Heat transfer medium

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A heat transfer medium (also: heat transport medium ) is a medium that transports heat from a place of higher temperature to a place of lower temperature in a heating or cooling circuit . Depending on the application and temperature range, heat carriers are also called

  • Heating medium , less often than heating medium (see other meanings under heating medium ), or as
  • Coolant (also: coolant )
  • Coolant, when used below 0 ° C

designated.

Heat carriers are transported in pipes. In order to minimize energy losses, short, insulated pipelines are sought.

conditions

Heat transfer media should ideally meet the following requirements:

media

water

Due to its very high specific heat capacity of around 4.2 kJ / kg K or its high specific evaporation enthalpy of around 2000 kJ / kg and its melting enthalpy of 333 kJ / kg, water is a very good heat and cold carrier. It is used in particular for water cooling : As cooling water, water is the most common coolant. Water has a corrosive effect in connection with ions or oxygen. However, fully demineralized water should not be used either, as it cannot deposit a protective layer and, on the other hand, it does not buffer changes in the pH value and is therefore particularly corrosive and also attacks sealing material.

Water is used not only in the liquid, but also in the gaseous or vaporous state as a heat carrier and in the partially solid or ice-shaped state as a cold carrier.

The use as steam offers the advantage of being able to carry out the transport in district heating lines in the form of superheated high pressure steam with relatively low energy losses. At the destination, the superheated steam (superheated steam) is then converted into saturated steam in steam conversion stations (pressure reduction with water injection). The saturated steam can then very effectively give off its heat again through condensation in the downstream heat exchanger (analogue: high-voltage line, transformer station). With shorter distances from the heat generator to the consumer, lower transport pressures can be used (here too, analogous to the transport of electricity).

The use as ice offers the advantage of being able to transfer relatively high powers with low volume flows (cf. specific heat capacity and melting enthalpy). In order to remain free-flowing (ice slush), however, the water must be mixed with anti-freeze.

Water is used in both open and closed circuits.

Alcohol-water solutions

Coolants made from water and alcohol are not or hardly corrosive. It is essential, however, to keep them free of oxygen, since alcohols in combination with oxygen produce highly corrosive organic acids (for example, from ethanol, ethanoic acid, " acetic acid ").

Glycols such as ethylene glycol , propylene glycol and higher alkylene glycols are mainly used as antifreeze agents. Glycols taste sweet ( glycos = sweet), which was abused in the glycol wine scandal and are slightly toxic.

Ethylene glycol has the best physical properties, but is not approved for use in food technology. Propylene glycol is used when the coolant can come into contact with drinking water or food, as it is approved as a food additive in the EU (E 1520). Higher alkylene glycols are used when resistance is required even at higher temperatures. Since the glycols have a significantly lower specific heat capacity and a higher viscosity than water, the lower the proportion of glycol, the worsening of the coolant properties of the mixture.

Salt-water solutions

Salt-water solutions can be divided into chloride-containing and chloride-free salt-water solutions. Salt-water solutions containing chloride are, for example, aqueous solutions of sodium chloride , magnesium chloride and calcium chloride . The aqueous solution of potassium carbonate , for example, is chloride-free .

In contrast to solutions containing chloride, coolants made from solutions of chloride-free salts are well compatible with stainless steel.

Salt solutions (brines) can also be used in food technology. Their viscosity is low.

air

Air is available almost everywhere and is mostly used in open circuits. It is used for air cooling , but also in the air collector and in some solar seawater desalination plants .

Thermal oils

Thermal oils (also: thermal oils) are used to cool oil and to heat industrial systems and processes in closed circuits. They can have different properties depending on their chemical composition. They can be subdivided into low-viscosity, easily flammable oils with low solidification and boiling points and higher-viscosity, flame-retardant oils with higher solidification and boiling points. According to their origin, they can be differentiated into

Mineral oils, better heat transfer oils, are hydrocarbons and are obtained from petroleum . They are relatively inexpensive, but usually flammable and chemically aggressive to many sealing materials.

The statement about flammability has to be put into perspective. Heat transfer oils based on mineral oil are difficult to ignite at room temperature in the delivery condition; low-boiling decomposition products are only formed after thermal exposure, which lower the flash point of the oil. The low-boiling components can be discharged through suitable care measures so that the high flash point is retained.

Mineral heat transfer oils do not attack seals comparable to IT. But there are synthetic oils that require special sealing systems.

Due to their molecular structure, aromatic hydrocarbons have a particularly high thermal stability and are used for applications up to 400 degrees.

Silicone oils are polymeric methylsiloxanes or polymeric phenylsiloxanes. Silicone oils are non-corrosive and non-toxic; the higher viscosity silicone oils are also approved for use in food technology. However, the specific heat capacities are relatively small.

Oils from the peel of citrus fruits (trade name e.g. D-Limonene) are safe for use in food technology and may be used. a. used for cooling molds in chocolate production.

Molten salt

As molten salts use z. B. Mixtures of sodium nitrate and potassium nitrate , up to about 550 ° C

During salt bath hardening , the steel parts to be heated are enclosed by cyanides at approx. 800 to over 1000 ° C.

Liquid metals

Liquefied metals are used for special applications, for example sodium in the fast breeder . Other metals in use are lead-bismuth alloys, which are also said to be used in some Generation IV reactors.

Special applications

Heating means

Heating means are used in circuits in which equipment is heated by a heat transfer medium circulating in the circuit.

Coolant

Coolant

Heat carriers which are intended for use below 0 ° C are usually referred to as cooling media. Due to the high specific heat capacity, water is also often used in this temperature range. However, pure water already freezes at 0 ° C, which means that the polar water molecules organize themselves into ice crystals. In order to be able to use the good heat transfer properties of water even at lower temperatures, this order must be disturbed. The addition of polar or amphoteric substances is suitable for this. In this way, the 0 ° C limit can sometimes be well below the limit, and the mixture of water and antifreeze can be used as a single-phase coolant.

As the temperature continues to fall, partial separation takes place first, and small ice crystals with a diameter of around 0.01 to 0.1 mm are formed, surrounded by an increasingly concentrated anti-freeze solution, so that an ice slurry is present just above the solidification temperature . This represents a two-phase coolant. In order to counteract solidification through connection of the ice crystals, the ice slurry must always be kept in motion; for example, storage vessels are to be equipped with stirring devices. The lower the application temperature, the higher the required proportion of antifreeze (of course only up to the eutectic ). The behavior of mixtures of anti-freeze and water can be represented by the freezing point curve. In order to protect the cooling system, the admixed antifreeze agent is mixed with inhibitors; the water used as the coolant should be degassed if possible.

Alcohols and salts are used as anti-freeze agents.

Web links

Individual evidence

  1. ↑ Thermal oil boiler fired with biomass ; As of March 27, 2011.