Hydrofluoroethers as low temperature refrigerants

Abstract

A method and apparatus are provided for using certain hydrofluoroethers (HFEs) as low temperature heat transfer media in secondary loop refrigeration systems. These materials exhibit a low, somewhat linear Temperature Difference Factor over the temperature range of -15° C. to -65° C., making them ideal for use as low temperature heat transfer media. The HFEs of the present invention are nonflammable, nontoxic, environmentally benign, and have a high heat transfer capacity and low viscosity over the required operating temperatures. Furthermore, since these materials have high boiling points and low freezing points, they are not prone to phase changes over the required operating temperatures, and do not require pressurized systems.

Description

FIELD OF THE INVENTIONThe present invention relates to heat transfer media, and in particular to the use of hydrofluoroethers (HFEs) as low temperature heat transfer media.BACKGROUND OF THE INVENTIONVarious materials are known to the art which can be used as heat transfer media in refrigeration systems. These materials include water, aqueous brines, alcohols, glycols, ammonia, hydrocarbons, ethers, and various halogen derivatives of these materials. While many of these materials are effective as heat transfer media under certain conditions, practical considerations eliminate many of them from use in key commercial settings, such as in refrigeration systems in grocery stores. In these applications, only a fraction of the class of known heat transfer agents are of commercial significance.One factor that eliminates many heat transfer media from consideration is their environmental impact. Many known heat transfer media are being phased out because of their environmental persistence, or...

AI Technical Summary

Benefits of technology

The present invention provides a method and apparatus for using certain hydrofluoroethers (HFEs) as low temperature heat transfer media in secondary loop refrigeration systems. Surprisingly, these materials exhibit a low, somewhat linear Temperature Difference Factor over the temperature range of -15.degree. C. to -65.degree. C., making them ideal for use as low temperature heat transfer media. The HFEs of the present invention are nonflammable, nontoxic, environmentally benign, and have a high heat transfer capacity and low viscosity over the required operating temperatures. Furthermore, since these materials have high boiling points and low freezing points, they are not prone to phase changes over the required operating temperatures, and do not require pressurized systems.

Problems solved by technology

Another factor that removes many heat transfer agents from consideration is their toxicity.

The toxicity of these materials, by ingestion, inhalation, or transdermal absorption, makes them dangerous to handle and unsuitable for commercial food handling environments.

Still other heat transfer agents are disfavored because of their flammability.

The risk of flammability is particularly great where the heat transfer agent is subject to large positive pressures within the refrigeration cycle.

Other heat transfer agents are disfavored because they are gases at normal operating temperatures.

Furthermore, high pressure systems are prone to leakage.

Still other heat transfer media are not preferred because of their corrosive nature.

Like gaseous media, corrosive agents require special handling provisions, such as Teflon.RTM.-lined conduits and interfaces, which add significantly to the overall cost of the system.

Furthermore, restrictions on the selection of materials usable with corrosive agents decreases the overall efficiency of these systems.

Hence, the total energy savings offered by secondary loop refrigeration systems in the supermarket sector alone is enormous.

Also, since the primary loop running through the compressor is segregated from the secondary loop used to cool the goods being refrigerated, the primary loop may utilize ammonia and other high efficiency refrigerants that are unsuitable for use as direct refrigerants in many applications.

While secondary loop systems have many clear advantages over conventional refrigeration systems, the commercial use of secondary loop systems has been limited by the unavailability of suitable secondary refrigerants.

The disadvantages of high pressure systems have already been noted.

Unfortunately, absent a high positive pressure, most refrigerants that perform suitably at normal temperatures no longer perform well at the low temperatures required by secondary loop systems.

See, e.g., E. Granryd, A. Melinder, "Secondary Refrigerants for Indirect Refrigeration and Heat Pump Systems", ScanRef 14-20 (April 1994), which considers a variety of secondary refrigerants, but concludes that it is difficult to nominate good candidates for low temperature applications.

Other refrigerants, such as silicone oils and hydrocarbon based fluids, have a poor heat transfer capacity at low temperatures.

As a result, systems utilizing these refrigerants suffer a marked decrease in energy efficiency at lower temperatures.

Often, the drop off in performance of refrigerants at lower temperatures is precipitous.

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