6 replies to this topic

[sandyhj]

hello All, I want to design a coil which is used in heating a amine solution in the tank. The tank is not supplied with agitator. Actually i got many references for heating coil design for tank with agitator. However, i am unable to find any reference for designing a coil submerged in the tank. So if you are having good artical or book reference, please let me know. Thanks in advance. Regards, Sandip.

[kkala]

Although http://www.cheresources.com/invision/topic/16647-teach-me-heating-coil-pipe-sizing and therein referenced http://www.cheresources.com/invision/topic/13699-how-to-get-value-of-heat-transfer-coefficient -mainly post 4, para A2 and post 5, para 2- give potentially useful introductory information, they also point out that reliable formulas for heat transfer coefficient of coils in non agitated tanks have not been traced (by kkala). There must be such published data, reference to an article by D. Stuhlbarg is mentioned there; other articles may be relevant. Clarifications on this useful topic would be appreciated, if possible by more information than a reference to the source (especially if not easily accessible).
Besides, can sandyhj clarify heating medium inside coil? Is it steam (quite small heat transfer resistance), or something else?

Edited by kkala, 12 December 2012 - 08:14 AM.

[srfish]

Sandyhi,



There is a section in the Heat Exchanger Design Handbook called "Helically Coiled Tubes of Circular Cross Sections". It gives the equations for the inside of the coil. For the outside of the coil, use a natural convection heat transfer coefficient. For a starting coil diameter visit www.gulleyassociates.com, click on Engineering Tips and look in the Calculations Section. It gives the coil diameter to use to start the design.

[sandyhj]

Thanks kkala and srfish..!!! i will go through the links and let you know if i need further help.

I have one more doubt that which equation to be used if steam used for heating is condensing in the tube. since in that case, Nu equals to 0.023 * Pr^1/3 * Re ^0.8 can not be useful. And which properties (gas / liquid) to be used in the calculation.

[kkala]

Heat transfer coefficient in the condensing steam side is so high (low resistance to heat transfer) that its calculation is not usually required. E.g. McCabe, Smith, Harriott (Unit Operations of Chemical Engineering, McGraw-Hill, 5th edition, 1993, Table 11.2, p. 327) report for steam and water.
Dropwise condensation (rare, not to be considered) 30000 - 100000 W/m2/oC
Film type condensation (usual) 6000 - 20000 W/m2/oC
Boiling water 1700 - 50000 W/m2/oC
Water (heating or cooling) 300 - 20000 W/m2/oC
Superheated steam 30 - 100 W/m2/oC
One can adopt an arbitrary value, e.g. 8500 W/m2/oC (post No 2, 2nd web reference), or 10000 kcal/m2/h/oC (university book); its effect on overall heat transfer coefficient will not be significant anyway.
It is clarified that
(α) the overall heat transfer coefficient will be much lower, since the heat transfer in the side of amine solution is controlling
(β) fouling factor in the side of condensing steam is not included in mentioned 8500 w/m2/oC or 10000 kcal/m2/h/oC; its value is small, e.g. 1.8E-3 / 5E-4 ft2*h*oF/Btu for oil bearing / non oil bearing steam as per http://www.cheresources.com/invision/topic/12692-fouling-factor-for-steamcondensate/.
(γ) air in condensing steam will reduce its (side) heat transfer coefficient, e.g. from 9500 W/m2/oC (no air) to 5700 W/m2/oC (10% air v/v), or to 1400 W/m2/oC (20% air v/v);
this per C R Branan's "Pocket Guide to Chemical Engineering", Gulf 1999, Heat Transfer.

Edited by kkala, 15 December 2012 - 04:11 PM.

[sandyhj]

Thanks.. I believe it will help me..!!!

[kkala]

I happened to find in a local university book (J. Maragotzis, Chemical Reaction Engineering, 1969, in greek) that the heat transfer coefficient inside the coil can be estimated by the corresponding heat transfer coefficient inside straight tubes (*) multiplied by the factor (1+3.5d/D), to account for the increased turbulence due to spire curve. d=inside coil diameter, D=spire diameter.
(*) not for condensing steam; but for fluid, the heat transfer coefficient of which can be estimated e.g. by Sieder-Tate formula.
E.g. 1.5" Sch 40 coil of a spire of D=1 m would give a factor of 1+3.5×40.9/1000 = 1.14, which would be insignificant for large values of D.
This is noted as an alternative for the Reader searching for information on "heating coil design". For condensing steam inside the coil, please refer to post No 5.