Paul, My comments are interspersed, as some things in this post are not fully
correct.
Bill.
> > My point was centrifugal pumps can be stalled by too much back
> > pressure and cavitate.
A centrifugal pump WILL stop giving flow at some pressure, the term stalled
is a good one, basically the pump is churning the water, and heating it some.
However, it is not cavitating. Cavitation occurs on the suction side when
the pressure is brought below the vapor pressure of the fluid, and vapor
bubbles form. Dissolved air can have the same effect, but usually it is
vapor. One would expect cavitation if the INLET were restricted, not the
outlet.
>> A positive displacement pump will excessively
> > load down the driving mechanism if the restriction is too high. In
> > some cases an over loaded positive displacement pump can shear the
> > input shaft or do other damage to the mechanism. I have had it happen.
> > I suspect that is the reason for the pressure relief valve on the
rotary
> > oil pump. I digress. In any event air conditioner *compressors* are
> > designed to drive the fluid through the highly restrictive evap
> > cores or coils.
> >
> > Paul Lamar
> Your point is correct Paul, I missed that point.:-)
> Yes the compressor is pulling a low pressure area at the evaporator, and
the high side is pressurizing the refrigerant up to the metering
> device where it goes thru the restriction and flashes into a low temp
vapor and pickes up the heat.
> Royce
A small but important point. The restriction on the metering device at the
inlet of the evaporator core causes a pressure drop. At steady state, the
refrigerant is not always "flashing" as it goes through the metering
restriction, liquid is flowing through, and assuming a lower pressure, but
the evap core has liquid, which is then boiling in response to the heat load
and the compressor suction value. The core is running in a partially
"flooded" mode, unless you are very low on refrigerant, in which case the
liquid does flash to vapor across the restriction, and you end up icing up
the evaporator core.
The over all process is the reverse of boiling water. In that case
you add heat to convert the liquid to a steam vapor.
No, the process is *identical* to boiling water, just at a different pressure
and temperature. You are adding heat (from the air) to boil a
low-boiling-point liquid. The same cycle can work with water as the working
fluid, just not as efficiently.
Any time you
have a state change, liquid/vapor or liquid/ice, a tremendous
exchange of heat is involved.
True.
To add a bit of clarification on the process the evaporator core's
job is to convert the cold vapor back to a warm vapor extracting heat
from the air passing through the core in the process.
The vapor leaving the core is very close to the equilibrium temperature that
is set by the boiling temperature of the refrigerant at the pressure
maintained by the compressor suction.
Then the hot
vapor is passed through another heat exchanger at the front of the
car to get rid of the excess heat before it goes back to the compressor
to be converted back to a liquid.
Actually the compressor works on the vapor coming from the evaporator core
(which is still cold), compresses it to a high pressure (which raises its
temperature) and then passes the high pressure vapor through the condensor
coils to convert it back to a liquid.
The reason the core is so dense compared to a water rad is the heat
transfer coef. between a metal and a vapor is not as good as the
heat transfer between a liquid and metal. The tubes in evap core
can be small as it is easier to pump vapor through small tubes than it
would be to pump liquid through small tubes. Therefore for a given
core volume you can have more tubes. The reason the core is thicker
is solely due to severe space constraints under the dash. It would work
better, just like a water rad works better, if it is thinner. You
can see that phenomenon in room air conditioners that enjoy no
such severe space constraints.
Paul Lamar
Bill Schertz
KIS Cruiser #4045
Thanks Bill for the excellent explanation and corrections.
Sounds like you are very knowledgeable and involved in engineering
air conditioners.
Paul Lamar
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