Performance evaluation of a thermal oil heater 1

How could we determine the correct efficiency of a thermal oil heater utilizing two vertical helical coils with a radiation and convection sections and uses a diesel fuel? Could anybody also extend some information why there are abnormalities with regards to the emission of flue gas, i.e. exceeding the designed stack gas temperature? And lastly how could I be able to determine the temperature of combustion gases? Thanks for any help.

 

[Jonkatz]

From the way you phrase the question, I assume that you have an operating furnace, and are trying to benchmark the performance? If that is the case, the you can determine the efficiency of the heater by accurately measuring the following with thermocouple and Flue gas analyzer:
- Flue Gas temp Exiting Convection bank
- Flue gas composition (%oxygen)
- Combustion Air temperature
You also need to know the Diesel fuel's Lower Heating Value(LHV) With the collected data, you then need to estimate the enthalpy (h of FG) of the flue gas at the stack temp and the enthalpy of AIR. You'll need to calculate the Air/Fuel ratio (AF) and the Flue GAS/Fuel ratio. (FG/FL)

Then Fuel Efficiency (%)can be estimated from:
[((LHV + (h-air x AF))- (h of FG x FG/FL)) / (LHV) ] x 100

(You should then subtract a loss percent from that to account for heat loss through the casing).
Assuming I understood the question, hope that helps.
As for reasons why the stack gas temp may exceed design values? It can depend on alot of factors: How close are you operating to the "design point" of the heater? Fouling in the convection section could raise stack gas temperature. Poor gas distribution over the convection tubes could also cuase this. We need to look into more detail to determine what was causing high gas temps.
- j. katz

 

Mr.Jon Katz,
Thanks for giving your precious time for my question.Actually getting the efficiency is just one of our basis on whether the thermal heater is functioning properly and efficiently, but the real problem is actually the high emission of the flue gas temperature. The heater is supposed to be rated from 1000-3900 kw and at present, it is supposed to supply only 3000 kw but the heat users cannot functions at its rated capacity because the heater exceeds the designed flue gas temp of about 390C which results to the frequent shutdown of the operations.For further information we had just done the recent flushing of our thermal heater for we thought there might be some coking that is happening inside. We had also conducted an X-ray testing of our pipes for possible cracks and defects on the tubes and inspect them for soot deposits that may be contributed by the combustion of the diesel fuel. Can you please tell me how can I get an accurate calculations in getting the enthalpy of air and the flue gas. Again thanks hope to hear from you soon. Best regards!

-tequilaalloy_15(archela/chemical)

 

[Jonkatz]

What is the function of this heater? What type of OIL is being heated? Is this an OLD heater?
Air and Flue gas enthalpies can be taken from any number of Eng. handbooks and will be equally accurate enough for what your trying to do. Maxwell's Hydrocarbon HAndbook has curves for both. If you can get your hands on an API databook, they should have correlations for enthalpy of all the component gases of AIr and flue gas.
-j. katz

 

Mr. Jonkatz,
Good Day! The vertical heater with dual tangentially wound helical coil, has a radiation-convection sections.The burner is placed at the top and the combustion air is not preheated.We are currently using a Geka type burner(THZ 25-THZ 60) with fabrication No. 5827.We are using this equipment since 1982, and it was only lasy year that we are experiencing such abonormalities like high flue gas(T >390C)emissions; another information is that the dt of inlet (255C) and outlet temperatures (265C) of thermal oil was only ranging from 10-15C instead of the usual (250-275 or 260-280C)and even though the rated capacity is 3900 kw the fatty acid section cannot function at its rated capacity which requires only 3100 kw.
This heater supplies heat at the heat exchanger and distillation stills of the fatty acids distillation section.The fuel used in the burner is the diesel fuel with LHV of about 10,200 kcal/kg.Hope you could enlighten us more regarding this matter.By the way do you know any calculations in determining the approximate temperature of combustion gases? Is that also similar in computing for adiabatic flame temperature. Do you have any idea what should be the possible range? Thank you.

regards,
tequilaalloy_15(archela/chemical)

 

[Jonkatz]

I am trying to understand your problem better. If I interpret correctly, the arrangment of this heater is:
a Dual Helical coil heater, fired from the top, THe radiant section is the contained by the Helical coils? Where is the "convection section" - is there a seperate bank of tubes? or is the convection section "BEHIND" the radiant helix tubes? So you are saying that the heater used to work fine until last year? and then the flue gas exit temperature increased AND the temp. increase of the oil was reduced to only 10 deg C? and it used be 250 deg C?
ARE you sure you are supplying enough flowrate of thermal OIL? You can do a heat balance to determine if your measurements are correct. You can measure the fuel heat input from the flowrate of diesel fuel. the Heat output from the flue gas measurements. THe difference is absorbed by the oil. SEE if it balances using the properties and flowrate of the heating oil. ALSO, measure the pressure drop! if it has increased since the time when it worked properly, that could be a sign of coking or fouling.
Calculateing the Flue gas temp in radiant section is not that simple. You have to perform thermal hot gas radiation heat transfer calcs to predict the Avg. radiating temp. THe adiabatic flame temperature is not appropriate because the gas/flame is loosing heat. THAT number will only give you a maximum possible flame temp in the area of the flame just exiting the burner. This THEORETICAL number usually ranges from 2500-3000 deg F depending on the LHV and percent excess air. THE best thing to do to figure out whats going on inside that heater is to put thermocouples into the FLUE gas & radiant section.
- hope this helps you.
j. katz

 

[Cormoran]

Dear Tequilalloy-15, I suggest the following.
Check flow of thermal oil. (Jonkatz)
Send one liter (or two) to thermal oil manufacturer for test and recomendations. Urgent!
Check for soot in radiation and convection section of thermal heater.
Check short-circuit of combustion gases through radiation section and outlet. (Damage in refractories). Hope this helps you.
Alfredo