Freeze 5000 kg of mangoes

[nh3simman]

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.

[NoNickName]

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).

[nh3simman]

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?:rolleyes:

[Josip]

Hi,


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


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:confused:

Can we do it, within 1 hour?

Did I miss something or this was a tricky Q:confused:

Best regards, Josip :)

[nh3simman]

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.

[chabby]

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

[ernestlin]

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.

[Bones]

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)

[nh3simman]

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

[ernestlin]

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.