TECHNOLOGY: THE STATE OF THE ART

The current state of the art provides users with a choice of thermal regulators generally classified according to the type of heat transfer fluid used (Diathermic oil or Water) and the type of heating system applied (electricity or gas).    

The ideal features of a heat transfer fluid are:

•  High boiling point;
• High thermal conductivity;
•  High flammability point (for oily fluids);
•  Resistance to oxidation;
•  Non corrosive;
•  Non toxic.

Heat regulating fluids generally used are water or diathermic oils.
Water heat transmission coefficient for convection and for conduction is twice as high as oil.

On the minus side though, it has a low boiling point and a tendency to leave scaly deposits, and it is likely to trigger corrosion.
These three issues can be overcome by pressurizing the loop, so that water maintains its liquid state even at high temperatures, and by using suitable corrosion-resistant components (AISI 304, 316 etc. stainless steel) and by treating the process water, if it is harder than 20°F.

Synthetic oils and fluids do not have the same heat conduction properties as water: an increase in temperature reduces internal conductivity while specific heat increases. Furthermore, these fluids are expensive to buy and dispose of; they are also contaminants and flammable above their critical temperature.

Oily fluids, in certain environment conditions (temperature > 60°C and in the presence of oxygen) or when in direct contact with heating elements, are liable to chemical cracking, which becomes apparent with a severe reduction in fluidity, compromising heat exchange.

The heating system can be powered by electricity or fuelled with methane gas (IECI Patent). The heating elements can be immersed directly in oil when using the electric heating system, or else they can be external, for indirect heating (IECI-IHCS System on heating regulators Climat Series PF-Z and PF-ZH).

HEAT REGULATORS WITH DIATHERMIC OIL

With this type of thermal control, the die is brought to the required temperature by circulating heated diathermic oil into the die channels.

There is a difference between mineral oils, obtained by distillation from crude oil, and synthetic fluids: both are mainly composed by hydrocarbons, but the latter have better ageing properties.

In fact, fluid decomposition produces volatile substances, called “low boilers” and high viscosity polymers called “high boilers”. The former, in both cases, must be vented out regularly to avoid pump cavitation, vapour locks or mechanical damage, while the latter are soluble only in synthetic fluids up to a given ratio (10-15%).

Due to its high boiling point, diathermic fluid allows die preheating to temperatures up to 300°C.   

On the contrary, it should be stressed that when temperature increases, thermal conductivity (W/m*°K) decreases and specific heat (Kcal/Kg*°K) increases, thus inhibiting heat transmission.

SUPER HEATED WATER FOR HEAT REGULATION

At normal pressure (760 mm Hg) water stays in a liquid form within temperature limits between 0° and 100°C; at 0°C it solidifies; at 100°C it evaporates.    

It is necessary to maintain water in a liquid state even at higher temperatures (see figure above), to obtain an effective and efficient thermal regulation and to avoid scale formation.

Precipitation of calcium carbonate and magnesium carbonate (main components of the incrustation commonly known as LIME SCALE) is directly linked to temperature increase and is at its maximum near the evaporation point.

Water thermal regulation plants must be equipped with especially reliable devices to ensure the lowest possible pressurization in relation to the maximum temperature attainable.

The most significant water properties for thermal regulation include:

• the highest specific heat among common substances (1 Kcal/Kg/°C), which remains constant as temperature changes;

• heat transmission coefficient by convection is nearly double the value of oily fluids;

• best heat transmission by conduction, compared to oily fluids;

• heat transfer properties remain constant throughout a wide range of temperatures (unlike oily fluids, whose specific heat increases and whose heat transfer capacity decreases as temperature rises);

• non flammability;

• low cost: the water to be used is not required to have specific chemical properties except for a total hardness between 10 and 20°F (average hardness).

It is therefore possible to utilise suitably filtered water from cooling towers, wells, closed cooled loops.