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Old Oct 1st 2018, 08:30 PM   #1
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Post Homework problem. Does something missing???

Good Evening forum members,

Okay here is the situation, We have a water tank with 200 liters of water heated at 372K, in addition, the water in this tank is stirred and this process inserts 0.5KW of power to the water. The water tank is used to heat a residential space and maintain its temperature at 293K.

What we know is the heat transfer coefficient is 12W/m^2 K and the area of the wall is 5 square meters. In addition, the water tank stops transmitting heat to the system when the temperature reaches 300K.

So the questions are 1) For how long does the water tank will provide heat water to the residential space. and 2) what is the total heat flow that exerted into the room.

So i mentioned that in order to maintain the same tempertaure into the room the Qin=Qout?? but the Qin of the room is the Qout of the water tank.

So what I did was to calculate the ΔU of the tank water from the formula ΔU=mc(T2-T1) and the heat flow for 336K which is the median heat flow by using the formula q=hA(T2-T1). Then I used the formula PxT =ΔU x (qxt) , and i solved it for t.

Is that right???
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Old Oct 5th 2018, 02:51 PM   #2
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I'm not sure if I understood the problem correctly, but working with a system on the surface of your tank, we have the lumped formulation:

$\displaystyle \frac{dU}{dt}=\dot{Q}_{net}-\dot{W}$

$\displaystyle \frac{dU}{dt}=mc_p\frac{dT}{dt}$

$\displaystyle \dot{Q}_{net}=-Ah(T-T_{\infty})$

$\displaystyle \dot{W}=-500 W$

$\displaystyle mc_p\frac{dT}{dt}=-Ah(T-T_{\infty})500$

$\displaystyle \frac{dT}{T-T_{\infty}}=-mc_pAh500dt$

$\displaystyle \int_{372}^{300}\frac{dT}{T-T_{\infty}}=-\int_{0}^{t}mc_pAh500dt$

Now a think you can solve the questions
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Last edited by mscfd; Oct 5th 2018 at 02:57 PM.
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Old Oct 5th 2018, 08:43 PM   #3
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Look at all those lines of equations from mscfd ...mind numbing ... this approach stops science from being comprehensible by the average person ...makes it elitist and more complex than necessary...

We can gently walkthrough this problem using simple english that everyone can understand...

But first we need to get the question correctly ... this is a simple thermal conduction problem. We are told "The water tank is used to heat a residential space and maintain its temperature at 293K..." ... but then latter in the question " the water tank stops transmitting heat to the system when the temperature reaches 300K. "

So I guess 'system' and 'residential space' are the same??? using different terms like this only confuses things !!... But the main point is why has the tank stopped transmitting heat when there is still a temperature difference?

Have you given us the question precisely ...word for word ???
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Old Oct 5th 2018, 09:12 PM   #4
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oz93666,

the only approach that stops science from being comprehensible is when you call an user by "average person". First of all, be polite. Second place, when making a comment, do it contructively. Judging a way to solve a problem doesn't make any difference.

Answering your question: the point of the question is not why the heat transfer is interrupted at a given temperature. There are several control mechanisms to make the convection coefficient zero or virtually zero, especially if there is forced ventilation.
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Old Oct 5th 2018, 10:17 PM   #5
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Originally Posted by mscfd View Post
oz93666,

the only approach that stops science from being comprehensible is when you call an user by "average person". First of all, be polite. Second place, when making a comment, do it contructively. Judging a way to solve a problem doesn't make any difference.

Answering your question: the point of the question is not why the heat transfer is interrupted at a given temperature. There are several control mechanisms to make the convection coefficient zero or virtually zero, especially if there is forced ventilation.
Don't take my post personally .. I was criticising the overuse of equations when dealing with problems ... the average physics student when he finishes education , soon forgets equations , and has not developed logic and common sense and is soon unable to solve the simplest thing .

I wasn't referring to anyone here as "the average person" , but the public in general...

The question says " the water tank stops transmitting heat to the system when the temperature reaches 300K. " (the system temp is 293K) this does not make sense ..and will effect all calculations relating to this problem.

I suggest the question has not been given correctly.

But your post did make me smile , you give 7 lines of formulas , then say "Now I think you can solve the questions"!!!

I ask kiris91 ...has that helped you solve the questions??

Last edited by oz93666; Oct 5th 2018 at 10:20 PM.
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