When choosing a process heat system, the selection of the heat transfer medium is one of the key decisions alongside the system technology itself: suitable heat transfer medium = temperature level + pressure level + process requirement + existing system + economic efficiency.
In this article, water is used as a collective term for two heat transfer media: warm water and hot water. These differ mainly in their temperature application range and pressure level. The safety requirements for operating hot water systems must be assessed significantly higher according to the recognized rules of technology.
Example: Why operating hours influence the system choice
For a preliminary assessment, it is not only the purchase cost of a system that matters. The decisive factor is also how many hours per year the system operates and what costs arise during operation.
A simplified example calculation:
500 kW heat output × 4,000 operating hours per year = 2,000,000 kWh heat demand per year
Assuming an energy price of $0.094/kWh, this results in:
2,000,000 kWh × $0.094/kWh = $188,000 energy costs per year
Even a difference of 5% in efficiency, heat losses, control behavior or availability can become relevant:
5% of 2,000,000 kWh = 100,000 kWh per year
Additional costs / savings: 100,000 kWh × $0.094/kWh = $9,400 per year
In industrial process heat, the investment alone is therefore not the only decisive factor. Over several years, energy use, maintenance, downtime risk, inspection and monitoring effort, operator supervision effort, spare parts inventory and integration into the existing system all matter.
Quick overview: The key points in 60 seconds
Anyone planning, modernizing or economically reassessing process heat should not choose the heat transfer medium simply out of habit. What matters is which temperatures are actually required, under which pressure the system needs to operate and how well it fits into the existing plant.
As a general rule:
- Water is often the obvious choice when lower to medium process temperatures (95 °C to 105 °C / 203 °F to 221 °F) are sufficient. At higher temperatures, for example up to 180 °C (356 °F), simple warm water technically becomes a hot water system with corresponding pressure maintenance.
- Steam is interesting when heat transfer through condensation, an existing steam infrastructure or several consumers are important. During operation, however, water treatment, operator supervision effort and inspection or monitoring issues must also be evaluated.
- Thermal oil is strong when high process temperatures, for example up to 320 °C (608 °F), are required at a comparatively low pressure level.
- The right solution results from process, temperature requirement, existing system, operating hours and safety requirements, but of course also with consideration of the investment costs.
Heat transfer media at a glance
| System | Particularly interesting for |
| Warm water Hot water | lower to medium temperature ranges, simple heat supply, existing water-based infrastructure |
| Steam | condensation, steam network, high heat transfer rates, several consumers |
| Thermal oil | high process temperatures, indirect heat, comparatively low pressure level |
Who is this decision relevant for?
This question mainly concerns companies that are planning new industrial process heat systems, expanding existing systems or modernizing existing plants.
The comparison is particularly relevant for:
- production managers
- project managers in plant engineering
- technical purchasers
- managing directors in industrial companies
- operators of existing process heat systems
Typical starting points are new heat processes, rising energy costs, expansions of existing networks or new production lines, modernizations or the replacement of older steam, water or thermal oil systems.
A careful assessment is also worthwhile when waste heat integration, frequent maintenance costs or downtime risks are involved.
Three typical scenarios for heat transfer medium temperatures: 100 °C (212 °F), 180 °C (356 °F) and 280 °C (536 °F)
The choice of heat transfer medium becomes easier to understand when viewed through specific temperature ranges.
Scenario 1: Process heat up to 100 °C (212 °F)
At process temperatures up to 100 °C (212 °F), water is often the obvious solution. In this range, a warm water system is usually sufficient. In terms of system and safety technology, the system remains comparatively simple, water is readily available and the technical effort is usually manageable.
Steam or thermal oil can still be useful up to this temperature if they already exist in the plant, if several consumers need to be supplied or if the overall process is designed around them.
| Temperature range | Often obvious choice | Decisive question |
| Up to 100 °C (212 °F) | Water / warm water | Is a simple warm water system sufficient for the process? |
Scenario 2: Process heat from 100 °C to 180 °C (212 °F to 356 °F)
At temperatures up to 180 °C (356 °F), hot water systems are predominantly used. At temperatures above 180 °C (356 °F), the real system comparison begins. Depending on the application, steam or thermal oil may be suitable.
The decisive factors are pressure level, controllability, existing infrastructure, safety concept, water treatment, operator supervision effort, maintenance and the required temperature stability at the consumer.
| Possible solution | When particularly interesting? |
| Steam | when condensation, a steam network or several consumers are relevant |
| Water / hot water | when steam is not required, but higher water temperatures are useful |
| Thermal oil | when indirect heat, stable control and a lower pressure level are important |
Scenario 3: Process heat around 280 °C (536 °F)
At 280 °C (536 °F), thermal oil becomes significantly more interesting. High process temperatures can be achieved with heat transfer oil without the correspondingly high steam pressure level that would occur with water or steam.
That does not mean thermal oil is automatically always the right choice. Here too, design, output, type of system technology, consumers, installation conditions, safety and operating costs matter.
| Temperature range | Often obvious choice | Why? |
| approx. 280 °C (536 °F) | Thermal oil / heat transfer oil | high temperature at a comparatively low system pressure |
Public Rothos references show such fields of application: thermal oil heaters in chemicals, packaging and food, as well as thermal oil heat supply in industrial plants:
The three heat transfer media at a glance
The following classifications are deliberately simplified. In practice, the selection always depends on temperature, output, consumers, control, existing system and safety requirements.
Water
Water is usually the simplest solution when low to medium process temperatures are sufficient. Typical applications include preheating, simple heat exchangers, building-related heat supply or processes in the lower temperature range.
The technical boundary between warm water and hot water is important: If temperatures up to around 95–105 °C (203–221 °F) are sufficient, warm water is often the obvious choice. If higher temperatures are required, pressure maintenance, safety technology and system design become much more important.
In short: Water is strong in lower to medium temperature ranges, but above the classic warm water range it must be considered as a pressure-relevant hot water system.
Steam
Steam is strong when heat is transferred through condensation. In this process, a large amount of energy can be released at a relatively constant temperature. This is why steam is established in many industries, for example in sterilization, cleaning, drying or when supplying several consumers.
Water treatment, condensate management, pressure level, fittings, inspection and monitoring obligations and safety must be taken into account.
During ongoing operation, steam can also require more supervision than other systems. Depending on the system and operator concept, this may include a boiler logbook, regular inspections, documented monitoring values, specialist personnel and additional operating materials for water treatment.
In short: Steam is strong in condensation and with existing steam infrastructure, but in operation it is often demanding in terms of supervision, water treatment and safety technology.
Thermal oil / heat transfer oil
Thermal oil is particularly interesting at high process temperatures. The central advantage is that high temperatures can be transferred at a comparatively low system pressure.
Here too, there are clear requirements: The heat transfer oil ages, must be monitored and the system must be properly designed against leaks, overheating and fire risks. With only limited exceptions, thermal oil systems are subject to inspection obligations. In Germany, thermal oil is subject to the German Water Resources Act (WHG). Thermal oil must be regularly checked for continued usability.
In short: Thermal oil is a strong candidate when high temperatures, indirect heat transfer and a comparatively low pressure level are important.
Practical reference: Rothos
In this context, Rothos is primarily to be classified as a specialist for thermal oil and heat transfer systems. Public references show, among other things, thermal oil heaters, thermal oil heat supply systems and combinations such as thermal oil-fired steam generators.
Comparison: Temperature, pressure, controllability, maintenance and safety
The following table serves as a first orientation. It does not replace system design.
| Criterion | Water | Steam | Thermal oil |
| Typical use | low to medium process temperatures | condensation, steam network, several consumers | high process temperatures |
| Pressure level | low with warm water, increasing with hot water | increases significantly with temperature | comparatively low |
| Controllability | good | good, but system-dependent | very good with indirect heat transfer |
| Maintenance | water quality, pressure maintenance and corrosion protection must be considered | water treatment, condensate, fittings | oil care, analysis, monitoring |
| Safety | at higher temperatures, pressure and safeguarding become more relevant | pressure, steam, condensate, water hammer, inspection obligation | oil temperature, leakage, overheating, fire protection |
| Economic efficiency | strong at suitable temperatures and with an existing water network | good with suitable infrastructure | strong at high temperatures and many operating hours |
It is also important to note that inspection and monitoring obligations may be relevant not only for steam systems, depending on the specific design. Hot water and thermal oil systems may also have to be classified accordingly. Pressure, temperature, medium, volume, output and the specific design are among the decisive factors. The applicable regulations provide information on the type and scope of monitoring obligations.
The higher the required process temperature, the more important pressure level, safety technology, maintenance and life cycle costs become. The decision should therefore never be made based on temperature alone.
Building regulations are also a criterion when selecting between different heat transfer systems.
Temperature and pressure: Why the difference is technically decisive
Temperature and pressure are closely connected in heat transfer systems. This relationship is especially decisive for system design, safety and operation when using water and steam.
Water and steam: Higher temperature means higher pressure
At normal pressure, water boils at around 100 °C (212 °F). If water is to be used above this range as a liquid heat transfer medium, the system must be pressurized. This is exactly where the distinction between warm water and hot water becomes technically relevant.
The same applies to steam: As the saturated steam temperature rises, the required steam pressure also increases.
Thermal oil: High temperature in the liquid phase
Thermal oil works differently. Heat transfer oil can transfer high temperatures in many industrial applications without creating a steam pressure level comparable to water or steam.
This is one of the main reasons why thermal oil often becomes interesting at higher process temperatures.
Why “pressureless” should be used with caution
In practice, thermal oil is sometimes described as “pressureless”. Technically, the cleaner formulation is usually: comparatively low system pressure level.
Because thermal oil systems also involve pump pressure, expansion, safety requirements, temperature limits and system-specific design.
As a basic rule:
Rising temperature requirement → with water/steam, pressure relevance increases significantly → with thermal oil, a different system logic applies with a comparatively lower pressure level.
Economic classification: Investment is only one part of the decision
In process heat, the first focus is often on investment costs. For an economic assessment, however, this is not enough. What matters is what the system costs over years of operation: energy, maintenance, downtime, inspection effort and possible modifications to the existing plant.
| Question | Example calculation |
| Annual energy demand | 500 kW × 4,000 h = 2,000,000 kWh/year |
| Energy costs | 2,000,000 kWh × $0.094/kWh = $188,000/year |
| Downtime costs | $5,900/h × 4 h = $23,600/year |
| Amortization | $141,000 / $21,200/year = 6.7 years |
These calculations are simplified, but they help with an initial classification. In practice, construction costs, buildings, foundations etc., maintenance costs, outage risks, energy price development, inspection and monitoring effort, operator supervision effort, operating materials for water treatment and possible expansions should also be taken into account.
For operators, this means: The cheapest solution is not automatically the one with the lowest initial investment. The decisive factor is which system operates safely, is easy to control and remains economically sensible over its entire service life.
Which solution fits which application?
After temperature, pressure and economic efficiency, the practical question is: Which solution fits the specific plant case?
| Starting point | First direction to examine |
| low temperatures | water / warm water |
| medium temperatures | compare water, steam or thermal oil |
| higher water temperatures without a steam process | examine water as a hot water system |
| steam is required by the process | steam |
| high temperatures, stable indirect heat | thermal oil |
| existing network available | evaluate the existing system technically and economically |
The existing system must not be ignored. A theoretically better solution can be economically worse if the piping network, fittings, consumers, safety technology and operating personnel are already designed for another system.
Decision questions for operators
Before selecting the heat transfer medium, operators should clarify the most important technical and economic key data:
- Which process temperature really has to arrive at the consumer?
- How constant does the temperature need to be?
- Are there load changes or many part-load phases?
- Does a steam, water or thermal oil system already exist?
- How many operating hours per year do the system and consumers run?
- What downtime costs arise in the event of a failure?
- Which inspection, maintenance and safety requirements must be considered?
- What operator supervision effort arises during operation, for example through inspections, documentation, specialist personnel or water treatment?
- Which expenses for construction costs, such as buildings, must be considered?
- Is the plant being expanded, modernized or completely newly planned?
- Is waste heat to be integrated?
- Which solution is economically most robust over 10–15 years?
- The possibility of obtaining public funding or subsidies should be examined.
The more clearly these points are answered, the more reliable the technical preliminary selection becomes.
Practical reference: What Rothos references show about typical systems
In practice, selecting the heat transfer medium is rarely a purely textbook decision. Public Rothos references show various typical applications.
Thermal oil heater in the chemical industry
One example is a thermal oil heater with 1,500 kW thermal oil output for a chemical plant. Such projects show why thermal oil is frequently used in industry: for high, stable process temperatures and indirect heat transfer.
Thermal oil-fired steam generator
Another example is a thermal oil-fired steam generator with 15,000 kW output, 24,000 kg/h steam output and 22 bar steam pressure.
This shows that heat transfer systems do not always have to be considered in isolation. Thermal oil can also be part of an overall solution in which steam is required elsewhere.
Water-based process heat in container design
Water-based process heat can also be relevant. The Rothos references include, among other things, a hot water heater in container design for the textile industry.
| Reference type | What is technically interesting about it |
| Thermal oil heater | high, stable process temperatures with heat transfer oil |
| thermal oil-fired steam generator | combination of thermal oil system and steam generation |
| Hot water heater | water-based process heat at suitable temperature and design |
Why the specific design should always be application-related
Tables, temperature ranges and comparison values help with initial orientation. But they do not replace technical design.
In practice, several factors must be considered together:
- required process temperature
- heat output and operating hours
- pressure level and safety requirements
- consumers and heat transfer
- controllability and load changes
- installation location and space conditions
- approval of the system with regard to emissions law and building law
- existing infrastructure
- maintenance effort and operating costs
A table can make preliminary selection easier. But the reliable decision only emerges through design, heat balance, interface review and economic assessment.
Conclusion: The heat transfer medium does not decide, the process does
Water, steam and thermal oil each have their place. Water is strong at suitable low to medium temperatures, steam for condensation and existing infrastructure, thermal oil for high process temperatures with a comparatively low pressure level.
The best solution therefore does not result from a general system preference. The specific process is decisive: temperature requirement, output, pressure level, existing system, safety, maintenance, operator supervision effort, operating hours and economic efficiency.
Anyone planning new industrial process heat or modernizing an existing system should therefore always consider the heat transfer medium as part of the overall system.
Rothos supports you already in the planning phase in developing the right plant concept while observing all applicable regulations.
Useful links / sources
- TLV: Steam tables and technical steam basics
Technical overview of saturated steam, pressure, temperature and other steam parameters. - BAuA: TRBS 2141 “Hazards caused by steam and pressure”
Technical rule for operational safety on hazards caused by steam and pressure in pressurized work equipment. - European Commission: Pressure Equipment Directive 2014/68/EU
Basis for the Pressure Equipment Directive. It applies to stationary pressure equipment with a maximum permissible pressure above 0.5 bar. - Rothos Energy Systems: References
Practical examples of thermal oil heaters, thermal oil-fired steam generators, hot water heaters and other industrial heat systems.