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If I were to remove all the air from a canister of WD40 so that it was a vacuum inside, and reinforced the walls so that it won't collapse in from atmospheric pressure, what would happen when I pull the trigger? Will it now suck like a vacuum cleaner?

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  • $\begingroup$ Does the valve function in noth directions or will it allow air into the vessel? $\endgroup$ – Solar Mike May 8 at 17:59
  • $\begingroup$ “In both directions” ... $\endgroup$ – Solar Mike May 8 at 19:14
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Figure 1. A typical aerosol valve. Source: Wikimedia.

An Internet image search will show that most aerosol valves are of the type shown in Figure 1. Here it should be clear that if the differential pressure across the valve is reversed (pressure inside < pressure outside) that the valve will tend to open. The internal spring opposes the opening and so the ΔP at which the valve opens depends on the spring pressure.

  • If the spring force is high enough then the valve may not even open at ΔP = -1 bar and your vacuum could be maintained.
  • If the spring force is lower then the pressure inside the can will settle at that ΔP that just balances the spring force.

... what would happen when I pull the trigger? Will it now suck like a vacuum cleaner?

Once you depress the valve the pressures will will begin to equalise at a rate determined by the ΔP, the resistance of the air path and the viscosity of the air. Since the rate is proportional to ΔP the pressure equalisation will follow an exponential curve rather than linear.

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The valve on such a can may be engineered to resist internal pressure in such a way as to seal when the can's contents are in place. If the can is completely empty and placed in a vacuum chamber, it's possible that you can evacuate any additional gases within, but once the vacuum is removed, the can will "suck" in air until at near-ambient pressure.

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