Subject: Re: Free hot water
From: ghg@cidmac.ecn.purdue.edu (George Goble)
Date: Oct 20 1996
Message-ID: <54duas$1co@mozo.cc.purdue.edu>
Newsgroups: alt.energy.renewable,sci.energy

In article <54ank5$477@thrush.sover.net> tooie@sover.net (Tooie) writes:
 >WalRog ("walrog@GULF.NET"@pcola.gulf.net) wrote:
 >: Someone correct me if I'm wrong, but the cooling coils heat up because
 >: of the mechanical energy put into the coolant (pressurizing it) to force
 >: it into liquid form so that it may expand (vaporization) allowing it to
 >: absorb heat from the inside of the freezer.  Any device attached on the
 >: outside of the cooling coil directly will just make the compressor have
 >: to work harder to fully ensure that the refrigerant is in its liquid
 >: (vice gaseous) phase.  The extra work done by the compressor will be
 >: more than the energy recouped from the cooling coil (2nd law of
 >: thermodynamics).
 >
 >Close but no cigar, the condenser take the superheated gas and (as the 
 >name implies) and condenses back to a saturated liquid where it then expands 
 >yada yada yada.  Therefore the compressor will *not* work harder.  It may 
 >actually cycle less frequently and for shorter times since you are 
 >removing more heat on the "hot" end and -Qh = Qc - W so you would get 
 >more cooling for your work input (compressor).
 >
 >There was a protracted refrigerator discussion about 2 months ago and I 
 >remember the statement "Refrigerator makers caution against putting your 
 >fridge in an unheated garage, etc."  I don't remember the rest but 
 >someone might.
 >
 >:      IOW (in other words), give it up.  Its not feasible.  If it were,
 >: the frig would already have a device like that installed as an energy
 >: conservation feature.
 >
 >Not if it wasn't cost efficient.
 >
 >The current issue of Popular Science (I know it's my second plug for the 
 >mag but it was in the shitter at work) had a blurb about Lennox? having a 
 >similar type system for a heat pump. 
 
 
A fridge is much too small to mess with.. 1/8 Ton (HP) or so...

Central air can be from 2-5 tons (HP).

There is some "free" heat, which can be recovered right off
the compressor discharge line.  The initial hot gas off the
compressor can be 150F or more... What you do is to cool
down this gas, but not enough to condense it (desuperheat it).
If you condensed it, and needed 120F hot water, then your head
pressure would be over 400 PSIG, and a cause great load on the compressor,
the same as running on a 120F day outside.

Therefore by not "condensing", you can still recover enough heat for
domestic hot water (but not for space heating), and still use
an external condenser which operates at outside temp (or better
still on colder ground water or loop) to keep the head pressure
down (compressor load) to what it was before.  Desuperheating
the freon, but not condensing it rejects only a very small amount
of heat compared to the complete condensation process.

Various companies make "desuperheating" attachments, to get
excess superheat into hot water.
--ghg