From: ghg@cidmac.ecn.purdue.edu (George Goble)
Newsgroups: sci.engr.heat-vent-ac,alt.hvac
Subject: Re: In The Presence Of
Message-ID: <4hvcn2$d6n@mozo.cc.purdue.edu>
Date: 10 Mar 1996 20:05:54 GMT

In article <4htcvh$rh8_001@grail717.nando.net> pjm@nando.net (paul
milligan) writes:

[SNIP]
>	Fill a blender with water.  Turn the beasty on, and watch where the
>bubbles _start_.  Before any air can be sucked in, there are bubbles down
>there at the impellor.  Why ?  Can this be described as a boiling process ?
>I don't think so.  BTW, I have no idea why this phenomena occurs, and I
>really hope someone can answer it here.  Comments on the nature of 'Frappe'
>will not be considered elucitory ! :~)

Cavitation of the impeller.  The pressure around parts of the impeller
has dropped below the boiling point of water at the current temperature,
so some water flashes into vapor (the bubbles).  There also may be dissolved
air in the water which comes out..  Try this with freshly boiled or distilled
water and see if you still get bubbles?  If you shut off the blender, the
bubbles should "vanish" instead of rising very far to the top.

Liquid pumps for propane (and refrigerants) are very easy to cavitate, since
the vapor/liquid are at or close to equalibrium, and the slightest pressure
drop (a pump trying to "suck" liquid) will create huge amounts of flash
gas and and pump cavitation.  Any thing like undersized pump input lines,
or trying to suck liquid uphill or thru fine screens, etc, leads to
extreme cavitation and quickly destroys a pump.  I once had to build a
3 ton refrigeration unit to subcool liquid 40-50 degrees due to someone's
dumbass attack of running liquid refrigerant lines uphill into the pump
inlet and it was easier to build the subcooler than to get the piping
changed.  Now they listen.  THe subcooling reduced the pump cavitation.

--ghg, Inventor R-406A.
http://worldserver.com/R-406A
http://ghg.ecn.purdue.edu