1、4845 (RP-1239) Distribution of Water between Vapor and Liquid Phases of Refrigerants Andy Gbur ABSTRACT This research determined the vapor-liquid equilibrium distribution ratio (K-value) for the refrigerants 22, 123, 134a, 32,125,143a, 404A, 407C, 410A, and 507A. These data were derived from measuri
2、ng the moisture content of the vapor and liquidphases of specijically contaminated refrigerant samples at temperatures of 40F: 75“e and 120F (4“C, 24“C, and 49C). INTRODUCTION It is well documented that moisture is an undesired contaminant in refrigeration systems. High levels of moisture may cause
3、physical and chemical damage to the system. Excessive moisture can lead to icing of capillary tubes, expan- sion valves, and other evaporating devices. In addition, water can expedite chemical reactions in a system, leading to corro- sion of metal surfaces and copper plating, hydrolysis of POE lubri
4、cants, and breakdown of motor winding insulation mate- rials. The best way to prevent moisture contamination in a system is to ensure proper care is taken in the installation and design of the refrigeration system. However, prevention alone cannot protect against all possible field conditions that m
5、ay introduce moisture into the system. In these situations, an understanding of the behavior of moisture in refrigerant will allow technicians to accurately diagnose problems, implement corrective actions, and ultimately prevent system failures. BACKGROUND Previous moisture saturation and solubility
6、 data published in the ASHRAE Handbook has been limited to chlo- rofluorocarbon (CFC) and a few hydrochlorofluorocarbon John Senediak (HCFC) refrigerants and has been questioned as to its validity. As a precursor to this research, four laboratories used various methods to veri the ratio of moisture
7、distribution between the liquid and vapor phases for R-134a (0.58) stated in the ASHRAE Handbook. The determined ratios were 1.19, 1.08, 0.93, and 0.89 at room temperatures. These results were considerably higher than those published. The first three results were determined experimentally by contami
8、nation of various amounts of refngerant. The last value was calculated using the Peng-Robinson equation-of-state. The suspect ratio published in the ASHRAE Handbook was determined with the rcfrigerant in a state of saturation. Typically refrigeration systems are below these saturation values, yet th
9、ese levels of moisture still have the potential for degradation of the system, thus the need to experimentally measure ratios at conditions below saturation. EXPERIMENT There were many environmental interferences and proce- dural challenges that had to be addressed prior to developing the procedure
10、for this project. Some of these are sampling technique, ambient humidity, accurate quantification of trace levels of moisture in the stock refrigerant, and moisture adsorption by the metal surfaces of the cylinder walls. The plan was to reduce the impact of each of these interferences to maximize th
11、e measurement accuracy of the moisture equilib- rium constants. The recognized standard for the measurement of moisture in refrigerant is the ARI Standard 700, appendix C. The AR1 Standard 700 moisture procedure dictates that aliquots of refrigerant be transferred to small sample cylinders prior to
12、analysis. The liquid sample is then evaporated through a capil- Andy Gbur is business manager and John Senediak is manager of the Refrigeration Chemistry Laboratory at Intertek, Columbus, Ohio. 02006 ASHRAE. 241 lary tube into a Karl Fischer vessel for titration. This method unnecessarily introduces
13、 several variables into the uncertainty contamination was seen, whereas afterward the recoveries were between 93% and 98%. equation, such as transfer efficiency and weighing errors. Additionally, several assumptions are made that cannot be readily supported. First, an assumption is made that a hot e
14、vac- uated sample cylinder is moisture free. Second, the assump- tion is made that moisture will not remain behind in the sample cylinder upon evaporation of the refiigerant. This directly relates to and is affected by the distribution of moisture between the liquid and vapor phases of refrigerants,
15、 which is what the project attempted to quantify. To eliminate as many variables as possible, the choice was made to introduce the refrigerant directly into the Karl Fischer vessel from the refrig- erant batch cylinder utilizing a capillary tube restriction. This ensured that a homogeneous mixture o
16、f moisture and refrig- erant was introduced and eliminated the possibility of using a contaminated sample cylinder. The next challenge was to minimize the effect of environ- mental moisture contamination. Two Liebert environmental conditioners control the laboratory temperature and humidity. The hum
17、idity remains at a constant 40% (*2%) with a consis- tent temperature of 74F (23”C), *2”F. This stable environ- ment allows us to predict and compensate for the infiltration of moisture into the Karl Fischer vessels. In addition, the Accumet Model 100 coulometric titrators being used were equipped w
18、ith a mechanism that monitors and reports the moisture background or “drift rate” of the vessels. All tests were conducted with a “drift rate” of less than 0.05 micro- grams per second. Furthermore, the samples were introduced at a rate of 5 grams per minute. This high rate of sample deliv- ery prod
19、uces a short sample introduction time, thus minimiz- ing the effect of environmental moisture. Moisture adsorption by the metal surfaces ofthe cylinders posed the final challenge. Initial attempts to contaminate a batch of refrigerant demonstrated this effect. After a calcu- lated 50 ppm water conta
20、mination of the refrigerant, analysis showed only 40 ppm. We concluded that the poor recovery was due to the absorption of some of the added water by the steel walls of the new, unconditioned cylinder (standard cylin- der constructed of 1008/1010 cold rolled steel). Our solution was to precondition
21、testing cylinders with wet refrigerant to allow the walls to saturate. First, we heated the cylinders to 100C for 24 hours. The cylinders were then removed from the oven, and pulled into a 200 micron vacuum. Next the cylinders were charged with R- 134a refrigerant contaminated with 120 PPM of moistu
22、re and allowed to react for a minimum of 24 hours. The refrigerant was then recovered and once again the cylinders were heated to 100C for a minimum of 24 hours and then evacuated. The cylinders were now ready for contam- ination. The validation of this technique was seen in the higher recovery numb
23、ers received from the moisture contaminations. Prior to this conditioning, 75% recovery of the moisture The contamination process begins with the precondition- ing of two cylinders with the above procedure and connecting them together, one inverted with respect to the other, using a steel cross fitt
24、ing. Next, the cylinder assembly was pulled into a 200-micron vacuum, and the precalculated weight of water was added through a septum into the cross fitting. The cylinder assembly was then placed on a scale and the required amount of prequantified stock refrigerant was added. To achieve an 80% full
25、 cylinder (1.6 L), 1700 to 1800 grams of refrigerant were added, depending on refrigerant density. The assembly was inverted multiple times to allow equal mixing of the refrigerant and water and was then placed on its side at room temperature for 24 hours to allow equal exposure to both cylin- ders.
26、 Finally, the cylinder assembly was placed upright to allow all liquid to settle in the lower cylinder and all vapor to migrate to the top cylinder. The final steps prior to testing were to confirm the contamination level and veri the required time to reach equi- librium. To verify the contamination
27、 level, the liquid phase was analyzed for moisture content at room temperature. Finally, the cylinder assembly was taken to the testing temper- ature *1”F until equilibrium was established. To verify that equilibrium was obtained, the contaminated tank was analyzed for its moisture content every day
28、 for three days and then every three days thereafter until the repetitive data were within measurement uncertainty of the experiment. All mois- ture analyses were conducted on an Accumet model 100 coulometric titrator. Sample sizes for the moisture determina- tion were approximately 25 grams. This a
29、mount was sufficient to provide an accurate value from the Karl Fischer titrators and also a relatively insignificant amount to the total sample amount, 1.5%. The individual analysis of the samples was performed by the following procedure. After an initial 24-hour period at conditions, the cylinder
30、assembly was separated and each cylinders pre-analysis weight was recorded. The cylinder containing the liquid refrigerant was then inverted, and 20 to 30 grams of liquid refrigerant were introduced into the Karl Fischer vessel. The moisture content value in micrograms was determined from the Karl F
31、ischer titrator, and the weight of sample introduced was determined by a post-analysis weight subtracted from the pre-analysis weight. The micrograms of water divided by sample weight produced the ppm value. Next, the cylinder containing vapor was tested. Ten to twenty grams of vapor were introduced
32、 into the Karl Fischer titrator and, using the same calculations as for the liquid moisture values, the moisture content was determined in the vapor phase. The two cylinders were then reassembled and placed- back into their respective environments for further tempera- ture exposure. This process was
33、 repeated at a minimum of 24 hours of exposure until three successive analyses came within 5% standard deviation of each other. 242 ASHRAE Transactions: Research Table 1. Moisture Content in VaporlLiquid Phase Refrigerant 20 ppm Range 60 ppm Range 120 ppm Range K-Value Ref Temp Vapor Liquid Vapor Li
34、quid Vapor Liquid (“Pl (PPW (PPW (PPW (PPm) (PPW (PPm) R-22 40 R-22 75 R-22 120 R-32 75 R-123 75 R-123 90 R-123 120 R-125 75 R-143a 75 R-404A 40 R-404A 75 R-404A 120 R-407C 40 R-407C 75 R-407C 120 R-4 1 OA 40 R-4 1 OA 75 R-4 1 OA 120 R-507A 40 R-507A 75 R-507A 120 10.4 11.4 14.8 8.2 132.8 114.5 124.
35、4 11.0 27.0 23.6 19.5 13.9 10.5 11.2 12.9 9.3 10.3 10.4 12.2 17.7 6.6 19.9 17.4 20.2 21.5 21.7 23.6 26.5 19.3 27.0 30.2 21.8 15.1 20.1 17.8 19.4 17.1 21.0 18.0 18.8 25.4 7.4 38.4 37.1 44.9 25.3 391.0 3 14.0 304.5 39.2 49.4 42.3 42.2 50.2 29.8 35.8 37.9 24.6 25.6 23.8 41.0 38.8 50.2 70.1 63.3 61.9 62
36、.1 60.0 65.4 71.8 63.7 49.7 52.9 46.4 54.3 59.6 60.6 59.7 57.9 57.9 56.3 63.5 55.1 55.4 62.2 66.9 82.6 47.5 718.0 584. i 542.0 73.2 120.0 88.9 94.6 103.0 63.8 76.6 80.5 57.4 60.0 56.1 78.5 106.8 i 16.0 113.0 108.5 116.5 119.3 122.2 120.5 118.2 118.0 114.0 107.5 108.0 108.5 132.6 134.4 131.6 109.7 11
37、2.9 112.0 130.3 136.0 119.0 0.55 0.6 1 0.71 0.40 5.90 4.84 4.52 0.62 1 .O6 0.83 0.88 0.95 0.48 0.57 0.61 0.51 0.53 0.49 0.60 0.79 0.97 DATA Table 1 shows the average moisture values determined for each contaminated batch at the varying temperatures. Accord- ing to the scope of this project, data wer
38、e not required for all refrigerants at all the moisture levels and temperatures. For readability purposes, R-134a data are shown in Table 2. Due to sampling issues and repeat contamination, the same refrig- erant batch may not have been used for the entire temperature range, thus the variance in som
39、e of the liquid phase moisture values. The units used are ppm or mg water/ kg of refrigerant. The calculation of the equlibrium constants was obtained by plotting a curve of the vapor and liquid-phase moisture concentrations. The point (O, O) is included in the line and a linear plot is fit to the p
40、oints. The plot is not forced through the origin. The slope of the line is the ratio of vapor to liquid and becomes the calculated equilbrium constant. ERROR ANALYSIS An estimation of the uncertainty for this work was deter- mined by analyzing a batch of R-134a contaminated to 120 ppm of water. The
41、batch had its liquid and vapor phases analyzed for the moisture content five times. Each sampling occurred on two-day intervals with intermittent shaking and rolling of the cylinder in between sampling. Theoretical 121.0 ppm in the Liquid Contamination Liquid Phase Vapor Liquid Constant (PPm) (PPm)
42、Measurement i 102.1 112.5 0.908 Measurement 2 103.8 113.3 0.916 Measurement 3 103.4 117.0 0.884 Measurement 4 107.6 113.0 0.952 Measurement 5 100.5 116.4 0.863 Average 105.5 114.6 0.905 Std Dev 5.8 2.1 .034 ASHRAE Transactions: Research 243 Table 2. Moisture Values in VaporlLiquid Phase for R-134a M
43、oisture Contamination Range for R-134a (ppm) Temp K-Value 20 60 120 300 450 600 900 Vap Liq Vap Liq Vap Liq Vap Liq Vap Liq Vap Liq Vap Liq 40F 0.97 25.3 26.9 48.8 47.5 108.3 124.8 256.8 304.2 390 383 - 75F 0.95 16.2 17.6 49.6 57.7 104.2 115.5 259.6 299.2 390 416 506 586 904 935 120F 0.87 23.9 26.2
44、57.3 62.8 108 114 259.1 298.5 - SUMMARY AND DISCUSSION The objective of this research was to determine the equi- librium constants for a select list of new HCFC and HFC refrigerants and blends. This was accomplished. R-22, R-123 and R- 134a are the only refrigerants studied in this research that cur
45、rently have published data in the ASHRAE Handbook. From the Handbook, the refrigerants R-22, R-123, and R-134a had vaporhiquid ratios of 0.404,4.69, and 0.585, respectively, at 70F and 0.405, 4.53, and 0.585 at 80F. The values found for these refrigerants at 75F were 0.61 for R-22, 5.90 for R- 123,
46、and 0.95 for R-134a. The slight temperature variation is insignificant for this variability. A more reasonable explana- tion is that the data in the ASHRAE Handbook, chapter 6, Table 2, are calculated from Equation 1 in the chapter, which is a very simple model assuming ideal behavior. The data are
47、not measured. The calculation according to the more sophis- ticated Peng-Robinson model is much closer to what was measured in this project as you can see from the R- 134a value derived from the Peng-Robinson model (0.89) vs. our exper- imental value of 0.95. The relative reproducibility of our meas
48、urements was calculated to be 9.4%. See Figures 1 through 8. Further analysis of the results shows differences in the effect of temperature on the equilibrium constants for some refrigerants, while others exhibit no effect at all. This behavior may be related to the ability of water-to-hydrogen bond
49、 with the liquid rehgerant. The moisture is held in the liquid phase by refrigerants that hydrogen bond well, such as R-32 and R- 22. The moisture is found in the vapor phase of refrigerants that hydrogen bond poorly, such as R-123 and the CFCs. R- 143a is unusual in that it may exist as strongly hydrogen- bonded dimmers in the liquid phase, resulting in an equilib- rium constant closer to unity than may be predicted based on the chemical structure alone. In conclusion, the research provided the information necessary to better assess and correct moisture control prob- lems in
