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thank you that was helpful.

OK, I agree with all that and I know I need NFPA 79. But I am asking if it sounds like my machine needs some sort of hazardous location certs. I would like to know if these certs would be needed to address the very concern you are talking about: certified safety. But if I have obviated the need...

Hi, I would be cool with any cert that I would need to do, if I knew if I needed it, and which one. I am building a machine, and I will certainly need to follow NFPA 79, becuase its a machine with a control box. fine. Here is my question: The machine will not be used in a hazardous location...

Prex, Yeah, but the way that I set it up is that the longer the tube the more power goes into it. I just picked a length a found a qdot that I used for whatever length. so I'm not sure I missed the point, I just used a guess at a particular instance and extrapolated form there. I know its not...

I think if you go back to my 11:35 posat on oct 22, you'll see that I understand about the temperatures and picking htem. Which is fine. I was having trouble regardless of what they were, that is why I kept everything as variables. Here is what I ended up doing (for those that may search for a...

you know what? never mind http://www.watlow.com/literature/specsheets/files/heaters/batcx20000605.pdf

Still being dense and still not getting my answer: Ok, I looked at incropera book again. I see the example you are talking about but it is not answering what I need. As per MintJulep, I know the amount of heat going into my gas stream. I need to know how long a tube needs to be to get my temp...

MintJulep, Ah, OK it was just a misunderstanding. I have a 300W heater. I dont know how much will go into the gas (yet). But yeah, for the purposes of this analysis I think either one is fine. unotec, I'll take a gander hopefully it is there. thanks.

mintjulep I dont get that. the only power input to the system is to the heater, this heat must be shared between the loss into the flowing gas stream and the loss through the insulation to the ambient air. So why would you assign the entire 300W to q2? Compositepro I have control over the...

more htoughts: So, I can find the heat going into the fluid by this formula: q=mdot*Cp*(Tout-Tin) then to find the pipe temp q=hA(tpipe-Tgas) where tgas is the mean temp of the gas Sound right? This will let me choose a length that I like and find the pipe temp and the power input. Please...

I sort of agree, except as I mentioned I have a fixed power input and fixed insulation thickness. The way I am attacking this right now, is to set up a lumped model. Looks like this: <-q1 q2-> Tinf-/\/\/\/\-Tins-\/\/\/\-Ta-\/\/\/\/\-Tpipe-\/\/\/\/\-Tgas...

OK here we go: unotec q is heat flux in watts heat is a quantity of energy in joules heat rate,q, is a quantity of power in watts heat flux,q", is a quantity of power through a surface W/m2 volumetric power generation or flux, qdot is a measure of power generated over an entire volume. ok...

ah.. sorry irstuff. No I will be controlling the temp to within +/- 3 degrees so around 2%

"If you have a flow, and know the gas characteristics, you can calculate how much residence time inside the exchanger you require" I totally agree. I am asking how.

"assuming you have sufficient length" this is what I am trying to determine. For the details I posted above, if the answer is 5cm then yeah +/-90% is just fine. But if the 'sufficient length' is 200 cm, then my accuracy must be better. Let me ask a different way: Is 90cm sufficient? is 30cm...

Actually, now that I htink about it.... even if all the heat goes into the pipe, why doesnt it matter what the length is? it takes power to raise a temperature of a substance and time to raise it based on its heat capacity. So if the differential volume of gas isnt in the pipe long enough to...

yeah see, that is the problem, clearly that never happens, so where do I go from there? the thing in insulated, but of course heat leaves, but I dont really have a good way to measure how much. Even if I did, it would just tell me that 5W is leaving through the insulation and 295 must be going...

I've searched for this throughout the forums. Its seems so simple, but I am simply not getting it: I have a pipe with gas flowing in it. I know the input temp and output temp I want. I know how much power I am going to put to the heater I know the diameter. I know the flow rate. I think I know...