|
|
re: ice chest effective conductance
19 may 1997
wrote:
>a friend, who is going on a long rafting trip, asked me about the optimal
>way to pack a very large ice chest with dry ice.
suppose the very large chest is 2'x2'x 8' long, with 72 ft^2 of r10 walls,
eg 2" of styrofoam, with a thermal conductance of 7.2 btu/h-f.
>the general consensus was that the dry ice should be packed in a small
>styrofoam ice chest inside the big one. optimally, we could get just the
>right gradient so that the food stayed frozen near the dry ice, and cold
>but not frozen at the far end.
a correction:
dry ice sublimates at about -109 f. here's an equivalent thermal circuit:
r 1/7.2
-109 f -------www-------www------- 70 f
|
40 f
making the larger cooler 40 f, 40 = 70 - 1/7.2 x (70-(-109)/(1/7.2 + r), so
r = 0.685, ie the dry ice cooler needs to have walls with an r-value of about
0.685x16ft^2=11, eg r10 walls with a few thick spots, or it could be smaller,
with r10 walls.
peter carmel wrote:
>i'd divide up the chest into three compartments. put the dry ice on one
>side, frozen stuff next to that, and cold food farthest away. use an
>insulating material to divide the compartments, such as wood.
this sounds less efficient, since the dry ice container loses some heat
through its walls directly to outdoors, vs through the walls of the outer
and warmer cooler, and the warmer frozen food stores about 0.5x(20-(-109))
= 64 btu/lb less coolth in this arrangement, although freezing the food (on
the night before the trip?) takes less time and energy. in either arrangement,
the frozen food to be used each day could be thawed in the 40 f cooler,
without any thermal penalty.
the first arrangement needs about 24(70-40)7.2 = 5k btu/day to stay cool,
21 pounds of dry ice per day at 245 btu/lb. local dealer dry ice corp. sells
10.5x10.5x12" blocks weighing 50 lb for $21.15 each.
the second arrangement might look like this:
.---------------------------------------.
/ / / / |
/ 2' / 2' / 4' / |
---------------------------------------- | 2'
| | | | |
| -109 f r 20 f r 40 f | |
| dry ice d frozen f refrigerated | /
| | | | / 2'
---rdw------rfw-------------rrw---------
with a thermal circuit like this:
(20 f)
rd | rf 0.28
-109 f ---www------www-------www------- 70 f
| | |
w w (40 f)
w 1 w 1
w w
| |
------------------------------ 70 f
if this were my rafting trip, i might add an extra 2" of styrofoam inside the
dry ice compartment, on every side except the one facing the frozen food (rd),
making the thermal conductance from dry ice to 70 f outdoors 5x4ft^2/r20 = 1
= rdw. an inch of foam on the inside of the freezer makes rfw = r15/16ft^2
~ 1, and rrw = r10/36ft^2 = 0.28. the heatflow through rf is (70-40)/0.28 =
108 btu/h, so rf = (40-20)/108 = 0.185, which corresponds to an r-value of
0.185x4ft^2 = 0.74, eg 3/4" plywood. another (70-20)/1 btu/h flow through rd,
so rd = (20-(-109))/158 = 0.816, corresponding to an r-value of 0.816x4ft^2
= 3.3, eg a 2x2' piece of 1/2" foam glued to another piece of 3/4" plywood.
it looks like the dry ice in this arrangement gains (70-(-109))/1 = 179 btu/h
through rdw and another 158 through rd, a total of 337 btu/h or 8,088 btu/day,
ie about 33 pounds of dry ice per day.
nick
|
|
