Freon

"Freon" is a trade name for a family of haloalkane refrigerants manufactured by DuPont and other companies. These refrigerants were commonly used due to their superior stability and safety properties: they were not flammable nor obviously toxic as were the fluids they replaced, such as sulfur dioxide. Unfortunately, these chlorine-bearing refrigerants reach the upper atmosphere when they escape. In the stratosphere, CFCs break up due to UV-radiation, releasing their chlorine atoms. These chlorine atoms act as catalysts in the breakdown of ozone, which does severe damage to the ozone layer that shields the Earth's surface from the Sun's strong UV radiation. The chlorine will remain active as a catalyst until and unless it binds with another particle, forming a stable molecule. CFC refrigerants in common but receding usage include R-11 and R-12. Newer refrigerants that have reduced ozone depletion effect include HCFCs (R-22, used in most homes today) and HFCs (R-134a, used in most cars) have replaced most CFC use. HCFCs in turn are being phased out under the Montreal Protocol and replaced by hydrofluorocarbons (HFCs), such as R-410A, which lack chlorine. However, CFCs, HCFCs, and HFCs all have large global warming potential.

Newer refrigerants are currently the subject of research, such as supercritical carbon dioxide, known as R-744.[4] These have similar efficiencies compared to existing CFC and HFC based compounds, and have many orders of magnitude lower global warming potential.

The thermodynamics of the vapor compression cycle can be analyzed on a temperature versus entropy diagram as depicted in Figure 2. At point 1 in the diagram, the circulating refrigerant enters the compressor as a saturated vapor. From point 1 to point 2, the vapor is isentropically compressed (i.e., compressed at constant entropy) and exits the compressor as a superheated vapor.

From point 2 to point 3, the superheated vapor travels through part of the condenser which removes the superheat by cooling the vapor. Between point 3 and point 4, the vapor travels through the remainder of the condenser and is condensed into a saturated liquid. The condensation process occurs at essentially constant pressure.

Between points 4 and 5, the saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure. That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid (typically, less than half of the liquid flashes). The adiabatic flash evaporation process is isenthalpic (i.e., occurs at constant enthalpy).

Between points 5 and 1, the cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the evaporator. The evaporator operates at essentially constant pressure. The resulting saturated refrigerant vapor returns to the compressor inlet at point 1 to complete the thermodynamic cycle.

It should be noted that the above discussion is based on the ideal vapor-compression refrigeration cycle which does not take into account real world items like frictional pressure drop in the system, slight internal irreversibility during the compression of the refrigerant vapor, or non-ideal gas behavior (if any).

Low Temperature Freezing

The low-temperature freezing (for frozen food) and the medium-temperature cold (for chilled food) secondary loops use theoretically noncorrosive and nontoxic brines with high heat capacities and low viscosities at low temperatures. They eliminate the use of refrigerants on both cold and freezing sides of the system and are separately pumped to the display cases and cold storage rooms. The display case heat exchangers were designed to use secondary fluids and, consequently, the temperature differences between the brines and the air were minimized.

As secondary fluid on the low-temperature freezing loop, an inhibited potassium formate solution with a concentration of 100%--a nontoxic product compatible with the majority of most common metals and alloys--was chosen. Some analytical studies indicate thermal and pressure drop advantages for HFE-7100 over other potassium formate solutions for low-temperature applications (below -20[degrees]C or -4[degrees]F) (IEA 2003). However, it was recently demonstrated that corrosion had been a problem associated with the use of potassium salts, particularly when galvanized materials were used in display cases. At the same time, the HFE-7100 is an unnatural global warming substance and is expensive. For future applications, a valuable deep-freeze coolant would be carbon dioxide (C[O.sub.2]). As a two-phase secondary coolant, C[O.sub.2] has no ozone degradation potential and negligible GWP. It is universally available, uses very little energy for pumping, and has low costs. In the presented system, the low-temperature secondary fluid (pure potassium formate) is circulated through the freezing secondary loops by two 37.5 hp parallel pumps. Although propylene glycol (35%) has a relatively high level of viscosity at low temperatures, it was chosen for the medium-temperature cold secondary loop. It is also circulated through the loop by two 37.5 hp pumps installed in parallel.

Two warm secondary loops reject the condensing excess heat to the outdoor air by means of remote air-cooled liquid coolers located on the roof (Figure 1). Both loops use ethylene glycol (with a concentration of 50%) as warm secondary fluid. This fluid presents certain environmental risks, but they are minimal compared to the risks associated with common refrigerant leakages.

Carbon dioxide as a natural refrigerant

Alberto Cavallini and Claudio Zilio

In the beginnings of mechanical refrigeration, at the end of the nineteenth century, carbon dioxide was one of the fi rst refrigerants to be used in compression-type refrigerating machines, later gaining widespread application mainly onboard refrigerated ships, but common in other sectors of refrigeration as well. It was only immediately after World War II that CO₂ was rapidly eclipsed as a refrigerant, due to the advent of the synthesised halogenated working fluids, addressed as safe and ideal refrigerants at that time.

Because of the stratospheric Ozone depletion environmental issue, CFC and HCFC working fl uids are now in the process of being phased out of use under the Montreal Protocol. The Global Warming environmental issue casts concern over the use of the new HFC fluids as substitute refrigerants, because of their high GWP values, which make them subject to regulations under the Kyoto Protocol.

In this mixed situation, CO₂ is being revisited as a fully environmentally friendly and safe refrigerant. An intense research activity on its prospective applications is underway in many research establishments in Europe, Japan and North America, and important results have already been reached in exploiting the peculiar characteristics of this high-pressure fl uid operated with a transcritical cycle. In some applications CO₂ systems have already been commercialised; this applies to heat pump water heaters, as a brine in indirect systems and in the low temperature stage of cascade systems.

The paper critically analyses the prospects for the future return of CO₂ as a working fl uid, or sometimes as a brine with change of phase, in important application areas. These include air conditioning and heat pump systems in the residential and commercial sectors, commercial and transport refrigeration and mobile air conditioning.

Source: http://ijlct.oxfordjournals.org/cgi/content/abstract/2/3/225