Compressor Question
Compressor Question
(OP)
When compressing a saturated gas, what happens exactly?
The temperature must rise to keep it at a saturated state - but what keeps the gas from simply condensing instead of the temperature increasing?
The temperature must rise to keep it at a saturated state - but what keeps the gas from simply condensing instead of the temperature increasing?





RE: Compressor Question
Molecularly, I say it this way, some energy put into a gas makes them move faster, some energy makes the molecule vibrate more. Movement is measured by pressure and vibration by temperature.
Once you compress the gas to a pressure above its criticle pressure, it's always a gas, a dense gas with physical properties close to that of its liquid state under it's criticle pressure, but still a gas.
RE: Compressor Question
There is no rule that the gas will stay saturated. As pressure increases, the gas's ability to hold water-vapor decreases (so condensation in the cylinder is possible). To more than offset this, the gas will heat up significantly while being compressed, which increases the water-carrying capacity (way more than the pressure decreased it). Typically you have gas at less than 30% of RH leaving the cylinder.
A positive-displacement compressor (like a recip or screw) will act very much like an adiabatic process (i.e., it happens without transferring heat to or from the environment) so the temperature change will be pretty close to:
T(out) = T(in) * (P(out)/P(in))^((k-1)/k)
Where temperatures and pressures are in absolute units and "k" is the ratio of specific heats.
David Simpson, PE
MuleShoe Engineering
www.muleshoe-eng.com
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RE: Compressor Question
RE: Compressor Question
HOWEVER, you will have a broken compressor (well a screw might survive). a pump or a compressor will be damaged by a phase change during compression. These things are not designed to handle the physical properties.
Now what is your application?
RE: Compressor Question
RE: Compressor Question
From the standpoint of energy into and out of a compression, the postulated phase change of vapor to liquid is exactly the opposite of what would be expected. Liquification implies a flow of energy (heat) out of the process. Compression is energy (work) into the process.
A phase change seems reasonable only because the concept of vapor pressure can trick us into reverse reasoning. A chemical engineer must never see a phase change without thinking about energy the change that accompanies it- this is the nature of our craft. Example: the above posts (entropy, temp explanations) instictively recognize that adiabatic compression must result in an increase in internal energy.
best wishes always,
sshep
RE: Compressor Question
In a depropanizer with a heat pump the tower is operated at 50 psig. The overhead is a vapor that enters a compressor. The compressor compresses the C3 to 250 psig. The hot propane gas is condened to a liquid at 120 F by reboiling the tower.
Some of the liquid C3 at 120 F and 250 psig is then sent to the top a reflux. Very efficient and the surface tension of C3 is lower making the tower smaller.