Working Fluids (Heat Transfer Media)

In practice, working fluids such as water, water–glycol mixtures, or thermal oils are commonly referred to as heat transfer media. They transport thermal energy between the source and the sink without themselves being part of a thermodynamic refrigeration cycle involving phase change. Heat transfer media are circulated in closed heating and cooling circuits and couple cooling or heat generation units to technical processes or building systems.

Their physical properties have a decisive influence on the applicable temperature range, system efficiency, hydraulic design, and operational safety of heating and cooling systems – particularly in the integration of industrial energy systems and in industrial high-temperature heat pump applications.

Water as a universal heat transfer medium

Due to its high specific heat capacity, wide availability, and ease of handling, water is the most widely used heat transfer medium.

Typical applications include:

• supply and return circuits of heat pumps,
• chilled-water circuits in chiller systems,
• heat transfer in preheating and hot-water applications adjacent to steam generation processes.

Operating limits:

• typically up to approximately 130 to 160 °C, depending on pressure control and system design
• corrosion control, water quality, and water treatment must be taken into account.

Water–glycol mixtures

Water–glycol mixtures are used when frost protection is required or when low supply temperatures must be operated safely.

Properties:

• frost protection down to approximately −30 °C, depending on concentration,
• lower specific heat capacity and higher viscosity compared to water,
• consequently increased pumping power demand and the need for adapted hydraulic design,
• established standard in industrial cooling, chilled-water, and recooling systems.

Thermal oil as a high-temperature heat transfer medium

Thermal oils are used when required process temperatures cannot be achieved with water in a technically or economically viable way.

Typical temperature ranges:

• approximately 150 to 320 °C, depending on the type of oil and the permissible film and operating temperatures.

Advantages:

• high thermal stability within the specified operating temperature range,
• operation at atmospheric pressure without boiling,
• uniform and reliable heat transfer even at elevated temperatures.

Limitations:

• oxidative and thermal ageing requires regular monitoring,
• strict compliance with permissible operating temperatures is essential,
• increased safety requirements regarding fire and leakage risks.

Additional heat transfer media in specialized applications

Depending on process requirements and temperature range, other media may be used, including:

• brine solutions (e.g. calcium chloride) for low-temperature applications,
• silicone oils for particularly high thermal stability,
• water-free specialty fluids for extreme chemical or thermal conditions.

Media with an intentional phase change, such as steam, are also used for heat transfer in industry; however, from a systematic perspective, they are not classified as conventional heat transfer media but rather represent independent working and process media.

The selection of suitable working fluids depends on temperature requirements, chemical stability, safety regulations, material compatibility, hydraulic design, and control strategy.

Relevance for high-temperature heat pump systems

Heat transfer media have a decisive influence on how efficiently heat is transferred between the heat source, the heat pump, and the heat sink. Key influencing parameters include:

• temperature differential between supply and return,
• thermal conductivity and specific heat capacity,
• viscosity as a relevant factor for pumping energy and pressure losses,
• thermal and chemical stability at elevated temperatures.

In industrial applications, the selected working fluids therefore play a crucial role in determining the performance, operational reliability, and economic efficiency of high-temperature heat pump systems, as well as their integration into existing processes.