Heat transfer media and refrigerants can be distinguished according to their range of application. The latter are capable of operation between –100°C and +150°C, the former between –50°C and +400°C. The great variety of such substances for industrial purposes is shown in Table 1.
A heat transfer medium in the most general sense may be present in the solid, liquid and/or vapor phase; it can be used to store heat in a reversible form and can be circulated within the installation, e.g., in pipes. With multiphase substances, a change of phase will occur during heat exchange with the surroundings, considerable amounts of enthalpy being exchanged. Whereas specific heat capacity c varies between 1.6 and 4.2 kJ/(kgK) for organic compounds or water, respectively, the amount of vaporization/condensation enthalpy ranges from 80 to 2500 kJ/(kgK). The heat exchanged (dq) can be determined from the sensible heat (c· dT) and the phase change enthalpy (r). For example, with vaporization (t,p = const), the added heat for heating to boiling point and complete vaporization is found by dq = c·dT + r = dh + r.
In order to calculate the heat exchange between a medium and a surface, the law of heat transmission has to be taken into account, for example, as nondimensional Nusselt number. For forced convection, the valid equation is Nu = f(Re, Pr), for free convection Nu = f(Gr, Pr), where Nu = (α · D)/λ, D signifying characteristic length. For the calculation of the heat transfer coefficient α[W/(m2K)] . Therefore, the following temperature-dependent properties are indispensable:
density ρ [kg/(m3)],
specific heat capacity c [kJ/(kg K)],
heat conductivity λ [W/(m K)] , and
kinematic viscosity ν [m2/s] or dynamic viscosity η [Ns/ m2], where ν = η/ρ.
From these are calculated the thermal diffusivity κ [m2/s] [κ = λ/(ρc)] and the Prandtl number Pr = v/κ.
In successfully selecting a suitable heat transfer medium, a great many criteria besides the four properties mentioned have to be considered. Concerning the installation, these are permissible operating temperature and pressure range. At the same time, a multitude of marginal and secondary conditions have to be maintained, each of which is necessary but far from sufficient in itself.
The most important heat transfer medium may well be water, which exhibits thermodynamic properties favorable to heat transfer. Moreover it is nontoxic, physiologically harmless, nonflammable, mildly corrosive in a way that can easily be controlled, thermally stable, readily available, and cheap. These advantages are compromised by a freezing point which is far too high, at 0°C, and an accompanying increase in volume of 9%, as well as by a rapid increase of vapor pressure with temperature. Water, therefore, may be employed as a heat transfer fluid in a limited range of applications only. For higher temperatures, substances with lower vapor pressures are substituted, mainly mineral oils and synthetic heat carriers.
Criteria to be considered in chosing a heat carrier include: closed flash point, water content, neutralization value (for aquaeous acids), sulfur content chlorine content, and corrosive activity, for example, with respect to copper. In addition the supplier has to provide information on various properties including: kinematic viscosity ν at 0, 40 and 100°C; pour-point (DIN ISO 3016); density at 15°C; mass percentage of oxide ash; inflammability point in °C; mass fraction of coke residue in mineral oil based heat carriers and data on thermal stability. The properties required for the design of heat transmission installations have to be given, stating the test procedure for the temperature range intended. Thermal stability values of unused heat transfer liquids can be determined according to DIN 51 528 (01.94). In addition to the initial treatment of the samples the evaluation of the boiling range by gas-chromatography simulation is set out there. For the evaluation of used heat transfer media a similar testing procedure is being devised.
New products in this field include substitutes for refrigerants that destabilize the ozone layer and developments of suitable lubricants for compressors (Rl34a as a mixture with other substances is viable).