OIL MIGRATION IN R-12 & R-22 SYSTEMS By: Norman Sharpe Professor, Air Conditioning & Refrigeration Dept., California State Polytechni College

Transcription

1 Service Application Manual SAM Chapter Section 1M OIL MIGRATION IN R-12 & R-22 SYSTEMS By: Norman Sharpe Professor, Air Conditioning & Refrigeration Dept., California State Polytechni College INTRODUCTION Abstract: Diverse published recommendations, usually based on field experience, made closer research necessary in order to properly teach refrigeration system design courses. Such research is now possible with the use of Pyrex extra strong glass pipe. Three modes of oil migration were discovered: (1) as a rippling oil film on the inside surface of the pipe, (2) as a mist, and (3) as a transparent colloidal dispersion in the refrigerant vapor. Little difficulty was encountered at any flow condition if the concentration of oil in the circulating refrigerant was low. Compressors can be designed which will put so little oil in circulation that its return will no longer be a problem. Without transparent pipe which could stand refrigerant pressures and which could be suitably shaped to resemble fittings, the refrigeration field was literally working in the dark. Extra strong Pyrex glass pipe now offers a means of investigating the oil migration problem. It successfully stands the pressures encountered in suction lines; it can be bent (with the aid of heat); and it is highly transparent. Fittings and sections of pipe of this excellent material were extensively used in our studies. A REVIEW OF THE STUDY PRESENTED AT THE 1959 RSES WORKSHOP AT CALIFORNIA STATE POLYTECHNIC COLLEGE Preliminary testing using copper pipe and fittings, with a balanced pressure sight gauge in the horizontal run approaching the riser, showed that, if vapor velocity could be considered a criteria, there was greater danger of oil settling in the horizontal run of the suction line than of the discharge line. This was fortuitous since the use of glass pipe at discharge pressures is questionable from a safety standpoint. All testing so far has, therefore, been confined to the suction line. Further investigation using the balanced gauge method of testing showed that the shape of the fitting at the bottom of the riser was one of the most important factors in the avoidance of oil accumulation. At low vapor velocity there was considerable settling in the horizontal run if a long turn ell were used. This accumulation was greatly reduced by the use of a short turn ell. Finally, there was no accumulation at all, at any velocity we were able to test, if a P shaped trap constructed of a street ell and a return bend were used at the bottom of the riser. These results could not be explained from previously published literature so we decided that a study using transparent fittings and lines was in order. The glass trap was constructed with dimensions shown in Figure 66F22A. The horizontal and vertical legs were of sufficient length to observe the horizontal and vertical oil migration patterns. The trap was installed in the suction line of a 3 hp R-12 system. Riser heights of 3 0 and 12 6 were used. Glass P Shaped Trap used in the Tests Copyright 1959, 2009, By Refrigeration Service Engineers Society. -1-

2 The compressor operated at about 1730 rpm during all tests. Its lubrication was partially forced feed and partially splash. It had a counter balanced crankshaft, an internal suction line strainer and an internal return-oil check valve. No discharge line oil separator was used. A naphthene base oil of 300 SSU viscosity was used. The system used a direct expansion coil evaporator. The suction pressure at the coil was approximately 50 psia (38 F) and the superheat leaving the coil was approximately 9 F during all tests. Capacity regulation was achieved by simultaneously throttling the air quantity and the suction vapor. Two lengths of suction risers were used: 12 ft. 6 in. and 3 ft. 0 in. Both risers were constructed of 1-1/8 in OD type L copper tubing. The glass P trap at the bottom of the riser had an inside diameter of 1.0 in. Its other dimensions are shown in Figure 66F22A. RESULTS: 1. The height of the riser had no effect on the oil migration pattern in any portion of the trap. Figure 66F22B shows the oil film at a vapor velocity slightly above 600 fpm. The oil flowed along the bottom of the horizontal leg; in various patterns depending upon the position in the U bend; and in a continuous upward creeping movement of a rippling liquid film in the vertical leg. Upward movement of oil film in R-12 riser with 625 fpm upward vapor velocity Figure 66F22C shows the oil film at a vapor velocity slightly below 500 fpm. The uneven ripples in the vertical leg appeared to move downward at a low velocity. A small pool of oil appeared at the bottom of the trap but the size of this pool remained constant after an equilibrium had be reached. The oil flowed along the horizontal leg at low velocity. The oil film in the riser must have been quite thin, since upon turning off the compressor, the oil accumulation at the bottom of the trap when using the 12 ft 6 in riser became only about 0.5 in deep. 2. The oil film moved along horizontal and vertical lines at a slow rate of travel. When the vapor velocity was 600 fpm the ripples appeared to move upward in the riser at less than 1.0 fpm while the ripples in the horizontal leg appeared to move at about 6.0 fpm. It must not be inferred, however, that the oil film moves at the same velocity as the ripples. In the riser, no doubt, the ripples are formed by two following opposing forces: (1) the upward viscous drag of the refrigerant oil film on the vapor and (2) the downward force of gravity on the oil film. The velocity of the ripples, however, do have some significance. Since they are undoubtedly caused by opposing forces, the oil must flow in the film at a slower rate than the ripples, and if the oil flow Downward movement of oil film in R-12 riser with 490 fpm upward vapor velocity Copyright 1959, 2009, By Refrigeration Service Engineers Society. -2-

3 were by film travel only, the return of oil to the compressor crankcase would be very slow indeed and could be best described as a migration. 3. The P trap satisfactorily drained the horizontal suction line under all conditions of operation. Also, satisfactory oil return to the compressor was achieved during all conditions of operation; however, if the suction line had been long and branched with the system operating at suction vapor velocities below 500 fpm, and at non-stabilized conditions, oil return difficulty might have been encountered because of the slower movement of the somewhat thicker oil film than when operating at higher velocities. 4. Since the size of the small oil pool at the bottom of the trap remained constant at velocities below 500 fpm and since the oil ripples moved steadily downward toward this pool, it was concluded that there must be more than one mode of oil migration. The oil not only traveled as an oil film, but also with the vapor. OIL MIGRATION WHEN ITS CONCENTRATION IN THE CIRCULATION REFRIGERANT IS LOW For many years we have received reports on difficulty of oil-return where long horizontal suction lines were involved. Most of these reports were concerned with refrigerating systems for super markets. Also, the first study indicated that, because of low velocity of oil travel in either horizontal or vertical lines, considerable time could elapse on long lines between the time oil left the compressor crankcase and was returned to it. Because of these circumstances we have set up a test system, as indicated in Figure 66F23, with a 115 ft. long suction line constructed of 1-in nominal diam. schedule 40 steel pipe, with 1.0-in inside diam. extra-strong heat-resistant glass sections, as indicated. Schematic of Test System A riser, 3 ft. 0 in high, preceded by a long-turn ell constructed by bending a piece of schedule 40 steel pipe on a 3.5 in radius, was included in the suction line, as indicated. The purpose of this riser was to give as poor a flow condition for the oil as possible. Copyright 1959, 2009, By Refrigeration Service Engineers Society. -3-

4 A 2 HP, 2 cyl. R-12 compressor running at 575 rpm was used. It had trunk type pistons with one compression ring above the entry of the suction vapor and two scraper rings below the entry of the vapor. It had splash lubrication. The crankshaft was not counter-balanced. One peculiarity of this compressor was that it had no internal nor external connection between the suction vapor and the compressor crankcase. The compressor was in good condition although it was 14 years old. It has never had any parts replaced. In spite of its age it pumped but little oil. In order to vary the oil quantity in circulation an external connection was made from the bottom of the compressor crankcase through a needle valve to the suction line. A naphthene base lubricating oil of 300 SSU viscosity was used. The capacity of the system was regulated by regulating the air quantity flowing through the blower coil and throttling the suction vapor at the compressor. A preliminary study showed no difficulty of oil return at suction pressures ranging from -20 F to + 20 F at refrigerant vapor velocities down to 375 fpm. It was decided that the amazing performance of this system warranted closer study of the percentage concentration of oil in the liquid refrigerant, a closer study of the compressor performance, and a photographic study of the oil circulation in the lines. First examined was the amazing performance of the compressor. In spite of its age, it pumped but little oil. The percentage of oil in the liquid refrigerant was 0.6 of 1% or less during all tests in which the needle valve in the line between the compressor crankcase and the suction line was closed. This low rate of oil pumping is comparable to that when a discharge line oil separator is used with most compressors. When oil was injected into the piping system by opening the needle valve, this oil returned to the compressor crankcase in less than 15 minutes although the crankcase pressure was considerably above that of the suction vapor. Figure 66F24A charts the crankcase pressure and the suction pressure over a 6 hour period of operation. Notice that the crankcase pressure exceeded the suction pressure by 13 psi shortly after start-up. This pressure difference increased to 27 psi about one hour after start-up, then slowly decreased until the pressure difference was only 3.0 psi after six hours of operation. No noticeable difference in crankcase oil level occurred during the period. In order to compare the flow of oil in the suction line of this system with that of the previous study, the P shaped trap was substituted for the long turn ell and photographs were taken both of it and the horizontal glass sections. The photographs of the horizontal glass sections showed little that is not shown here in those of the trap; therefore, only the photographs of the trap are reproduced here. These photographs were for tests with the needle valve between the crankcase and the suction line closed. The concentration of oil in the liquid refrigerant did not exceed 0.6 of 1% by weight. Three tests were run at Crankcase and Suction Pressures vapor velocities in the suction line respectively at 400, 480, and 670 fpm. The suction pressure was maintained at approximately 22 psig (approximately 22 F), and at the trap, the vapor was super-heated approximately 40 F above the saturated condition during all tests. An unusual flow pattern appeared in the trap when the suction line vapor velocity was 400 fpm. After more than three hours of continuous operation, oil drops appeared in the horizontal leg of the trap, but these drops were stationary. Above these drops the line was completely clear showing no indication of an oil mist. A still pool of oil about 3/8 in thick lay at the bottom of the trap. A slight fogging of the glass appeared at the bottom of the vertical leg, but almost immediately beyond the bend, the vertical leg was entirely clear. The system was operated for about three additional hours with no apparent change in crankcase oil level nor oil quantity in the pool at the bottom of the trap. Since the concentration of oil in the liquid Copyright 1959, 2009, By Refrigeration Service Engineers Society. -4-

5 refrigerant was 0.6 of 1%, it must be concluded in this case that this oil traveled as a transparent colloidal dispersion. Figure 66F24C shows the flow pattern in the transparent trap when the suction line vapor velocity rose to 480 fpm, with suction temperature and superheat approximately the same as in the previous test. The thickness of oil at the bottom of the trap was about the same thickness as before, but in this case the surface of the pool was turbulent. Some oil condensed at the surface of the pipe and just beyond the bend. No film appeared in the riser beyond this condensation. The system operated several hours without apparent change in crankcase oil level or without change in quantity of oil in the pool at the bottom of the bend. Again it must be concluded that the small quantity of oil in circulation must have traveled in the horizontal and vertical legs of the trap as a transparent colloidal dispersion. After more than three hours of continuous operation at a vapor velocity of 670 fpm, ripples in the oil stream in the horizontal leg were moving at about 2 in/min. Only a small pool of oil still Oil flow pattern with 480 fpm vapor velocity remained at the bottom of the trap, but this pool was blown to the riser side by the vapor. In the upward leg of the trap, oil ripples moved slowly upward. At this velocity, the vapor stream is sufficiently turbulent to cause condensation of oil by impingement along the surface of the pipe. CONCLUSIONS 1. The study of oil flow in long horizontal suction lines was conducted at lower suction pressure, higher superheat at the trap, and lower concentration of oil in the refrigerant than that of the performance of a P shaped trap at the bottom of a suction riser. The combination of these factors affect the oil flow pattern considerably and further work needs to be done to adequately separate their effects. It appears, however, that the concentration of oil in the refrigerant is the most significant. 2. At the low concentration of oil in the refrigerant of the second study 0.6% or lower it would be quite difficult to encounter an oil return problem, except by tapping into the bottom of a horizontal suction main or by improper drainage at the evaporator. 3. If the concentration of oil in the refrigerant is sufficiently low it may be carried through a horizontal or vertical suction line as a transparent colloidal dispersion. 4. R-12 compressors can be designed which will put such a small quantity of oil in circulation that it would be exceedingly difficult to encounter an oil return problem except by tapping into the bottom of a horizontal suction main or by improper drainage at the evaporators. LUBRICATION OF LOW TEMPERATURE REFRIGERANT - 22 SYSTEMS When requested to prepare this report, it was suggested that information should be included which would apply to the use of R-22 at evaporating temperatures of -35 F or lower these evaporating temperatures are commonly used with ice cream cabinets. Testing within this region with glass pipe offers difficulties. The line would frost on the outside unless two concentric pipes with a vacuum between these two pipes were used. We have not performed such tests; however, we have worked with two stage R-22 systems at suction temperatures as low as -90 F and our experience with these systems may be of some help. Copyright 1959, 2009, By Refrigeration Service Engineers Society. -5-

6 I further believe that we should be thinking in terms of two stage systems when dealing with such low evaporating temperatures (-40 F). While such temperatures can be maintained with a single stage system with a water cooled condenser, operating at a condensing temperature below 100 F, its efficiency will be deplorably low. If an air cooled condenser is used, the condensing temperature often rises to 120 or 130 F. Under such conditions the compression ratio becomes so great that most compressors practically cease to pump. The compression ratio at -40 F evaporating and 120 F condensing temperature is approximately 18. Pumping at such a compression ratio is unthinkable without two stages. With two stages, each stage could pump at a compression ratio of 18 or At this compression ratio a volumetric efficiency of 80% or higher could be expected. A small compound two-stage R-22 system (with a 1.0 hp motor on each compressor) was set up to study equalization of crankcase oil levels and other lubrication problems. The compressors were of the open type, using shaft seals. Crankcases were on the same level. A 3/8 OD copper oil-equalizing line, tapped in at the desired oil level, connected the two crankcases. A 3/8 OD copper vapor equalizing line tapped in well above the oil level was also used. The oil return check valve in the low stage compressor was removed and the oil return port blocked off so that the low stage crankcase could operate at the same pressure as the crankcase of the high stage. The system was equipped and valved so that it could operate without the use of a discharge line oil separator, with one oil separator, with two oil separators, with or without a vapor equalizing line between crankcases, and with or without an oil equalizing line between crankcases. A naphthalene base oil of 300 SSU viscosity was used. For each test the system was allowed to operate seven days. It was found that the use of two oil separators, one for each compressor, was superfluous. The use of a single oil separator on the high stage with the use of the oil equalizing line between crankcases gave slightly better equalizing. It was also found that the vapor equalizing line was superfluous. Satisfactory oil equalizing was achieved without the use of an oil equalizing line, but with the high stage oil separator discharging into the low stage crankcase so long as both compressors were run continuously. Without the use of an oil separator, but with the use of the oil equalizing line, the levels were satisfactory but somewhat lower, particularly in the low stage crankcase. There was no difficulty with oil return at evaporating temperatures down to -80 F, but at -60 F the rate of heat transfer in the evaporator dropped to about 2/3; of that when an oil separator was used. The percent concentration of oil in the circulating refrigerant was between 2 and 3% when the separator was not used, and so low that it could hardly be tested with the use of an oil separator (approximately 0.1%). For best all-round results, the most suitable arrangement turned out to be the use of a single oil separator on the high stage compressor, discharging the oil back to the high stage crankcase, with an oil-equalizing line at the high stage crankcase oil level, allowing the oil to flow back to the low stage crankcase. Subsequently, another compound two stage R-22 system was set up using 3.0 h. p. motors on each stage. Compressors were of the open type. As before the oil return port to the low stage compressor was blocked off. The high stage compressor was mounted directly above the low stage compressor. A discharge line oil separator was used on the high stage discharging the oil back into the high stage crankcase. A 3/8 OD copper vertical oil equalizing line was used between the two crankcases. This line was tapped into the higher stage crankcase with its lower edge at the desired oil level. A 1-1/8 OD type K copper suction line was used from the evaporator to the low stage compressor. A naphthene base oil of 300 SSU viscosity was used. Suction temperatures down to -90 F were maintained. Test runs were normally for the period of one week. Oil levels in both crankcases remained nearly constant at all conditions of operation. The oil equalizing on this system (with the high stage compressor mounted directly above the low stage compressor) was equal in all respects to the former one in which the crankcases were mounted at the same level. Although the suction line was sized for a low pressure drop, no difficulty was encountered on oil migration. At -90 F suction temperature, a pressure change of 1.0 psi results in an evaporating temperature change of approximately 9 F; therefore, low friction is almost necessary. This system has a capacity of approximately 0.5 ton at -90 F. With a 1-1/8 OD type K copper line this results in a pressure drop due to friction of 0.37 psi per 100 feet of pipe. It is probable that the oil travel through the system was as a mist and as a colloidal dispersion since the concentration of oil in the circulating refrigerant was small. The study showed that suction lines may be sized for low pressure drop on low temperature systems if a suitable discharge line oil separator is used. Since excessive entrained oil in the refrigerant greatly reduces the heat transfer rate in the evaporator, it may be concluded that an oil separator is a necessary part of a low temperature system. Copyright 1959, 2009, By Refrigeration Service Engineers Society. -6-

7 This may not apply to systems operating at -40 F if the compressor is of such a design that it puts little oil in circulation. Further studies of heat transfer rates at this temperature, as a function of oil concentration, are desirable. It has been general practice to use an oil of lower viscosity on a low temperature system than one operating on a normal suction temperature (say, 0 F or above) because of the fear of oil congealing in the evaporator. In view of our tests this would seem neither necessary nor desirable. Compressors operating at low suction temperatures often have a higher discharge temperature than those operating at normal suction temperatures; therefore, the use of lower viscosity oils than that recommended by the manufacturer for normal suction temperatures is questionable. SUMMARY OF TEST RESULTS 1. The shape of the fitting at the bottom of a riser is a very important factor in avoiding oil accumulation in the horizontal run approaching the riser. At low vapor velocity the setting was the greatest with a long turn ell; less with a short turn ell; finally, there was no settling at all at any vapor velocity which we were able to test when a P trap was used at the bottom of the riser. 2. Three modes of oil travel were observed: A. as a rippling oil film in vertical risers or as a rippling oil stream in horizontal runs. B. as a mist, and C. as a transparent colloidal dispersion. The transition region for rippling film movement from downward to upward in a 1.0 inch diameter suction riser on a R-12 system operating at 50 psia was between 500 and 600 fpm. No critical values were found for oil travel as a mist; however, this mode of travel requires a much lower vapor velocity than is necessary to insure film travel in the proper direction. It is probably that oil will travel as a transparent colloidal dispersion at any vapor velocities. At a high concentration of oil in the circulating refrigerant film travel will be the main mode of oil migration, while at low concentrations the major portion of the oil will travel with the vapor. 3. The height of the riser following the P trap had no effect on oil film travel, either when the ripples appeared to be moving upward or downward. Evidently a continuous equilibrium was established in the riser between the film travel and the other two modes of travel in the riser. 4. The migration of oil by film travel is very slow. We have not determined its rate of flow under the many conditions encountered in a piping system; however, in the 1.0 inch diameter suction line carrying R-12, at 50 psia, with a vapor velocity of 600 fpm, the ripples appeared to move at about 1.0 fpm in the riser and 6.0 fpm in the horizontal run. Since ripples are the result of opposing forces (viscous drag by the vapor, viscous resistance at the surface of the pipe, and resistance by the force of gravity) the rate of migration would be considerably slower than that of the ripples. Therefore, if the concentration of oil in the circulating refrigerant were high, any instability in the operation of the system that would vary the amount of oil put in circulation would result in a variation over a considerable period if the lines were long. 5. If the concentration of oil in the circulating refrigerant is maintained at a low level (say 0.6% by weight or lower) there will be little difficulty with oil migration since at low vapor velocity this small concentration will travel through the lines as a mist or as a colloidal dispersion. This low concentration can be achieved by suitable compressor design or with the aid of a discharge line oil separator. 6. Various methods of equalization of the oil levels in the two crankcases of two stage compound systems were found. The most suitable, when using open type compressors, was the maintenance of the low stage crankcase at the intermediate pressure by blocking off the oil return port to the low stage crankcase; using an oil equalizing line between crankcases tapped into the high stage compressor crankcase at the desired oil level; and using a discharge line oil separator discharging the separated oil into the high stage compressor crankcase. Compressors may be Copyright 1959, 2009, By Refrigeration Service Engineers Society. -7-

8 mounted at the same level or the high stage compressor may be mounted above the low stage compressor. 7. No oil return difficulties were encountered on a compound two stage R-22 system without the use of a discharge line oil separator at an evaporating temperature of -60 F; however, the heat transfer rate in the evaporator was low. We, therefore, consider a discharge line oil separator to be a necessary part of a low temperature system. 8. If the concentration of oil in the circulating refrigerant is maintained at a low level on a R-22 system, oil with a viscosity of 300 SSU may be satisfactorily used at evaporating temperatures down to -90 F. PIPING AND LUBRICATION RECOMMENDATIONS No attempt will be made to cover all situations. Only such recommendations will be offered that can be directly inferred from our test results. 1. The maintenance of low oil concentration in the circulating refrigerant can simplify piping design. If the concentration of oil in the circulating refrigerant is maintained at a low level (say 0.6% by weight or lower) little difficulty will be encountered with oil migration in the system since at low concentrations the two main modes of travel are as a mist and as a colloidal dispersion. The main precaution to be observed is that of tapping suction line branches into the top of horizontal mains since some oil, even at low concentrations, does appear at the bottom of horizontal runs. When the branch is tapped into the top of the horizontal run, no oil drainage will then occur into the branch when vapor is not flowing in the branch. With low oil concentration suction lines may be sized for low pressure loss so that the capacity loss due to friction will be small. 2. The concentration of oil in the circulating refrigerant may be maintained at a low level by suitable compressor design or by the use of a discharge line oil separator. 3. Maintain low concentration of oil in the circulating refrigerant on low temperature systems. At present this will usually require the use of a discharge line separator. While suitable oil return may be achieved on low temperature systems without the use of a discharge line oil separator, the use of the separator will insure a better rate of heat transfer in the evaporator. 4. Maintain low concentration of oil in the circulating refrigerant if the lines are long and branched and if instability of operation of the system is expected. Instability can be caused by sudden changes of capacity, improper drainage at bulb of the thermal expansion valve, oversize of the thermal expansion valves either at a full or partial loads, sudden changes of suction pressure due to improper action of cylinder unloaders, etc. At a high oil concentration the oil circulates largely by film travel. While the rate of this travel could not be accurately measured, it was found to very slow. Any instability in the operation of the system which would vary the oil concentration in the circulating refrigerant would vary the crankcase oil level which would be slow in resuming its normal level if the lines were long. 5. If the oil concentration in the circulating refrigerant is expected to be appreciable (say above 0.6 %) and if low vapor velocity is to encountered during any normal operating condition of the system, use P shaped traps at the bottom of all suction risers to drain the oil from the horizontal runs approaching these risers. Copyright 1959, 2009, By Refrigeration Service Engineers Society. -8-

9 Critical vapor velocity for upward oil film travel in all sizes of riser was not determined; however, for 1.0 inside diameter glass riser, the transition region from a downward rippling movement of the oil film to an upward rippling movement was between 500 and 600 fpm. Satisfactory oil return was achieved with both downward and upward rippling movements when a glass P shaped trap was used at the bottom of the riser without excessive accumulation of oil at the bottom of the trap. 6. If appreciable concentration of oil is the circulating refrigerant is expected, special attention should be given to a suction riser following a horizontal evaporator connection on which the bulb of a thermal expansion valve is clamped. If the horizontal run is not properly drained, erratic operation of the expansion valve may result. Such unstable operation will result in varying oil concentrations in the circulating refrigerant with consequent variation in the crankcase oil level. A P shaped trap should, therefore, be used to connect the horizontal connection to the riser. 7. If the concentration of oil in the circulating refrigerant is low, a naphthene base oil with a viscosity of 300 SSU may be satisfactorily used with R-22 at evaporating temperatures down to -90 ;. It is desirable that the compressor use the same viscosity oil at low evaporating temperatures, since the temperatures of the compressor may be equal or higher than that when operating at normal evaporating temperatures. Low temperature systems should be designed so that there will be a low concentration of oil in the circulating refrigerant to insure a proper rate of heat transfer in the evaporator. As mentioned before, this low concentration can be achieved with the use of a discharge line oil separator or by compressor design. Copyright 1959, 2009, By Refrigeration Service Engineers Society. -9-