When we have a blended refrigerant entering the expansion valve, does only the refrigerant with the lower boiling point flash (assuming there is a large difference between the boiling points of each component)?
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When we have a blended refrigerant entering the expansion valve, does only the refrigerant with the lower boiling point flash (assuming there is a large difference between the boiling points of each component)?
Quote:
Originally Posted by SES Utah
I doubt it.
The largest glide that I know of in common use is nearly 7deg C with 407..
So if the refrigerant boiled at 10 deg lower than the air off it would all flash off.
cool runnings.
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The refrigerant is not one of the standards, we are making our own blend for a very specialized case. I'm not even sure if glide is the proper term because the boiling points are so far apart...
But my assumption is that the lower boiling point fluid has the be the one to flash because the lower saturation temperature is so much lower that if the upper liquid flashes it cannot stay a gas for long at this cold temperature...
Quote:
Originally Posted by SES Utah
I know of ultra low temp applications that used special blends that boil at
different temps with in the same system to create Evaporation temps of about
-160 degC. Bit complicated for me ;) but the science behind it is quite simple
and it is proven to work. I'm not sure what the blends are but when set up correct
the refrigerant evaporates at different temps within the system and it runs like a
cascade system, except it uses only one compressor?? Like I said a bit complicated for me :p.
But in answer to your question yes blends can boil at different rates if in the right
application, just don't ask me to explain how because its black magic to me. Just like
heat absorption is :D
taz
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Is that an autocascade you are describing, taz? I've been looking into that as they sound interesting but very complicated. To accurately model the behavior of the mixture takes some intense thermodynamics, which I will pass on to my Chemical Engineers!
Assuming that the blend is Non Azeotropic, the large difference in boiling points would indicate as such, the quick answer is that some of ALL components of the blend will boil. The thermodynamic answer is how much of each refrigerant will 'boil' or 'flash' will depend on the conditions at the inlet and outlet of the expansion valve. Assuming a normally operating system the temperature and pressure at the inlet of the XV will be such that all components of the blend arrive at this point subcooled. Although both pressure and temperature changes occur at all points in the XV it is easier to think of the changes that occur as being in distinct steps. Pressure change occurs first (Pressure can be rapidly adjusted) and then flashing occurs due to instability and finally temperature falls (Temperature changes by nature occuring at a slower rate than pressure. Thinking step by step then through the XV.Quote:
Originally Posted by SES Utah
Step 1 - Pressure is dropped from discharge to evaporator pressure, temperature remains constant. At this point all components exist as an unstable liquid as the temperature is above the saturation temperature of all the blend components.
Step 2 - Micro boiling 'flashing' of all components starts which requires heat which is drawn from the unstable liquid.
Step 3 - As heat is drawn from the liquid the temperature drops until the saturation temperature of the highest boiling point component is reached. At this point this component stops boiling.
Steps 2 and 3 are repeated the temperature continuing to fall which subcools the higher saturation temperature components until the saturation temperature of the lowest boiling point component is reached at which time the mixture exits the XV into the evaporator. In other words the operation is identical to how a single component refrigerant such as R134a performs in a TXV using Steps 1-2-3 once but repeating 2 and 3 for the number of components in the blend
Thanks for the walk through. As for step 3, I'm thinking of a situation where we are entering the expansion valve below the saturation temperature of the higher boiling compenent at the low pressure.
I attached a simple drawing with a very idealized TQ diagram showing what I anticipate to happen. This is what I imagine an autocascade system is...
Attachment 7315
agree with most of what aducus says, except as the system settles the latent heat exchange has not only been cooling the remaining liquid but also the txv itself, eventually will come a point where the abient temperature being transfered to the valve will not be able to keep up with the latent heat exchange from within, as time progesses there will be less and less flashing due to external temperature transfer until the point it is negligable to a subcooled liquid and flashing/boiling of a subcooled liquid moves on from the txv and into the evaporator distributor tubes and then eventually into the evaporator itself, where heat transfer can be maintained
I also think latent heat exchange is not required to lower the liquids temperature as this is simultaneous with a dropping pressure