JR wrote:
> Yes, higher pressure is very relevant.
> R-134A has a higher pressure because it is not as efficient as R-12, period.
> An AC system designed to use R-134A has a compressor that creates the
> necessary higher pressure. The higher pressure is required to achieve the
> required pressure drop in the orfice tube to get the desired cooling effect.
> Refrigerants do not carry heat.
HUH!!! That is exactly what they do. They pass through the system and
carry heat from the evaporator out to the condenser.
They are compressed into a liquid, then
> passed through an orfice tube or other device that quickly drops the
> pressure thus dropping the temp and creating lack of heat (cold) which flows through
> the evaporator and is transferred to the air passing through to the cabin.
They do not "create cold" They absorb the heat from the air. This is the
common reaction of refrigerants during the phase change from liquid to gas.
> At this point in the cycle the refrigerant is mostly a gas that is then sent through a
> condenser, recompressed into a liquid, and starts the cycle over again. The
> system requires a SMALLER orfice to create the larger pressure drop needed at the lower
> compressor pressures created by city driving or idling.
You are correct that the majority of refrigerant leaving the evaporator
core is a gas. That is because it changes state from a liquid into a gas
inside the evap. core. (Amazingly it EVAPORATES from liquid to gas)
Then the now hot gas goes through the compressor. The compression of the
gas makes it denser and heats it more. Now it travels into the condenser
core which removes the heat and allows the gas to condense from a
gaseous state into a high pressure liquid. Then the liquid passes into
the receiver/receiver-drier where it removes any water vapor or
contaminants which may have gotten drawn into the system through any
seals. It also collects liquid refrigerant so that any remaining gas can
condense out.
The liquid now passes through either an orifice tube or a TXV (dependent
on the system design) The purpose of either one is to regulate the flow
and pressure of liquid refrigerant into the evaporator core. The idea of
both is that given a working pressure and temperature of the liquid it
allows only enough into the system that it can ALL convert to a gas and
extract the heat. The TXV is MUCH better for this but it is also more
expensive. Either way the liquid exits the metering device and begins
changing into a gas as it extracts heat.
In the event you are using an orifice tube you will also need an
accumulator. This allows any liquid that makes it through the evap. core
to collect and prevents it from entering the compressor.
Any material that will change state from a solid to a liquid or liquid
to a gas at a temperature lower than the desired cooling temperature can
be used as a refrigerant. Some are much more practical and cost
effective than others. For instance you can compress carbon dioxide from
a gas to a liquid to a solid and it makes a dandy refrigerant. It is
also not easy to deal with when it comes to the environmental
regulations. Propane is another one that works well. However it doesn't
play well when mixed with air and heat. The results can be really
spectacular...
Water can also be used, However the solid to liquid phase is the only
one with a temperature low enough to be practical for air conditioning
in a vehicle. The conversion temperature to steam is a little higher
than folks would like to have inside the cabin.
It works well in evaporative cooling but you also require a continuous
supply of water to replace the amount that is lost. Not real practical
for a vehicle. However it does carry heat rather well and as such is
used to cool the engine. However it is kept under pressure to prevent it
from converting to a gaseous state.
> You have it exactly
> backwards, Stormin, but
> you are not alone. It's called the "Combined Gas Law", and I wish I could
> recover the time I've spent explaining it with this very simplified version
> to customers who want to understand.
> Regards,
> JR
>
> "Stormin Mormon" <cayoung61***
spam...@hotmail.com> wrote in message
> news:Y8gfr.129111$6P3.1...@news.usenetserver.com...
>> 134a does have noticably higher discharge pressure. Not sure that's
>> relevant. What is relevant, is that 134a carries less heat. So, the system
>> needs a larger orifice, to deliver more refrigerant.
>>
>> --
>>
>> Christopher A. Young
>> Learn more about Jesus
>>
www.lds.org
--
Steve W.