Jump to content



Featured Articles

Check out the latest featured articles.

File Library

Check out the latest downloads available in the File Library.

New Article

Product Viscosity vs. Shear

Featured File

Vertical Tank Selection

New Blog Entry

Low Flow in Pipes- posted in Ankur's blog

- - - - -

Heat Transfer Coefficients For Air


This topic has been archived. This means that you cannot reply to this topic.
5 replies to this topic
Share this topic:
| More

#1 Steve Hall

Steve Hall

    Gold Member

  • ChE Plus Subscriber
  • 167 posts

Posted 23 November 2012 - 03:56 PM

I am calculating exit temperature of a high temperature vent stream (air). The vent line runs indoors, within 0.5 m of the building roof. The pipe is stainless steel.

Accepted correlations for the Nusselt number include the Sieder and Tate equation:

Nu = 0.023 Re^0.8 Pr^(1/3) (ub / uw)^0.14

And since the Reynolds number follows:

Re = D V ro / u

the Nusselt number (and therefore the inside heat transfer coefficient) varies by the velocity to the 0.8 power.

After converting from Nusselt to inside coefficient, I obtained the following data:

Velocity hi Re
3 m/s 12 W/m2-K 5,200
5 19 8,800
8 28 14,000
12 38 21,000
20 56 35,000

The Rule of Thumb for the outside heat transfer coefficient ranges from about 30 to 50 W/m2-K (quiet to 5 m/s wind) when considering heat transfer from a pipeline to the environment (see Chris Haslego's article, http://www.cheresour...with-insulation).

Now, my question: Why are the calculated inside heat transfer coefficients so much lower than the Rule of Thumb for outside coefficient? Does this make sense? If the flow rate drops into the laminar range, the calculations (using correlations for laminar conditions) give much lower results.

Calculations were done at a bulk temperature of 135 C, atmospheric pressure, 50 mm inside diameter. See attached Excel spreadsheet.

Secondary question: I can calculate the wall temperature, which for this case is around 40C (room air temperature is at 30C). I can also compute a radiation factor (see Chris's article) using the wall temperature. But is this really valid indoors, not knowing anything about the various surfaces that are within sight of the pipe? It does make a difference -- the radiation heat transfer is about 20% of the convective transfer -- but I'm leaning toward neglecting it because I think the calculation is overly optimistic.

Attached Files



#2 breizh

breizh

    Gold Member

  • Admin
  • 6,292 posts

Posted 24 November 2012 - 10:36 PM

https://docs.google....iZATx6wswpcfjUA

Hi Steve ,
Will it help?
Breizh

#3 kkala

kkala

    Gold Member

  • Banned
  • PipPipPipPipPip
  • 1,939 posts

Posted 25 November 2012 - 01:45 PM

Below is an interpretation on the topic; critical comments are welcomed.
1. Inside heat transfer coefficient (hi).
Calculations and air properties seem correct, http://www.engineeringtoolbox.com/air-properties-d_156.html. Going into the spreadsheet, Nu numbers result in slightly higher hi (14, 21, 31, 43, 64 W/m2K), which does not practically affect conclusions. It is the outside heat transfer coefficient to be looked into.
2. Outside heat transfer coefficient (ho).
2.1 Attached "HlossP.xls" estimates about 8 W/m2K for 2" bare pipe at operating conditions, regarding heat losses to ambient air by natural convection (wind speed = 0 m/s).
2.2 Attached "windheatloss.doc", taken from Chris Haslego's article reported in post No 1, seems to estimate about 5.3 Btu/(hft2oF) = 30 W/m2K for 2" bare pipe at operating conditions and 1.5 m/s windspeed. Even if radiation is included, result 2.1 is at lower base than 2.2.
Nevertheless hi and ho as above are close to each other (same order of magnitude).
Formula to estimate ho in function of wind speed from others would be appreciated.
2.3 Concerning the article mentioned above:
2.3.1 Value of 50 W/m2K in mentioned article seems to represent heat losses to ambient air for bare 3" steel pipe, wind velocity =20 ft/s, 300 oF temperature difference (metal wall - environment). See page 4, Figure 6.
2.3.2 Table 5, surface resistances, seems to indicate 1/ho; for 100 oF (55 oC) stainless steel wall temperature difference, it seems ho = 7.9 W/(m2K), versus 4.5-6 W/(m2K) per para 2.1 (the latter steel pipe, without radiation). Agreement is rather good between the two, but 2.2 would give a higher value, at least ho = 17 W(m2K). Values of ho reported in the article may be on the high side, yet advice is needed for this (welcomed).
2.4. It is noted that http://www.cheresour...ninsulated-pipe '> http://www.cheresour...ninsulated-pipe roughly gives an overall heat transfer coefficient (U) to ambient air of 7-9 W/(m2K) for bare pipe (wind speed 1-2 m/s); ho is anticipated to be actually smaller than hi, so closer to the value of U.
3. I have not distinguished where radiation is in the article; but would include it, even if relevant data were vague. Metal wall temperature is anticipated to be higher than 40 oC, rather close to 135 oC (than to 30 oC), since there is flow inside the pipe.
Note: When heating a room during a cold winter day, we can still feel cold at 22 oC air temperature, if the room walls are still "frozen". Our body radiates heat to walls.

Attached Files


Edited by kkala, 25 November 2012 - 02:40 PM.


#4 Steve Hall

Steve Hall

    Gold Member

  • ChE Plus Subscriber
  • 167 posts

Posted 25 November 2012 - 05:23 PM

Thank you Breizh and koala. Your information and data are spot on. I'll reevaluate my analysis paying attention to ho, and use the example in the google docs presentation to help validate my model.

#5 kkala

kkala

    Gold Member

  • Banned
  • PipPipPipPipPip
  • 1,939 posts

Posted 26 November 2012 - 03:16 AM

One could say that hi is anticipated to be higher than ho (see post No 3), since both refer to air and velocity in the vent pipe is significant, while that of ambient air is close to 0. Heat transfer resistance is higher in the side of ambient air; thus pipe metal temperature has to be closer to 135 oC than to 30 oC (on the condition that fouling factor is insignificant).
Mentioned points of Cheresources article "Making Decisions with Insulation" may be worthy of clarifications by knowledgeable members; same for secondary queries expressed in above posts.
Conclusions would be pleasantly read, when Steve Hall has accomplished his analysis

#6 Steve Hall

Steve Hall

    Gold Member

  • ChE Plus Subscriber
  • 167 posts

Posted 26 November 2012 - 11:24 AM

Attached File  Vent air temperature calculation.xlsm   64.65KB   1056 downloadsThe completed spreadsheet is attached. I didn't pull out the intermediate values into the spreadsheet, only the answer, which is calculated in a VBA subroutine. However, if you want to review intermediate values you can do so (tediously!) by going to the last line of the Function Temperature() macro where Temperature = Tout. Change to any of the other variables (U, hi, ho, etc.), return to the spreadsheet and recalculate. The reported result (in Cell B30) is then the variable that you've set Temperature equal to.




Similar Topics