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.
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.
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.
