What metal (or other fairly common substance) retains COLD the longest?
I was thinking of experimenting this hot Summer with placing various chunks of
metal in my freezer and...more or less using them like ice. Putting them in my
cooler and seeing if they kept items nice and cold --without the side effect
which ice has (melting to liquid).
If I'm not making myself clear, I mean...would iron or steel or brass or lead
or what metal would likely "hold its cold" the longest?
Surely your cooler should keep things cool without needing lumps of metal in
it.
However, to answer your question, Heat absorbed = specific heat x mass x
temperature rise
The metal with the largest specific heat will remain cool the longest
assuming that it's shape and thermal conductivity are not limiting things.
A better way is to involve a phase change like freezer blocks. The advantage
of ice is that it absorbs a lot of heat just by melting. Get a flexible
container, put some water in it and use that.
John
On 20 May 2004 09:27:51 GMT, in sci.engr.mech snipped-for-privacy@wmconnect.com
(AlWahrabi) wrote:
Excluding phase change, for a given volume, it will be the material with
the highest value of density times specific heat. Copper is a good one.
________________________________________________________
Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)
http://EdwardGRuf.com
Excluding phase change, for a given volume, it will be the material with
the highest value of density times specific heat. Copper is a good one. Ed
Ruf>>
So, you're saying: copper (once placed in the freezer and chilled overnite) is
a metal that will take a LONG time to lose its cold temperature? Longer than,
say, iron or steel?
I appreciate the several replies, and in the meantime I am reviewing old
postings. It seems like at least 2 parameters are at work: Specific Heat AND
Thermal Conductivity.
Each year, when I buy dry ice for cooling in my vehicle, I notice that dry ice
is a very POOR conductor of cold. Dry ice appears to have to be directly ontop
of something, in order to chill it. Something one inch away from the dry ice is
warm, while...whatever it is directly in contact with it is super-chilled...but
only at that one point of contact with the dry ice.
On Fri, 21 May 2004 01:17:06 GMT, in sci.engr.mech Brian Whatcott
Ahem, got a frog in your throat? If you actually read my post, I said per
unit volume. Given the density difference between copper and aluminum, it
wins hands down. One of the reasons the heatsink copper scramjet models we
test at work are made of copper.
________________________________________________________
Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)
http://EdwardGRuf.com
Ed,
I cut out the personal invective from your interesting response above,
and offer this explanatory note. Hope it helps.
I'd like you to take the opportunity to think a little more about your
response.
1) TERMS
"Specific" is a keyword in physics for "per unit mass"
so...
Specific heat times density means
heat capacity / mass X mass/volume
heat capacity per volume.
Your sentence "For a given volume, the material with highest specific
heat times density" [gives the highest heat capacity]
means
"For a given volume, the material with highest heat capacity per
volume" gives the highest heat capacity.
I think you can see that this statement is not a deduction or
conclusion, but a recasting of terms.
2) FIGURE OF MERIT.
... more importantly, if you are going to fly something as an
engineering proposition, you work out a figure of merit.
Where the desired objective is maximizing heat capacity, it costs
something to get the item aloft, and that cost is mass.
So the reasonable figure of merit in your ram-jet case is heat
capacity per mass which is - guess what? called specific heat
capacity.
That's why recip valve stems are sodium filled, and engine cases not
exposed to high temperature are...aluminum. They are not ever copper,
which has a lower figure of merit for the application.
Sincerely
Brian Whatcott Altus OK
On Sat, 22 May 2004 00:19:45 GMT, in sci.engr.mech Brian Whatcott
and
Yes, it is a tautology. I was trying to point out in the freezer problem
the major constraint one has is volume, not mass (within reason, of
course). Which your other reply suggesting water filled bottles would seem
to follow. Given a freezer filled with food, one is space limited, no?
If this didn't come across in my posts in the wee hours of the morning, it
is my fault.
FWIW, Though Cu is better than Al, as I said in this regard, carbon steel
is a bit better than copper as well. The OP asked about metals, but even
neglecting the phase change, water is still better than common metals in
this regard, too. Adding the phase change and water wins hands down. Which
is why the original icebox came to be, no?
As to your figure of merit. You have chosen this as your figure of merit.
It need not be the figure of merit for every application, even if it is
that for some.
As far as our models, volume limited total heat sink capacity is of major
importance. Another is the ability to survive very high heat fluxes. You
will find OFHC (Oxygen Free High Conductivity) Cu is used as a standard
material in environments such as this. Where more strength is needed
Glidcop, a alumina dispersion strengthened copper alloy is used.
For example see:
http://hapb-www.larc.nasa.gov/Public/Engines/3-strut/3-strut.html
http://hapb-www.larc.nasa.gov/Public/Engines/Engine_A/Engine_A.html
http://hapb-www.larc.nasa.gov/Public/Engines/Sxpe/SXPE.html
http://hapb-www.larc.nasa.gov/Public/Engines/CDE/CDE.html
http://hapb-www.larc.nasa.gov/Public/Engines/Hxem/Hxem.html
http://hapb-www.larc.nasa.gov/Public/Engines/Hxfe/Hxfe.html
Yes, the above is ground test hardware. I did not necessarily say fly in my
post. However, the HXFE test hardware is a spare flight engine. The actual
flight engine hardware flown at Mach 7 on March 27, 2004 was the same,
almost entirely made from Glidcop, as can be seen at:
http://www.dfrc.nasa.gov/gallery/photo/X-43A/HTML/EC99-45265-11.html
http://www.dfrc.nasa.gov/gallery/photo/X-43A/HTML/EC99-45265-22.html
http://www.dfrc.nasa.gov/gallery/photo/X-43A/HTML/EC99-45265-23.html
It performed as designed, with the vehicle accelerating while in a climb.
As with any problem, one must be sure of the design requirements and
constraints, as these will define the bounds of the possible solution
space. Surely, sodium is outside the solution space of the problem posed by
the OP, no?
________________________________________________________
Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)
http://EdwardGRuf.com
replying to Ed Ruf, Johnathan Standridge wrote:
Everyone is debating how long any metal will stay cold, but I wonder, will
copper leach out into the drink, and if so, is that really safe? Just to be on
the safe side, I'd say iron or someone said porcelain. Personally, I'd love to
see someone make some neat porcelain drink coolers.
longest...John
Using google advanced and searching for
all words: density metal table
exact phrase: specific heat
URL:http://metals.about.com/gi/dynamic/offsite.htm?site=http://www.apo.nmsu.edu/Telescopes/SDSS/eng.papers/19950926%255FConversionFactors/19950926%255FMProperties.html
They call specific heat "heat capacity", for some reason. As John Manders
said, you would look for some mass times the specific heat, or on a unit
volume basis, density times specific heat.
You may also want to consider "metering" the cold out slowly (thermal
conductivity), so that it feels cool longer, but doesn't freeze you out for
a short period of time.
David A. Smith
you won't be impressed with the results of your experiment--it's the phase
change from solid to liquid that makes ice a good source of cooling.
to put things in perspective, assume that you cool 1 pound chunks of copper
and ice to 0F.
Question: how much energy do you have to put into each of them to heat up to
60F??
Answer: Copper: 5.64 BTU; Ice: 188.1 BTU
The ice absorbs about 33 times as much energy as the copper.
you won't be impressed with the results of your experiment--it's the phase
change from solid to liquid that makes ice a good source of cooling. MBush>>
Yes, I understand that frozen water (ice) would be best, cause it requires
calories of heat to change state from ice to water. But, it is sloppy and
always leaks (in the back of my truck, each Summer).
phase
MBush>>
requires
If you have good ventillation, you could then use solid carbon dioxide,
which sublimes from solid direct to gas...
David A. Smith
Huh?! I've never, ever, had a plastic pop bottle (either 600 mL or 2 L)
leak on me when refilled with water. Bags of ice always leak--there's
nothing you can do about that, but put water in pop bottles and freeze them
the night before your trip (just as you would do with your metal block), and
you'll have something far more effective, taking up less volume, and
weighing less, while working faaaaar better. Plus when they melt, you've
got a source of fresh water--often useful when you venture into the
boondocks!
-Paul
High specific heats
sodium, then aluminum.
But better yet is a material that gets its coolth from a change of
state: you'd never guess:
a plolythene bottle filled with water and frozen.
Brian W
On 20 May 2004 05:50:44 GMT, snipped-for-privacy@wmconnect.com (AlWahrabi)
wrote:
On Fri, 21 May 2004 01:11:54 GMT, in sci.engr.mech Brian Whatcott
Really, who would have thought?.... Wait, maybe there was a reason behind
the design and name of the original invention called an icebox, you think?
:-)
Seriously, one of the first hurricane preps I take is to fill my freezers
to the brim with any kind of plastic jug/bottle filled with water. This
came in quite handy in the aftermath of Isabel were we were without power
for almost two weeks and had water supply systems issues in the days
immediately following the storm. Keeps the fridge/freezer contents cold
longer and you have drinking water after the ice melts. Which at the time
were not such trivial matters.
________________________________________________________
Ed Ruf Lifetime AMA# 344007 ( snipped-for-privacy@EdwardG.Ruf.com)
http://EdwardGRuf.com
Polytechforum.com is a website by engineers for engineers. It is not affiliated with any of manufacturers or vendors discussed here.
All logos and trade names are the property of their respective owners.