of water varies with altitude and how this can affect people who live or climb at high altitude.
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