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Heat Loss From Fluid Flowing Through Uninsulated Pipe


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#1 process85

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Posted 05 August 2008 - 11:18 AM

Hi everybody

This is my first time in this forum.
I have a problem. During detailed engineering I did not apply heat conservation to a pipe which is used to heat a fluid.

I need to find out reduction in temperature during cold days when ambient temperature is expected upto 15 degC.

I am attaching the whole scenario and my analysis. Please help me find find the temperature at the inlet of exchanger.

Attached Files



#2 ankur2061

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Posted 05 August 2008 - 02:10 PM

Process85,

First of all I cannot understand your logic of not having any insulation on a 500 m length piping with a process temperature of 220 deg C. This is certainly not good engineering practice. You definitely need to provide heat conservation insulation. To me, providing insulation doesn't seem to be such a big problem. The heat loss calculation for insulated pipe is calculated as follows:

q/l = 2.PI*Km*delta T / Ln (Do/Di)

where

q/l = heat loss in Btu/h/ft

Km = Thermal conductivity of insulation material, Btu/ft*h*degF @ avg temperature

delta T = Process Temperature, deg F - Acceptable Surface Temp at top of insulation cladding, deg F
(Note: In your case process temp is 220 deg C or 428 deg F. An acceptable surface temperature would be 49 deg C or 120 deg F. However, the end user can decide the surface temperature at the insulation cladding surface).

Do = Diameter of pipe including insulation thickness, feet = OD of bare pipe + 2*insulation thickness

Di = OD of bare pipe in feet.

It is important to note that for a given insulation thickness, higher the delta T i.e. lower the surface temperature selected at the cladding surface, higher the heat loss.

An industry thumb rule is to select an insulation thickness to have a heat loss less than equal to 100 kcal/h/m2 (36.9 Btu/h/ft2).

Hope this information helps.

Regards,
Ankur.

#3 ankur2061

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Posted 05 August 2008 - 02:42 PM

process85,

Also check this out for heat loss through steel pipes with surrounding air:

http://www.engineeri...-loss-d_53.html

Regards,
Ankur.

#4 process85

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Posted 06 August 2008 - 10:49 AM

Thanks Ankur

But i think u haven`t understood my problem.
My problem is that we cannot provide the insulation at this stage(AFC) as the pipe runs over piperack(which i have clearly mentioned"See Attached file") and to provide insulation will require space which cannot be accomodated inside piperack as it requires shifting all pipes on the piperack.

I wanted to know the heat loss through the uninsulated pipe over a length of 500m so that i can find the temperature of the fluid at exchanger inlet and the heat duty given by this fluid to heat the shell side fluid.
And the excess duty required could be provided by the other exchanger in series with this exchanger, which uses steam as heating medium.

I hope u will understand the problem now and reply asap.

#5 ankur2061

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Posted 06 August 2008 - 12:20 PM

Hi,

My 2nd posting gives the answer for estimating heat loss from bare steel pipe in W/m for a given pipe size and for a given delta T. In your case the delta T is 220-15= 205 deg C and the pipe size is 12" or 300 NB.

Regards,
Ankur.

#6 JoeWong

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Posted 07 August 2008 - 12:21 AM

process85,
I can not download your file...

This link could be interesting to you...
Insulation Calculation Programs...http://www.cheresources.com/inssoftwarezz.shtml

#7 siretb

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Posted 07 August 2008 - 01:18 AM

Hi Process85. I cannot download the file/scenario. Link missing or broken. Please send again

#8 process85

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Posted 07 August 2008 - 08:15 AM

Sorry everybody while modifying the file i forgot to upload the file.
Attached is the file for ur reference .

Attached File  Heat_loss.xls   33KB   729 downloads

#9 khor0023

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Posted 02 October 2011 - 10:38 PM

Hi, I would like to ask if the formula provided includes conduction and convection heat transfer - or is it used as a rough estimation?

Thanks!

#10 ankur2061

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Posted 02 October 2011 - 11:10 PM

Jason Khor,

The above formula is basically Fourier's law for thermal conduction. The convective heat transfer (loss) would also include wind currents (wind velocity).

This formula gives a fairly good approximation of the heat loss considering that the surface temperature on the cladding over the insulation remains nearly constant over a given time duration. This is specially true when you are considering heated insulated piping inside a building or enclosure.

One of the practices which we used to follow while performing heat loss calculations for equipment / piping was to provide a factor of safety of 1.5 to 2 to the calculated heat loss to account for any uncertainties while designing a heating system. This approach ensured that we never underdesigned the heating system.

Hope this helps.

Regards,
Ankur.

#11 DB Shah

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Posted 03 October 2011 - 01:09 AM

Dear process85

My experience which relates to your case-
We have ~ 500 mts of 4" uninsulated CO2 line. CO2 at source is 20 Barg & 205°C & at ambient conditions of 30°C, guess CO2 temperature at exit, It was 34°C, APPROACH OF JUST 4°C, Both temps have indication on DCS, I physically touched the line at exit plant battery limits.

Considering the line as a single tube exchanger I backcalculated the Ud as 8 kcal/hr-m2C (In Aspen you have an option to plug ambient temp & U value for pipes to match outlet temp).
We have another uninsulated CO2 line of 12" 600 mtrs where CO2 at -5°C is sent to another plant. First 10 mts of the line is SS and rest is CS. Here also I back calculated the Ud & it was in the range of 6~7 kcal/hrm2C.

I think you can safely take the values of ~8 kcal/hrm2C to cal your heat loss and exit temp.

#12 breizh

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Posted 03 October 2011 - 01:23 AM

Hi ,

Consider this resource .

Hope this helps

Breizh

Attached Files



#13 kkala

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Posted 05 October 2011 - 01:47 AM

Following post by DB Shah, I remember we were advised to use an overall heat transfer coefficient of 5 kcal/(m2hoC) for quick rough estimate of heat losses from uninsulated liquid tanks in a fertilizer factory (1979). This complies with his recommendation.
Exposure to wind seems the strongest factor. For instance see http://www.actahort....ks/76/76_55.htm; it concerns heat losses from greenhouse, but it gives an idea.
In an old article about heat losses & insulation (now lost), external heat transfer to ambient air increased exponentially with wind velocity. It is worth while searching for such articles in litterature.

#14 DB Shah

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Posted 05 October 2011 - 05:02 AM

@kkala
I forgot to mention that the prevailing wind velocity was in the range of 4~ 8 kmph = 1~2 m/s.

#15 kkala

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Posted 05 October 2011 - 06:47 AM

I forgot to mention that the prevailing wind velocity was in the range of 4~ 8 kmph = 1~2 m/s.

This makes recommendation of 6-8 kcal/(m2hoC) more useful, for quick estimates of heat loss from uninsulated pipe.
Note: by the way, reported heat loss from bare pipe to environment may not be precise in the book "Heat transmission" by W. McAdams (3rd edition, McGrawHill, 1954). I read it in the Web in about 2002, though this does not lower the whole book value (hard, systematic work, badly needed nowadays).

Edited by kkala, 05 October 2011 - 06:48 AM.


#16 jithin

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Posted 16 February 2012 - 03:50 AM

Got the heat loss formula and i thank everybody for publishing these informations. I would like to know about the thickness selection of the insulation of both hot and cold type. If you have any formulas for that please wite it down for me.

#17 S.AHMAD

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Posted 16 February 2012 - 04:51 AM

Dear process85

1. You are interested to determine the temperature at the HE inlet, right?
2. Since DBShah has given you the overall heat transfer coefficient and use this value for your iteration/calculation.
3. i suggest following steps
Step 1 - assume temperature T2
Step 2 - calculate LMTD
Step 3 - calculate Q1 = U.A.LMTD
Step 4 - calculate Q2 = M.C.(T1 - T2)
Step 5 - repeat step 1 until Q1 = Q2
I hope the above helps.

Edited by S.AHMAD, 16 February 2012 - 04:52 AM.


#18 JMW

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Posted 27 February 2012 - 07:09 AM

Apart from energy conservation a very good reason for insulation is health and safety.
Anyone working in any space, especially confined or difficult to access space where there are uninsulated pipes carrying very hot or very cold fluids, is at risk.
Are there no safety standards that cover this where you are?

#19 pjsit

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Posted 18 August 2012 - 06:08 AM

I am looking for the specific formula used to determine BTU loss per linear foot per hour on a piece of uninsulated steam or process pipe. Is there a complete formula that takes into account both heat transfer through conduction and has has the option to account for wind speed as a factor. Ultimately, I would like to determine savings through insulation in terms of natural gas. So a comparison of a pipe that is uninsulated and insulated and the differences would be helpful. There is a piece of software called 3E but it can only calculate one line at a time. I would like to be able to use the formula as an addition to my estimating for pipe insulation and determine a payback on insulation upgrades.

Thanks in advance.
Patrick

#20 kkala

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Posted 25 November 2012 - 03:10 PM

It could be helpful to look at http://www.cheresour...air/#entry66564 '> http://www.cheresour...air/#entry66564 . Heat insulation article is referenced and discussed, together with heat losses. In post No 3, para 2.2, formula to specify influence of wind velocity on heat losses has been requested. More contributions may be realized.

#21 ElSid

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Posted 11 January 2013 - 11:05 AM

pjsit
The formula is very basic heat transfer. Being a Mechanical in a CHE forum, the easiest example I can link to is http://www.raeng.org...2_SteamPipe.pdf
I have worked with Dr. Stevens on the MathCAD forum.

#22 kkala

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Posted 11 January 2013 - 11:55 AM

If anybody could advise a formula to indicate effect of wind velocity on heat loss, this would be helpful (post no 20).

#23 breizh

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Posted 11 January 2013 - 08:25 PM

Kostas,

You may consider this resource regarding the effect of the wind .
Hope this helps

Breizh

Attached Files



#24 Janarthanan

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Posted 16 January 2013 - 06:42 AM

For heat loss through uninsulated or bare pipe is found using ASTM C 680... which uses the langmuir's equation and also please check the text book "Thermal Insulation by John F Malloy", because i have calculated the heat loss for a steam pipe line using the above resources. ASTM C 680 you can find in the following link

http://www.hrsgdesign.com/design0.htm

Check in the publications link on that page !!!!

Edited by Janarthanan, 16 January 2013 - 06:43 AM.


#25 kkala

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Posted 20 January 2013 - 03:25 PM

Thanks breizh for the nomograph of heat loss from tanks (insulated or not) to ambient air.
1. In the example of 17000 gallon tank, total heat transfer coefficient (no insulation) is 250 kW / (1025 ft2 ×220 oF) = 18.5 kcal/(hm2oC) for still air. This is higher (that is more conservative) compared to previous posts, probably incorporating some safety factor.
2. Wind increases heat losses by a factor up to 3.3, for velocities up to 56 km/h, also depending on surface temperature (the latter considered for the case of still air). Actually wind substantially lowers surface temperature, which may explain why mentioned factor gets so low (even lower than wind velocity =0) at strong wind combined with high surface temperatures: temperature difference (between surface and ambient) is much smaller now compared to still air.
Other interpretation welcomed.

Edited by kkala, 20 January 2013 - 03:26 PM.





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