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A FOUR COMPONENT NON-CFC/HCFC MIXED REFRIGERANT CHARGE FOR USE IN ULTRA-LOW TEMPERATURE AUTOCASCADE REFRIGERATION SYSTEMS
Inventor: Michael St. Pierre (Santa Rosa, CA)
1st Publishing: January 13th, 2013, Revised: December 31st, 2013

Abstract: The invention is a specific formula of mixed non-CFC/HCFC, and where flammability is not an issue, non-HFC refrigerants requiring no more than 4 individual components, that when used in a Kleemenko-Cycle cryogenic refrigeration system (also known as an autocascade), will enable reliable operation of -150° C or colder in the final evaporator cryocoil (dependent upon heat load and the ratio of individual refrigerants being used). And furthermore through the use of refrigerants not containing any chlorine, a safer non-ozone depleting charge can be produced that meets both US EPA and European F-Gas requirements.

Summary: Kleemenko-Cycle cryogenic refrigeration systems work on the single stream mixed refrigerant cascade principle (developed by Russian scientist A. P. Kleemenko in 1959, which he called "One Flow Cascade"). Where liquid vapor phase separation, refrigerant fractionation, throttle and subsequent expansion, sequentially cool each cascade heat exchanger to ever colder temperatures. Critical to making this system work, is the proper selection of refrigerants having boiling points that are far enough apart to maximize heat exchanger efficiency, while still satisfying freezing and chemical compatibility concerns. Also due to the evolution of environmental laws, the choice of refrigerants has been greatly narrowed. This has often been catered to by using even more refrigerants in the mix, thereby complicating both the charging and tuning process. It is the primary goal of this invention to not only satisfy all the environmental laws, but to minimize the total number of refrigerants required and still reliably achieve temperatures of -150° C or colder.

Figure 1: Example of Basic Single-Stage Kleemenko-Cycle (autocascade) Refrigeration System
Compressed mixed refrigerants release their heat energy while passing through the H2O (water-cooled) Condenser, are sub-cooled by the Auxiliary Condenser, and then undergo liquid/vapor phase separation where the liquid is throttled by a cap tube and expanded in the Cascade Condenser, which further cools the vapor exiting the phase separator (hopefully condensing it) and then throttles and expands it in the final Evaporator via the 2nd cap tube.

Figure 2: Example of Mytek Controls Water-Cooled "CryoBUG" Autocascade Refrigeration System
Similar to system shown in Figure 1, but with addition of Sub-Cooler stage, which provides additional  condensation of final refrigerant stream via colder gas returning from evaporator, as well as integration of R740 gas into solution with R14 condensate, thus creating a lower boiling point component.
The latest CryoBUG refrigerant charge is now a Hydrocarbon/Argon composition.

Please note that the original "CryoBUG" was air-cooled. Water-cooling is an unexplored option. 


Background: The first truly successful -150° C test of this invention was conducted on July 21st 2012, utilizing a miniature Kleemenko-Cycle refrigeration system loosely based on Andrija Fuderer's patented design having but one liquid/vapor phase separator (US Patent No. 3203194). The mixed refrigerant charge consisted of R600a, R170, R14, and R740. At an approximate static heat load of 10 watts, the evaporator on this system achieved a temperature of -156° C.
The first tests on this unit that I named CryoBUG (reference Figure 2), began on December 17th, 2011 utilizing a two component R134a/R14 HFC charge. In these tests I duplicated Andrija Fuderer's temperature results that he achieved with a R12/R14 combination. Further tests, using additional refrigerants and hardware changes, eventually broke through the -150° C barrier, resulting in the concept and execution of a 4 component non-CFC/HCFC charge. Please note; that in the latest iteration, CryoBUG now employs an HC/Argon based refrigerant charge.
Later in November of 2012, a larger commercially made Kleemenko-Cycle cryogenic refrigeration system having multiple counter-flow heat exchanger stages, preceded by three liquid/vapor phase separators, and driven by a 37 CFM compressor was used as the experimental test bed (system design similar to US Patent No. 3768273). For this test of the invention, it was desirable to have a charge entirely composed of nonflammable components. And since this particular refrigeration system had extensive market penetration, creating a simplified non-CFC/HCFC charge for it came with financial incentives.

Two baseline tests were conducted utilizing commercially made mixed refrigerant charges, the first using an older 5 component HC/HCFC charge, and the second using a present day patented 5 component HFC charge (US Patent No. 6502410 / RE40627).

The 3rd test was with the invention formula consisting of a non-flammable HFC mixture of R227ea, R23, R14, and R740. Table 1 and Table 2 show the test data collected from all three mixed refrigerant charge tests.
TABLE 1
150 Watt Static Heat Load
Cryocoil = 72 feet x 5/8” od
w/8 foot line set
Commercial
5 component
HC/HCFC Charge
Commercial
5 component
HFC Charge
Invention
4 component
HFC Charge
Compressor Suction (psi) 12 19 20
Compressor Discharge (psi) 138 193 210
Compressor Suction (°C) -5.2 9.2 11.1
Compressor Discharge (°C) 96.9 100.5 99.9
Liquid Line (°C) 18.2 20 19.9
Coldest Liquid (°C) -131.8 -126.7 -126.6
Cryocoil Feed (°C) -136.3 -132.4 -134
Cryocoil Return (°C) -134.1 -130.9 -132.3
Cryocoil Average (°C) -135.2 -131.7 -133.2
Compressor Current (amps) 10.8 13.4 14.1
Compressor Voltage 440 440 440

TABLE 2
3600 Watt Total Heat Load
Cryocoil = 72 feet x 5/8” od
w/8 foot line set
Commercial
5 component
HC/HCFC Charge
Commercial
5 component
HFC Charge
Invention
4 component
HFC Charge
Compressor Suction (psi) 39 40 40
Compressor Discharge (psi) 327 340 347
Compressor Suction (°C) 1.4 11.8 14.2
Compressor Discharge (°C) 115.6 113.2 113
Liquid Line (°C) 20 22 21.8
Coldest Liquid (°C) -91.4 -101.7 -100.8
Cryocoil Feed (°C) -106.4 -110.9 -112.1
Cryocoil Return (°C) -94.7 -95.7 -92.2
Cryocoil Average (°C) -100.6 -103.3 -102.2
Compressor Current (amps) 22 22 21.9
Compressor Voltage 440 440 440
Note: Coldest Liquid references the temperature of the condensate feeding the final evaporator throttle device.

As can be seen by looking at the table data, the cryocoil feed and return temperature gradient is larger with the invention formula as compared to the other two commercially produced refrigerant mixtures. This is attributed to the higher ratio of R23 being used in the invention formula, which results in a greater glide as the individual components evaporate out at their respective boiling points. This higher ratio of R23 (as compared to the commercially produced charges) is desirable when using fewer individual components in the make up of the mixed refrigerant charge.

Due to the invention formula also having a slightly greater percentage of R740, a colder cryocoil feed temperature is achieved under heavy load. The R740 promoting colder evaporation temperatures of the R23 and R14 by what is believed to be a partial pressure effect, thus creating a lower evaporation pressure. It is also theorized that some of the R740 vapor dissolves into the subcooled R14, thus influencing the boiling point of this refrigerant for even colder temperature then R14 alone.

When the cryocoil feed is averaged with the cryocoil return, the invention formula yields a similar temperature to what was achieved by the other two commercially produced mixed refrigerant charges, and a very close match to it's HFC counter part (see chart in Figure 3 below).


Figure 3: Comparison of the Cryocoil Performance
with the Commercial vs. Invention HFC Charge

What is claimed is:

A non-CFC/HCFC four component refrigerant mixture, capable of producing temperatures of -150° C or colder, comprising the steps of blending;

1st component: any single refrigerant from the group R134a, R227ea, R236fa, R600, R600a
2nd component: any single refrigerant from the group R23, R170, R1150
3rd component: any single refrigerant from the group R14, R50
4th component: any single refrigerant from the group R50, R740, R784

Table 3 shows the charge ratio of the individual components, which would result in a functioning Kleemenko-Cycle cryogenic refrigeration system. Due to the wide variation in hardware, refrigerant selection, and application requirements, it has been broadly defined.

TABLE 3
Refrigerant
Range (% by Weight)
1st component
20-60
2nd component
15-40
3rd component
15-40
4th component
2-15

The farther apart the 1st component's boiling point is from the 2nd component's boiling point, the greater will be the required proportion of the 2nd component in the final make up of the mixed refrigerant charge.

When selecting refrigerant components for use in single phase separated designs, it is important to select ones with the lowest freezing points, or below that which you intend to operate at.

To obtain reliable compressor oil return, either POE or PVE lubricant having an ISO viscosity rating of 32 should be used. On single phase separated designs, PVE is the preferred choice in compressor lubrication, due to it's slightly better cold temperature characteristics, and less problems with cap tube clogging or turning acidic. And on systems incorporating a non-hydrocarbon mixed refrigerant charge, coalescing or other equally effective oil separators are an essential hardware component in order to better augment proper oil return to the compressor.

Note: ISO 32 viscosity oils have a colder pour point as compared with the more commonly used ISO 64 rated oils, thus making them better suited for ultra-low temperature autocascade refrigeration systems.


References:
Refrigerant Conversion of Auto-Refrigerating Cascade (ARC) Systems, Dale J. Missimer (Polycold Systems International, Inc.)
Heat Transfer Efficiency of Kleemenko Cycle Heat Exchangers, W.A. Little (MMR Technologies, Inc.)
US Patent No. 2041725, Art of Refrigeration, W.J. Podbielniak, July 14th 1934
US Patent No. 3203194, Compression Process for Refrigeration, Andrija Fuderer, Aug 31st 1965
US Patent No. 3768273, Self-Balancing Low Temperature Refrigeration System, Dale J. Missimer, Oct 30th, 1973
US Patent No. 5702632, Non-CFC Refrigerant Mixture, Chaun Weng, Dec 30th 1997
US Patent No. 6631625, Non-HCFC Refrigerant Mixture for an Ultra-low Temperature Refrigeration System, Chaun Weng, Oct 14th 2003
US Patent No. 6481223, Refrigerant Blend Free of R-22 for use in Ultralow Temperature Refrigeration, Flynn et al., Nov 19th 2002
US Patent No. 6502410, Nonflammable Mixed Refrigerants (MR) for use with Very Low Temperature Throttle-Cycle Refrigeration Systems, Podtchereniaev et al., Jan 7th 2003 (Reissued No. RE40627, Jan 27th 2009)

Websites: www.mytekcontrols.com, www.cryobug.comwww.mmr-tech.com/kleemenko.php, Hermawan's Blog - Auto-Cascade Refrigeration