|
|
re: dry ice: anyone know about it?
27 jul 2000
rick matthews wrote:
>would you explain this sentence to me please?
>---------------------
>"a 2' 32 f cubical cooler with 3" us r15 styrofoam walls containing an
>l' dry ice cube with 1.5" r7.5 walls absorbs less heat than a 2' -109 f
>cube."
>--------------------
>it appears to be double-speak to me. does the cooler have 3 inch walls
>or 1.5 inch walls? what is a "l' dry ice cube"?
the cooler is a 2' cube with 3" walls. a smaller cooler inside contains
some dry ice, so the space between the two coolers can be about 32 f vs
-109 f. using two coolers makes the dry ice last longer. the temperature
between the coolers can be lowered by decreasing the thermal resistance
of the inner cooler by increasing its surface or decreasing its wall
thickness. i suggest "an l foot cube with 1.5" walls," then calculate l
to make the space between the coolers 32 f when it's 90 f outdoors.
>would you please explain this:
>--------------------
> r7.5/(6l^2) r15/24ft^2
> -109 f -------www-------www------- 90 f
> |
> 32 f
>--------------------
that's an analog dc electrical circuit called a "resistor divider,"
2 resistors (---www---) in series. high school physics (easy for an
11 year old girl i met at a science fair, who was quietly sitting
beside her physics project, reading a college textbook :-) the one
on the right (15/24 "ohms") is the outer cooler. the one on the left
is the inner cooler.
>now the final sentence:
>--------------------
>"on a 90 f day, (90f-32f)/(15/24) = 92.8 btu/h of heat flows into the
>cooler,
that's ohm's law for heatflow. the heat that flows in through the outer
cooler is the temperature difference divided by the thermal resistance.
>so (32-(-109))/(7.5/(6l^2)) = 92.8 and l = 0.91',
that same heat also flows into the inner cooler, which determines l,
after a little high-school algebra.
>with a 7.88" id, enough to contain 18.4 pounds of dry ice,
if the outer dimension is 0.91 feet, ie about 10.88 inches, the inner
dimension is about 7.88 inches, after subtracting 2 1.5" walls, no?
and the volume is 0.29 ft^3, which can hold 0.29ft^3x65lb/ft^3 = 18.4
pounds of dry ice containing 18.4x245 btu/lb,
>ie 4509 btu of coolth,
>which can keep the ice cream frozen for 4509/92.8 = 48.6 90 f hours."
>--------------------
>this refers to a cooler; is this the 2' cooler previously mentioned?
yup.
>is there room in that cooler for 10 half gallons of ice cream with
>the dry ice? it doesn't sound like it.
let's see. the outer cooler contains 18^3 = 5832 in^3, and the inner one
uses up 10.88^3 = 1288 in^3, the difference being 4544 in^3 or 2.63 ft^3,
enough to hold about 2.63x8 = 21 gallons or 42 half-gallons of ice cream.
(the answer is always 42.) you might reduce the size of the cooler and
adjust the dimensions to make the half-gallons fit neatly inside, using
a cut $16 4'x8'x1.5" styrofoam sheet held together with deck screws and
aluminum duct tape.
nick
|
|
