Water as a heat transfer fluid

In temperature control units, water is used almost exclusively for operating temperatures of up to 90 °C. At temperatures above 90 °C, the operator must choose between water and thermal oil. In the past few years, pressurised water units have become more and more popular for temperatures up to about 250 °C. The following article provides information on the advantages and disadvantages of water as a heat transfer fluid and discusses measures required for its optimal use.

Water’s excellent thermal properties make it an exceptional heat transfer fluid. Its specific heat, the property that determines how much thermal energy a heat transfer fluid can transport, is about twice that of thermal oil. In addition, its heat transmission coefficient is also at least twice as high as that of thermal oil. This is a measure of the transmission of thermal energy from walls to the heat transfer fluid.

A further advantage of water is its low viscosity, which, in contrast to thermal oil, remains almost constant throughout its application range. Water also has ecological advantages; it is completely safe to handle and dispose of. Cost can also play a significant role.

Yet another advantage of water is its low expansion, which becomes important primarily when using consumers with large quantities of heat transfer fluid.
Example: When 100 litres of water are heated from 20 to 140 °C, the volume increases by about 7.4 litres; in the case of oil, it would be about 10.2 litres. Oil units therefore require a larger expansion vessel due to their greater volume increase.

Additional positive properties: Water is not subject to coking. This fact makes it possible to achieve significantly higher specific heating capacities, i.e. smaller, more economical units. Last, but not least, water is not flammable.

These many advantages have led to the greatly increased usage of water as a heat transfer fluid.

One reason that some operators do not use pressurised water units is safety. At an operating temperature of 250 °C, the system pressure is more than 38 bar. The pressure in the outlet can easily be 50 bar, as the pump pressure (dependent of the connected consumer) is added to the system pressure. It is therefore important to use suitable hoses and connections and to check the temperature control circuit daily.

If these conditions are fulfilled, safety is no reason to avoid using water as a heat transfer fluid.

For consumers that allow only low pressures (e.g. double-sleeved containers), water cannot be used as a heat transfer fluid at higher temperatures, as often the system pressure alone exceeds the allowable pressure. In this case, there is no choice other than to use thermal oil.
The significant advantages of water mentioned practically force its use, but the following point must be considered: As a rule, the use of water is only up to about 60 °C completely without difficulty.

With increasing temperatures, the problems listed in section 2 can occur. For this reason, most temperature control unit manufacturers limit their maximum outlet temperature to about 200 °C, although there are a few units rated to 200 °C. These temperatures produce a very high system pressure (not counting the additional pump pressure) in the unit of approximately 40 bar. This value is with respect to safety not entirely unproblematic. In addition, water behaves differently at high temperatures as it does at 60 °C. Operators are often unaware of this fact.

As long as drinking water was used in the temperature control circuit and for the cooling of the temperature control unit, problems such as corrosion cropped up only seldom. One reason was the relatively low operating temperatures of the units earlier. With the introduction of high-quality plastics, higher mould temperatures became necessary, which led to an increase in problems.

As practically no facility can afford to use drinking water, for reasons of cost and environmental protection, cooling water systems are installed, in which the heat is dissipated by means of cooling towers. The circulating water is generally treated, and most operators are then convinced that the quality of the water more than fulfils the requirements. This may be true for temperatures up to about 40 °C, for which the cooling water is primarily used.

The circulating water is, however, used as a circulating medium in addition to its task of cooling the temperature control unit. This double use of the water leads to a problem:

The water treated for use as cooling water is now also used for temperatures up to 180 °C or even higher. As a rule, it is not suited for these temperatures.
In order to avoid the problems described below, water used as a circulating medium must fulfil the following requirements:

Total hardness4–18 °dH
Carbonate hardness2–12 °dH
pH value7–9 KKG
Conductivity, max.max. 1000 µS/cm

 

The calcium carbon dioxide balance (CCB), determined by the pH value, is the best indication
of the quality of the water. Water that is in calcium carbon dioxide balance is well suited for use as heat transfer fluid, regardless of whether it is soft or mediumhard water.

Unfortunately, even if the water falls within these values, it doesn’t mean that all associated problems are solved. In units with open bath heating, heavy vaporisation occurs, especially at temperatures about 90 °C. In the process, chemically pure water is lost in the form of steam. All of the other substances in the water, however, remain in the circuit. The bath is refilled with water from the cooling circuit, so that over time, the water thickens. The degree of thickening can be determined most easily by measuring the conductivity of the water.

Example: When the refill water (from the cooling system) has a conductivity of 500 μS/cm and the water in the temperature control circuit has a conductivity of 1000 μS/cm, the thickening factor is two. Rule: if the conductivity exceeds 1500 μS/cm, the water should be replaced. 
Thickening also increases the hardness of the water. Hard water is the cause of scale in hoses, temperature control channels and, primarily, heating elements in the temperature control unit. Because the surface temperature of the elements is very high in the heating phase, incrustation (Fig. 1) forms, and, at temperatures above 100 °C, boiler scale forms.

Both types of deposits are very hard and practically insoluble. The deposits form an insulation layer, strongly inhibiting the heat transfer between the heating element and the circulating water, leading to pronounced overheating of the heating elements, which can in turn lead to their destruction.
Scale in the temperature control channels also hinders heat exchange and fluid flow, so that the optimal temperature control of the consumer is no longer assured.

A high degree of thickening can lead to salt enrichment and thereby to corrosion. Oxygen and carbon dioxide in the water, however, can also lead to corrosion. Oxygen creates localised craters and hole-shaped corrosion (Fig. 3). Carbon dioxide causes flaking.

Material destruction (e.g. to components such as impellers or pumps) can also be caused by cavitation. In cavitation, the damage is caused by microjets, at pressures up to 100,000 bar (105 bar), hitting the material.

The pH value is a very important criterion for evaluating water, as it determines the corrosion behaviour for different materials. A good compromise is a pH value between 8.5 and 9.5. For steel, a higher value would be advantageous, but that would be bad for copper. If aluminium is used, the pH value must be reduced, as its corrosion resistance is only guaranteed between 5 and 8.3.

Measures implemented by the temperature control unit manufacturer
The selection of proper materials and design procedures can make a temperature control unit suitable for use with water. With regard to corrosion resistance, stainless steel would be the best choice for all components coming in contact with water. For reasons of cost, however, the use of stainless steel is generally limited to the container. For pumps, tubes, and coolers, other non-rusting materials such as brass, bronze, and copper are used. In high-quality pressurised water units, however, stainless steel pumps with ceramic shafts, impellers made of PEEK, and bearing of SiC are sometimes used.

It is not just the materials used, but also the design of the components that has a significant influence on the lifespan of the temperature control unit. For example, the design of the unit can prevent the damage done by cavitation to a great extent.

Measures implemented by the operator
The operator is often of the opinion that he simply has to connect the unit to the existing cooling water circuit and let it run. Unfortunately, for the reasons discussed in section 2, that seldom works. That the reason for the problems is usually the water used as the circulating medium must be restated here. As a rule, an external specialist must be called in for treating the cooling water. In our experience, however, this specialist often assumes that the water will be used only for cooling purposes at correspondingly low temperatures.
For the proper treatment of the water, it is essential that the specialist know that the cooling water is also used as a circulating medium in the temperature control unit at correspondingly higher operating temperatures (> 40 °C). As previously mentioned, higher temperatures require different treatment of the water.

For various reasons, the required quality of the water often cannot be realised. In this case, the temperature control unit can either be filled by hand, or a separate filling system can be mounted on the unit for the circulating water in the temperature control circuit.

The circulating water of the facility would then be used only to cool the unit. Specially prepared water that would be adjusted to the requirements of the temperature control unit would be used for filling the unit. As it is a relatively small quantity of water, its treatment in comparison to the treatment of the entire amount of cooling water is significantly more economical. In addition, it is simpler to maintain a small amount of water at the same level of quality.

If the quality of the cooling circuit cannot be made to meet requirements and it is not possible to create a separate circuit for filling the unit, the only solution remaining is to treat the water in the unit itself. For this purpose, there are special corrosion inhibitors that, in addition to preventing corrosion, provide a series of other advantages. One of these products will be discussed in the following section.

The following section refers to Regloplas’ RK93 corrosion inhibitor. RK93 has a wide range of effects, and functions as follows. A gas-impermeable film that functions as a dielectric insulator is applied to the tube walls and surfaces. This film prevents substances contained in the water from attacking the metal, including nonferrous metals. It prevents pitting (oxygen corrosion) and the formation of deposits. The protective film remains even after the circulating water is drained. Rust film in the cooling channels of the mould is thus also prevented in storage.

RK93 stabilises the components of cooling water, prevents deposits on tube walls, cooling channels, heating elements and hinders the precipitation of lime. When the capacity of the water is exceeded, lime and other components are discharged as sludge, which can be removed easily by rinsing with water. Boiler scale no longer occurs.

RK93 improves the heat transfer to the mould, prevents localised overheating and extends the service life of the temperature control unit and connected consumer. RK93 makes it possible to use normal drinking water as cooling water, without the need for special treatments. Distilled water can also be used. Due to its aggressiveness, distilled water is not used without treatment in temperature control units. On the other hand, it has practically no components (lime, salts), so it does not contribute to scale or the discharge of sludge, keeping the circuit clean. Adding RK93 takes away the aggressivity of distilled water.

RK93 was developed specifically for temperature control units. It is therefore also suitable for high temperatures up to 180 °C, and, thanks to the synergetic effect of the individual components, it is much more effective than conventional additives.

As RK93, like many other corrosion inhibitors, increases the pH value, caution is required with aluminium, as aluminium is corrosion resistant only within the pH range 5 to 8.3. If aluminium is in the circuit, the pH value must be tested. If it is over 8.5, the aluminium will be attacked. In this case, the dosing of the corrosion inhibitor must be reduced.

For temperatures up to approximately 250 °C, water is to be preferred over thermal oil as a heat transfer fluid for the advantages discussed (significantly better heat transfer properties in comparison to oil, ecologically supportable, low costs). However, the operator must be aware of the possible problems associated with water also discussed. The decisive point is the quality of the water as the circulating medium in the temperature control circuit. “Cooling water quality” is not sufficient; the water must also be suitable for use at the high operating temperatures of the temperature control unit. As the unit manufacturer has no direct influence, the operator must take on the role of ensuring that suitable water is available. It is in the interest of the temperature control unit manufacturer or supplier to work with the operator to find an optimal solution.