In many refrigerating applications, heat is transferred to a secondary coolant, which can be any liquid cooled by the refrigerant and used to transfer heat without changing state. These liquids are also known as heat transfer fluids, brines, or secondary refrigerants.
These solutions are implemented in both the air conditioning equipment and refrigeration systems used in cold storage, where they substitute the direct expansion plants allowing for a better control of the temperature and relative humidity of the cold rooms. Moreover, the more and more growing attention devoted to the natural fluids, ammonia and hydrocarbons, as refrigerants makes their implementation even more frequent and, sometimes, necessary.
Using indirect systems, it is possible to confine the refrigerant charge in a technical room, equipped with safety systems, far from populated areas. Therefore indirect systems have two important advantages over direct systems: it is possible to use the natural refrigerants, which are efficient and eco-friendly, moreover,
the control of the temperature and relative humidity of the ambient is improved. However, an indirect system with a secondary circuit means an extra cost for pump and heat exchanger, as well as an added temperature difference, which may lead to somewhat higher total energy consumption than for a direct system.
Finding a suitable fluid for a given application is therefore an interesting challenge. Water is an excellent secondary fluid for air conditioning and other applications where temperatures down to about 3 °C are enough. When the temperature is lower than 0 °C, the problem is to find a suitable fluid that presents a freezing point under the operating temperature.
There are several requirements that any secondary refrigerant must meet; it should exhibit good thermophysical properties, making possible to transport large refrigeration capacity with small temperature change and at small volume flow, to use small temperature differences in heat exchangers (i.e. to give high heat transfer coefficients) and to show low pressure drops in order to limit the pumping power consumption.
Finally, it must present a freezing temperature below the operating one. In the table, the freezing temperatures of the aqueous solutions and of the non-aqueous fluids used in the refrigeration and air conditioning systems are listed.
During the selection of the proper secondary fluid, other properties must be taken into account, in particular, the fluid should be non-corrosive, non toxic, non-flammable, safe to handle and cheap.
Unfortunately, all the secondary fluids present at least one or more drawbacks; for the aqueous solutions we can list the followings:
- Ethylene glycol: high toxicity with risk of environmental pollution
- Propylene glycol: high viscosity at low temperature, some risk of environmental pollution
- Ethyl and Methyl alcohols: flammability risk, low boiling point, health hazard
- Glycerol: high viscosity at low temperature
- Ammonia: flammability risk, health hazard, very low boiling point
- Potassium carbonate: risk of eye damage in event of contact, low operative limit
- Calcium chloride: highly corrosive when oxygen is present, corrosion inhibitors with chromates may cause risk
- Magnesium chloride: same as calcium chloride, low operative limit
- Sodium chloride: same as calcium chloride, low operative limit
- Potassium acetate: long term effects not yet so well known.
The non-aqueous liquids present low transport properties and they are quite expensive; in particular, corrosion may be a problem with terpene from citrus oils.
It is clear that there is not an ideal secondary fluid suitable for all the applications, but every time the different characteristics of each fluid must be compared in order to find the best solution. The properties that control the transport capabilities of a secondary refrigerant are: the freezing temperature, the density, the dynamic viscosity, the specific heat at constant pressure and the thermal conductivity.
In almost all the cases, the density ρ permits to know the concentration of the solutions; this method is not very accurate with alcohols and glycols (propylene glycol aqueous solutions), as the density is near that of pure water, therefore, more reliable techniques should be adopted.
The dynamic viscosity µ is linked to the pressure drops of the fluids; the viscosity should not be too high at the operating temperature of the secondary refrigerant to perform well. A high value of specific heat at constant pressure cp is favourable to limit the volume flow rate needed for a given heat flow rate and for high heat transfer coefficients. A high value for the thermal conductivity λ is desirable as it contributes to good heat transfer coefficient. Another important thermophysical property is the thermal volume expansion that permits to design the expansion vessel of the refrigeration system.
A simple comparison can be carried out considering two properties, the density ρ and the specific heat at constant pressure cp, which contribute to determine the heat flow rate exchanged by a certain volumetric flow rate Ṿ , as:
Thus, for a given flow rate equal to 0.001 m3 s-1, which evolves at a mean temperature of 0 °C, for a temperature change of 5 K, we can calculate the heat flow rate exchanged by several different aqueous and non-aqueous secondary refrigerants. The results are reported in the next table.
The aqueous solutions present better heat transfer properties that the non aqueous ones (DEB, MS2), this means that, a constant heat flow rate and temperature gain, the non-aqueous fluids need of a higher flow rate (2-3 times higher) than the aqueous solutions.
Figure 1 reports two diagrams relative to specific heat at constant pressure of a propylene glycol aqueous solution and of several non-aqueous fluids as a function of the temperature. Considering the aqueous solution, it can be seen that the specific heat decreases as the concentration increases. For the nonaqueous fluids, we can state that the highest specific heat, exhibited by HCM, is however, lower than that of the worst aqueous solution (60% in weight) at the same temperature.
Figure 1: Specific heat at constant pressure for a propylene glycol aqueous solution and for several non-aqueous fluids as a function of the temperature. Data by Melinder (1997).
Melinder (1997) presents the thermophysical properties of the aqueous and non-aqueous secondary refrigerants briefly presented in this discussion. Data are presented using: tables, diagrams and equations.
What it has to be highlighted again is that the selection of the proper secondary fluid for a given application must be based on a critical analysis of the different thermophysical and technological properties, considering also the safety and cost issues of each fluid; keeping always in mind that there is not any ideal secondary refrigerant.
- Melinder, 1997, Thermophysical Properties of Liquid Secondary Refrigerants, International Institute of Refrigeration, Paris, FR.
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