When high temperatures are needed in an industrial process, two common options are steam and thermal oil systems. As steam condenses, it transfers a large amount of heat; thermal oil, on the other hand, can reach high temperatures with relatively low system pressure. The right choice should be made based on target temperature, process control, facility distance, safety, maintenance and total cost of ownership.
No single system is superior in all applications. Steam and thermal oil can also be used together for different processes in the same factory.
Operating character of steam heating
Water is evaporated under pressure in the steam boiler. As steam condenses at the point of use, it transfers its latent heat. Since the temperature of saturated steam depends on the pressure, a stable process temperature can be achieved by pressure control.
Steam; It is common in food, textile, paper, chemistry, sterilization and many heating processes due to rapid heat transfer, long-distance energy transportation and direct use. System; It requires water preparation, condensate return, blowdown and periodic control.
Operating character of thermal oil heating
In the thermal oil boiler, the thermal fluid is circulated by a pump in a closed circuit. The oil heats up in the boiler, releases its energy to the process and returns to be reheated.
The thermal oil system can provide high temperature at lower pressure compared to the water-steam system. This feature; It can create an advantage in chemical reactors, dryers, presses, asphalt-bitumen, wood panel, textile and industrial ovens.
Basic comparison
| Criteria | Steam system | thermal oil system |
|---|---|---|
| Heat transfer | High with latent heat of condensation | With sensible heat; depending on oil flow and temperature difference |
| Temperature control | Pressure related, can be stable | Input-output temperature can be controlled precisely |
| Pressure at high temperature | Pressure increases as temperature increases | High temperature is possible at relatively low pressure |
| Pumping | Steam moves forward with pressure; condensate is pumped back | Oil is pumped continuously |
| Fluid preparation | Water conditioning and deaerator are important | Oil selection, filling and condition analysis are important |
| Loss points | Leakage, blowdown, flash steam, condensate | Pump electricity, surface loss, oil degradation |
| Safety risk | Pressure vessel, hot steam and water hammer | Flammable oil, leakage, excessive temperature and low flow |
| Direct process use | Possible with appropriate quality | Usually indirect heating |
Selection in terms of temperature and pressure
As the process temperature increases, the pressure required for saturated steam increases. High pressure; affects the pressure class of the boiler, piping, valves and control equipment. thermal oil can carry high temperature with lower circuit pressure; However, the film temperature and maximum operating limit of the oil should not be exceeded.
Just looking at the target product temperature is not enough. The approach temperature on the heat transfer surface, oil film temperature, vapor pressure loss and process load should be calculated.
Energy efficiency and operating cost
Steam can provide high heat flux in the compact exchanger thanks to condensation. Water and heat can be reused if the condensate is recovered. On the other hand, blowdown, leakage and flash steam losses should be managed well.
thermal oil operates in a closed loop and does not require blowdown. However, circulation pumps constantly consume electricity; As oil ages, viscosity and heat transfer can change. High surface temperature accelerates thermal decomposition.
The comparison should be made on the total cost per year of useful heat:
- Fuel and electricity,
- Water, chemicals and wastewater,
- Thermal oil renewal/filtration,
- Maintenance and inspection,
- Insulation and distribution loss,
- Unplanned downtime and redundancy.
Process control
On the steam side, the pressure control valve, trap and heat exchanger selection determines the process temperature. Failure of condensate to discharge from the exchanger at low load may cause temperature instability.
Three-way valve, pump flow rate and boiler modulation are used in thermal oil. The thermal inertia of the system may be high; Processes requiring rapid cooling require separate evaluation.
Security differences
High pressure, low water level, safety valve and water hammer in the steam system are the main risks. Cutting off the flow of thermal oil, oil reaching excessive film temperature, oxidation, expansion tank errors and oil leakage to the hot surface pose the risk of fire.
It cannot be said that “thermal oil is completely safe because it is at low pressure” or “steam is risk-free because it uses water”. Both systems must be operated with appropriate design, automation, training and maintenance.
In which situation does steam stand out?
- If the process uses steam directly,
- If fast and high heat transfer by condensation is required,
- If more than one remote user will be fed from the same energy center,
- If sterilization, humidification or reaction with steam is required,
- If condensate can be recovered effectively.
In which situation does thermal oil stand out?
- If high temperature is desired with low system pressure,
- If precise and homogeneous surface temperature is required,
- If the process does not come into direct contact with the fluid,
- If a closed loop without blowdown is desired,
- If appropriate pump, expansion and fire safety infrastructure can be installed.
Process data required for decision
Target inlet-outlet temperatures, thermal power, daily operating time, load change rate, distribution distance, available fuel, water quality, electrical infrastructure, maintenance possibility and security requirements should be evaluated together.
Hisarmak deals with steam and thermal oil systems according to the process need before the product name. Submodels can be examined on steam boilers and thermal oil boilers pages.
Frequently asked questions
Is the thermal oil system more economical than steam?
Not always. Annual cost including temperature, load, pump electricity, condensate recovery, oil life and maintenance must be calculated.
Is there no pressure in the thermal oil system?
There is pump pressure, static load and thermal expansion. The fact that high temperatures can be achieved with lower pressure compared to steam does not mean that the system is pressureless.
Can’t the steam system reach high temperatures?
It may come out; In saturated steam, as the temperature increases, the pressure increases. Superheated steam or different boiler designs can also be used.
Can two systems be used in the same facility?
Yes. If the temperature and usage patterns of the processes are different, a hybrid facility setup can increase the total efficiency.