COMPRESSOR OILS WITH HIGH VISCOSITY INDEX

Abstract

Polyalkyl (meth)acrylates are useful in compressor oils. A method of increasing the energy efficiency of a compressor involves operating the compressor with a compressor oil containing a polyalkyl (meth)acrylate-based viscosity index improver.

Claims

1: A method of increasing the energy efficiency of a compressor, the method comprising: operating the compressor with a compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 30 wt. % of a polyalkyl methacrylate-based viscosity index improver comprising: (a) 0 wt. % to 25 wt. % of methyl methacrylate; (b) 75 wt. % to 100 wt. % of at least one straight-chained or branched C10-18 alkyl (meth)acrylate; and (c) 0 wt. % to 2 wt. % of at least one straight-chained or branched C5-9 alkyl (meth)acrylate or at least one straight-chained or branched C20-24 alkyl (meth)acrylate, wherein a weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/mol to 400,000 g/mol; (ii) 70 wt. % to 99 wt. % of a base oil, wherein the base oil is an oil of American Petroleum Institute (API) group II, III, IV, or V, or a mixture thereof; and (iii) optionally, up to 2.5 wt. % of a performance package comprising one or more further additives, wherein the compressor oil has a viscosity index of at least 140.

2: The method according to claim 1, wherein the polyalkyl methacrylate-based viscosity index improver comprises: (a) 0.2 wt. % of the methyl methacrylate; (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate; and (c) 0 wt. % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate.

3: The method according to claim 1, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/mol to 200,000 g/mol.

4: The method according to claim 1, wherein the performance package (iii) comprises at least an antiwear agent, an anticorrosion agents and an antioxidant.

5: The method according to claim 1, wherein the compressor is selected from the group consisting of household or domestic refrigeration units, air compressors, and carbon dioxide compressors.

6: The method according to claim 1, wherein the compressor is a household or domestic refrigeration unit, the base oil (ii) is an API group IV or V oil, or a mixture thereof, and the compressor oil has a kinematic viscosity at 40 C. in a range of 2.88 and 7.48 cSt.

7: The method according to claim 6, wherein the household or domestic refrigeration unit uses isobutane or propane as refrigerant.

8: The method according to claim 1, wherein the compressor is a household or domestic refrigeration unit using isobutane or propane as refrigerant, the method comprising: operating the refrigeration unit with the compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 10 wt. % of the polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; and (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/mol to 200.000 g/mol; (ii) 90 wt. % to 99 wt. % of an API group IV or V base oil, or a mixture thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent, and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 2.88 and 7.48 cSt and a viscosity index of at least 140.

9: The method according to claim 6, wherein the base oil (ii) is selected from naphthenic base oils having a C.sub.N value of more than 42%.

10: The method according to claim 6, wherein the compressor oil has a pour point of 60 C. or lower.

11: The method according to claim 1, wherein the compressor is an air compressor, the base oil (ii) is an API group II, III, or IV oil, or a mixture thereof, and the compressor oil has a kinematic viscosity at 40 C. in a range of 28.8 and 74.8 cSt.

12: The method according to claim 1, wherein the compressor is an air compressor, the method comprising: operating the air compressor with the compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt. % of the polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate; and (c) 0 wt. % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the range of 5,000 to 400,000 g/mol; (ii) 80 wt. % to 99 wt. % of an API group II, III, or IV base oil, or a mixture thereof; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent, and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 28.8 and 74.8 cSt and a viscosity index of at least 140.

13: The method according to claim 11, wherein the compressor oil has a pour point of 33 C. or lower.

14: The method according to claim 1, wherein the compressor is a carbon dioxide compressor, the base oil (ii) is an API group III, IV, or V oil, or a mixture thereof, and the compressor oil has a kinematic viscosity at 40 C. in a range of 41.4 and 110 cSt.

15: The method according to claim 1, wherein the compressor is a carbon dioxide compressor, the method comprising: operating the carbon dioxide compressor with the compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 20 wt. % of the polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; and (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in the g range of 5,000 g/mol to 100,000 g/mol; (ii) 80 wt. % to 95 wt. % of a polyolester base oil or g mixture of different polyester base oils; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent, and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 41.4 and 110 cSt, and a viscosity index of at least 140.

16: The method according to claim 1, wherein the compressor is a carbon dioxide compressor, the method comprising: operating the carbon dioxide compressor with the compressor oil, wherein the compressor oil comprises: (i) 1 wt. % to 30 wt. % the polyalkyl methacrylate-based viscosity index improver comprising: (a) 0.2 wt. % to 25 wt. % of the methyl methacrylate; and (b) 75 wt. % to 99.8 wt. % of the at least one C10-18 alkyl (meth)acrylate, wherein the weight average molecular weight (M.sub.w) of the polyalkyl (meth)acrylate-based viscosity index improver is in a range of 5,000 g/ml to 100,000 g/mol; (ii) 80 wt. % to 99 wt. % of a polyalphaolefin base oil or mixture of different polyalphaolefin base oils; and (iii) 0 wt. % to 2.5 wt. % of a zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent, and an antioxidant, wherein the compressor oil has a kinematic viscosity at 40 C. in the range of 41.4 and 110 cSt and a viscosity index of at least 140.

17: The method according to claim 1, wherein the compressor oil has a viscosity index of at least 180.

18: The method according to claim 2, wherein the polyalkyl methacrylate-based viscosity index improver comprises: (a) 4 wt. % to 16 wt. % of the methyl methacrylate; (b) 84 wt. % to 96 wt. % of the at least one C10-18 alkyl methacrylate; and (c) 0 wt. % to 2 wt. % of the at least one straight-chained or branched C5-9 alkyl (meth)acrylate or the at least one straight-chained or branched C20-24 alkyl (meth)acrylate.

19: The method according to claim 4, wherein the performance package (iii) is zinc-free.

20: The method according to claim 4, wherein the performance package (iii) is ashless.

Description

[0151] FIG. 1 illustrates the test settings used to determine the effects on energy consumption in an air compressor.

[0152] The invention is further illustrated by the following non-limiting examples and comparative example (reference oil). The examples below serve for further explanation of preferred embodiments according to the present invention but are not intended to restrict the invention.

EXPERIMENTAL PART

Abbreviations

[0153] Synesstic5 alkylated naphthalene base oil from ExxonMobil with a KV.sub.40 of 29 cSt [0154] Berylane 230 naphthenic base oil from Total with a KV.sub.40 of 2.3 cSt and a CN value of about 45% [0155] KV kinematic viscosity measured according to ASTM D445 [0156] KV.sub.40 kinematic viscosity measured @40 C. to ASTM D445 [0157] KV.sub.100 kinematic viscosity measured @100 C. to ASTM D445 [0158] M.sub.n number-average molecular weight [0159] M.sub.w weight-average molecular weight [0160] NS3 naphthenic base oil from Nynas with a KV.sub.40 of 2.9 cSt and a C.sub.N value of about 57% [0161] PAO6 Group IV base oil with a KV.sub.100 of 6 cSt [0162] PAO8 Group IV base oil with a KV.sub.100 of 8 cSt [0163] PDI polydispersity index [0164] PP pour point [0165] T3 naphthenic base oil from Nynas with a KV.sub.40 of 3.6 cSt and a C.sub.N value of about 52% [0166] T9 naphthenic base oil from Nynas with a KV.sub.40 of 9.1 cSt and a C.sub.N value of about 45% [0167] VI viscosity index

[0168] Test Methods

[0169] The polyalkyl methacrylate-based polymers according to the present invention were characterized with respect to their weight-average molecular weight.

[0170] The compressor oils including the polyalkyl methacrylate-based polymers according to the present invention and the comparative examples were characterized with respect to their kinematic viscosity at 40 C. (KV.sub.40) and 100 C. (KV.sub.100) to ASTM D445, their viscosity index (VI) to ASTM D2270, their pour point to ASTM D5950, their flash point ASTM D92 and their viscosity shear loss.

[0171] Determination of effects on energy consumption in a household or domestic refrigeration unit A standardized performance test rig measured the power consumption of the compressor at specified rating conditions. It allowed to ensure the same operating conditions for a number of tests. Furthermore, the performance test comprised the calculation of the coefficient of performance (COP; corresponding to the ratio of cooling power to electric drive power) at the specified rating conditions and the volumetric efficiency, the ratio of real volume flow to geometrically possible volume flow. The latter indicated the sealing properties of the working chamber of the compressor.

[0172] The test-rig setup was designed for performance tests of small capacity refrigerant-compressors in accordance with the ASHRAE standard 23.1 (2010), resp. DIN EN 13771-1 (2017). Based on a standard vapor compression cycle, the test bench included a calorimeter evaporator and a flow meter to determine the refrigerant mass flow rate. Besides the main components like the compressor, the condenser, and an electronic expansion device, the cycle was additionally equipped with an oil separator, a filter dryer, sight glasses, and an accumulator.

[0173] The compressor was a hermetic reciprocating piston compressor of type Embraco VEMX 7C, refrigerant was R600a (isobutane). The compressor was operated at three speeds: 50 Hz, 100 Hz and 150 Hz. CECOMAF (Comit europen des constructeurs de matriel frigorifique) conditions were applied: gas temperature on suction side=32 C., dew point on suction side=25 C., dew point on pressure side=+55 C., ambient temperature=35+2 C.

[0174] The general processing of the acquired data for this experimental investigation followed the European standard on compressor rating (DIN EN 13771-1, 2017).

TABLE-US-00002 TABLE 1 Formulations and results retrieved with inventive and comparative refrigeration compressor oils. Composition CE 1 Ex 1 Ex 2 Ex 3 CE 2 Polymer 1 3.40 2.60 6.20 [wt. %] Performance 0.8 0.8 0.8 0.8 package*.sup.) [wt. %] Nynas T3 96.60 93.00 32.40 [wt. %] Nynas NS3 95.80 [wt. %] Nynas T9 66.80 [wt. %] Genuine oil, 100 Alkyl benzene based [wt. %] Total [%] 100 100 100 100 100 KV.sub.40 [mm.sup.2/s] 4.90 4.12 4.94 7.08 7.08 ISO VG 5 4**.sup.) 5 7 7 KV.sub.100 [mm.sup.2/s 1.59 1.63 1.79 2.55 1.99 VI 64 240 164 238 58 PP [ C.] 57 87 81 78 69 Shear loss 7.3 6.2 11 ASTM D5621 [%] Efficiency (COP) 1.26 1.39 at 50 Hz [%] Efficiency (COP) 1.42 1.47 at 100 Hz [%] Efficiency (COP) 1.33 1.37 at 150 Hz [%] Volumetric 62 65 Efficiency at 50 Hz Volumetric 61 64 Efficiency at 100 Hz Volumetric 59.8 61.3 Efficiency at 150 Hz *.sup.)As performance package, a commercially available zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant was used to protect the compressor. **.sup.)The value of KV 40 = 4.12 mm.sup.2/s is slightly below the defined range for ISO VG 5; an ISO VG 4 is not defined.

[0175] Polymer 1 consists of 13 wt. % of methyl methacrylate, 86.5 wt. % of C10-16 alkyl methacrylates and 0.5 wt. % of C11-18 alkyl methacrylates (Mw=77,000 g/mol, 80% solids dissolved in highly refined mineral oil).

[0176] As Comparative Example 1 (CE 1) was used a commercially available alkyl benzene based oil having a KV.sub.40 of 4.90 mm.sup.2/s (corresponding to ISO VG 5). Comparative Example 2 (CE 2) is a mixture of different naphthenic base oils, having a KV.sub.40 of about 7 (corresponding to ISO VG 7). The comparative examples do not contain any polyalkyl (meth)acrylate.

[0177] Working examples 1-3 (Ex 1-3) are also based on naphthenic base oils and Polymer 1 as a polyalkyl (meth)acrylate. Ex 1-3 are formulated to a KV.sub.40 of 4 mm/s (Ex 1), 5 mm/s (Ex 2) and 7 mm/s (Ex 3), corresponding to ISO VG 4, VG 5 and VG 7, respectively.

[0178] Conclusions:

[0179] The inventive oil has shown an improvement of the volumetric efficiency and the coefficient of performance at all driving speeds (50/100/150 Hz). The compressor oils with high VI show good compatibility (no detrimental separation and accumulation was observed) with the refrigerant and allow an improvement of equipment performance.

[0180] Determination of Effects on Energy Efficiency in Air Compressors

[0181] Another aspect of the invention was the improvement of air compressor efficiency.

[0182] Compressor oils with VI 140 and higher were tested in a Kaeser SX4 screw compressor and were compared with the commercially used mineral oil-based monograde fluid of Kaeser having a VI of 106.

[0183] A second air compressor of larger size was used to determine energy efficiency benefits, Atlas Copco GA75VSD.

[0184] The test settings used are described in FIG. 1.

[0185] Characterization of air compressors as used in relevant test procedures:

TABLE-US-00003 (1) KAESER SX4 Date of Manufacture: 2019 September Manufacturer: Kaeser Compression Medium: Air Reference Frequency: 50 Hz Maximum Air Volume Flow Rate: 0.36 m.sup.3/min Presurre Stages: 1 Maximum discharge pressure: 11 bar Motor Capacity: 3.0 kW (2) Atlas Copco GA75VSD P A 13 MK5 Date of Manufacture: 2019 January Manufacturer: Atlas Copco Compression Medium: Air Reference Frequency/ 73/20 Hz Lower limit Frequency: Maximum Air Volume Flow Rate: 14.76 m.sup.3/min Presurre Stages: 1 Maximum discharge pressure: 13 bar Motor Capacity: 75 kW

[0186] The following parameters were measured: oil sump temperature, air temperature at the suction and discharge side, ambient air temperature, pressure and humidity; air pressure on suction and discharge side, air flow rate, and the power demand of the equipment. On the discharge side, a condensation air dryer was used to maintain dry air with less than 0.1% water in the compressed air.

[0187] Stationary operating conditions with two different oil temperatures and four different air pressures were adjusted. Air flow rates and power demand resulted in specific power demand values in W/(bar*L/min).

[0188] The following Table 2 shows the formulations and results retrieved with inventive and comparative air compressor oils.

TABLE-US-00004 TABLE 2 Formulations and results retrieved with inventive and comparative air compressor oils (AirEx and AirCE). Composition AirCE 1 AirEx 1 AirEx 2 AirEx 3 AirEx 4 AirEx 5 AirEx 6 Polymer 2 [wt. %] 0 0 0 9.5 13.6 0 11.8 Polymer 3 [wt. %] 0 0 0 0 0 5.0 0 Polymer 4 [wt. %] 0 1.0 10.5 0 0 0 0 Performance 0.8 1.5 0.8 0.8 0.8 0.8 package*.sup.) [wt. %] PAO6 [wt. %] 10.0 PAO8 [wt. %] 89.0 Kaeser genuine 100 fluid [wt. %] Group III oil with 29.3 54.2 **) KV.sub.40 of about 4 mm.sup.2/s [wt. %] Group III oil with 88.0 60.4 31.4 94.2 **) KV.sub.40 of about 6 mm.sup.2/s [wt. %] Synesstic5 [%] 6.0 Total [%] 100 100 100 100 100 100 100 KV.sub.40 [mm.sup.2/s] 46.0 45.99 46.27 45.96 46.81 46.34 55.0 ISO VG 46 46 46 46 46 46 KV.sub.100 [wt. %] 6.92 7.83 8.17 9.0 9.73 9.66 10.3 VI 106 140 151 181 200 200 180 PP [ C.] 30 54 45 45 45 45 42 Shear loss at <1 <1 <1 4.8 6.4 >20 5.8 100 C., ASTM D5621 [%] *.sup.)As performance package, a commercially available zinc-free performance package comprising at least an antiwear agent, an anticorrosion agent and an antioxidant was used to protect the compressor. **) mixture of Group III oils adding up to 81.4% by weight

[0189] Polymer 2 consists of 13 wt. % of methyl methacrylate and 87 wt. % of C10-16 alkyl methacrylates (M.sub.w=56,000 g/mol, 74% solids dissolved in highly refined mineral oil).

[0190] Polymer 3 consists of 11.3 wt. % of methyl methacrylate, 88.3 wt. % of C10-18 alkyl methacrylates and 0.4 wt. % of C20-22 alkyl methacrylates (M.sub.w=375,000 g/mol; 42% solids dissolved in highly refined mineral oil).

[0191] Polymer 4 consists of 0.2 wt. % of methyl methacrylate and 99.8 wt. % of iso C12-15 alkyl methacrylates (M.sub.w=13,800 g/mol).

[0192] As comparative example 1 (AirCE 1) was used a genuine fluid (commercially available from Kaeser) having a KV.sub.40 of 48 mm.sup.2/s (corresponding to ISO VG 46). It does not contain any polyalkyl (meth)acrylate.

[0193] Working examples 1-6 (AirEx 1-6) arm based on different Group III base oils and contain a polyalkyl (meth)acrylate. AirEx 1-5 were formulated to a KV.sub.40 of about 48 mm.sup.2/s, corresponding to ISO VG 48; AirEx 6 was formulated to a KV.sub.40 of about 55 mm.sup.2/s.

[0194] The effects on energy consumption in an air compressor were received by using the compressor oils according to the present invention are summarized in the following Tables 3a, 3b and 3c.

TABLE-US-00005 TABLE 3a Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of 8.39 to 9.43 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [W] [ C.] [L/min] ([W*min)/L] [(W*min)/(bar*L)] [%] AirCE 1 8.39 91.9 3111 70 294.5 10.56 1.26 9.11 73.4 3242 59 310.5 10.44 1.15 AirEx 4 8.85 93.7 3226 70 307.8 10.48 1.18 4.3 9.43 74.9 3321 60 320.5 10.36 1.10 2.9 AirEx 5 8.85 92.7 3215 70 298.6 10.77 1.22 1.5 9.39 75.0 3310 60 314.6 10.52 1.12 1.0

TABLE-US-00006 TABLE 3b Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of 7.06 to 7.67 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [M] [ C.] [L/min] [(W*min)/L] [(W*min/(bar*L)] [%] AirCE 1 7.06 87.9 2872 67 304.8 9.42 1.34 7.44 70.7 2949 57 318.4 9.26 1.25 AirEx 4 7.27 89.0 2948 68 318.2 9.26 1.27 3.9 7.67 70.7 3013 58 329.7 9.14 1.19 3.4 AirEx 5 7.22 87.8 2926 67 309.6 9.45 1.31 1.3 7.63 70.7 3003 57 325.1 9.24 1.21 2.0

TABLE-US-00007 TABLE 3c Effects on energy consumption and efficiency in an air compressor by using compressor oils according to the present invention at an air pressure p.sub.air in the range of of 4.89 to 5.15 bar. T.sub.Air Air flow Efficiency p.sub.Air T.sub.Oil P.sub.Total outlet rate P.sub.specific Power ratio improvement Ex # [bar] [ C.] [W] [ C.] [L/min] [(W*min)/L] [(W*min)/(bar*L)] [%] AirCE 1 4.89 81.2 2539 63 318.1 7.98 1.63 5.04 68.1 2585 55 328.6 7.87 1.56 AirEx 4 5.00 81.1 2596 63 331.3 7.84 1.57 3.4 5.15 68.3 2613 55 337.1 7.75 1.51 3.0 AirEx 5 4.92 80.6 2559 63 322.5 7.93 1.61 0.8 5.15 68.4 2613 55 334.2 7.82 1.52 2.2 p.sub.Air: air pressure at air discharge T.sub.Oil: Compressor oil temperature P.sub.total: total power demand of compressor Air flow rate: air flow at air discharge side (dry air at pair) P.sub.specific: power demand of compressor unit divided by air flow rate Power ratio: power demand of compressor unit divided by (air flow rate air discharge pressure)

[0195] The efficiency improvement was calculated from P.sub.specific, suction pressures and the individual compression ratios at test conditions vs reference conditions (correction factor):

[00001]${\mathrm{Efficiency}}_{\mathrm{relative}}=\frac{{}_2}{{}_1}-1=\frac{P_{\mathrm{specific},1}}{P_{\mathrm{specific},2}}\frac{p_{\mathrm{suction},2}}{p_{\mathrm{suction},1}}\mathrm{correction}-1$

[0196] Additional tests were run on Atlas Copco GA75VSD. The oil temperature was controlled to 90 C. Three different discharge air pressures were investigated at 8 bar, 10 bar and 12.5 bar.

[0197] The following Table 4 shows the results retrieved with using Atlas Copco GA75VSD.

TABLE-US-00008 TABLE 4 results retrieved with using Atlas Copco GA75VSD rel. efficiency KV90 T.sub.Oil P.sub.air, out P.sub.specific improvement Fluid VG VI [cSt] [ C.] [bar] [W*min/L] [%] mineral-based 46 105 8.69 90 8 7.11 VG46 - 90 10 7.92 Reference 90 12.5 9.11 AirEx3 46 180 11.05 90 8 6.99 1.7 90 10 7.77 1.9 90 12.5 8.91 2.2 AirEx6 55 180 12.55 90 8 7.00 1.6 90 10 7.75 2.2 90 12.5 8.87 2.7

TABLE-US-00009 TABLE 5 Shear loss of oils during test procedure after 1 day testing at various conditions: KV40 KV100 KV40 KV100 AVI AKV40 VI (cSt) (cSt) VI (cSt) (cSt) (%) (%) Fluid Fresh oil After test AirCE1 106 46.1 6.9 106 46.2 6.9 0 +0.2 AirEx3 181 45.9 9.0 181 45.9 9.0 0 0 AirEx4 200 46.8 9.7 198 46.7 9.7 1 0.2 AirEx5 200 46.3 9.7 177 40.6 8.1 11.5 12.4

[0198] Conclusions:

[0199] The electric power demand was measured for at least 15 minutes after stationary operating conditions were achieved at various discharge pressures and oil temperatures.

[0200] The power ratio was defined by the ratio of the measured electric power demand and the output power, measured in air volume flow rate in liter per minute multiplied by the pressure at the compressor air discharge side. Constant and repeatable ambient conditions were achieved by operating the equipment in a controlled air-conditioned room.

[0201] The investigations on the air compressor test rigs have clearly shown an efficiency advantage of compressor oils with a VI of at least 140 and high shear stability. The efficiency was significantly improved at all investigated operating conditions. At oil temperatures of about 75 C., a reduction of the power ratio from 1.15 (W*min)/(bar*L) to 1.10 (W*min)/(bar*L) was achieved with changing the compressor oil from AirCE1 to AirEx4, the fluid comprising Polymer 2 and having a VI of 200. At an oil temperature of 92 to 94 C., an even stronger improvement from 1.26 (W*min)/(bar*L) of AirCE1 to 1.18 (W*min)/(bar*L) of AirEx4 was observed. The corresponding efficiency improvement was calculated to 4.3%. The fluid AirEx5 comprising Polymer 3 and having a VI of 200 also allowed to increase the efficiency. The improvement at oil temperatures above 90 C. and an air discharge pressure of about 9 bar was about 1.5%. The molecular weight of the polymer used in AirEx5 was higher and the shear stability of the oil was lower compared to compressor oil AirEx4. A higher shear stability is advantageous for the efficiency improvement and for the lifetime of the oil. The inventive fluids had a maximum KV.sub.100 shear loss of 40% in the 40 minutes sonic shear test method according to ASTM D5621. Preferred is a lower shear loss of maximum 20% and more preferred a shear loss of less than 10% according to ASTM D5621.

[0202] Table 5 shows the viscosities of oils before and after the testing on the compressor test rigs. Viscosities of AirEx3 and AirEx4 have not changed over time of the test duration, however, the viscosity of oil AirEx5 with Polymer 3 dropped down by more than 10% under real life conditions. The molecular weight of polymer 3 is relatively high and shear stability is not good enough for a long-term efficiency improvement of air compressors.

[0203] The pour point of the compressor fluids according to the present invention were 33 C. or lower. High VI, low pour point and high shear stability were achieved by blending Group II, Group III or PAO base oils with the polyalkyl methacrylate-based viscosity index improvers according to the present invention having a defined composition and a maximum molecular weight of 400,000 g/mol, preferably below 200,000 g/mol and more preferably below 100,000 g/mol. It was recognized that the equipment can be operated at lower temperatures with higher VI and more shear stable lubricants. When using more efficient fluids it became necessary to block the cooling units to achieve higher oil operating temperature levels of 90 C. as requested for the test runs. The investigations have shown that overheating can be avoided by using compressor oils according to the present invention, as a more efficient air compressor has the tendency to run at lower temperatures.