part 2 of question

[Smokey2007]

this question is in addition to my previous one. i am really trying to understand heat transfer and after much research that has not helped me, your help would be greatly appreciated.

so lets say 1,000,000 BTU/hr are being transferred from a liquid to a solid.

1. how does flow rate affect this? i realize there is a point where it can flow so slow that the fluid no longer has BTUs to transfer, and it can flow too fast where the fluid does not have time to accept BTUs from heat exchanger/give off BTUs to solid.

so how would using 10 GPM(gallons per minute) effect the BTUs differently than 20 GPM?

2. when using the law Q=h*A*delta T(heat transfer coefficient) it gives me an answer in Watts which I can convert to BTUs. as delta T decreases so will Q, so

does this mean that if i calculate for Q and get 1,000,000 BTUs/hr, after one hr of time, will 1,000,000 BTUs have been transferred? because as the BTUs are transferred, the delta T will decrease so therefore so will the Q.

or does this mean that 1,000,000 BTUs will be required to get the initial temp to equal the desired temp where delta T = 0??

 

[Unotec]

I am a little confused in the way your question is, I will try to answer, though. But... First and foremost, if you are trying to understand heat, picture it as water, being the heat=volume of a reservoir (a dam) and T=P. The higher the P differential the more flow you get out of the dam.

1.- Yes, the flow will effect the heat transfer in an indirect way. The heat transfer is ruled by the efficiency of it. If the liquid can take the heat load very quickly, it will do so until the flow is faster than the rate of heat transfer.

2.- Q will be reduced if the delta T is reduced, but you have a flowing system. Typically the delta T is somewhat constant. If thhere is no flow, Q will decrease until they reach thermal equilibrium.

I tried to explain it as simply as I can from what I understand of your question. I hope it helps

<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Smokey2007]

unotec, thanx for your response.

1. so to the first point, if i can calculate the efficiency of this system, then i can calculate the optimal Flow rate. any guess how to do this??


2. i am still confused on this part.

the purpose is to transfer heat from an oil through the stainless steel, and into the substance on the other side to heat it. so as time goes by and heat is applied, the delta T will have to decrease.

the oil is in a closed system. and i would assume that eventually if the oil is 350 F, then the stainless steel will reach 300 F. therefore, a decrease in delta T.

i care about the amount of BTUs transferring because I need to get a heat exchanger that can supply enought BTUs to the oil. and since currently my BTUs is rather high for such a small item(i posted another question on this today), something just doesnt make sense.

 

[dvd]

It is not directly the information you want, but here is a pretty corny analogy. Imagine a farmer in the field, sowing corn. If he has one hour to sow one acre, then he better have the capability to sow one hour/acre. But, if after he has sown the field a flock of crows flies in and eats all of the corn in five minutes, then the farmer needs to replace the corn at a rate of one acre/five minutes (or 12 acres/hour). This is pretty much the case with your process. The initial rate to heat up the platen is determined by the requirements to heat up the equipment, but the sustained rate to run the equipment is determined by how much heat is taken away by the material being warmed on the platen.

 

[Unotec]

I am hoping some of the guys that are really good at explaining things jump onto this. But I will give it a shot.

1.- Q=mCp detlaT. The m (mass) will be determined by the flow. From what I read, you must have a closed loop system. What are you heating the oil for? It must loose its heat somewhere, right? What will the losses be? Do a heat balance here. The mass flow will take the heat from the exchanger.
As for the flow, if it is a closed system, you will continuously feed heat to it until you reach thermal equilibrium. Is your intention to heat the loop system and reach a determined temperature and just stay there? Where in all this heat going to from the oil? Are your requirements to reach this temperature right away, in a single pass? Or can you do a couple cycles? If it is a closed system the "optimal flow" might be a little tricky.
What is your heat source? An electric heating plate? A steam heat exchanger? This will determine what your heating capacity is.
2.- After 1hr you will have transferred 1MBTU, providing you have the capacity to take them both, from the oil and the heat exchanger efficiency. Your delta T and Cp will determine this. But you must be loosing them somewhere else, right?


<<A good friend will bail you out of jail, but a true friend
will be sitting beside you saying ” Damn that was fun!” - Unknown>>

 

[Smokey2007]

i plan on using a tube and shell heat exchanger to heat the oil via steam. i will have another shell heat exchanger that will serve as a water cooling tower to cool the oil when needed. the goal is to keep the oil at about 350 F when it starts its journy.

the oil will be at a temperature such as 350 F, and will be in a closed system that is being pumped for circulation. at one point, there will be no insulation and the intent is to transport heat from the oil, through stainless steel, and into the substance on the other side. this substance is a solid.

this process can take many cycles to get the solid to a close temperature to what is desired. say 10 minutes is fine.