convection question?? MechE or Civil???


by Trooko
Tags: civil, convection, meche
Trooko
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#1
May8-06, 07:51 AM
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I'm just asking out of curiosity, and I'm just a first year engg student.....so it maybe non-senses....

In an enclosed system, with heat being generated. Let's say a very fancy toast factory, where bread becomes toast. The factory itself have already installed thermal insulation underneath the roof, shallow pond with movable insulation. etc..so it can be assumed that no external heat is being transdered in. However, due to the toasting process the factory has become extremely hot. Active cooling systems like air conditioning system are not possible due to the cost of energy....since no one will pay $10 for toast...The factory have lot of fancy ventilation equipments that are all over the place but without proper planning.

the question is : By properly placing the ventilation equipments will it significantly lower the temperature within the factory. And to do it properly, will the toast-guy/girl need to have a good understanding of convection/ air flow design?

Is this a MechE or Civil problem? If I'm the toast guy/girl, what will I need to know.....(name a book or some specific terms... I found stuff like forced convection and natural convection.etc).
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Astronuc
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May8-06, 08:06 AM
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This is a mechanical engineering question, and perhaps industrial engineering.

Active cooling systems like air conditioning system are not possible due to the cost of energy . . . . The factory have lot of fancy ventilation equipments that are all over the place but without proper planning.
These two statements seem a bit inconsistent.

Anyway, proper planning would preclude the need for expensive ventilation systems.

First of all, the use of thermal energy should be not more than necessary to get a part to temperature and hold it for the necessary period of time. In other words, use no more energy than necessary.

Secondly, thermal systems are insulated, so that ventilation need only be used local, not in an entire building. I have stood outside a furncace where the inside temperature was 1650 - 1800C (>3000F) while the outside temperature was ~25C.
Mech_Engineer
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May8-06, 09:02 AM
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Quote Quote by Trooko
In an enclosed system, with heat being generated. ... The factory itself have already installed thermal insulation underneath the roof, shallow pond with movable insulation. ...The factory have lot of fancy ventilation equipments that are all over the place but without proper planning.
Heat transfer is a useful topic for deciding how efficient a cooling fin is, or where heat is going in a system, but for a simpler answer with a few strategic "fudge factors," thermodynamics may help with an easier answer.

I would say the best route to go would be to first decide how much power is being dissipated into the air from the "toasters" (you could use heat transfer for this part probably, perhaps natural convection). Based on this power amount, then you should decide what temperature is desired inside the factory. Then, using an average outside temperature, and the specific heat capacity of air, you can decide how much air you would need flowing into the factory to maintain temperature. The problem with a straight ventilation system is that if it's 90 degrees outside, the "coldest" the inside of the factory can be is 90 degrees.

You might look into evaporative coolers, they are usually pretty inexpensive, can cool large buildings (such as factories) and blow a lot of air, which would be good for a heavy cooling application like this. The workers in the factory would probably be more interested in putting up with a little humidity than the "blazing heat" of the toasters.

As for placement of the "ventilation system," just moving a couple of fans around might help (but probably not that much, and you have to move the ducting, power suplpies, etc... not much cheaper than getting some active cooling). Make sure you are running close to neutral pressure (equal number of fans blowing in and out of the building). Also, try to make sure all of the air blows in the same direction, from one side of the building to the other, or bottom to top, to minimize the occurence of flow dead sopts. Overall, this is really more of a fluids problem than Heat Xfer or Thermo because it's more important to know where the air is flowing than anything else.

Of course, it would be a little easier to answer the problem if it was more obvious what you were trying to accomplish...

Trooko
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#4
May10-06, 08:38 AM
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convection question?? MechE or Civil???


Thank you both for your replies. I found them to be very insightful.

I've just completed my first general year and recently chose MechE as my specialization. However, I am not absolutely certain that MechE is my "thing" because I didn't understand the particular differences between the specializations, since some of them overlapse. And if I change my mind now, I may still be able to change my choice.

After reading your replies, I feel more confident about my choice of MechE so thank you very much

Also, I'm under the impression that thermodynamics, fluid dynamic and moving motion are important part of MechE. Is this correct.
Astronuc
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May10-06, 11:26 AM
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Quote Quote by Trooko
Also, I'm under the impression that thermodynamics, fluid dynamic and moving motion are important part of MechE. Is this correct.
That is correct.

There are overlaps or similarities in civil and mechanical engineering, mainly in the area of structures, mechanics of materials, and fluid mechanics. And in other areas, the two disciplines are complementary.

Civil engineers design water supply and waste water transportation systems with the same methods that a mechanical engineer would design a pipe flow for a cooling system. Generally the civil engineer would be looking at cold water or room temperature systems, whereas the mechanical engineer would also design high temperature liquid or vapor flow for power systems.

Structural engineering developed from Civil engineering, but is essentially Mechanical engineering for structures.

Geotechnical engineering is another discipline evolving from Civil engineering which is essentially geo-mechanical engineering.

Civil engineering is essentially Mechanical engineering applied to civil structures, like roads, bridges, dams, buildings, transportation systems, . . .

Similarly, Aerospace Engineering is Mechanical engineering applied to spacecraft, rocket and missiles, and aerospace systems.
russ_watters
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May10-06, 11:37 AM
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Quote Quote by Astronuc
Secondly, thermal systems are insulated, so that ventilation need only be used local, not in an entire building. I have stood outside a furncace where the inside temperature was 1650 - 1800C (>3000F) while the outside temperature was ~25C.
To expand just a little, consider an open grille in a restaurant. You go for brunch, the chef asks you how you want your eggs, then turns and starts cooking them on a grille right behind him and just 2 meters from you. You may not even notice, but there is an exhaust hood above the grille and a supply grille above the chef. In cheap restaurants, the air blowing down at the chef isn't conditioned and he may be standing there sweating under 25C air, while just 2 meters away, you're nice and cool in your 18C air. Simply by segregating the two airstreams, you can keep one area cool while the other area is allowed to get hot. The same principle can be applied to your toast factory.


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