Heat Transfer Issue

[Smokey2007]

Something does not appear right with my following calculations. I am trying to determine how many BTUs will be transferred in on hour through a material and I am getting a huge number. I understand as "delta T" decreases so will the number of BTUs transferred, but in its initial state, this doesnt seem right. Can somebody please help me out.

given four 24" * 12" rectangle plates of 304L (stainless steel) that is 1/2" thick, how much heat will be transferred through it.

I used the following formula to calculate:

heat transfer coefficient
delta Q=h*A*delta T

k = thermal conductivity of the material = 18 W/(m*K)
t = thickness of material = .25" = .001588 meters

h = k/t = 11338 W/(m^2*K)

A = surface area = 4*height*length = 4" * 12" * 24" = 4 * .3048meters * .6096 meters = .74322 meters^2

delta T = 350F - 80F = 450K-300K=150K

delta Q = 11338W/(m^2*K)*.74322m^2*150K
delta Q = 1265751 W = 1265751 J/s

convert to BTUs (times by 3.41)
delta Q = 4316214 BTU/hr

did i do something wrong?? this number sounds way too high since the device is small and most heat exchangers I can find cannot supply this amount of BTUs.

 

[vpl]

There's no way to tell, because you didn't give the temperatures and flow rates.

BTW, this reads like a homework problem. If it's not, you need to provide more details. If it is, you need to go see your professor.

But it's not the highest heat flux I've seen.

Patricia Lougheed

Please see FAQ731-376: Eng-Tips.com Forum Policies for tips on how to make the best use of the Eng-Tips Forums.

 

[Smokey2007]

definitely not a homework problem. i just got out of the military after attending West Point, and this is a current problem I have in my new field of work.

i am designing a flusher used in the plastics business and want to be sure that the i buy a heat exchanger that will work well.

the temp of the heat transfer fluid is 350F, while the initial temperature of the steel will be room temp, about 80F.

I am hoping to get the outside of the steel(the part not touching the oil) to around 300F.

the viscosity at 300F is 1.085cP.

the system will have a flow rate of 10 GPM.

 

[MintJulep]

Smokey,

I suggest that you download, read and understand this free text on heat transfer before you attempt to go any further.

http://web.mit.edu/lienhard/

http://www/ahtt.html

I know that this may sound unkind, but unfortunately it appears to be true. At the moment it seems that you don't have a sufficient understanding of some basic fundamental concepts. The level of of help that you seem to need is beyond what may be reasonably expected in a "tips" forum such as this.

 

[Smokey2007]

i will agree that at the moment i dont know much about heat transfer, kinda since i havent had a thermo class since 2000 when i was at West Point. and i specialized in managing communication networks while in the Army, so not really thermo related, but alas my job requires me to be an expert in all engineering fields, so im trying to quickly learn. thanx for the book though, other than wilkipedia, its a pain finding such on this topic.

 

[MintJulep]

Here are some things to look for and think about while looking at the book.

There are steady-state processes and transient processes.

Every system must maintain a heat balance. Heat in = heat stored + heat out.

Your steel plate has 6 surfaces. Heat transfer is occurring at all of them.

Remember that there are three modes of heat transfer.

The coefficient of convection between a fluid and a solid is a function of Reynolds, Nesault, Prandtl and a few other dimentionless numbers, and a bunch of other factors.

 

[IRstuff]

Your math seems to have some problems. You state 0.5" initially, but use 0.25" in the calculation. Moreover, 0.25" is 0.0064 m, not .001588 m.

Net result is that your heat flow calculation is about 8 times higher than it should be, based on your given numbers.

But, frankly, the calculations are not unreasonable, for a 300ºF delta across a 0.5" gap, particularly for your given thermal conductivity for steel. The high end is more like 70 W/m-K.

You're also confusing capacity with actuality. Your calculation explicitly assumes that both sides of the steel are HELD at those temperatures, and essentially are an infinite source and sink.

In actual operation, the 350ºF is quickly reduced to a lower value, and the 50ºF is quickly increased to a higher value, since the limiting factor will then be the convection cooling of the outer surface. For example, let's say at the forced air cooling provides only 25 W/m^2-K cooling. That's a 56 times higher thermal resistance, so the total cooling problem is, as it should be, dominated by the convection cooling. Additionally, the interior of the steel is not being supplied by an infinite heat source at 350ºF, so it's also limiting the heat flow to some degree. The correct temperature delta will be had with a 3 simultaneous equation solution, that would result in a much smaller temperature delta across the steel, but a substantially higher exterior surface temperature. But, even with a 150ºF surface to air delta, the maximum heat flow would be limited to about 1500 W, much smaller than your original calculation. What that means is that the temperature delta across the 0.5" steel would be on the order of 3ºF, as it should be in an operating HX.

TTFN

FAQ731-376

 

[Smokey2007]

good observation, i did accidently change the thickness of the stainless steel, so .5" is correct.

you said, "But, frankly, the calculations are not unreasonable, for a 300ºF delta across a 0.5" gap, particularly for your given thermal conductivity for steel. The high end is more like 70 W/m-K."

how are you coming up with this 70W/m-K value? looking up 304L stainless steel on the internet has a thermal conductivity of 16.2 at 100 C and 21.4 at 500 C, so shouldnt this value be between them?

in the last paragraph you mention "convection cooling" will dominate this process, so does this equate to "conduction" and "radiation" effecting this system minimally?

thanx for all the help, im am quickly trying to understand heat transfer in a couple of days, even going over that book in the previous hyperlink

 

[Smokey2007]

the more i look at a heat transfer book, the more i think im not approaching this right (duh, huh?).

the goal is to have a 350 F heat transfer oil flow through a .5" stainless steel pipe. this pipe will tranfer energy from the oil to a solid on the other side and heat it. i would like to get the solid between 200-300 F. and i am just trying to prove that this will work. commen sense says it will, but i like facts.

 

[IRstuff]

A very low allow steel will have about the same thermal conductivity as pure iron, at 73 W/m-K, but that's quite different than 304 SS.

Your original posting didn't specify, so I made assumptions about what was on the other side of the steel. Your posting talked about 12"x24" surface areas, which didn't sound much like a pipe.

Your last posting is still very nebulous as far as detailing the actual requirements. This "solid" is contained in what? What are the thermal losses from this "solid?" How much solid? What's its thermal conductivity, etc? How long do you have do warm up this "solid?" What's the contact time between the oil and the "solid?"

I understand that you haven't done much of this, but you need to describe the problem fully to get meaningful answers.

TTFN

FAQ731-376