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  1. #1
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    Few questions on rate of freezing in freezer



    Hi

    I have a few questions relating to slow freezing in a freezer. I would be very grateful if somebody could kindly answer them for me.

    1. In freezer engineering 'freezer capacity' refers to the weight of items that can be frozen at normal speed over a certain time period. This is different from the volume of space a freezer can hold e.g. a freezer which can hold 100litres of product. Freezer capacity is where for example a freezer can freeze 2kg of food in 12 hours. Is this correct?

    2. I am told that the rate of freezing of an item depends on freezer capacity and product load. If you increase the product load over its' capacity significantly this will lead to slow freezing of the products. Is this correct and why?

    3. I am told that the temperature in the freezer does not determine the rate of freezing but rather the final equilibrium temperature the product will reach.
    However the same person (who I can no longer find) also told me changing the temperature alone will cause slow freezing but on the condition that the capacity of the freezer is big. He said slow freezing can be achieved by increasing product load over capacity or capacity over load.
    Greatly increasing capacity over load doesn't make sense can somebody explain why this would lead to slow freezing?
    I was told that if you set the temp of a domestic freezer to -5c this will usually cause slow freezing. However why would this be the case if -5c is only the equilibrium temperature. Assume the freezer is set to -18c and the freezer passes -5c in 2 hours, then surely if you lower the temperature to -5c it will reach that equilibrium in 2 hours also? Therefore the rate of freezing wouldn't be affected, just the final temperature reached?
    Or am I missing something here where the professor told me temperature will assure it(i.e. lowering the temp to -5c) only if the capacity of the freezer is very high?


    4. If somebody leaves say meat in a freezer for 6 hours at -1c, and then increases and leaves for 6 hours at -2c, and then increases and leaves for 6 hours at -3c, am I correct in saying the freezer wouldn't have slowly frozen the food over a period of 18 hours?
    Rather what will happen is that if the freezer when set to say -18c normally gets food to -3c in 3 hours, then it will bring the food to -1c in an hour(assuming) and then the remaining 5 hours is stationary. The same thing will then happen after the 6 hours when the temperature is increased to -2c. Is this the case in which case it doesn't matter if you leave the freezer for long periods at higher temperatures since the rate of freezing will be the same? Or am I wrong?

    Thanks.



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    Re: Few questions on rate of freezing in freezer

    btw here is my original question/answer from the person I spoke to, not sure if he is referring to size of unit or capacity, please clarify dear experts:

    "1) I believe you said the rate at which the meat will freeze will depend upon the 'size and load' of the freezer. I thought the temperature of the freezer alone e.g. -5c would be enough to ensure slow freezing regardless of size and load of freezer. Can you explain if this is not the case and if so how, technically speaking, the freezers size and load affect the rate of freezing? I note you said that a normal freezer may achieve slow freezing at -5c wheareas a deep freezer may achieve it at -10c.

    Temperature alone will assure it only if the capacity of the freezer is very high. If the load is high in a refrigerator situation, it will not be able to maintain the -5C. It can warm even to + tempeatures in order to cool down the product. The rate of freezing depends on the rate of heat removal and hence they are dependent on the cooler capacity and product load. By increasing one over the other you can make it depend either on the cooler capacity or product load."

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    Re: Few questions on rate of freezing in freezer

    http://uk.rs-online.com/web/p/data-loggers/4901064/
    saves listening to he who tells you things

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    Re: Few questions on rate of freezing in freezer

    I will answer the easy one
    1) Yes most freezers are designed to hold product at frozen ie storage and capacity is storage capacity.
    Last edited by hookster; 07-09-2013 at 07:59 PM.
    I love the smell of Ammonia in the morning!

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    Re: Few questions on rate of freezing in freezer

    Quote Originally Posted by iceman31 View Post
    Hi

    I have a few questions relating to slow freezing in a freezer. I would be very grateful if somebody could kindly answer them for me.

    1. In freezer engineering 'freezer capacity' refers to the weight of items that can be frozen at normal speed over a certain time period. This is different from the volume of space a freezer can hold e.g. a freezer which can hold 100litres of product. Freezer capacity is where for example a freezer can freeze 2kg of food in 12 hours. Is this correct?

    2. I am told that the rate of freezing of an item depends on freezer capacity and product load. If you increase the product load over its' capacity significantly this will lead to slow freezing of the products. Is this correct and why?
    Yes.

    Heat flows from high to low.
    It flows faster if there is a greater difference.



    Quote Originally Posted by iceman31 View Post

    3. I am told that the temperature in the freezer does not determine the rate of freezing but rather the final equilibrium temperature the product will reach.
    However the same person (who I can no longer find) also told me changing the temperature alone will cause slow freezing but on the condition that the capacity of the freezer is big. He said slow freezing can be achieved by increasing product load over capacity or capacity over load.
    Greatly increasing capacity over load doesn't make sense can somebody explain why this would lead to slow freezing?
    I was told that if you set the temp of a domestic freezer to -5c this will usually cause slow freezing. However why would this be the case if -5c is only the equilibrium temperature. Assume the freezer is set to -18c and the freezer passes -5c in 2 hours, then surely if you lower the temperature to -5c it will reach that equilibrium in 2 hours also? Therefore the rate of freezing wouldn't be affected, just the final temperature reached?
    Or am I missing something here where the professor told me temperature will assure it(i.e. lowering the temp to -5c) only if the capacity of the freezer is very high?


    4. If somebody leaves say meat in a freezer for 6 hours at -1c, and then increases and leaves for 6 hours at -2c, and then increases and leaves for 6 hours at -3c, am I correct in saying the freezer wouldn't have slowly frozen the food over a period of 18 hours?
    Rather what will happen is that if the freezer when set to say -18c normally gets food to -3c in 3 hours, then it will bring the food to -1c in an hour(assuming) and then the remaining 5 hours is stationary. The same thing will then happen after the 6 hours when the temperature is increased to -2c. Is this the case in which case it doesn't matter if you leave the freezer for long periods at higher temperatures since the rate of freezing will be the same? Or am I wrong?

    Thanks.
    If you have a difference then nature will try and even it out.
    The greater the difference the more heat that moves.

    Imagine a glass of warm water at 20 degc in a freezer at -20 degc
    and compare it to a glass the same size at 10 degc next to it.

    Which do you think would freeze first?

    When heat starts moving it is like a round stone rolling down a steep hill,
    it might take a while to get moving but the steeper the hill and the greater
    distance it has to roll only causes the stone to accelerate, until it meets it's
    finish point.

    Heat is similar, it might take a while to get moving but the greater the difference
    the greater the speed it will accelerate to (within limits)

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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    Re: Few questions on rate of freezing in freezer

    Quote Originally Posted by Rob White View Post
    Yes.

    Heat flows from high to low.
    It flows faster if there is a greater difference.





    If you have a difference then nature will try and even it out.
    The greater the difference the more heat that moves.

    Imagine a glass of warm water at 20 degc in a freezer at -20 degc
    and compare it to a glass the same size at 10 degc next to it.

    Which do you think would freeze first?

    When heat starts moving it is like a round stone rolling down a steep hill,
    it might take a while to get moving but the steeper the hill and the greater
    distance it has to roll only causes the stone to accelerate, until it meets it's
    finish point.

    Heat is similar, it might take a while to get moving but the greater the difference
    the greater the speed it will accelerate to (within limits)

    Rob

    .

    Hi Rob

    You have said that basically leaving at a higher temperature will cause slower freezing.

    However what about the capacity question? The quote I gave, when he says increasing capacity over size leads to slow freezing, is he referring to freezer size(volumes/litres storage) or is he referring to cooler capacity(e.g. 2kg in 24 hours). Which is he referring to and why would, technically speaking, increasing freezer capacity greatly over freezer product load lead to slow freezing?

    Hope you will be able to answer. Many thanks.

  7. #7
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    Re: Few questions on rate of freezing in freezer

    .

    I'm not too sure on freezer capacity / volumes but freezers are designed
    to either remove heat from the product (blast freezers) or maintain the
    product temperature.

    So a domestic freezer will have a capacity / volume in litres which is
    the physical size of the product holding ability. If the product is at
    -20 degc and the freezer is at -20 degc then the heat transfer is minimal.

    if you where to put 20 Kg of pork in at +5 degc then the freezer
    will pull the pork down to -20 degc and the speed of this will depend
    on the size of the freezer system or the freezer capacity.

    The larger the system the more heat it will move.

    Heat transfer is a funny thing, if you move too much from the product
    too quick, then it can damage the product in the form of ice crystallisation,
    so the skill is to remove the correct amount over the correct time.

    Domestic freezers do not have the ability to control this capacity so it
    is a balancing act to design a freezer that is large enough to maintain
    the temperature of the product and have extra capacity to be able to
    freeze fresh products if and when required.

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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    Re: Few questions on rate of freezing in freezer

    Quote Originally Posted by Rob White View Post
    .

    The larger the system the more heat it will move.

    Heat transfer is a funny thing, if you move too much from the product
    too quick, then it can damage the product in the form of ice crystallisation,
    so the skill is to remove the correct amount over the correct time.


    .
    Didn't you mean moving too much too quickly will cause less ice crystal damage? i.e. isn't quick removal of heat the same as fast freezing?

    Actually, believe it or not, the thing I want to do is create a freezer which slow freezes the product and causes maximum ice crystal damage I.e. the opposite of what most freezer manufacturers would want to do. This is why I am trying to work out what I need to do in terms of temperature, load, size and capacity in order to create a slow freezer of meat. Can you give me any input on this? The crucial thing is that I want the meat to spend say 12-24 hours at -2 to
    -4. This is why I was asked the question about if I leave something meat at -2c for 6 hours woud it keep freezing(ice crystal getting larger) or would it reach minus 2 in say a shorter time and then just stop freezing.

    Thanks
    Last edited by iceman31; 08-09-2013 at 04:52 PM.

  9. #9
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    Re: Few questions on rate of freezing in freezer

    Quote Originally Posted by Rob White View Post
    Yes.

    Heat flows from high to low.
    It flows faster if there is a greater difference.





    If you have a difference then nature will try and even it out.
    The greater the difference the more heat that moves.

    Imagine a glass of warm water at 20 degc in a freezer at -20 degc
    and compare it to a glass the same size at 10 degc next to it.

    Which do you think would freeze first?

    When heat starts moving it is like a round stone rolling down a steep hill,
    it might take a while to get moving but the steeper the hill and the greater
    distance it has to roll only causes the stone to accelerate, until it meets it's
    finish point.

    Heat is similar, it might take a while to get moving but the greater the difference
    the greater the speed it will accelerate to (within limits)

    Rob

    .
    Mpemba effect

  10. #10
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    Re: Few questions on rate of freezing in freezer

    Quote Originally Posted by r.bartlett View Post
    Mpemba effect


    I had to google him

    It has caused great debate on here over the years.

    Rob

    .
    .. ... -. .----. - / -- --- .-. ... . / -.-. --- -.. . / --. --- --- -..

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