From: John De Armond
Subject: Re: Recreational Vehicle Type Refrigerators
Date: Wed, 28 Jul 1999 22:37:42 EDT
Message-ID: <379FBE65.A1186E9C@bellsouth.net>
Newsgroups: sci.engr.heat-vent-ac

crebel@pop.chyn.uswest.net wrote:

Funny you'd mention that.  Just go through working on one.

> 
> Does anyone have any info on these types of refrigerators; I
> assume they are some type of absorption system?  

yep

>What
> typically goes wrong when they stop working?  

They quit getting cold?  :-)

>What is the
> refrigerant type?  

Ammonia, water and nitrogen under high pressure.

>Are there any Web sites devoted to this
> type system?

Dunno.  go look.

 
> I had one guy tell me that to fix one that isn't working you
> simple remove the refrigerator from the RV and turn it
> upside down for a couple of hours then reinstall
> it....sounds to good to be true to me.  Anyone ever heard of
> this kind of refrigeration gymnastics?

Well there are no moving parts inside the refrigerant loop so if all
else fails, that's worth a try. Don't try to open the refrigeration
system unless you'd like a bath of concentrated ammonia water under
high pressure.  Unless the thing rusts through and loses its charge
(you'd know that if it happened), there's not much to go wrong
inside the refrigeration system. 

These things have to be level to work.  Older ones have to be REALLY
level.  On multi-fuel units, propane provides the most cooling,
120vac the next best and 12vdc almost none, barely enough to hold
existing cold while on the road.  I'd check for obstructions in the
flame circuit, the flue and the ventilation ducts.  The gas flame is
small, little more than a large pilot light, so a large dirt
dauber's nest could block the flame channel.  If it will cool on gas
but not on electricity, then you need to check out the electrical
circuit.  

Most of the RV fridges are swedish made Domitec brand.  If yours is
a Domitec, it has the capability of modulating the gas flow to
control the temperature.  Make sure you have a vigorous flame on the
burner.  If it is barely burning, it is on minimum fire and won't do
much in the way of cooling.  later models have electronic controls
but I'm not terribly familiar with them.  but regardless, if you
have heat on the boiler, and good airflow on the condenser you
should get cold inside.

From: John De Armond
Subject: Re: Frig repair--$600?  Why
Date: Tue, 08 Feb 2000 16:33:17 EST
Message-ID: <38A08B73.9A41CDED@bellsouth.net>
Newsgroups: rec.outdoors.rv-travel

Windrider wrote:
> 
> Fred thanks for the information. I will do that. I do want you to know that
> I don't mind paying, I just need to feel like I am not being taken
> advantage of just because I have a RV.

A conventional refrigeration service shop - even if they work on
large absorption systems - will have neither the specialized tools
nor the techniques to work on RV absorption refrigerators.  I have a
lot of experience with such systems since the company I used to own
commissioned large systems in nuclear power plants and for NASA.  I
tried recharging an RV unit once just to see if I could.  To make a
long story short, I couldn't, at least to achieve any meaningful
cooling.

The problem is the charges of ammonia, water, hydrogen and chromate
are arbitrary amounts determined by the manufacturer for the
specific configuration.  A tech cannot make a simple measurement of
pressure or temperature and know that recharging has been done
correctly.   I have never seen a listing of charge amounts and so
anyone attempting to service the unit for the first time would be
guessing.  Guessing works OK for other types of refrigeration.  It
does not for absorption units.  The second issue is, once a system
is opened to air, it is likely not salvageable.  Look at
http://www.rvmobile.com/ and click on "rebuilding process" over on
the left hand column.  They don't do all that work just to run the
cost of repair up.  They do it because experience has shown that it
is necessary in the majority of cases.

IMHO, the price these shops charge for the refrigeration package,
particularly if you do your own install, is quite reasonable.  Even
with my experience, I'd never give the slightest thought to trying
to rebuild the absorption system myself or try to cheap out have
have someone hack it together in their general service shop.

If you've followed my posts in this group, you'll know that I'll try
building or repairing almost anything.  The pressure system of an
absorption refrigerator is one of the few exceptions.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: I burped my Baby
Message-ID: <7r0dcugqa6ul4mp65ls0b0es62d5ia2vte@4ax.com>
Date: Wed, 24 Apr 2002 06:24:07 -0400

I'm proud to announce the successful burping of my baby.  RV refrigerator,
that is :-)

This is a 20 year old strictly manual three-way refrigerator.  It had
faithfully maintained the cabinet temperature at a balmy 32 degrees ever since
I've owned the rig.  A couple of weeks ago it started drifting upward.  By
last weekend, it had hit 50 inside and last night had risen to 55.

No sign of leakage so I decided to try the burp technique.  Itasca practically
embedded the cabinet in RTV as part of the mounting scheme so the couple of
hours it took to dig all this stuff out was the worst part of demounting the
fridge.

Once on the bench, the first was to disassemble the burner and chimney to
clean and inspect it.  The flue was clear but the burner tube and generator
were quite rusty.  I gave it a very good wire brushing.  Then to discourage
further rust, I gave it several coats of LPS cold galvanize.  I know from
experience that this stuff will withstand the heat and provides excellent
galvanic protection against rust.  The flue assembly was packed with very
crufty fiberglass insulation.  I replaced this stuff with some new fiberglass
insulation.

Next to the burping.  The objective is to break up any gas pockets that may be
blocking circulation and to re-dissolve any crystallized chromate solution.
There is a wide variety of suggestions on how to do this on the web.  All
seemed excessively complicated and time consuming so I devised my own.

What I did was to put the refrigerator on its side, left it there until the
gurgling stopped (NOT overnight as some of the procedures indicate), then
turned it upside down until the gurgling stopped, followed by placing it on
its other side until the gurgling stopped.

To aid the fluids' return to their proper places, I then rocked the 'fridge
between lying flat on its (looking from the rear) right side to just a bit
past upright.  All the plumbing is on the right on my unit so this was
designed to let the fluid flow freely.  On each move, I again waited for the
gurgling to stop.

The final step was to sit the cabinet upright and let the gurgling stop.  I
then hooked up propane and lit the burner.  I placed a data logger in the
cabinet along with a gallon jug of water, my standard test condition.

When I first got the rig, one of the tests I did on it was to run the data
logger and measure the cooling capacity (by looking at the delta temperature,
if anyone here cares).  I set up the same conditions and am proud to say that
the unit now cools a bit better than it did 4 years ago when I bought the MH.

Notice that I did NOT leave the thing lying on its side overnight, as some
burping procedures recommend.  I could see no logical reason for waiting that
long.  In any event, I decided to do the short program, knowing that I could
always go back and do the long program if the short one didn't work.

A half hour to stick the thing back in the rig, light that puppy off and set
the thermostat.  It's now sitting there right at 32 degrees like it's supposed
to.

I love it when a plan comes together....

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: I burped my Baby
Message-ID: <flafcus1pbn85c7rkgopptt40cjbd40mmh@4ax.com>
Date: Thu, 25 Apr 2002 03:09:48 -0400

On Thu, 25 Apr 2002 02:50:43 GMT, "No Spam" <No@Spam.com> wrote:

>Nice Job!
>I have read several times that this doesn't work.

Well, you know what they say about things you read on the net :-)

>Good to see it worked for you.  Especially after all you went through.

Actually I had called to see if the $CW$ in Nashville had a new one in stock
:-)  But either way the fridge had to come out so little labor lost.  I
figured it was worth a try.  It'll be interesting to see how long the burp
lasts.  The 'fridge is almost exactly 20 years old so I figure it's done its
duty.

>I'm so dam lazy I would have left the thing in the trailer and turned the
>trailer on it's side.
>I guess it pays to do it right.

I thought about that.  But in my MH, I'd have had to stand the darn thing up
on its nose and that might have turned into actual work.... :-)

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: I burped my Baby
Message-ID: <vhiscuohgqn2ilrf277afvv6evplqqhjro@4ax.com>
Date: Tue, 30 Apr 2002 03:40:19 -0400

On Sun, 28 Apr 2002 03:40:43 GMT, "TBFisher" <tbfisher@swbell.net> wrote:

>  John, do you think that it would be
>practical to build an absorption AC
>system for an RV? Looks like Honda
>and others have considered it:
>http://world.honda.com/news/1998/p980312.html
>http://www.pnl.gov/microcats/fullmenu/compheatpump.html
>Tom Fisher
>Dallas,TX

Possible?  Probably.

Practical? Probably not, at least not now.

First thing to understand is that very, very little of what comes out of the
national labs has any commercial viability.  Funding the nat labs now consists
of little more than scientific welfare.

Honda doesn't tell us enough about the unit to see if it is practical or not.
Conventional absorption systems are quite inefficient.  Not an issue when
running on exhaust heat but very important when fuel powered.

I have a friend who owns a farm up in Kentucky who has several gas wells.  In
other words, his energy is free.  He has two approx 3 ton absorption AC units
on his house and shop.  I can't quote figures from afar but I can say the
things are horribly inefficient.  Of no concern there, of course.

I think we'd be a lot better off if effort were to be put into a system
similar to what the trucking industry is gradually adopting to end idling.
These systems involve a small engine, a 12 volt alternator, a refrigeration
compressor and if operation on shore power is needed, an electric motor.
Always seemed odd to use a generator to turn rotary motion into electricity,
only to turn it back into rotary motion in the AC compressor.  From the units
I've seen at the trucking shows, the things aren't any larger than a typical
RV generator.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: I burped my Baby
Message-ID: <brbucusv55nqa14npi8c1n4ut3ag6ijgrj@4ax.com>
Date: Tue, 30 Apr 2002 20:07:34 -0400

On Tue, 30 Apr 2002 12:45:56 GMT, "TBFisher" <tbfisher@swbell.net> wrote:

>  So, how many HP are required to power
>an AC compressor for a 30KBtu/hr system?
>Tom Fisher
>Dallas,TX

The basic measure of efficiency is the COP or Coefficient of Performance which
is the number of watts pumped divided by the number of watts consumed.
Measured under conditions specified by ASHRAE.  A COP of 3 is pretty good for
an air conditioner and can roughly be equated to a SEER (seasonal energy
efficiency rating or COP modified by a bunch of fuzzy seasonal factors) of 10.
For a given refrigerant, the only way to improve COP is to make the condenser
larger and/or more efficient.

The problem for RV units is there is not enough space to achieve a high COP in
a package unit.  If we deviate from the package unit format so that a larger
condenser and higher air velocity could be used, we could improve the COP to
the equivalent of high efficiency home units.  That we could either get more
cooling capacity or else reduce the power drawn at the same cooling capacity.

I've never measured my AC's COP.  Guess that would be an interesting thing to
do sometime.  I'm going to bet that it's down around 2.25-2.5.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: I burped my Baby
Message-ID: <l4oucu4if0pq96lq0qcreb56u0qek90a6b@4ax.com>
Date: Tue, 30 Apr 2002 23:40:22 -0400

On Wed, 01 May 2002 01:35:20 GMT, "TBFisher" <tbfisher@swbell.net> wrote:

>  So assuming a COP of 3 would give 3.92594 horsepower
>needed to produce 30KBtu/hr of cooling.
>Tom Fisher
>Dallas,TX

Sounds about right.  Rule of thumb is 1 HP per ton of cooling.  BUT!  The
problem still remains of having enough condenser area to achieve this level of
efficiency.  Take a look at your home unit and see the size of condenser on
it.  Unless your unit is very new, it's likely not much better than SEER
10-12.  The ultra-efficient SEER 13-14 units are HUGE for the capacity.  I
think that obtaining a COP of 3 on an RV would be a major trick.  I'd have to
run some numbers to be sure but my gut tells me so.

The next issue is noise.  Given the size limitation on an RV, the other way to
achieve reasonable efficiency is to move more air across the condenser.  If
you've ever had to sit near a home central unit, you know what kind of noise
I'm talking about.  My last home had the condensing unit right under the deck.
I HATED that unit!!!!  I really don't like listening to the fan in my little
Coleman unit which means that it's turned off most of the time I'm not
actually inside.  I'd hate to think of the noise from a large, high efficiency
unit.

Absorption type cooling would be even worse.  Plus the absorption process is
much less efficient than the carnot cycle stuff.  At least for existing
technology.

The advantage to be had with an engine-driven compressor is that the engine no
longer has to run at a synchronous speed (3600 or 1800).  Experimenting I've
done with my cordless battery charger project has shown me that the sync
speeds are particularly annoying, probably because we all are subjected to 60
hz buzz so much in our daily life.  Dropping the speed of my CBC even a few
hundred RPM makes it a LOT less annoying.  You could use a low speed, high
displacement engine to get the necessary power.  Using a water cooled engine
would make it even quieter.  This is what I'm planning for when I start
building my custom coach.

John

From: John De Armond
Newsgroups: rec.outdoors.rv-travel
Subject: Re: Propane or AC for refrigerator
Date: Fri, 06 Jul 2007 23:56:52 -0400
Message-ID: <u93u83do78697ea605i0gs0b2ps90fhtng@4ax.com>

On Fri, 6 Jul 2007 20:34:21 -0700, "Sorobon" <Sorobon@AOL.com> wrote:

>"Robert Bonomi" <bonomi@host122.r-bonomi.com> wrote in message

>> The absorption process is somewhat more efficient than the
>> compressor-based way.
>
>I don't know if an absorption refrigerator is more efficient than a
>compressor type, I suspect it is not, but it is not as effective

Absorption is far less efficient by a factor of >5 or so.

The COP (coefficient of performance - BTU moved/BTU consumed) for absorption is
typically from 0.5 to 0.7*.  The COP of a typical compressor refrigerator operating
under ASHRAE standard conditions is between 2.5 and 3.0.

* http://geoheat.oit.edu/pdf/tp51.pdf

The only reasons absorption refrigeration is so prevalent in RVs are that a pound of
propane contains so much more energy than a pound of battery and because the
operation is silent.

John