BTU calc for heat generated from product

Snap25

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How would one calculate the BTU's needed for an enclosed conveyor with hot product (Food) as the heat source?

I'm going to go on the floor this afternoon to get the dimensions of the enclosed portion, the product temperature, and the qty enclosed at one time.
 
Since you say the food is the heat source, presumably you are trying to cool it as it passes through this enclosure. Here are a few things to consider.
One BTU is the amount of energy it takes to raise one pound of water one degree Fahrenheit, also the amount of energy released when a pound of water cools by one degree. Specific heat of any substance is the ratio of the amount of energy required to raise that substance's temperature by one degree to that of water. You will need to know the specific heat of the material you want to cool. Specific heat is not constant at all temperatures, so you need to use a value that's correct for the range of temperatures involved in your process.
If there will be a phase change (liquid to solid for example) you will also need to take that into consideration. The amount of energy released by a pound of something as it changes from liquid to solid is called the latent heat of fusion. For water that's 144 BTU per pound, so you can see it's not insignificant.
Also the rate of heat transfer is proportional to the temperature difference.
 
Since you say the food is the heat source, presumably you are trying to cool it as it passes through this enclosure. Here are a few things to consider.
One BTU is the amount of energy it takes to raise one pound of water one degree Fahrenheit, also the amount of energy released when a pound of water cools by one degree. Specific heat of any substance is the ratio of the amount of energy required to raise that substance's temperature by one degree to that of water. You will need to know the specific heat of the material you want to cool. Specific heat is not constant at all temperatures, so you need to use a value that's correct for the range of temperatures involved in your process.
If there will be a phase change (liquid to solid for example) you will also need to take that into consideration. The amount of energy released by a pound of something as it changes from liquid to solid is called the latent heat of fusion. For water that's 144 BTU per pound, so you can see it's not insignificant.
Also the rate of heat transfer is proportional to the temperature difference.


Sound advice, Thank you.

I may be over thinking the whole idea, I tend to do that.

The product is sliced but is gumming up the blades due to the product being to warm. We did some testing with placing a lot into a cooler prior to the finish equipment and found the results we longed for.

I don't think we need to drop the temperature to drastically but the exact temp is still TBD. Management wanted me to quickly enclosure the tunnel and us portable air conditioning units to trial the idea before building something permeant. I found a 14k BTU unit.

BTW, the product is 98 degrees. Like I said, i still have to determine what the target temperature needs to be.
 
Q = M + C + dT

M : 2000 units/hr @ 330g per unit = 1455 lb/hr
C : .597 BTU/lb °F
dT : 28 Degrees

So my BTU/hr (Q) would be 24,321 BTU/hr, in a perfect world with perfect heat transfer.

My question is, the cooling tunnel will run at a 10fpm speed with 70' of conveyor - so, 7 minutes the produce will be in there.

What calculation can I do to determine if the 7 minutes will suffice with the calculated BTU/hr.

Also, I may have done the above calculation incorrectly, I used the machines total run capacity (2000 units an hour).. Is that correct?

I'm just considered because the calculation doesn't know if my cooling tunnel is 20' long or 70' long! I don't know how to calculate that into the equation?

Muchas Gracias to anyone who can help me.
 
you also need to consider the following.

1. the oven itself is heated due to the process.
2. the conveyor is heated due to the process.
3. the air is heated due to the process.
4. the product is heated due to the process.

item 1 is fixed and does not move.

items 2,3,4 are moving and must be considered in your calculations.
i was involved in a project years ago in which the designer forgot some similar details and it was a mess.

james
 
is q = m Cp dT, the heat removed from the product? So the 24,321 BTU/hr is the amount of energy removed or the energy I need to remove the heat from the product?
 
That 24k BTU/hour is the energy that must be removed from the product to cool it from 98 degrees to 70 degrees.
That energy needs to get transferred to something which will heat up as the product cools down. You could accomplish that by blowing cooler air through the tunnel. You could line the tunnel with pipes carrying chilled water. The air or water will heat up as the product cools.
 
You are missing an important part of the calculation - the coefficient of convection - the factor that idntifies the rate of heat transfer from product to air.If varies with the air temperature, air velocity, and the characteristics of the product. https://www.engineeringtoolbox.com/convective-heat-transfer-d_430.html

This is probably a question best answered empirically. Do a smallbatch run with a fan and see what happens. If you want a first guess assume the product has the thermal properties of water, assume an air velocity, and use a fan with variable speed.
 
Heat transfer will be the challenge, as pointed out in earlier posts. Since the product is something that will be sliced, I am imagining a body without a lot of surface area relative to volume. So you will have the challenge of conduction within the object along with modes of heat transfer off the surface. Convection, along with likely evaporative cooling at the necessary air temperature and velocity, can probably get the outside cool at the rate you are describing (28 deg F in 7 minutes). But you may end up with a steep gradient from the inside (and part in contact with the conveyor) to the outside -- meaning it may require much more cooling capacity if the innermost part of the body needs to be at the specified temperature. Without knowing the specific characteristics of the material, it will be speculation on what this temperature difference will be. A food-like item will probably be difficult in this respect.
 
Something I just thought of.

Since this is FOOD, your cooling system must meet food purification standards in order to be used. it must also be cleaned to food standards also.

a regular a/c system will NOT be allowed since they can build up moisture, mold, dirt, etc, and will blow these harmful things onto the food. One case of food contamination linked to the cooling system is all it takes and the FDA will be in there on a regular basis.

james
 
Something I just thought of.

Since this is FOOD, your cooling system must meet food purification standards in order to be used. it must also be cleaned to food standards also.

a regular a/c system will NOT be allowed since they can build up moisture, mold, dirt, etc, and will blow these harmful things onto the food. One case of food contamination linked to the cooling system is all it takes and the FDA will be in there on a regular basis.

james


Good point James. I'm sure once I got further in the design phase, QA would of shut me down in the next meeting.

Regardless if this is the correct for this application, I'm still going to continue with the design to challenge myself and gain more knowledge. Thanks for your input and advice!
 
I used to work in food where we used CO2 to chill batches of meat before processing. It is quite effective, but there is the need for adequate ventilation. We had several truckloads of liquid CO2 brought on site daily and transferred to huge storage tanks, but we were using a boatload of it (~1 million pounds per day). The equipment was very simple, basically some solenoid operated valves were the only real moving parts aside from the exhaust fans on the roof...

Googling "CO2 for cooling food" brought up this:

http://www.praxairfood.com/products/chillers/carbon-dioxide-snowing

I have no experience with that sort of CO2 application, but it is food friendly so might be worth some research.
 

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