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Should ice box be drained ?

  1. Jun 3, 2012 #1
    I've obtained an insulated plastic cooler box. I want to use this when sea fishing to store my catch and keep it chilled as cold as possible until I can get it home. I intend to load a quantity of ice cubes into the bottom of it and drop the caught fish on top of the ice. My question is - Should I fit a spacer mesh platform to allow the meltwater to drain away from the ice, or should I allow the ice to sit in its meltwater ? - Would the effectiveness of the chilling be better using one method rather than the other ? - or would there be no difference ?
    Would the ice melt faster if it was allowed to float in its melted runoff ?
     
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  3. Jun 3, 2012 #2

    mfb

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    The energy ice needs to melt does not depend on the presence of water.
    Once the ice begins to melt, you have nearly 0°C everywhere until everything is molten. This gives a fixed heat capacity, just depending on the amount of ice. I don't know the details of your cooler box, but I would not expect that the thermal flux outside->inside depends significantly on the location of the ice inside.
     
  4. Jun 3, 2012 #3

    Integral

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    The ice submered in water will be held in good thremal contact with 32°F water, while the unsubmerged ice will be surrounded by air which will (due to enclosure) be at very near the temperature of the ice, which should be much below 32°F. A smaller temperature differential means a lower rate of heat loss so the ice in air should melt slower then the submerged ice.

    I think a water drain would be a very good idea.
     
  5. Jun 3, 2012 #4
    I'm thinking you shouldn't drain the water, because the water has higher heat capacity per volume than air. I assume the loss of heat through the ice box walls is about the same regardless of whether water or air is inside (since the rate of heat leaking out should depend on temperature difference), so you want higher thermal mass inside.
     
  6. Jun 4, 2012 #5

    russ_watters

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    Melting ice is at exactly 32F: The delta-T between the ice and the water is zero....
    Agreed.
     
  7. Jun 4, 2012 #6
    The meat of some species will be degraded if you put the fish on ice while still alive. But then once they are dead, you want to cool them down as quickly as possible. This is what the chilled brine tank on a commercial boat is for. You can approximate this by leaving the melt water in the cooler. The approximation is even better if you salt down the ice, because that will drop the temperature down below normal freezing. Heat transfer is much faster to the liquid than to drained ice. If it is a one day trip, then this is sufficient. But if a longer trip, learn another lesson from the commercial guys and have a second ice box that is drained to transfer the fish to after rapid chilling in the brine tank. But don't put them on top of the ice. Pack them down in the ice such that each fish is completely surrounded by ice. For this reason a commercial box is divided into two sections by boards. Put ice in one section and leave the other empty. Pack the fish in the empty section while drawing ice as needed from the other section.
     
    Last edited: Jun 4, 2012
  8. Jun 5, 2012 #7

    Integral

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    Except that the ice is NOT at 32F, it comes out of the freezer at much lower then 32. So the ice can be as much as 10-20F° below freezing. That is why it is better to drain to slow melting.
     
  9. Jun 5, 2012 #8

    sophiecentaur

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    There seem to be several factors here. However, I reckon it has to be true to say that is is a waste to just get rid of the melt water as it is cold and has a high heat capacity - to absorb heat entering the box from outside (which will be there with or without the presence of the water). Where, exactly, that melt water should sit inside the cooler is a more difficult question to answer. Perhaps an outer jacket for the melt water would be best.
     
  10. Jun 5, 2012 #9

    russ_watters

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    I must be misunderstanding something because it sure sounds like you are saying the 32F water and 12F ice ice can coexist. They can't, except for in a very short transient situation when ice is first added to water (which doesn't happen in a cooler). Here's what happens in a cooler:

    Stage 1: Ice is put into the cooler at 12F. It warms up to 32F, absorbing about 9.4 BTU/lb of ice. It starts and ends completely solid.

    Stage 2: Ice melts. Whether the water is drained or allowed to stay with the ice, the mixture is at 32F during this entire stage unless impurities are added. It absorbs 144 BTU/lb of ice.

    Stage 3: This applies only if the water is not drained. If not drained, the water starts at 32F and absorbs 10 BTU for every 10F temperature rise. If you need a 32F icebox, that doesn't help you any, but if you only need a 40F refrigerator, it gives you an extra few percent of cold box time. I should amend my previous statement though: I would think this isn't a major enough factor to be worth it most of the time. Ie, if you are transporting, you have to lug around all that water that isn't doing much for you.

    Certainly water will transfer heat more efficiently to the sides of the icebox than ice will - so that's a good point - but if the icebox is well insulated, I would be surprised if that is a major factor. Are you sure commercial fishermen don't drain it for another reason: like ballasting? An ice/water mixture is heavier than the same volume of ice or ice and fish.
     
  11. Jun 5, 2012 #10
    Commercial fisherman could drain due to osmosis affecting the quality of the meat - do they drain the water only for salt water fish, since they would be out at sea for weeks. Fresh water fishing wouldn't seem to have as much of a time factor, perhaps a day or two at most.
     
  12. Jun 6, 2012 #11
    Heat transfer considerations give us competing and conflicting objectives.

    If our objective is to make the ice last as long as possible, then drain the box. The liquid makes the heat transfer from the sides of the box to the ice much more rapid, which leads to quicker melting of the ice.

    If our objective is to cool the fish as quickly as possible, then don’t drain the box because the liquid makes the heat transfer from the fish to the ice much more rapid, hence more rapid cooling of the fish.

    In no case will the ice be at a temperature lower than the melt water if you have melt water. If you pull the ice out of the freezer at -50 degrees F, it must warm up to the melting point before you will have any melt water. For fresh water, that would be 32 degrees F. If you salt the ice down, it will be substantially less than that.

    The concern with the quality of the meat is a result of biological mechanisms completely unrelated to this discussion. Some fish will release enzymes that will cause their bodies to produce more heat if they swim into cold water. Blue Fin Tuna is an example of such a species. If the fish is still alive when you put it into the cooler, it will release these enzymes, which will degrade the quality of the meet.

    The meat will degrade very rapidly if it is not quickly cooled as soon as the fish dies, hence the reason for the brine tanks to cool them very rapidly. A commercial brine tank is typically between 4-28 degrees F, depending on the design of the tank. A boat that has a very cold brine tank will also have a dry freezer at -50 degrees F to transfer the fish to after the brine tank. A boat with a warmer brine tank is probably a low dollar operation for short trips, and they may transfer the fish to simple drained ice storage.

    All this concerns salt water fishermen. I’ve got no idea what the fresh water guys do; but they would not be concerned with the long duration storage times as the salt water fisherman who might be out for weeks or months. One friend who stays out until the freezer if full tells me that his shortest trip was three weeks and the longest a year, in his 35 years of operating that particular boat. A lot of short duration trip fishermen will simply leave them in the brine tank, but if they will get beat up from sloshing around if they leave them in too long.

    As it turns out, osmosis is not a concern that causes any problems for the storage of fish.
     
  13. Jun 6, 2012 #12
    Reminds me of an engineering student who told a co-worker that he shouldn't drain the water out of his ice chest because of it's heat capacity. The co-worker replied he didn't want his food to get too soggy.
     
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