swguru,
as usual, i have second thoughts on your question and my answer.
i assumed a constant pipe wall temperature at 22C, per your original data on soil temperature where the pipe is buried. my thinking is that (in a first-cut calculation) the air mass flow rate and net heat transfer...
swguru
check your viscosity .... should it be 1.66E-5 ?
also, i think k = .02625 (assume 35C)
vc66 is right about the velocity, but i am calculating h = 23 inside the pipe (probably due to our different viscosities). if i use your h = 13, i get Texit = 30F as you do.
for a 10C temperature...
after more minutes of thought...
during the transient, the heat transfer out of the insulated metal wall would be much much less than the heat into the wall from the internal side.
possibly you can rationalize that all the heat transfer (only) during the transient is simply from the hot...
if you doubt the FEA results, possibly you should talk thru the boundary conditions you used (maybe over on the finite element forum?).
if you check the incroprera text recommended by desertfox, section 3.3 gives the overall U for a multilayered pipe.
after calculating U, you could probably...
i agree with dvd .... get a reality check from the hardware people.
but, based on the numbers, i don't think you can get there from here...
assuming a volume of 1250cuft
time(sec) = 1250*60/CFM
mass = 1.2 * 1250 * .0283 = 42.5 kg (inside the box)
Q = 250 Kw = (mass / time) * Cp * DeltaT
=...
thermoD
the air is only heated while it's in contact with the heating element.
at 20,000 cuft/min = 333 cuft/sec * 10 seconds = 3330 cuft.
that's a cube about 15 ft on a side (IF it's a cube)..... and that does not include the volume of the heating coils.
is that about how big this heater is...
this topic is well covered in textbooks...... see...
the textbook Heat and Mass Transfer, Yunus Cengel, Section 9-4 .... deals with exactly this topic, equations and an example.
the textbook Fundamentals of Heat and Mass Transfer, Incroprera and Dewitt, Section 9.7.1 .... equations and...
if i drew the sketch correctly...
the wall at the bottom of the channel is 0.2cm thick (2 - 1.8) .... I'd use that as the plate thickness for heat transfer calcs.
need to calculate the convection coefficients on both sides (channel and utility stream) and the t / k for the 0.2cm wall.
looks...
virk
i wrote that yesterday and threw some numbers in there to get the spreadsheet running. in fact, i am still writing up that topic (just added more stuff this morning, if you check the web page again).
regarding the velocity ... note that the mass flow rate per hole (mdot) is a user input...
virk
i took this question a step further, and added impingement to my class notes (which i have wanted to do for years!)
this is a simplified version of the first paper that i quoted here, but it will get your first calculations in the ballpark.
i would NOT design critical parts to this...
before you decide what to do, find out what happened in the Boston "Big Dig" project.... epoxy was used to hold bolts in, and they let go in short order, part of a tunnel collapsed and killed some people.
now there is much legal / political / engineering turmoil (not to mention the deaths!)...
we used impingement cooling inside turbines.
the effects of "spent" flow (or "crossflow") are significant.
an important paper(maybe the best) is:
Steamwise flow and Heat Transfer Distributions for Jet Array Impingement with Cross Flow, by Florschuetz, Truman, Metzger, ASME, 1981. (specifically...
hi there
i cannot help with the building code issues or with the details of the required input values for this particular problem.
and i'll spare your client my tedious lecture on "do it right or don't do it".
but the basic equations for addressing this type of problem are here...
GBor
I stand corrected.
NRL7396 (that you refer to) is *not* the NRL1396 that I referred to.
They are separate documents.
regards,
magicme
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there's no place like gnome.
recent postings here on shock levels has me recalling DDAM shock analysis.
I did several DDAMs over the years, all applying to large shipboard system, and they were the worst experiences of my professional life. We used large scale FE models, and the criteria was always that no "critical" item...
