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Direct Fired Heater Or Indirect Fired Heater?

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#1 anand1194


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Posted 19 December 2018 - 08:46 AM

Is direct fired heater suitable for a slurry service needed to be heated to 350 deg C ? Which is better for this kind of service : direct fired heater or indirect fired heater (hot oil heater)?

#2 Art Montemayor

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Posted 19 December 2018 - 12:05 PM



Before answering your specific query, some basic data is required.  Otherwise, you will only obtain a general answer to your general question.


Working with a slurry is a special operation that entails some special problems.  For example, what kind, type or configuration of direct fired heater are you referring to?  There are many types available and it makes a difference if you are going to use it to heat a slurry (whose fluid properties and composition we are ignorant of) to as high a temperature of 350 °C - beyond the maximum temperature of most process metals.  Those of us who have had to deal with slurries know the potential problems they present.  The type of heater employed makes a big difference in a proper design.  Any potential dead spots, excessive pressure drops, low velocity, accumulations, etc. all are possible causes for equipment failure by metal burn-out and total spillage.


Knowing next to nothing regarding your slurry, I would recommend a special thermal fluid such as Dow’s SYLTHERM™ 800 stabilized HTF that is rated to work at the very high temperature you desire - 350 °C.  You are not going to find many thermal fluids that can take that high of a constant temperature without degrading.  With a thermal fluid you don’t have to fear a potential metal burn-out in spite of a complete fluid blockage - as long as you furnish a high temperature alloy as the heater’s material of construction.

#3 Bobby Strain

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Posted 19 December 2018 - 12:11 PM

What is the fluid?



#4 anand1194


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Posted 19 December 2018 - 11:29 PM

The slurry composition is : 20% "effluent treatment plant (ETP) sludge" in water by mass. Particle size is less than 100 microns. This slurry basically has to be thermally decomposed at 210 bar pressure and 350 deg C. So we require fired heater for heating the feed slurry going to the reactor. The viscosity of the slurry is very high (20 cP at 350 C & 400 cP at 200 C). Specific heat and thermal conductivity can be taken as for water at that temperature and pressure.

I am concerned about coking/charring because of the solids present.

#5 Bobby Strain

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Posted 20 December 2018 - 12:03 AM

Surely this application is nothing new. Have you talked with vendors who supply heaters for this?



#6 anand1194


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Posted 20 December 2018 - 12:36 AM

Bobby Strain:


I have talked with fired heater vendors. But none of them are comfortable with this kind of fluid service and have asked us to consider indirect fired heater (using hot oil, Dowtherm). They mostly provide products for known process fluids in refinery. If you could suggest few vendors who deals with slurry service, that would be really helpful. 

#7 Dazzler


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Posted 23 December 2018 - 07:05 PM

Hello anand, 


Could you advise the location country of the end user.? Then relevant OEM vendros might be able to be suggested.


Also the 350°C is on the limit of many typical hot oil systems and perhaps also the piping specs needed.  So if your normal operating temperature, maximum operating temperature were lower, then the mechanical design temperature could be chosen to be 350°C or lower.  Perhaps you state those also.  I work at a relevant OEM and we almost never see mechanical design temps higher than 350.  Is your 350 normal or max operating or mechanical design?



#8 thorium90


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Posted 24 December 2018 - 03:09 AM

Anand, if you are talking about thermal hydrolysis for ETP sludge, then it is not done at 210Bar and 350C. Typical range is 6Bar and 165C and double pipe heat exchangers are used.


If you are talking about SCWO, supercritical water oxidation, (which based on the conditions you described seems so) for sludge treatment, whereby the water is heated until it reaches supercritical, above 221Bar and 374C, then this experimental and potentially expensive, new age, wastewater/sludge treatment technology is an active research topic and scientists researching this technology can help you. There are some companies actively researching this novel treatment process. You might be able to ask them if they can share some of their know how privately with you.


As for the heater, it is true that direct fired heaters are used in the SCWO process to reach the 400+C temperatures required.. But I think you really need to talk with those companies to know more about the treatment process.


I am surprised that you have asked this question. It would be rare for anyone to be able to recognize that what you are asking is actually referring to a specific, experimental treatment process in the wastewater industry.

I hope you are not trying to build the system on your own. The SCWO process is not as simple as it seems. There's a reason why so many researchers are working on understanding and solving the problems associated with it and why its not a mainstream treatment process yet.



Are you doing all this for practical reasons, for an ETP in your facility? Are you designing a new plant?

I can suggest more practical sludge treatment methods for you rather than SCWO. For example, the THP process I mentioned in my first line is the most common method, and is a mainstream method for sludge treatment.

This topic as a whole is larger and way more advanced than your original question of whether direct fired heater is suitable for slurry service.

I can talk a lot more on this topic, but I think you get my point.


If you are just doing this for casual knowledge gathering, you can easily google, supercritical water oxidation. There are quite a number of research publications on this novel treatment process.


A few more points to note.

ETP sludge for feed to THP or SCWO process is 20% dry solids 80% water, not 20% water (which implies 80% dryness) as you have mentioned.

At 20%, the slurry is non newtonian. Normally compensation curves are used for the design rather than a specific value of viscosity.

20% implies that pre dewatering was already done, so 100 micron for solids is not correct since 20% is the agglomerated mass and there isn't a specific micron size anymore.

Edited by thorium90, 24 December 2018 - 08:19 AM.

#9 anand1194


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Posted 25 December 2018 - 01:48 AM





It’s not SCWO.  The process is called hydrothermal liquefaction.  The optimum operating condition is as mentioned.  The technology is not an issue here. The technology is working fine at pilot scale.  The challenge is scaling up.  And one of the main challenge is taking feed to temperature to 350 C.  We are just not sure about direct fired heater.  We are talking to different vendors but since the service fluid is somewhat unconventional as normally used in fired heater they are also facing difficulties.  That’s why I wanted to know if fired heater is at all common for heating slurry services or should we go for Dowtherm heat exchanger.

And the composition is as I mentioned : 20% ETP sludge in water.  It means 20% solid and rest is water. 

#10 Technical Bard

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Posted 26 December 2018 - 11:39 AM

A directed fired solution will have tube wall temperatures higher than 350ºC.  Will the material be detrimentally affected by such a temperature?  An indirect heater will require either a Syltherm 800 silicone thermal oil or a molten salt heating medium.    You will want to remain liquid or dense phase supercritical even at the tube wall - so your operating pressure will be > 22 MPa (3200 psi), with a design pressure probably closer to 24 MPa.  This is very high for commercial scale facilities.  Also, what metallurgy is this made out of?  Water containing sludges/salts/oxygen at these conditions can be VERY aggressive from a corrosion perspective.


Temperatures above 360ºC may begin to causing coking reactions, depending on the carbon forms in the feedstock, so based on your pilot data you may need a small temperature approach between heating medium and the fluid, which could result in large thermal fluid circulation rates and  large surface areas (which themselves may be detrimental to getting good fluid velocities).  

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