4 replies to this topic

[ashishpvnit]

Dear Experts,

I was going through design of bottom heater of HFO storage tank.There i have seen value of heat transfer coefficient was taken and further then heat load calculation & steam flow rate to heat the HFO oil was calculated.

My question is how can we get the value heat transfer coefficient for Steam & HFO oil.Is there any table , empirical chart or standard available.

Thanks in advance.

Regards

[ankur2061]

ashishpvnit,

For condensing steam applications in coil heaters for tanks, the recommended HTC is 1500 Btu / h-ft²-°F (8500 W / m²-K). This is as per the classical heat transfer book "Process Heat Transfer" by D. Q. Kern. Chapter 20 , example 20.1 of this book mentions this value.

Hope this helps.

Regards,
Ankur.

[breizh]

other resource :

http://www.cheresour...xchangers.shtml

Breizh

[kkala]

Following could be helpful in estimating overall heat transfer coefficient.
Before the estimate, specify the temperature of the stored fuel oil to flow freely in suction line (say ~ 220 cSt viscosity, to my opinion) and estimate max heat loss from tank to ambient for the already specified fuel oil temperature (http://www.cheresour...for-tankvessel/).
A1. Assume a fouling factor for fuel oil, say 0.0010 m2*h*oC/kcal, http://www.engineeri...ng_factors.html. This value is also given by Perry, 7th edition (1997), Table 11-4. If you have a specific value by experience, so much the better.
A2. Formula for partial heat transfer coefficient has not been found in Perry, only a reference (Stuhlbarg, D., "How To Design Tank Heating Coils," Pet. Ref, V. 38, No. 4, p. 143, 1959). Data for rough estimate is as follows (U = overall heat transfer coefficient).
-Perry 7th ed, Table 11-2, steam 110-140 psig / vegetable oil, no agitation, overall U = 110 - 140 kcal/m2/h/oC.
-http://www.spiraxsar...and-jackets.asp, steam 2-6 barg, heavy oils, overal U=70-96 kcal/m2/h/oC.
A3. Assume U=80 kcal/m2/h/oC, that is resistance 1/U = 0.0125 m2*h*oC/kcal. Even though fouling factor and U may not be based on exactly same area, it seems that fouling factor is the limiting stage in this heat transfer. Resistance from steam side 1/7320 = 0.00014 m2*h*oC/kcal; and (assuming 2" sch80 pipe) coil thickness / thermal conductivity =5.5E-3 m / 37 kcal/m/h/oC = 0.00015 m2*h*oC/kcal.
A4. Consequently U~80 kcal/m2/h/oC. depending on assumed fouling factor. When the coil has been "cleaned", much higher heat transfer rate will occur. Steam supply can be controlled by stored fuel oil temperature.
Notes: -Fins on the steam coil are not judged suitable, due to scales.
-Steam coil design, supply & installation is undertaken here by specialized firms, having the necessary design tools for precise data.

Editing note 7 Dec 11: Statement for fouling factor corrected, as it actually represents ~8% of overall resistance (not 80%).

Edited by kkala, 07 December 2011 - 03:27 PM.

[kkala]

“Process Heat Transfer “ by D. Q. Kern (McGraw-Hill, 1950) was found and example 20.1 in Chapter 20 was looked into. It is noted that a fuel oil tank has no mechanical agitation and is heated through a steam coil close to bottom by free convection.
1. Example 20.1 does not correspond to the fuel oil tank configuration, since it is about a small jacketed tank with paddle agitator. Kern's Fig 20.2 on previous page indicates heat transfer coefficient from coil, but for agitated tanks (sort of agitator?). Steam side reported heat transfer coefficient = 1500 Btu/ft2/h/oF = 7320 kcal/m2/h/oC can be generally used. It is not controlling heat transfer (especially in case of fuel oil), thus precise value is not of significance.
Kern's heat transfer coefficient to the liquid (forced convection) has the form Nu=αRe^(2/3)*Pr^(1/3)*(μ/μw)^0.14, α=0.36, μ= bulk liquid viscosity, μw= viscosity at metal temperature (Re=modified Reynolds No). A local University book extends the formula, giving α=0.36 - 1.50 for jacket or coil under agitation (formula by Chilton, Drew, Jebens, as generalized by Achley).
2. For free convection Kern (p. 721 & 215) advises approximate outside coefficient h=0.50(Δt/d)^0.25 for horizontal pancake coils (Δt oF, d=outside diameter in, h Btu/ft2/h/oF). This seems not appropriate for the case, giving low values of h (e.g. for Δt=60oC & d=2.375", h=6.3 kcal/m2/h/oC). One has to search for correlations developed after 1950, e.g. by Stuhlbarg, or roughly estimate overall heat transfer coefficients from examples for cost estimation only. Refer to post of 29 Nov 11 by kkala.
3. It is again noted that design & supply of heating coil(s) can be implemented by a specialized firm. The heating coils are often placed in pairs (one operating, the other standby), each one covering whole tank bottom. At any case estimation of necessary stored fuel oil temperature and heat losses from the insulated tank have to be made by the Engineers of the company owning the tank.
4. Several issues of fuel oil tank design and its heating can be found in http://www.cheresour...il-tank-design .
In addition to keeping constant fuel oil temperature balancing heat losses, tank heating may also intend to heat stored fuel oil from t1 oC to t2 oC in so many hours. This option usually requires higher heat supply compared to the conditions stated in para 3 above, or in the post of 29 Nov 11. See Perry, 7th edition (1997), p. 11-20 to 22, Thermal design of tank coils.

Edited by kkala, 12 December 2011 - 01:03 PM.