Pressure and temperature measurements of current and future low GWP pMDI propellants in the actuator sump

 

Ben Myatt1, Stephen Stein2 & Barzin Gavtash1

1 Kindeva Drug Delivery, Charnwood Campus, 10 Bakewell Road, Loughborough, United Kingdom, LE11 5RB

2 Kindeva Drug Delivery, 3M Center, St. Paul, MN, US, 55144-1000

 

Pressure and temperature of a pMDI formulation during actuation are influential parameters governing atomisation efficiency and therefore droplet size and velocity of the aerosol emitted by a pMDI. Current and future low-GWP propellants have varying thermodynamic properties which result in differences of formulation pressure and temperature and could influence atomisation efficiency and therefore droplet sizes and respirable doses produced by a pMDI.

A Kindeva actuator stem-block was instrumented with an Omega Ltd. fine wire thermocouple and a Kistler 601 series piezoelectric pressure sensor enabling simultaneous high-speed measurement of pressure and temperature of the two-phase pMDI formulation in the sump during a spray event. This diagnostic tool allows rapid screening of differences between propellants in real pMDIs to be undertaken.

An initial sharp drop of temperature due to cold vapour entering the actuator sump is followed by a recovery of temperature as the sump fills with warmer liquid propellent. Subsequent liquid evaporation causes self-cooling of the fluid and produces vapour which ejects the two-phase mixture via the spray orifice.

We found that HFA227ea had the overall warmest sump temperature and lowest peak pressure followed by HFO1234ze. HFA134a has a higher peak pressure than HFA152a, as predicted, but the initial temperature drop for HFA152a was significantly higher (colder) than that of HFA134a.

The final minimum temperatures measured correlate exactly with the saturation temperature of each propellant at ambient conditions. Peak sump pressures also correlated exactly with the saturation vapour pressure of each propellant at the measured temperature.

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