Monday, July 14, 2008

Sailboat Icebox Ice: The Definitive Post

I know you folks count on May Shrink Or Fade to raise, and answer, the tough, controversial questions that most people can't be bothered to think of, let alone think about. And I am not going to let you down. And by the time you finish reading this post, most of you are going to be saying to yourselves "O... M... G. That guy really is whacko." No matter. This must be written.

So, for years now I have been pottering up & down the coast of the Smallish State in sailboats with iceboxes. In case you are too young to remember how an icebox works, it's fairly simple: you put ice in the box, and then you put things that you want to stay cold in the box, and for a while the things stay cold. Eventually the ice melts and you need to put in more or the cold things get warm again. Luckily, though, the Smallish State is relatively cold to begin with, especially the parts of it that are below the waterline of a sailboat, so 20 or so pounds of ice can hold you nicely for week. (Also, you can chill a beer just by tying a line around it and dangling it 10 or so feet under water.)

Nonetheless, the perennial sailboat ice-user starts to ask himself some questions. Questions such as:

1) Is all ice created equal? Isn't it likely that one ice-vendors' freezer is colder than another's, thus providing colder ice? Cold ice will obviously not melt as fast as warmer ice, right? So how much better is colder ice than warmer ice?

2) Is it worth buying ice at all? How much does it cost to make ice in the freezer at home, compared to buying it?

3) As the ice melts, the cold water drains out of the icebox. But you could keep the ice instead in a container within the icebox, and thus hold on to the ice-cold melt water. That would be sort of a pain-- but would it be beneficial, or not?

I have given these questions a lot of ill-deserved thought. Here's what I've come up with for answers:

1) A: All ice is not created equal, but the differences are trivial. Discussion: The thermal value of ice, I came to realize, lies not in its coldness-- it lies in its solidity. Raising the temperature of liquid water absorbs relatively little heat: one calorie per gram per degree (centigrade). Raising the temperature of ice absorbs even less heat: half a calorie per gram per degree. But melting 0 degree ice to 0 degree water absorbs a great deal of heat: 80 calories per gram. So, the temperature the ice starts at is relatively irrelevant. It's the final melting moment that does most of the cooling work. In fact, to get just 10% more cooling than about-to-melt 0C ice, you have to get the ice wicked cold first: -16C (3F) So, it's not worth looking for cold ice. Just make sure it's solid.

2) A: It costs very little to make ice. If you have the freezer space, you should home-freeze. Discussion: A 5lb block of ice sells for, what, $2.50? And we learned above that turning 5lbs of water into ice absorbs 181,136 calories*, or 181 kcal. So to freeze 5lbs of water into ice, you need to cool your freezer by the same amount. 181 kcal is the same as 0.21 killowatt hours. Making a wild guess that a freezer is about 33% efficient, you would use 0.63 kwh of electricity to make a block of ice. And doing a little division with my latest electricity bill shows I'm paying about $0.16 per kwh. So that works out to roughly $0.10 per block. Such a deal!

3) A: Keep the meltwater if you're expecting to run out of ice. Drain it if you aren't. Discussion: Ice-cold water has the potential to absorb some additional heat as it warms to room temperature-- about 41 kcal for 5 lbs of water ("room temperature" around here being 65F). But what's really going on in the ice box? Theoretically, once things have stabilized, and as long as there is any solid ice in the box, everything in there exists at an equilibrium: the ice, the meltwater, and the food all sit at 0C. Heat gradually seeps in through the sides of the box; the ice gradually melts to absorb it. The meltwater is pretty much a passive zero-degree bystander-- it is neither absorbs heat from the food, nor from the ice, both of which are also at 0C. So at this phase, keeping the meltwater in the icebox is neither useful nor deleterious. However, once the last shred of ice melts, the story changes: now the temperature of the icebox will start to rise above 0C. The heat which creeps in will be evenly divided by the fridge contents, all rising in temperature together. And here is where having a large mass of cold water will be briefly useful, because it absorb some heat, will slow the overall temperature rise, and will help keep your food a little less warm, a little bit longer.

Next up for pointless thermodynamic consideration: the old question of whether you should empty your bladder to stay warmer when you're winter camping.

[* 1 block = 5lbs = (5lb) / (2.2lb/kg) *1000 = 2,274 grams. For melting ice, (80cal/g) * (2,274g/block) = 181,136cal/block.]

14 Comments:

Blogger brushfiremedia said...

A few points.

1) in terms of ice quality, you are correct, imo about the lack of importance of coldness. However, you understate the importance of the type of ice. I've run into certain brands of ice on the coast of smallish state which are clearly nothing more than freezer-frost scrapings which the vendor has placed into a form and then jumped on two or three times to compact it. This so-called ice is absolutely worthless and often times will melt out faster than a bag of cubes.

2) You didn't take into account the cost of the water itself, the cost of appropriate molds for freezing large chunks of ice, nor the percentage usage from the cost of the freezer itself. I'm just saying...

2a) As an aside here, home frozen ice is great for storing frozen meats. Fill a mold half full with water, and freeze. Take frozen vacum-sealed meat product and place in half-frozen mold. Top up with water. Freeze. The meat will stay frozen until the ice melts enough to expose some portion of the meat. At which time it will melt faster than the surrounding ice, but will still stay very, very cold. Works a peach.

3) One facto to consider about meltwater is the impact of the drain itself. Cold air is heavier than warm air. If you have a straight-out drain arrangement for the box, cold air will flow out the drain as well as the meltwater. Warm air will flow in. When I rebuilt my icebox, I incorporated a drain with a trap that holds a few ounces of the cold meltwater, thereby keeping more cold air in, more warm air out.

In summary, good post. And you can see that I too have spent far too much time pondering ice and boxes.

7/15/08, 8:19 AM  
Blogger Turboglacier said...

Brushfire's makes some good points, which I will address further:

1) I would maintain that so long as you acquire the same mass of ice, there should be no difference between different "types". That said, I have experienced the "low-quality", uncompacted "ice" Bfire refers to. I would not be surprised if unscrupulous ice merchants make a block of this "fluff ice" which has the same dimensions as a 5lb block, but in does note in fact does weigh 5lbs (due to all the air in it.)

2) Further investigation reveals that the TurboPalace's municipal water costs about $0.05 / gallon. A gallon of water being 8lbs, a 5lb block of ice needs only $0.03 of water. [To be even more precise-- the cost of the water is only $0.01. The other $0.02 is the "sewer charge", which I say should be waived if you are taking your block of ice to the boat and melting it back into the ocean. But I down the Smallish City Water Authority will want to hear from me on this matter.]

3) Bfire describes an ingenious idea. I have little interest in the bodily contortions which would be required to redo the icebox drain hose, but have been meaning to fit a round piece of sponge into the drain hole-- the theory being that this would let drainwater drip through, but keep the cold air from freely draining out.

7/15/08, 4:30 PM  
Blogger Weeble said...

This reminds me that the thermo part of my brain has many, many cobwebs in it.

7/15/08, 9:37 PM  
Blogger Ladyk73 said...

You are a dork...

My physics is 20 years old. Wasn't pressure another factor in the ice/water thing? Like the water at the bottom of the ocean is -10 centigrade or something.

They have pressure cookers, what about pressure freezers? Or you can use dry ice.

7/15/08, 11:13 PM  
Blogger brushfiremedia said...

Yes, I suspect retrofitting a trap into an existing box would be less than fun... Mine was built as an integral part of the new box, so access was excellent.

7/16/08, 7:30 AM  
Anonymous girltuesday said...

oh wow. i'm not sure what was funnier: TG's post? or the equally committed response from b'fire?

perhaps we can hash this out tomorrow night during the race, fellas? hopefully one of you has ice for my PBRs?

also, i am waiting for johanna to weigh in on the math . . . .

7/16/08, 3:53 PM  
Anonymous GoAround said...

Long time reader, first time poster.

Some semi-related everyday ponderings on fluid thermodynamics...

1) I once got into a heated debate with my mother-in-law regarding my preferred method of mixing a gin and tonic. I insist that the following steps create a colder "first sip" drink than one created by haphazardly mixing the ingredients simultaneously due to the volume of liquid to be chilled. a) Place ice in glass; b) pour room-temperature gin into glass and stir vigorously to rapidly chill gin; c) add refrigerator-temp tonic.

2) The airline I work for is considering a capital investment of roughly $125,000 in insulated ice-storage containers to reduce the volume of ice purchased/melted for each aircraft throughout the day. Payback in reduced ice expense and labor to board ice is estimated at approximately 3 months. Perhaps more on this later.

7/16/08, 10:32 PM  
Blogger Turboglacier said...

Go Around:

I *always* make a G&T just as you describe. I can't say I have a scientific reason, but it's habit.

As for ice on airplane-- it being something like -30F outside at 30,000ft, shouldn't it be pretty easy to make ice right on board, somehow? I'm picturing a small bit of fuselage with no insulation, where water pours into an ice cube mold that's in contact with the exterior aluminum...

Or, how about just reducing the amount of ice put into each drink? I would be quite happy with a reduction in the standard 80:20 ice:beverage ratio...

7/16/08, 10:47 PM  
Blogger brushfiremedia said...

I can see how real iceboxes could make up that cost expenditure so quickly. With my original from-the-factory-whoops-we-forgot-to-actually-use-insulation icebox, I went through two blocks of ice per 3 days. With my new highly-insulated box, that same amount of block ice lasts around 14 days.

TG, I suspect the problem with airline ice occurs during the endless on-the-tarmac delays, rather than at 30k...

7/17/08, 8:26 AM  
Blogger Turboglacier said...

Bfire-

Yeah, but I seen the pictures of what you went through to redo your icebox... it doesn't look like so much fun...

As for the airplanes-- they never serve you drinks/ice on the tarmac anyway. So they could just wait until "cruising altitude" to turn on the naturally-cooled icemaker, make as much as they need for the flight.

7/17/08, 10:14 AM  
Blogger The MSILF said...

In Berkeley, I actually lived in an APARTMENT so old it had an icebox.

Actually, a 29 foot sailboat would have been a big upgrade from that apartment.

Apropos ice...and iceboxes.

7/17/08, 11:11 AM  
Blogger Johanna said...

girltuesday -
the math seems fine to me, though I am far too lazy to wrap my head around this mixture of SI and silly units which introduce an error of about 7 grams but irrelevant. I'm just happy that sailors understand latent heat of fusion.

I am waiting for the winter camping and bladder post, as it is directly applicable to my own life. :) J.

7/23/08, 11:15 AM  
Blogger Raggi Thor said...

This was quite interesting :)
I am planning for a fridge or a cold cupboard with shelves for butter and milk on our mountain cabin with no electricity.

A "normal domestic fridge" will spend 300Kwh/year or maybe 30/Kwh per month. So, with 0.1kwh per kg of ice, I need 300kg per month? Would I need a ton of ice below the floor to keep the milk cold from June to September?

5/25/09, 7:42 AM  
Anonymous Anonymous said...

Put the Gin in the freezer.

11/2/09, 1:14 PM  

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