careful, Glenn, the "heat removal" figure you see listed for marine reefers are
a.) TOTAL heat removal *****including****** the heat added by the compressor
(in other words NOT the heat removed from the ice box), and
b.) **IF** you believe the cooling figures given you ALSO believe you can make
26 pounds of ice per 8 hours and use just 70 amps to do it.
75 pounds of ice per day?????????? On a total of 9 amps per hour?????????
those are the figures you quote below. Really. Sound like you have a cruising
income source ready at hand. just buy a Briggs and Stratton engine
(HarborFreight.com) for a couple hundred, and a used alternotor from an auto
junk yard for fifty bucks and start churning ice you can sell to other cruiser.
Sell them the canvas bags to carry all that ice back to their boats and make
some extra money as well.
N. Bruce Nelsen did a number on ya, Glenn. Sorry.
Actually $1,000 of vacuum panels will just about completely insulate my 4.5
cu. ft. freezer to R50 and ajoining 7 cu. ft. fridge to R28. (Plus another
R12 for the iso sheets that I am wrapping the panels in.)
Why think six packs??? I am thinking in the case range. A case of brew
takes 750 BTUs to get it from dock side 80F to whistle wetting 40F. That
was built into my calculations from the start along with another 800 BTUs
for opening and closing the box to get to the beer. No insulation is going
to help that. But the base heat gain without adding anything or opening at
an ambient temperature of 95F with vacuum insulation will be about 2,300
BTUs. Total load right at 3,800 BTUs worst case. That is about 8 hours
and 70 amp hours on a BD50F compressor. I don't have room for more than 2"
of insulation so foam would increase the heat load to about 6,400 BTUs plus
the 1,550 BTU beer load.. That would be 14 hours and 110 amp hours on a
BD50.
To add enough foam insulation to equal the vacuum panels would increase the
volume of my box by about 40 cu. ft. Considering that the interior volume
of my boat will cost about $60/ cu.ft. not counting my labor, saving 40 cu.
ft. is worth more than twice what the vacuum insulation will cost.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
"JAXAshby" wrote in message
...
Glen, I was trying to say that $1,000 a square foot "vacuum" panels don't
stand
up to close scientific scrutny. Buy 'em as is your wish.
btw, have you checked just how much cooling power is required to chill a
air
temp six-pack or two as compared to how much cooling power is required to
remove heat passed into the reefer from the outside. hint: the difference
in
total BTU's of cooling required is almost nothing when comparing R-4 with
R-8,
and is virtually nothing when comparing R-8 with R-100. It ain't the heat
going through the side that gets you, it is the heat you put into the
reefer in
the form of food.
something N. Bruce Nelsen kinda neglects to make mention of.
but if you feel you want to hug P. T. Barnum, have at it.
Jax, you still don't understand the principle that it is better to keep
your
mouth shut and have people think you are stupid than opening it and
confirming the fact.
Most vacuum panels are filled with Instil, an inert open celled silica
based
foam board from Dow. It has a crush strength of about 50 PSI, well above
the 14.7 atmospheric pressure, and an R value at standard pressure of
about
5. At 1 milibar vacuum the R value is about 25 and at .1 milibar it is
about 30. Glacier Bay uses a special reinforced aerogel material called
Nanogel made by Cabot (the Cabosil people) with an R value at standard
pressure of about 15. While the bare material is extremely delicate its
latticework structure makes it very strong under even compression. At 10
milibar the R value is about 30 and at .1 milibar it is about 50. The
curve
of vacuum to R value is flatter with Nanogel than with Instil. The
obvious
advantage is that as the panel loose vacuum the Nanogel will maintain
more
insulation value. The disadvantage is that Nanogel is much more
expensive
and harder to work with.
The problem with all vacuum insulation is that it is impossible to
maintained a high vacuum with a low conductivity flexible membrane. Air
molecules will slowly find a way in. Back in the 80s
a vacuum panel made of a stainless envelope packed with precipitated
silica
was popular in refrigerated containers and some high end European
refrigerators. It was only effective in large sizes because the steel
conducted a lot of heat around the edges. In the mid 90s Toyo and Dupont
developed films consisting of several layers of various UHMW plastics
coated
with a very thin layer of aluminum that made smaller panels practical and
easier to fabricate. Those films have been greatly improved over the
last 5
years. Around the same time SAES introduced a room temperature getter
material to absorb stray gas molecules and packages it in small
inexpensive
pucks to be inserted in the panels.
The net result is that you can reasonably expect 10 to 15 years of R
values
better than 25 per inch from almost any well constructed vacuum
insulation
panel. The Glacier Bay Panels will last about 30% longer and have the
distinct advangate of maintaining a reasonable level of insulation even
with
no vacuum. The down side is that they are about twice as expensive.
Regardless, marine refrigerators made with vacuum insulation should be
built
with the need to eventually replace the panels in mind. I am counting on
8
to 10 years and will probably replace them even if they are still
reasonably
effective. At the present rate of improvement by then the technology
will
be far better and the prices significantly lower.
BTW, you should NOT use two part pour in place foam to fill gaps between
vacuum panels. Two part foams produce a lot of heat as they cure. The
vacuum panels are so effective that they will trap the heat and possibly
damage the plastic film. Moisture curing spray foam like Great Stuff is
a
lot safer.
--
Glenn Ashmore
I'm building a 45' cutter in strip/composite. Watch my progress (or lack
there of) at: http://www.rutuonline.com
Shameless Commercial Division: http://www.spade-anchor-us.com
"JAXAshby" wrote in message
...
Rusty, you take measurements for a living, but are not a design eng. a
couple
of things to make note of:
1.) "shiney" is shiney from both sides, as far as radiation is
concerned.
shiney out or shiney in, same same.
2.) shiney on the outside does NOT make for greater (or lesser)
conductivity
or convectivity. shiney on the outside makes for reflection of the
radiant
heat **from the outside** (where heat is in a reefer system). shiney
on
the
inside means some of the radiant heat is absorbed on the way through
the
insulation (makes for warmer insulation) and then is reflected back
into
the
insulation where some of it is also absorbed (making for even warmer
insulation).
3. You, Rusty, sound like a shill for N. Bruce Nelsen of Glacier Bay, a
man who
over the years has made one hell of a lot of claims that don't stand
close
examination.
4.) "vacuum" panels are not vacuum at all, but rather are panels with a
plastic
latice inside (to hold the sides of the panel apart) with much, but by
no
means all, of the air removed. (air pressure is 14.7 pounds PER SQUARE
INCH,
so a 1 square foot panel encasing a true vacuum would have over 2,000
pounds
pressure trying to collapse the sides.