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Size An Indirect Fired Water Bath Heater With Force Draft Burner

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#1 M.Shafahi


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Posted 14 August 2015 - 12:02 AM

Hi everyone,

I am working on indirect fired bath water heater for CGS&TBS. From past up to now we used natural draft burner on the heaters according to API12k, but now we are trying to replace forced draft burner with atmospheric ones. I appreciate it if you help me how to resize our heater to match with forced draft burner. I read in articles that size of heater reduses noticeably by using forced draft burner.

Mehdi Shafahi

#2 Bobby Strain

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Posted 14 August 2015 - 12:45 PM

Let me see if I can interpret your query.


You have a fired water bath heater for CGS&TBS, something unknown to me. It has a natural draft burner. You want to replace the burner with a forced draft burner. Correct me if my interpretation is wrong, to make it clear to others who might help you.



#3 M.Shafahi


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Posted 20 August 2015 - 03:12 AM

Hi Boby, ok, I make it clear, I mean from CGS is city gate station which used to reduce the pressure of natural gas from 250psi to 60 psi before each city.

In this station a water bath heater designed according to API 12k is used to compensate temperture reduction of gas in regulator. This heater regularly equipped with natural draft burner but now we are asked to replce it with forced draft burner for energy saving.

Now I'm trying to resize heater becuase rate of heat transfer by using forced draft burner is increased and therefore size of the heater is expected to decreased. Unfortunately I don't have much experience about boiler sizing calculation, I appreciate it if any friend help me about it.




#4 Bobby Strain

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Posted 20 August 2015 - 09:13 AM

Are you replacing burners on an existing unit? And, you never design such equipment. If it is a retrofit, talk to the heater vendor. If it is new, simply provide vendors with a duty spec.



#5 Art Montemayor

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Posted 20 August 2015 - 10:26 AM



Like Bobby, I’m also confused; but I’m also curious about what you state because it seems to contradict itself.  For example:


A natural draft burner is the simplest method of heating up a batch of static water - which is what a water bath heater basically is.  The hot water heater that is in every residence in the USA has exactly the same basic design.  It works, and has no moving parts (which identifies its simplicity).  But it is also inefficient because it can’t control the stoichiometric oxygen requirement for the fuel and establish an efficient combustion.  However, for the results expected, it is the correct design.


A forced draft burner requires a driving force to feed air and fuel that atomize, mix, and create turbulence within the combustion chamber - factors that are important in establishing efficient burning.  However, the combustion process requires power in the form of an air blower - which requires MORE energy than a natural draft, not less energy.


Although we don’t know the size of the water bath heater in heat load or heat transfer area, my guess is that you should normally be able to convert the natural draft system to a forced draft keeping the same water bath vessel.  You’ll wind up with excess heat transfer area for a forced draft application, but what of it?  You have essentially a bath heating operation some of the time and continuous when there is large gas demand.  The larger water bath will transfer heat faster, be more combustion efficient (little or no CO and NOx in the flue).


I don’t understand why you have to resize the water bath heater.  I would not replace the existing heater vessel unless it was mechanically deficient or damaged.  It should result as “oversized” when modified with a forced draft burner.

You will have to work with a burner manufacturer for the modification and I am sure he/she will tell you the same thing.  But check with them before going forward with engineering plans to modify the water bath.  Depending on the size of the heater, you may or may not see any efficiency difference - except that your flue will be cleaner.  With the forced draft you can insert spiral tube packings in the tubes to increase turbulence and the convective heat transfer coefficient.  It is important to work directly with your burner supplier to establish the correct air blower and burner controls required for the unit and the method(s) of adapting them to the existing vessel.

#6 Varghese


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Posted 01 November 2015 - 04:04 AM



You must know how much volume of natural gas your existing burner is consuming per minute.

You will need a blower than can deliver air volume approx 10-11 times the gas volume.

Considering the existing burner has an auto igniter and a flame stabilizer. You can mount the blower

such that the air is enveloping the flow of gas ( in the same axis over the gas pipe ).

You may want to use a damper at the exhaust vent of the heater to restrict slightly the escaping hot gases.

Thus forcing the hot gases, but without causing backing up of gases.

You could take help from a local burner supplier if its possible for safety reasons.

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