Aerosols and pressure. The filling line.

1. How aerosols work

The filling line

P.6

Picture 1.7 The start of the production line. The empty cans are ready to be filled.

	The parts of an aerosol
		Aerosols are assembled and filled on a single production line. The operation of this line relies on many principles of Physics to keep it working quickly and safely. On this page we will take an overview of a filling line for an antiperspirant in a single compartment aerosol, giving links to more details of the Physics later in the e-source. One of the overriding safety concerns is that there are a number of flammable materials – most particularly the propellants. It is, therefore, essential to reduce all the possibilities of gas escaping or being exposed to any sparks.

	Loading
		The cans arrive in palettes with the cone and dome already in place. They are lifted off the palettes and put into a bay, from where they join the production line. The line itself is a series of conveyer belts. These moving belts are specially designed so that they do not charge sufficiently to cause a spark (see page 26).

A transparent can showing the product at the bottom

Picture 1.8 The contents of a can. You can see the product at the bottom.

	Product
		The product is injected into the top of the can, which, as yet, has no valve attached. For an antiperspirant, a machine drops the product in paste form into each can. Once this viscous paste has gone in, the can is sealed; the valve assembly is crimped onto the hole in the cone section on the top of the can. Now the can is ready for the propellant.

Propellant

The propellant is usually a simple mix of hydrocarbons (usually a mixture of butane & propane - called LPG). Although these are both gases at room temperature, they can be turned into a liquid by putting them under pressure (see page 9) (At room temperature, propane turns into a liquid at 8.5 x 10⁵ Pa and butane at 2.1 x 10⁵ Pa).

The propellant is squirted in through the valve under pressure of about 50 atmospheres. This means that it stays as a liquid throughout the filling. Given that the LPG is so flammable, special precautions have to be taken whilst putting the propellant in the can. One of these is to fill the cans in special ‘gassing rooms’.

	Picture 1.9. Structures of butane and propane.
What’s this?

Picture 1.10. The gassing room is often separate from the main factory. The production line (behind the screen on the left) carries the cans through the gassing room to have the propellant put in under pressure.

	Gassing rooms
		The gassing room is kept isolated from the rest of the factory floor: it is specially designed to withstand explosions and, preferably, it is built outside of the factory. The production line passes through the gassing room where the LPG propellant is injected in through the valve. There are a number of safety features that can be designed into the gassing room such as: explosion proof drivers and motors: the drivers are pneumatic and the motors are encased in explosion proof casings it is unmanned and is monitored by CCTV cameras there are gas sensors at working level and floor level there is efficient ventilation – the air is changed about once a minute the ventilation will be increased if gas is detected there are spark sensors it has a loose roof.

Summary Close

the product is put in on the production line
flammable propellants are injected under pressure in a separate gassing room
the gassing room is equipped with explosion proof devices