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Thread: Freeze 5000 kg of mangoes
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22-03-2007, 08:13 AM #1
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.
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22-03-2007, 09:49 AM #2
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).
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22-03-2007, 01:30 PM #3
Re: Freeze 5000 kg of mangoes
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22-03-2007, 02:26 PM #4
Re: Freeze 5000 kg of mangoes
Hi,
Originally Posted by nh3simmanOriginally Posted by nh3simman
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.
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22-03-2007, 02:33 PM #5
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.
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23-03-2007, 03:07 AM #6
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
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23-03-2007, 05:39 AM #7
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.
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23-03-2007, 06:09 AM #8
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!"
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23-03-2007, 06:47 AM #9
Re: Freeze 5000 kg of mangoes
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
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23-03-2007, 08:08 AM #10
Re: Freeze 5000 kg of mangoes
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.