Quote:
Originally Posted by jeff5may
Ok, so I revisited the articles to gain clarity. I was thinking like BradC and things just didn't make sense. More suction pressure = more mass flow = more capacity, right? Hmmm...
Then I read this:
"...the high solubility of mineral oils in propane could represent a problem, especially for applications such as heat pumps displaying high suction pressures... the experimental results reported in the present work can therefore be explained by means of the strong influence that the propane solubility could have on the sealing effect of the lubricant between piston and cylinder... the solubility is shown to strongly decrease by increasing the superheating; as a consequence, the viscosity of the oil strongly increases with superheating until reaching a maximum at around 30-40K superheating... the suction superheat would have a positive influence on the volumetric efficiency."
and from the other article:
"Refrigeration capacity and COP differs only slightly from R22
data. However, the suction superheat with R290 has essentially
a greater influence (see Fig. 4). In other words: R290
profits in capacity and efficiency from useful superheat, the
use of a heat exchanger between the suction and liquid lines
is therefore an advantage.
R290 has mainly favourable thermodynamic properties and
low energy requirements during compression. Pressure levels
and volumetric refrigeration capacity values are very similar
to R22, however, there is a large difference in enthalpy,
density, mass flow and isentropic compression exponent
(operating temperatures)...
R290 has an extraordinary high solubility with conventional
lubricants and Ester oils. This characteristic is of course
desirable for the oil circulation in the system. However, it
can lead to a considerable decrease of the oil viscosity in
the compressor especially at low oil temperature and high
suction pressure. In addition to this, there is a strong degassing
effect in the crankcase and lubricating spaces
which is, amongst other things, due to the enormous volume
change with the evaporation of R290. This leads to
high oil carry over (foaming), reduced performance and
stronger wear on the moving parts (also see para. 3.2)...
Expansion valves should be specifically designed for R290.
The use of R22 valves is of course possible but at higher
evaporating temperatures it can lead to insufficient superheat
(different pressure / temperature relationship). A corrected
superheat setting would then be necessary.
Attention!
The minimum discharge gas temperature should be at
least 20 K (try to attain 30 K) over the condensing temperature.
(see information in para. 2.1)"
Then it hit me. R290 has MUCH more heat capacity than R22. How else could it transfer the same heat with only 40% of the mass? It also expands more than R22 when it evaporates. So in my case, running at 10-15 psi less suction pressure is actually giving the propane more time and dT to pick up heat in the evaporator. Meanwhile, my beer oil is now flat once it reaches the compressor. Both effects add to efficiency in their own way.
Since hardcore research into propane and butane refrigerants is just now being approved, there are a lot of blanks in the lanscape. But the wizards are all saying that the standard mass flow enthalpy density energy flux capacitor application models are going to be different for R290 systems than their current refrigeration law books dictate. What this means to me is that they will be hammering out new laws for propane in the not so distant future. But their crystal balls and cauldrons are beginning to show some images to interpret and conspire upon.
Meanwhile, ima hack.
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I was wonderng that maybe this doc. thesis could give some more ideas
rgd. r290- stuff and design parameters of the equipment at small size.
Not easy to dig up but pls. google first:
ISRN KTH/REFR/07/58-SE
and then you get this kind of view:
[PDF]
Experimental Investigation of Refrigerant Charge ... - DiVAkth.diva-portal.org/smash/get/diva2.../FULLTEXT0... - Käännä tämä sivu
Annoit tälle +1. Peruuta
Tiedostomuoto: PDF/Adobe Acrobat - Pikakatselu
kirjoittanut
W Fernando - 2007 - Viittausten määrä 2 - Aiheeseen liittyviä artikkeleita
SE-100 44 Stockholm, Sweden. Stockholm, February 2007. Trita REFR Report No. 07/58. ISSN 1102-0245.
ISRN KTH/REFR/07/58-SE. ISBN 978-91-7178-569- ...
.... and click again ... and
allow the file get loaded!