# Vapor Pressure Measurement Problem

## Main Question or Discussion Point

Hi all,

Just have a quick question regarding the measurement of vapor pressure.

Firstly, let me explain my scenario. I am carrying-out some measurements of steam condensation in a vacuum. The steam is being condensed in a air-cooled heat exchanger.
Initially, I let in a quantity of steam (at atmospheric pressure) into my heat exchager. I then close off the inlet & exit valves to trap a quantity of steam in the heat exchanger (creating a closed system) and commence the air flow. Thus there is a fixed mass of steam in my heat exchanger. The energy from this steam is transferred to the surrounding air flow and eventually, the temperature of the steam (vapor) decreases until it reaches the ambient air temperature. With no air leaks occuring, the pressure I measure also reduces to the saturation pressure for that given ambient temperature.

Firstly, am I right in assuming that at this point I have a vapor-water interface in my heat exchanger, which is in equliibrium (rate of evaporation=rate of condensation) at the given ambient temperature? Once the system has reached steady-state of course?

Secondly, if I recommence the steam flow into the system, I see that the pressure I am measuring increases...why is this? Should the steam entering the system not be under a vacuum and have a temperature and pressure determined by the ambient air temp?

Thanks for any help

## Answers and Replies

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SteamKing
Staff Emeritus
Science Advisor
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By definition, if your heat exchanger has a liquid-vapor mixture inside before you admit more steam, there is no vacuum present. The pressure may be small, relative to atmospheric, but it is not a vacuum. Admitting more steam can only raise the pressure until further condensation occurs.

Obviously there is no vacuum, to achieve a vacuum is impossible. What I meant is that the system is operating in sub-atmospheric conditions, which some people refer to as vacuum conditions.