August 26, 2008

Thermic Fluid Vs Steam for Heating

Recently I came across an article on above topic suggesting the use of thermic fluids in place of steam as heating medium even for lower temperature range where usually steam is employed. The article was from Hydrocarbon processing Oct'1991 issue.

The article suggest that we should consider thermic fluids in place of steam for new projects as they are more efficient & cost effective.

Do you agree with this?

I also doubt, so I decided to do some calculations as they are presented in the paper.

Let us assume that we need steam for a process heating load of 1,000,000 Kcal/hr i.e. 1.0 Million Kcal/Hr or 1.0 Gcal/Hr. Steam temperature required is 200°C.

So the corresponding pressure & other parameters are given below in the table.



In the above table it is clear that though the fired heater duty requirement is more for boiler, the efficiency is ultimately playing the role in higher fuel consumption for thermic fluid heaters. Even if we consider other system components the cost of operation will be higher for TF heaters as shown below.



The said article also shows that area requirement for TF based exchangers for process heating is almost similar due to gain in LMTD, but if I consider reasonable temperatures then the following table suggest that area requirement increases by at least 2 fold.



The other factors which should be considered in the selection between the two are.

  • Higher Flow rates are required for TF (as shown you need only 2000 Kg / Hr steam but 66 M3/hr TF) resulting in higher power consumption in pumps.

  • Need more heating fuel due to low efficiency of fired heaters @60% compared to large boilers having efficiency range from 88 - 92%

  • Steam can be economical after utilizing its pressure energy for power generation which has conversion efficiency of ~94%. In such cases even if we consider the overall steam generation station efficiency as said in the article, it will be economical

  • Generally heat transfer coefficients are large for phase change compared to sensible heat transfer & hence you need more surface area for TF heaters

  • Practically fouling is the biggest problem for TF heaters as they degrade easily due to temperature fluctuations. This is rare with steam

  • Careful design & selection of TF is needed for each application. Sometimes they can be dangerous & hazardous also

  • For large heating loads steam is more economical compared to TF as shown above

  • TF may be economical at very high temperature requirements >250°C where steam system hardware becomes uneconomical or where steam facility is a limitation.

  • Capital cost wise you need to replace TF after a specified time limit which add to your capital cost

  • TF system can not be used where live steam can be used.

  • In the process where cooling & heating cycles are required its very complex & response time is very low. It is always better to use steam in such cases.

  • Safety is an issue with the use of TF as in case of leaks they easily absorbed by insulation & cause sudden fire due to higher temperatures.

  • Since only sensible heat transfer is there & because of low specific heat of TF, the temperature control in the narrow range is quite difficult.

The above discussion suggest that one should very carefully consider the option of using TF system in place of steam system. In fact, the pressurized hot water system is more safer & better if high temperature is required. So always compare TF with HP hot water system as benchmark before suggesting any TF.

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9 comments:

Anonymous said...

Dear Writer,

I agree with your statement. I recently found that for my plant the total cost/ton for hot oil (TF) is much higher compared to steam. Of course I did not compared it apple to apple like what you did in term of producing the same supply pressure and temp. But in term of real cost, my results showed the same trend, ie it is more expensive to use TF instead of steam.

profmaster said...

Yes Dear
Thats the purpose of this blog to make you aware of certain facts which otherwise are very cost ineffective.

In fact, if you consider the different cost items including maintenance & reduction in efficiency over a period of time, the overall life cycle cost impact will be further higher.

profmaster said...

This also gives a learning that never consider any article whosoever has published & whichever magazine is publishing as perfect with out evaluating your own case.

devil^2 said...

Sir can u please give the exact reference of the article which u hv dis-proven i mean ... Hydrocarbon
processing Oct'1991 issue...

i searched it on the net but couldnt find it ... can u giv the link or the exact copy of the article ... please i need it ... my email id is anup.iit@gmail.com

Bhadresh Joshi said...

I was planing to introduce TF, replacing the steam... in my process vessels of 1000 / 2000 LTRS. I am confused now. Please help me.

Sambhav Chowdhary said...

Hi,

The comparison is considered on the basis of 60% efficiency. However the scenario changes with increase in efficiency. Do you have anything on HAG v/s Steam heating.Basis: Air is heated to a temp of 200 C .Heat load, 10,00,000 Kcal/Hr. In HAG, Air is indirectly heated M.S. being the heat transfer media.

Mahesh Kadre said...

From where this 60% efficiency came I don't understand. Today practical fact is that most of heater even running on solid fuel are giving 68-75% efficiency, whereas in liquid or gaseous fluid it is more than 80%. So please consider the same. You have not considered water treatment cost & chemical cost. It seems that you have considered 100% condensate return, Let me know where you get 100% condensate? Cost of condensate polishing and treatment chemicals? How could steam temperature inlet and outlet in process would be 200 Dg (same).
Above all in steam only latent heat is used so sensible heat cost is straightaway loss.
Degradation of oil is not every year like in case of steam where most of people are doing descaling or boil out every year or so. I have plenty of thermic fluid unit where oil is not changed from last 15 years and no carbonisation also. Can you show me boiler where in last 15 years no descaling is done and no regular treatment is given?
Above all thermic fluid system are installed in such a process or plant, where installing high pressure boiler or power plant boiler is not possible. It is all inclusive decision, not on one or two issues. Temperature control is more precise in thermic fluid as compared to direct fired heating or steam heating and there are so many things which can explain utility of thermic fluid heater over steam.
Live example is many plywood mfg who shifted from TF to steam had already again reverted back on TF system

Techkasamba Agarwal said...

Mahesh I can understand your point & confusion but you are taking this article as crticism against TFH suppliers if I am right....Please note that purpose is different...Now let me prove it again for your better clarity....

1. When you say 68-75% efficiency on solid fuel...please do some basic calculation for heat balance that for a TFH of 200 C what will be the flue gas temperature & how eff will be more than 60-65%...This can be 75% for some other heating where you can do preheating but not for oil of 200 C which returns at 175C as considered in this example post.

2. If you know basics water treatment cost today is hardly 1-2% of total cost, so you can add it if you feel so....result will not change.

3. 100% condensate return...possibly again you have not read it properly. I have considered only quantity not energy. if you check it the steam balance is based on only latent heat. This is also same 200 C in HEX area calculation.

4. If you evaluate you will find that boiler efficiency is higher due to recovery of flue gas heat in sensible form only. So the statement that sensible heat is lost is a general sentence. Moreover it is less if process temperature level is less.

For argument like this I may use this energy anywhere why it should be a loss.

5. I can not argue on your personal experience about no change of oil in 15 years as I can not challenge it. Just to clarify for the benefit of others that theory can not be challenged so if at 200C hydrocarbons do not break at all.......Petrol can not be formed in the crest of earth.

6. Again regarding Temperature precision I agree bcoz Pressure fluctuation causes variation in case of steam but as mentioned the response in case of emergency is very poor in oil.......Can you cool it quicker than steam???

7. Possibly the factors given for consideration are missed out by you OR I could not make it clear.

As a business for Plywood manufacturer may not afford a good boiler OR operation may not be suitable for them...I don't know but this does not govern or change the theory that THF is costlier than steam for some.......This is what I said.....I already mentioned that THF is good where high T is required.

Possibly another point missed by you is the comment section where first user says that his finding on cost are same.

I hope it clarifies.

Anonymous said...

I am yet to see a thermic fluid heater with 60% efficiency. Most of not all thermic fluid heaters have an air preheater to pre-heat combustion air to the burner / furnace. Typically for thermic fluid supply and return temperatures of 280/250 Deg C, the flue gas temperature is say 220 - 250 Deg C.
Kersi

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