Results 1 to 10 of 10
  1. #1
    Join Date
    Mar 2007
    Location
    South Africa
    Posts
    230
    Rep Power
    18

    Freeze 5000 kg of mangoes



    Do you know the difference between the LOAD and the HEAT TRANSFER?

    You plan to freeze 5000 kg mangoes in 1 hour.

    No problem!

    Mangoes are about 81% water
    latent heat of water 335 kJ/kg
    Q = 5000 x 0.81 x 335 / 3600 = 376 kW

    Problem solved, not really.

    This is just the energy that must be removed from the mangoes to freeze them.

    What if the room is only -5degC?
    Freezing temperature is just under zero so it will eventually get there, but not in 1 hour!!!

    The HEAT TRANSFER must be used to calculate the rate.

    The rate of heat transfer depends on:
    Product packing
    Shape
    Size
    Thermal diffusity
    Air velocity over product

    One of the world experts on this subject is Prof Richard Earle, New Zealand. He has now placed his book on-line for free access.

    http://www.nzifst.org.nz/unitoperations/index.htm

    It is well written and free. Check it out.



  2. #2
    Join Date
    Mar 2006
    Location
    Merate (LC) - Italy
    Age
    52
    Posts
    2,554
    Rep Power
    24

    Re: Freeze 5000 kg of mangoes

    Well, no, Watts are measuring power, not energy.

    Anyway your calculation just considers the power required to freeze (or thaw) 5000Kgs of mangoes, not the energy (or power) required to cool them from any temperature to any other temperature.
    You just froze 5000Kgs of mangoes from 0°C to 0°C in one hour (supposing the mangoes are perfectly exposed to freezing agent).

  3. #3
    Join Date
    Mar 2007
    Location
    South Africa
    Posts
    230
    Rep Power
    18

    Re: Freeze 5000 kg of mangoes

    Quote Originally Posted by NoNickName View Post
    Well, no, Watts are measuring power, not energy.

    Anyway your calculation just considers the power required to freeze (or thaw) 5000Kgs of mangoes, not the energy (or power) required to cool them from any temperature to any other temperature.
    You just froze 5000Kgs of mangoes from 0°C to 0°C in one hour (supposing the mangoes are perfectly exposed to freezing agent).
    I think you missed the point. I intentionally left out all of the other loads so as not to confuse the problem.

    The point was, by load calculation, you may have enough installed power to do the job but can it do do the job in the time you expect?

  4. #4
    Join Date
    Jan 2006
    Location
    Croatia
    Age
    68
    Posts
    2,299
    Rep Power
    31

    Re: Freeze 5000 kg of mangoes

    Hi,

    Quote Originally Posted by nh3simman
    The HEAT TRANSFER must be used to calculate the rate.

    The rate of heat transfer depends on:
    Product packing
    Shape
    Size
    Thermal diffusity
    Air velocity over product
    Quote Originally Posted by nh3simman
    The point was, by load calculation, you may have enough installed power to do the job but can it do do the job in the time you expect?
    Agree, but what about to use freezing tunnel instead of freezing room.

    Freezing rooms usually we use to use to keep already frozen goods but not for freezing

    Can we do it, within 1 hour?

    Did I miss something or this was a tricky Q

    Best regards, Josip

    It's impossible to make anything foolproof because fools are so ingenious...

    Don't ever underestimate the power of stupid people when they are in large groups.

    Please, don't teach me how to be stupid....
    No job is as important as to jeopardize the safety of you or those that you work with.

  5. #5
    Join Date
    Mar 2007
    Location
    South Africa
    Posts
    230
    Rep Power
    18

    Re: Freeze 5000 kg of mangoes

    Hi Josip, The freezing tunnel is a good example.

    You agree that the freezing load is unchanged my the method?

    What the tunnel gives you is high velocity over the product surface and therefore enhanced heat transfer coefficient.

    If the conveyer belt is too fast, then the product will not be frozen.

  6. #6
    Join Date
    Jul 2006
    Location
    Trinidad
    Age
    63
    Posts
    15
    Rep Power
    0

    Re: Freeze 5000 kg of mangoes

    When A mango thaw out after freezing, It loses its taste and texture..I don't think freezing mangoes is a good idea

  7. #7
    Join Date
    May 2006
    Location
    china
    Posts
    98
    Rep Power
    18

    Re: Freeze 5000 kg of mangoes

    yeah, chabby. frozen food isn't as delicious as fresh one, but it can be preserved for a long time.
    And as I know, the load of fruit refrigeratory includes heat transfer (conductive heat & radiant heat), air exchange heat, motors' heat, fruit's release heat (such as respiration action) etc. If you want to freeze food, it needs a chilling room or a deepfreeze room.

  8. #8
    Join Date
    Nov 2003
    Location
    NSW, Australia
    Age
    40
    Posts
    243
    Rep Power
    21

    Re: Freeze 5000 kg of mangoes

    i'm not sure that i really understand your question???

    but to work it out, you need to know what the specific heat of 1kg of mangoes is... what temperature they are entering into the freezer/tunnel at then this will tell you how many Kw you need to freeze down the mangoes to 0'c

    the equation for doing this is below.

    q = mc^t (q = mc delta t)
    "Old fridgies never die, they just run out of gas!"

  9. #9
    Join Date
    Mar 2007
    Location
    South Africa
    Posts
    230
    Rep Power
    18

    Re: Freeze 5000 kg of mangoes

    Quote Originally Posted by Bones View Post
    i'm not sure that i really understand your question???

    but to work it out, you need to know what the specific heat of 1kg of mangoes is... what temperature they are entering into the freezer/tunnel at then this will tell you how many Kw you need to freeze down the mangoes to 0'c

    the equation for doing this is below.

    q = mc^t (q = mc delta t)
    Hey Bones, you gave the wrong equation. Freezing is a latent process. Your equation is for sensible heat and you also forgot the time.

    The correct equation for freezing is:

    q = m hfg / time
    where hfg is the latent heat of freezing

    since the stuff that freezes is the water content
    and mangoes has about 81% water,
    we can use instead

    q = (0.81 m) 335 / 3600
    335 is the latent heat of freezing of water

  10. #10
    Join Date
    May 2006
    Location
    china
    Posts
    98
    Rep Power
    18

    Re: Freeze 5000 kg of mangoes

    Quote Originally Posted by nh3simman View Post
    Hey Bones, you gave the wrong equation. Freezing is a latent process. Your equation is for sensible heat and you also forgot the time.

    The correct equation for freezing is:

    q = m hfg / time
    where hfg is the latent heat of freezing

    since the stuff that freezes is the water content
    and mangoes has about 81% water,
    we can use instead

    q = (0.81 m) 335 / 3600
    335 is the latent heat of freezing of water
    Hi, simman! Latent heat exchange occurs in the chilling(phase transiformation) process, not in the preservation process which is under zero. Those processes are different. Unless, the cold room tempreature remains at transmission point. So, your formula cann't be all-purpose.

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •