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Low loss measurement of oil concentration in a refrigerant-oil mixture in a liquid line
Low loss measurement of oil concentration in a refrigeram-oil mixture in a liquid line J. A. M c G o v e r n Department of Mechanical and Manufacturing Engineering, Parsons Building, Trinity College, Dublin 2, Ireland
A method of measuring the mass concentration of lubricating oil in a refrigerant-oil mixture in a liquid line within a refrigeration system is described. It is suitable for use where the oil and refrigerant are fully miscible in the liquid phase, e.g. for mineral oil and R12. The technique involves taking a small liquid sample, measuring its volume, venting the refrigerant back into the system and weighing the residual oil. Many measurements can be made, with negligible loss of refrigerant from the system. An application of the method to a split circuit bypass type compressor load stand is also described. (Keywords: oil concentration;refrigerationsystem; load stand)
Mesure de la perte faible de la concentration d'huile d'un m61ange frigorig6ne/huile dans une conduite de liquide On d~crit une mkthode de mesure de la concentration massique de I'huile de lubrification d'un m~lange frigorig~ne/huile dans une conduite de liquide, h l'int&ieur d'un systkme frigorifique. On peut utiliser cette m~thode lorsque I'huile et lefrigorig~ne sont complktement miscibles en phase liquide comme l'huile rain&ale et le R12 par exemple. Ia technique consiste gt prendre un petit ~chantillon de liquide, fi mesurer son volume, h transf&er le frigorigOne dans le syst~me et d peser l'huile rksiduelle. On peut effectuer beaucoup de mesures avec une perte n~gligeable de frigorigOne dt partir du systkme. On d~crit ~galement, pour l'application de la mkthode, le montage d'un compresseur dans le circuit avec by-pass.
(Mots cl6s: concentrationd'huile; syst6mefrigorifique;banc d'essail
A weight over volume method of determining the mass concentration of oil in a liquid refrigerant-oil mixture within a refrigeration system is described which, typically, requires a sample of about 10 ml of liquid to be taken from the system. The method has the particular advantage that only a minute quantity of refrigerant is permanently lost from the system for each sample taken. This is achieved by venting the sample to the suction side of the compressor in the system after its volume has been noted. The method requires that the oil and refrigerant are fully miscible in the liquid phase, e.g. liquid R12 and naphthenic refrigeration oil. An ASHRAE standard 1 describes a method, based on weighing, to determine the mass concentration of lubricating oil in liquid refrigerant. This method requires that a sample of about 0.454kg (1 lb) of refrigerant be taken from the system and, thus, it is not suitable for use in small refrigeration systems or compressor load stands where the total refrigerant charge is not much greater than this and where such a loss of charge would affect the operation of the system. A similar method, also based on weighing, is described in a DIN standard 2 but, in this case, no particular sample size is prescribed. Both the ASHRAE and DIN methods would normally involve total loss of the sample from the system. The ASHRAE and DIN methods and that described here all rely on the fact that at temperatures close to ambient the vapour pressures of standard refrigeration oils are very low. For a typical naphthenic refrigeration 0140-7007/89/060310-04503.00 © 1989 Butterworth & Co (Publishers) Ltd and IIR 310 Int. J. Refrig. 1989Vol 12 November
oil the vapour pressure is estimated as 0 . t p m H g (0.013Pa) at 26.7°C (80°F) 3. A typical single stage vacuum pump, as used in refrigeration applications, has a blank-off pressure of 75/anHg (10 Pa). Equipment details
The connections to the sample vessel, the liquid line, the suction line and the necessary valves are shown in Figure 1 The three-port valve which is used to take the sample is permanently open in the straight through direction and is of a type which contains no dead space, where an unrepresentative liquid sample could lodge. The last characteristic is essential for the satisfactory operation of the measurement system. The bleed valve to the suction line is sized to ensure a low velocity within the sample vessel of the refrigerant vapour which evolves from the liquid surface of the sample as evaporation takes place. This is to prevent eruption of large bubbles from within the sample and possible carry-off of liquid droplets containing oil. The sample vessel is of thick-walled glass tubing, closed at the bottom. Figure 2 is a diagram of the vessel and its mounting fixture. The glass sample vessel fits over a brass spigot on the fixture and sealing is by means of an O-ring. The vessel is held in place by means of a support bracket and a wing nut. A scale is fitted to the back plate of the mounting fixture and the volumetric capacity of the sample vessel is calibrated in terms of this. A transparent hinged housing
Low loss measurement of oil concentration." J. A. McGovern Liquid line
Sampling valve
To vent or
...
J
vacuum pump
T BI
2Y
valve
Vent valve
>
Suction line
Figure 1 Connections to the sample vessel, the liquid line and the suction line Figure 1 Connextions au rOcipient d¥chantillonnage, gtla conduite de liquide et d la conduite d'aspiration
il
Tapped hole for C u s h i o n
O - r i n g
il
II II II q
flare fitting
I[
pI II II 1.1 h-q"
k.L.i.
seal
B B D _ e -
_
F
r a m m
m m
-
S
Scale attached to back plate
sample vessel is mounted on its holding fixture and evacuated through the vent valve. A sight glass in the liquid line is viewed to ensure that only liquid refrigerant oil mixture, free of bubbles, is flowing past the sample point. The vent valve is closed and a sample of up to about 12ml of liquid mixture is allowed into the evacuated vessel, by opening the sampling valve for a short time, perhaps 5s. After a settling period, during which the sample reaches approximately the known ambient temperature, the volume of liquid is determined from the scale reading and the refrigerant content of the sample is then allowed to evaporate slowly by just cracking open the bleed valve to the suction line. When the evaporation process appears to have finished and only oil appears to be left, the bleed valve is closed and the sample vessel is vented to atmosphere. The vessel is then connected to a vacuum pump, evacuated, and left under vacuum for a period to ensure the evaporation of any remaining refrigerant. Strictly, the oil will still contain a small concentration of refrigerant and, for maximum accuracy, the concentration of refrigerant in the oil refrigerant mixture under the known vacuum at the known ambient temperature could be calculated and used to correct the mass of oil. For R12 with naphthenic oil this correction was found to be unnecessary, e.g. at a temperature of 15°C and a pressure of 0.01 bar (1 kPa) the concentration of refrigerant in the oil refrigerant mixture would be 5.5 x 10 -3 (Bambach's equation4). The vessel is then opened to atmosphere, removed, and weighed to determine the weight including the sample. After cleaning, the vessel is reweighed and, by subtracting the second weight from the first, the mass of oil is calculated. The mass concentration of oil in the original sample is calculated from the measured oil mass, the mixture volume and the ambient temperature, making use of the known relationships between temperature and density for the oil and the refrigerant 5.
Removable glass sample vessel
B m m
Application of the method to a split circuit bypass type compressor load stand
w
m --2"~
m ~ --
i
-
. . . .
-1
II_.
Support bracket Figure 2 Diagram of the sample vesselused to measure the concentration of oil in liquid refrigerant and its mounting fixture (not to scale) Figure 2 Diagramme du r~cipient d¥chantillonnage utilisb pour mesurer la concentration d'huile du frigorigbne liquide et diagramme du dispositif de fixation (pas ~ l¥chelle)
made of tough plastic, which is not shown, is fitted around the sample vessel and mounting fixture as a safety device in case the glass should rupture under pressure. Procedure
With the sampling and bleed valves closed, the clean dry
The method described has been applied to measure the oil concentration in compressor discharge vapour in a split circuit bypass type load stand similar to a version J described in DIN 87992. Figure 3 is a diagram of the load stand circuit 6. The oil concentration measurement method was, in fact, developed to meet the oil concentration measurement requirements in this situation, where unfortunately, only part of the compressor mass flow rate, about one third, passed through the liquid line. The refrigerant used was R12 and the oil was a standard refrigeration naphthenic type (Suniso 3GS7). It is felt that a more suitable application of the method would be to a conventional refrigeration system or calorimeter load stand, where the entire mass flow rate of refrigerant -oil mixture passes through the liquid line. For the split circuit load stand, reasonably accurate measurements of the oil concentration in the refrigerant oil mixture were required in order to calculate thermodynamic properties at various points. Hughes et al. 8 have demonstrated how the properties of refrigerant oil mixtures differ from those of pure refrigerant. These differences are generally not very significant when the oil concentration is below a few per cent. However, the
Rev. Int. Froid 1 9 8 9 Vol 12 N o v e m b r e
311
Low loss measurement of oil concentration. J. A. McGovern Sampling valve
I
+
Condezmer
I
I
eomp~88~
Figure 3 Diagram of the load stand circuit. A, B, C and D are load stand control valves. The following are typical operating pressures within the system: compressor suction pressure, 2.19 bar (0.219 MPa); compressor discharge pressure, 9.62 bar (0.962 MPa); heat exchanger saturation pressure, 7.0 bar (0.70MPa); refrigerant pressure after mixing before valve D, 5.6 bar (0.56 MPa) (Reference 6) Figure 3 Diagramme du circuit d'essai. A, B, C et D sont des robinets de r~glage. Les pressions ci-apr~s sont des pressions de fonctionnement types du systdme: pression d'aspiration du compresseur, 2,19 bars; pression de refoulement du compresseur, 9,62 bars; pression de saturation de l'~changeur de chaleur, 7,0 bars; pression du frigorig~ne apr~s le m~lange effectu~ avant le robinet D, 5,6 bars. (R~f~rence 6)
differences can be significant, even with fractional percentage oil concentrations, when the 'apparent' superheat is low. The apparent superheat is the superheat which would exist if the fluid whose temperature and pressure are measured were pure refrigerant and not a refrigerant-oil mixture. In the load stand referred to in this paper low apparent superheat values were observed after the mixing process before valve D (Figure3) (full details of the load stand are given in Reference 6). The following values illustrate the effect of a very low oil concentration at a low apparent superheat on specific enthalpy. At - 3 . 0 3 ° C and 2.65 bar (0.265 MPa), for pure refrigerant 12:
oil concentration as the greater part of the flow, which bypassed it. The angle which the branch to the heat exchanger formed with the main pipe was made as small as possible (about 30 ° ) to minimize any effect of the tendency ofoil droplets to continue in a straight line. This junction was in the horizontal plane to minimize gravitational bias in the division of flow, particularly of liquid droplets or liquid film on the pipe walls. The diameter of the branch pipe to the heat exchanger was chosen so that the vapour velocity within it would be about the same as in the main-stream. The bleed valve used, which had a compliant stem tip for positive shut off, had an effective flow area of 1.44 mm 2 when fully open (2.5 turns) and an effective flow area of 0.07 mm 2 when 0.05 turns were open. (The effective flow area is the product of the area and the coefficient of discharge of an ideal orifice which would give the same flow rate.) The latter area was used in estimating the flow rate through the bleed valve when just cracked open. Based on dry saturated refrigerant vapour at a pressure of 4.91 bar (0.491 MPa) in the sample vessel and 2.19bar (0.219 MPa) in the suction line, the calculated mass flow rate for choked flow was 0.16x 1 0 - 3 k g s -1. For an internal diameter of 12.6 mm within the sample vessel this gave a calculated velocity of 0.05 m s 1. This was considered low enough to avoid entrainment of liquid droplets. During evaporation of the liquid refrigerant the temperature within the sample vessel dropped as the latent heat of evaporation was absorbed from the sample and the sample vessel. This had the effect of slowing down the evaporation process. A visible manifestation of this was a haze of water condensation which was observed to form on the outside surface of the glass vessel and which indicated that the surface temperature was below the dew point of the surrounding air. Although it was possible to increase the opening of the bleed valve progressively as the pressure in the sample vessel fell, this was found to have little effect on the evaporation rate. However, it was found that the evaporation process could be speeded up significantly by gently heating the vessel using a hair drier. 0.005
l
I
---r
......
~. . . .
I
superheat = 1.53 K specific enthalpy = 186.54 kJ k g - 1 0.004
and for an R12-oil mixture containing 0.33% oil: apparent superheat = 1.53 K actual dryness fraction of refrigerant = 0.986 specific enthalpy of refrigerant-oil mixture = 184.01 kJ k g - 1 Ideally, a sample should have been taken of the full flow of refrigerant in the split circuit load stand. However, as the oil concentration measurement method required the refrigerant to be in the liquid state, the liquid line coming from the heat exchanger was chosen as the location from which to take the sample. A major assumption underlying the technique in this particular application was, therefore, that the oil concentration in the liquid refrigerant coming from the condenser heat exchanger was the same as that in the refrigerant vapour coming from the compressor. In the long term no build-up of oil could occur in the heat exchanger due to the high solubility of the oil in liquid R12. It was necessary to ensure that, as far as possible, the refrigerant which went to the heat exchanger had the same
312
Int. J Refrig. 1989 Vol 12 November
c o
O.OO3 L
c3 0.oo2 0.001
0.0
i
I
10 Suction
i
20 temperature
3o
(°C)
Figure 4 Oil concentration (mass of oil over mass of mixture) in compressor discharge vapour as a function of suction temperature: test results Figure 4 Concentration d'huile (rapport masse d'huile/masse du mblange) dans la vapeur de refoulement du compresseur en fonction de la tempbrature d'aspiration: r~sultats des essais
Low loss measurement of oil concentration: J. A. McGovern
The disappearance of the water condensation was taken as the signal to discontinue the heating. The rate of evaporation of refrigerant from the sample was, thus, controlled by its temperature. The bleed valve was left in the just-cracked-open position and served as a flow limiting device. Bleed times were about 20 min. Figure 4 illustrates results of tests to assess the effect of suction temperature on the oil concentration in the discharge vapour of an open compressor (based on liquid line measurements in the split circuit load stand) operating at 600 rev min-1 with suction and discharge saturation temperatures of - 10 and 40°C, respectively. Some additional results and an interpretation of these are included in Reference 6.
methods have been developed 9'1°, this method has advantages of low cost, simplicity and sensitivity to very low oil concentrations, and measures mass over volume directly. References 1 2 3 4 5
Conclusions
A simple weight over volume method of determining the oil concentration in liquid refrigerant has been described. This requires only a small sample of liquid to be taken, the refrigerant content of which is almost all vented back into the system. Repeated samples can be taken without significantly affecting the charge of the system and loss of refrigerant to the atmosphere is virtually eliminated. This method was developed for use in a split circuit type of compressor load stand but it is felt that it would be more suitable for refrigeration systems, or load stands, where the entire flow passes through the liquid line. Although alternative instantaneous oil concentration measurement
6 7 8 9
10
Standard Method for Measurement of Proportion of Oil in Liquid Refrigerant ASHRAE Standard 41.4-84 (1985) Leistungspriifung yon Kfiltemittel-Verdichtern (Testing of Refrigerant Compressors) DIN 8977 (1973) Private communication from Castrol Research Laboratories, Berkshire, UK (May 1989) Bambaeh, G. Das Verhalten yon MineralSI-F12 Gemischen in Kfiltemaschinen Abhandlung No. 9 des Deutschen K/iltetechnischen Vereins, C. F. Miiller, Karlsruhe (1955) MeMullan, J. T., Hughes, D. W., Morgan, R. A suite of computer programs for calculating the thermodynamic properties of refrigerant-oil mixtures Heat Recovery Systems (1985) 5 181-194 MeGovern, J. A. On refrigerant compressors PhD Thesis University of Dublin (1988) Suniso Refrigeration Oils from Sonneborn Sonneborn Division, Witco Chemical Corporation, New York Hughes, D. W., McMullan, J. T., Mawhinney, K. A., Morgan, R. Pressure-enthalpy charts for mixtures of oil and refrigerant R12 lnt J Refrig (1982) 5 199-202 Bastian, J. J., Pate, M. B., Bergles, A. E. Properties of oil refrigerant liquid mixtures with applications to oil concentration measurement: part I - thermophysical and transport properties; part II electrical and optical properties ASHRAE Trans IA (1986) 92 55 92 Baustian, J. J., Pate, M. B., Bergles, A. E. Measuring the concentration of a flowing oil-refrigerant mixture: instrument test facility and initial results ASHRAE Trans 1 (1988) 93 167-177
Rev. Int. Froid 1989 Vol 12 Novembre
313
A method of measuring the mass concentration of lubricating oil in a refrigerant-oil mixture in a liquid line within a refrigeration system is described. It is suitable for use where the oil and refrigerant are fully miscible in the liquid phase, e.g. for mineral oil and R12. The technique involves taking a small liquid sample, measuring its volume, venting the refrigerant back into the system and weighing the residual oil. Many measurements can be made, with negligible loss of refrigerant from the system. An application of the method to a split circuit bypass type compressor load stand is also described. (Keywords: oil concentration;refrigerationsystem; load stand)
Mesure de la perte faible de la concentration d'huile d'un m61ange frigorig6ne/huile dans une conduite de liquide On d~crit une mkthode de mesure de la concentration massique de I'huile de lubrification d'un m~lange frigorig~ne/huile dans une conduite de liquide, h l'int&ieur d'un systkme frigorifique. On peut utiliser cette m~thode lorsque I'huile et lefrigorig~ne sont complktement miscibles en phase liquide comme l'huile rain&ale et le R12 par exemple. Ia technique consiste gt prendre un petit ~chantillon de liquide, fi mesurer son volume, h transf&er le frigorigOne dans le syst~me et d peser l'huile rksiduelle. On peut effectuer beaucoup de mesures avec une perte n~gligeable de frigorigOne dt partir du systkme. On d~crit ~galement, pour l'application de la mkthode, le montage d'un compresseur dans le circuit avec by-pass.
(Mots cl6s: concentrationd'huile; syst6mefrigorifique;banc d'essail
A weight over volume method of determining the mass concentration of oil in a liquid refrigerant-oil mixture within a refrigeration system is described which, typically, requires a sample of about 10 ml of liquid to be taken from the system. The method has the particular advantage that only a minute quantity of refrigerant is permanently lost from the system for each sample taken. This is achieved by venting the sample to the suction side of the compressor in the system after its volume has been noted. The method requires that the oil and refrigerant are fully miscible in the liquid phase, e.g. liquid R12 and naphthenic refrigeration oil. An ASHRAE standard 1 describes a method, based on weighing, to determine the mass concentration of lubricating oil in liquid refrigerant. This method requires that a sample of about 0.454kg (1 lb) of refrigerant be taken from the system and, thus, it is not suitable for use in small refrigeration systems or compressor load stands where the total refrigerant charge is not much greater than this and where such a loss of charge would affect the operation of the system. A similar method, also based on weighing, is described in a DIN standard 2 but, in this case, no particular sample size is prescribed. Both the ASHRAE and DIN methods would normally involve total loss of the sample from the system. The ASHRAE and DIN methods and that described here all rely on the fact that at temperatures close to ambient the vapour pressures of standard refrigeration oils are very low. For a typical naphthenic refrigeration 0140-7007/89/060310-04503.00 © 1989 Butterworth & Co (Publishers) Ltd and IIR 310 Int. J. Refrig. 1989Vol 12 November
oil the vapour pressure is estimated as 0 . t p m H g (0.013Pa) at 26.7°C (80°F) 3. A typical single stage vacuum pump, as used in refrigeration applications, has a blank-off pressure of 75/anHg (10 Pa). Equipment details
The connections to the sample vessel, the liquid line, the suction line and the necessary valves are shown in Figure 1 The three-port valve which is used to take the sample is permanently open in the straight through direction and is of a type which contains no dead space, where an unrepresentative liquid sample could lodge. The last characteristic is essential for the satisfactory operation of the measurement system. The bleed valve to the suction line is sized to ensure a low velocity within the sample vessel of the refrigerant vapour which evolves from the liquid surface of the sample as evaporation takes place. This is to prevent eruption of large bubbles from within the sample and possible carry-off of liquid droplets containing oil. The sample vessel is of thick-walled glass tubing, closed at the bottom. Figure 2 is a diagram of the vessel and its mounting fixture. The glass sample vessel fits over a brass spigot on the fixture and sealing is by means of an O-ring. The vessel is held in place by means of a support bracket and a wing nut. A scale is fitted to the back plate of the mounting fixture and the volumetric capacity of the sample vessel is calibrated in terms of this. A transparent hinged housing
Low loss measurement of oil concentration." J. A. McGovern Liquid line
Sampling valve
To vent or
...
J
vacuum pump
T BI
2Y
valve
Vent valve
>
Suction line
Figure 1 Connections to the sample vessel, the liquid line and the suction line Figure 1 Connextions au rOcipient d¥chantillonnage, gtla conduite de liquide et d la conduite d'aspiration
il
Tapped hole for C u s h i o n
O - r i n g
il
II II II q
flare fitting
I[
pI II II 1.1 h-q"
k.L.i.
seal
B B D _ e -
_
F
r a m m
m m
-
S
Scale attached to back plate
sample vessel is mounted on its holding fixture and evacuated through the vent valve. A sight glass in the liquid line is viewed to ensure that only liquid refrigerant oil mixture, free of bubbles, is flowing past the sample point. The vent valve is closed and a sample of up to about 12ml of liquid mixture is allowed into the evacuated vessel, by opening the sampling valve for a short time, perhaps 5s. After a settling period, during which the sample reaches approximately the known ambient temperature, the volume of liquid is determined from the scale reading and the refrigerant content of the sample is then allowed to evaporate slowly by just cracking open the bleed valve to the suction line. When the evaporation process appears to have finished and only oil appears to be left, the bleed valve is closed and the sample vessel is vented to atmosphere. The vessel is then connected to a vacuum pump, evacuated, and left under vacuum for a period to ensure the evaporation of any remaining refrigerant. Strictly, the oil will still contain a small concentration of refrigerant and, for maximum accuracy, the concentration of refrigerant in the oil refrigerant mixture under the known vacuum at the known ambient temperature could be calculated and used to correct the mass of oil. For R12 with naphthenic oil this correction was found to be unnecessary, e.g. at a temperature of 15°C and a pressure of 0.01 bar (1 kPa) the concentration of refrigerant in the oil refrigerant mixture would be 5.5 x 10 -3 (Bambach's equation4). The vessel is then opened to atmosphere, removed, and weighed to determine the weight including the sample. After cleaning, the vessel is reweighed and, by subtracting the second weight from the first, the mass of oil is calculated. The mass concentration of oil in the original sample is calculated from the measured oil mass, the mixture volume and the ambient temperature, making use of the known relationships between temperature and density for the oil and the refrigerant 5.
Removable glass sample vessel
B m m
Application of the method to a split circuit bypass type compressor load stand
w
m --2"~
m ~ --
i
-
. . . .
-1
II_.
Support bracket Figure 2 Diagram of the sample vesselused to measure the concentration of oil in liquid refrigerant and its mounting fixture (not to scale) Figure 2 Diagramme du r~cipient d¥chantillonnage utilisb pour mesurer la concentration d'huile du frigorigbne liquide et diagramme du dispositif de fixation (pas ~ l¥chelle)
made of tough plastic, which is not shown, is fitted around the sample vessel and mounting fixture as a safety device in case the glass should rupture under pressure. Procedure
With the sampling and bleed valves closed, the clean dry
The method described has been applied to measure the oil concentration in compressor discharge vapour in a split circuit bypass type load stand similar to a version J described in DIN 87992. Figure 3 is a diagram of the load stand circuit 6. The oil concentration measurement method was, in fact, developed to meet the oil concentration measurement requirements in this situation, where unfortunately, only part of the compressor mass flow rate, about one third, passed through the liquid line. The refrigerant used was R12 and the oil was a standard refrigeration naphthenic type (Suniso 3GS7). It is felt that a more suitable application of the method would be to a conventional refrigeration system or calorimeter load stand, where the entire mass flow rate of refrigerant -oil mixture passes through the liquid line. For the split circuit load stand, reasonably accurate measurements of the oil concentration in the refrigerant oil mixture were required in order to calculate thermodynamic properties at various points. Hughes et al. 8 have demonstrated how the properties of refrigerant oil mixtures differ from those of pure refrigerant. These differences are generally not very significant when the oil concentration is below a few per cent. However, the
Rev. Int. Froid 1 9 8 9 Vol 12 N o v e m b r e
311
Low loss measurement of oil concentration. J. A. McGovern Sampling valve
I
+
Condezmer
I
I
eomp~88~
Figure 3 Diagram of the load stand circuit. A, B, C and D are load stand control valves. The following are typical operating pressures within the system: compressor suction pressure, 2.19 bar (0.219 MPa); compressor discharge pressure, 9.62 bar (0.962 MPa); heat exchanger saturation pressure, 7.0 bar (0.70MPa); refrigerant pressure after mixing before valve D, 5.6 bar (0.56 MPa) (Reference 6) Figure 3 Diagramme du circuit d'essai. A, B, C et D sont des robinets de r~glage. Les pressions ci-apr~s sont des pressions de fonctionnement types du systdme: pression d'aspiration du compresseur, 2,19 bars; pression de refoulement du compresseur, 9,62 bars; pression de saturation de l'~changeur de chaleur, 7,0 bars; pression du frigorig~ne apr~s le m~lange effectu~ avant le robinet D, 5,6 bars. (R~f~rence 6)
differences can be significant, even with fractional percentage oil concentrations, when the 'apparent' superheat is low. The apparent superheat is the superheat which would exist if the fluid whose temperature and pressure are measured were pure refrigerant and not a refrigerant-oil mixture. In the load stand referred to in this paper low apparent superheat values were observed after the mixing process before valve D (Figure3) (full details of the load stand are given in Reference 6). The following values illustrate the effect of a very low oil concentration at a low apparent superheat on specific enthalpy. At - 3 . 0 3 ° C and 2.65 bar (0.265 MPa), for pure refrigerant 12:
oil concentration as the greater part of the flow, which bypassed it. The angle which the branch to the heat exchanger formed with the main pipe was made as small as possible (about 30 ° ) to minimize any effect of the tendency ofoil droplets to continue in a straight line. This junction was in the horizontal plane to minimize gravitational bias in the division of flow, particularly of liquid droplets or liquid film on the pipe walls. The diameter of the branch pipe to the heat exchanger was chosen so that the vapour velocity within it would be about the same as in the main-stream. The bleed valve used, which had a compliant stem tip for positive shut off, had an effective flow area of 1.44 mm 2 when fully open (2.5 turns) and an effective flow area of 0.07 mm 2 when 0.05 turns were open. (The effective flow area is the product of the area and the coefficient of discharge of an ideal orifice which would give the same flow rate.) The latter area was used in estimating the flow rate through the bleed valve when just cracked open. Based on dry saturated refrigerant vapour at a pressure of 4.91 bar (0.491 MPa) in the sample vessel and 2.19bar (0.219 MPa) in the suction line, the calculated mass flow rate for choked flow was 0.16x 1 0 - 3 k g s -1. For an internal diameter of 12.6 mm within the sample vessel this gave a calculated velocity of 0.05 m s 1. This was considered low enough to avoid entrainment of liquid droplets. During evaporation of the liquid refrigerant the temperature within the sample vessel dropped as the latent heat of evaporation was absorbed from the sample and the sample vessel. This had the effect of slowing down the evaporation process. A visible manifestation of this was a haze of water condensation which was observed to form on the outside surface of the glass vessel and which indicated that the surface temperature was below the dew point of the surrounding air. Although it was possible to increase the opening of the bleed valve progressively as the pressure in the sample vessel fell, this was found to have little effect on the evaporation rate. However, it was found that the evaporation process could be speeded up significantly by gently heating the vessel using a hair drier. 0.005
l
I
---r
......
~. . . .
I
superheat = 1.53 K specific enthalpy = 186.54 kJ k g - 1 0.004
and for an R12-oil mixture containing 0.33% oil: apparent superheat = 1.53 K actual dryness fraction of refrigerant = 0.986 specific enthalpy of refrigerant-oil mixture = 184.01 kJ k g - 1 Ideally, a sample should have been taken of the full flow of refrigerant in the split circuit load stand. However, as the oil concentration measurement method required the refrigerant to be in the liquid state, the liquid line coming from the heat exchanger was chosen as the location from which to take the sample. A major assumption underlying the technique in this particular application was, therefore, that the oil concentration in the liquid refrigerant coming from the condenser heat exchanger was the same as that in the refrigerant vapour coming from the compressor. In the long term no build-up of oil could occur in the heat exchanger due to the high solubility of the oil in liquid R12. It was necessary to ensure that, as far as possible, the refrigerant which went to the heat exchanger had the same
312
Int. J Refrig. 1989 Vol 12 November
c o
O.OO3 L
c3 0.oo2 0.001
0.0
i
I
10 Suction
i
20 temperature
3o
(°C)
Figure 4 Oil concentration (mass of oil over mass of mixture) in compressor discharge vapour as a function of suction temperature: test results Figure 4 Concentration d'huile (rapport masse d'huile/masse du mblange) dans la vapeur de refoulement du compresseur en fonction de la tempbrature d'aspiration: r~sultats des essais
Low loss measurement of oil concentration: J. A. McGovern
The disappearance of the water condensation was taken as the signal to discontinue the heating. The rate of evaporation of refrigerant from the sample was, thus, controlled by its temperature. The bleed valve was left in the just-cracked-open position and served as a flow limiting device. Bleed times were about 20 min. Figure 4 illustrates results of tests to assess the effect of suction temperature on the oil concentration in the discharge vapour of an open compressor (based on liquid line measurements in the split circuit load stand) operating at 600 rev min-1 with suction and discharge saturation temperatures of - 10 and 40°C, respectively. Some additional results and an interpretation of these are included in Reference 6.
methods have been developed 9'1°, this method has advantages of low cost, simplicity and sensitivity to very low oil concentrations, and measures mass over volume directly. References 1 2 3 4 5
Conclusions
A simple weight over volume method of determining the oil concentration in liquid refrigerant has been described. This requires only a small sample of liquid to be taken, the refrigerant content of which is almost all vented back into the system. Repeated samples can be taken without significantly affecting the charge of the system and loss of refrigerant to the atmosphere is virtually eliminated. This method was developed for use in a split circuit type of compressor load stand but it is felt that it would be more suitable for refrigeration systems, or load stands, where the entire flow passes through the liquid line. Although alternative instantaneous oil concentration measurement
6 7 8 9
10
Standard Method for Measurement of Proportion of Oil in Liquid Refrigerant ASHRAE Standard 41.4-84 (1985) Leistungspriifung yon Kfiltemittel-Verdichtern (Testing of Refrigerant Compressors) DIN 8977 (1973) Private communication from Castrol Research Laboratories, Berkshire, UK (May 1989) Bambaeh, G. Das Verhalten yon MineralSI-F12 Gemischen in Kfiltemaschinen Abhandlung No. 9 des Deutschen K/iltetechnischen Vereins, C. F. Miiller, Karlsruhe (1955) MeMullan, J. T., Hughes, D. W., Morgan, R. A suite of computer programs for calculating the thermodynamic properties of refrigerant-oil mixtures Heat Recovery Systems (1985) 5 181-194 MeGovern, J. A. On refrigerant compressors PhD Thesis University of Dublin (1988) Suniso Refrigeration Oils from Sonneborn Sonneborn Division, Witco Chemical Corporation, New York Hughes, D. W., McMullan, J. T., Mawhinney, K. A., Morgan, R. Pressure-enthalpy charts for mixtures of oil and refrigerant R12 lnt J Refrig (1982) 5 199-202 Bastian, J. J., Pate, M. B., Bergles, A. E. Properties of oil refrigerant liquid mixtures with applications to oil concentration measurement: part I - thermophysical and transport properties; part II electrical and optical properties ASHRAE Trans IA (1986) 92 55 92 Baustian, J. J., Pate, M. B., Bergles, A. E. Measuring the concentration of a flowing oil-refrigerant mixture: instrument test facility and initial results ASHRAE Trans 1 (1988) 93 167-177
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