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16/05/2019 10:30

Viscosity and Viscosity Index of Compressor Oils

 

Compressor oil plays a key role in the appropriate and long term functioning of compressors. As it is responsible for making a proper film layer between the moving parts (e.g. piston, cylinder), and for cooling the components, therefore it is very important that what kind of combination ability the specific compressor oil has with the used refrigerant, and what kind of viscosity index belongs to the given oil.

 

 

Viscosity of Compressor Oils

 

One of the important criteria in case of compressor oil is its viscosity. The term viscosity describes the given liquid’s resistance to flow, so an oil with bigger viscosity flows less, and an oil with lower viscosity is thin fluid. The kinematic viscosity describes the oil’s flowing time on a given temperature, its unit in the SI-system is mm2/s, or the commonly used centiStoke (cSt). Thus 1 cSt ≈ 1 mm2/s. In the base of this kinematic viscosity the ISO 3448:1992 standard defines a viscosity group system which determines the minimal and maximal kinetic viscosity point of a given oil on 40° Celsius. The average value derived from this two points indicates the name of the given group. Accordingly, e.g. an oil which has a minimal viscosity of 28 mm2/s, and a maximal viscosity of 35.2 mm2/s, takes place in the group ISO VG 32. (The VG means Viscosity Group). Though viscosity is affected by several factors (e.g. the kinetic speed of the components, and the quality of machining), essentially it needs to be striving after picking the lower but still satisfying viscosity classification.

 

Viscosity Groups based on 3348:1992 ISO standard
ISO 3448 Viscosity Group (40°C) Minimal Kinetic Viscosity (mm2/s) Maximal Kinetic Viscosity (mm2/s) Kinetic Viscosity Midpoint (mm2/s)
ISO VG 2 1.98 2.42 2.2
ISO VG 3 2.88 3.52 3.2
ISO VG 5 4.14 5.06 4.6
ISO VG 7 6.12 7.48 6.8
ISO VG 10 9 11 10
ISO VG 15 13.5 16.5 15
ISO VG 22 19.8 24.2 22
ISO VG 32 28.8 35.2 32
ISO VG 46 41.4 50.6 46
ISO VG 68 51.2 74.8 68
ISO VG 100 90 110 100
ISO VG 150 135 165 150
ISO VG 220 198 242 220
ISO VG 320 288 352 320
ISO VG 460 414 506 460
ISO VG 680 612 748 680
ISO VG 1000 900 1100 1000
ISO VG1500 1350 1650 1500

 

Determining the Viscosity Index of Compressor Oils

 

The other critical characteristic of compressor oils which is important in case of cooling the components too is the alteration of viscosity due to temperature changing. The alteration of viscosity is described by the viscosity index (VI) which shows that how the viscosity of the given oil changes due to temperature changing, so the index number indicates the alteration of viscosity, not the viscosity itself. If the temperature of the oil rises, its viscosity will decrease; and if the temperature of the oil is down, its viscosity will get larger. The viscosity of the oils (known as ’paraffinic oils’) that has the best viscosity index (= 100) alters less, while in case of the oils (known as ’naphthenic oils’) which has the worst viscosity index (= 0) the viscosity changes in a large measure. So in case of the paraffinic and of naphthenic oils, too, there is a viscosity index based sequence, by which the kinematic viscosity of the specific oil is given. To determine the viscosity index of a specific oil, it needs to be picking one from those paraffinic and one from those naphthenic base oils, which have the same value of viscosity index at 100°C as the wanted oil has at 100°C. The following table summarizes the kinematic viscosity of the base oils at 100°C, in addition it contains a value (L) for the base oils which have the viscosity index of 0 at 40°C, and another value (H) for the base oils which have the viscosity index of 100 at 40°C.

 

The base oil values (ν, L and H) used for determining the viscosity index of wanted oils
Kinetic Viscosity (ν) (100°C. mm2/s) L H
2.00 7.994 6.394
2.10 8.640 6.894
2.20 9.309 7.410
2.30 10.00 7.944
2.40 10.71 8.496
2.50 11.45 9.063
2.60 12.21 9.647
2.70 13.00 10.25
2.80 13.80 10.87
2.90 14.63 11.50
3.00 15.49 12.15
3.10 16.36 12.82
3.20 17.26 13.51
3.30 18.18 14.21
3.40 19.12 14.93
3.50 20.09 15.66
3.60 21.08 16.42
3.70 22.09 17.19
3.80 23.13 17.97
3.90 24.19 18.77
4.00 25.32 19.56
4.10 26.50 20.37
4.20 27.75 21.21
4.30 29.07 22.05
4.40 30.48 22.92
4.50 31.96 23.81
4.60 33.52 24.71
4.70 35.13 25.63
4.80 36.79 26.57
4.90 38.50 27.53
5.00 40.23 28.49
5.10 41.99 29.46
5.20 43.76 30.43
5.30 45.53 31.40
5.40 47.31 32.37
5.50 49.09 33.34
5.60 50.87 34.32
5.70 52.64 35.29
5.80 54.42 36.26
5.90 56.20 37.23
6.00 57.97 38.19
6.10 59.74 39.17
6.20 61.52 40.15
6.30 63.32 41.13
6.40 65.18 42.14
6.50 67.12 43.18
6.60 69.16 44.24
6.70 71.29 45.33
6.80 73.48 46.44
6.90 75.72 47.51
7.00 78.00 48.57
7.10 80.25 49.61
7.20 82.39 50.69
7.30 84.53 51.78
7.40 86.66 52.88
7.50 88.85 53.98
7.60 91.04 55.09
7.70 93.20 56.20
7.80 95.43 57.31
7.90 97.72 58.45
8.00 100.0 59.60
8.10 102.3 60.74
8.20 104.6 61.89
8.30 106.9 63.05
8.40 109.2 64.18
8.50 111.5 65.32
8.60 113.9 66.48
8.70 116.2 67.64
8.80 118.5 68.79
8.90 120.9 69.94
9.00 123.3 71.10
9.10 125.7 72.27
9.20 128.0 73.42
9.30 130.4 74.57
9.40 132.8 75.73
9.50 135.3 76.91
9.60 137.7 78.08
9.70 140.1 79.27
9.80 142.7 80.46
9.90 145.2 81.67
10.0 147.7 82.87
10.1 150.3 84.08
10.2 152.9 85.30
10.3 155.4 86.51
10.4 158.0 87.72
10.5 160.6 88.95
10.6 163.2 90.19
10.7 165.8 91.40
10.8 168.5 92.65
10.9 171.2 93.92
11.0 173.9 95.19
11.1 176.6 96.45
11.2 179.4 97.71
11.3 182.1 98.97
11.4 184.9 100.2
11.5 187.6 101.5
11.6 190.4 102.8
11.7 193.3 104.1
11.8 196.2 105.4
11.9 199.0 106.7
12.0 201.9 108.0
12.1 204.8 109.4
12.2 207.8 110.7
12.3 210.7 112.0
12.4 213.6 113.3
12.5 216.6 114.7
12.6 219.6 116.0
12.7 222.6 117.4
12.8 225.7 118.7
12.9 228.8 120.1
13.0 231.9 121.5
13.1 265.0 122.9
13.2 238.1 124.2
13.3 241.2 125.6
13.4 244.3 127.0
13.5 247.4 128.4
13.6 250.6 129.8
13.7 253.8 131.8
13.8 257.0 132.6
13.9 260.1 134.0
14.0 263.3 135.4
14.1 266.6 136.8
14.2 269.8 138.2
14.3 273.0 139.6
14.4 276.3 141.0
14.5 279.6 142.4
14.6 283.0 143.9
14.7 286.4 145.3
14.8 289.7 146.8
14.9 293.0 148.2
15.0 296.5 149.7
15.1 300.0 151.2
15.2 303.4 152.6
15.3 306.9 154.1
15.4 310.3 156.6
15.5 313.9 157.0
15.6 317.5 158.6
15.7 321.1 160.1
15.8 324.6 161.6
15.9 328.3 163.1
16.0 331.9 164.6
16.1 335.5 166.1
16.2 339.2 167.7
16.3 342.9 169.2
16.4 346.6 170.7
16.5 350.3 172.3
16.6 354.1 173.8
16.7 358.0 175.4
16.8 361.7 177.0
16.9 365.6 178.6
17.0 369.4 180.2
17.1 373.3 181.7
17.2 377.1 183.3
17.3 381.0 184.9
17.4 384.9 186.5
17.5 388.9 188.1
17.6 392.7 189.7
17.7 396.7 191.3
17.8 400.7 192.9
17.9 404.6 194.6
18.0 408.6 196.2
18.1 412.6 197.8
18.2 416.7 199.4
18.3 420.7 201.0
18.4 424.9 202.6
18.5 429.0 204.3
18.6 433.2 205.9
18.7 437.3 207.6
18.8 441.5 209.3
18.9 445.7 211.0
19.0 449.9 212.7
19.1 454.2 214.4
19.2 458.4 216.1
19.3 462.7 217.7
19.4 467.0 219.4
19.5 471.3 221.1
19.6 475.7 222.8
19.7 479.7 224.5
19.8 483.9 226.2
19.9 488.6 227.7
20.0 493.2 229.5
20.2 501.5 233.0
20.4 510.8 236.4
20.6 519.9 240.1
20.8 528.8 243.5
21.0 538.4 247.1
21.2 547.5 250.7
21.4 556.7 254.2
21.6 566.4 257.8
21.8 575.6 261.5
22.0 585.2 264.9
22.2 595.0 268.6
22.4 604.3 272.3
22.6 614.2 275.8
22.8 624.1 279.6
23.0 633.6 283.3
23.2 634.4 286.3
23.4 653.8 290.5
23.6 663.3 294.4
23.8 673.7 297.9
24.0 683.9 301.8
24.2 694.5 305.6
24.4 704.2 309.4
24.6 714.9 313.0
24.8 725.7 317.0
25.0 736.5 320.9
25.2 747.2 324.9
25.4 758.2 328.8
25.6 769.3 332.7
25.8 779.7 336.7
26.0 790.4 340.5
26.2 801.6 344.4
26.4 812.8 348.4
26.6 824.1 352.3
26.8 835.5 356.4
27.0 847.0 360.5
27.2 857.5 364.6
27.4 869.0 368.3
27.6 880.6 372.3
27.8 892.3 376.4
28.0 904.1 380.6
28.2 915.8 384.6
28.4 927.6 388.8
28.6 938.6 393.0
28.8 951.2 396.6
29.0 963.4 401.1
29.2 975.4 405.3
29.4 987.1 409.5
29.6 998.9 413.5
29.8 1011 417.6
30.0 1023 421.7
30.5 1055 432.4
31.0 1086 443.2
31.5 1119 454.0
32.0 1151 464.9
32.5 1184 475.9
33.0 1217 478.0
33.5 1251 498.1
34.0 1286 509.6
34.5 1321 521.1
35.0 1356 532.5
35.5 1391 544.0
36.0 1427 555.6
36.5 1464 567.1
37.0 1501 579.3
37.5 1538 591.3
38.0 1575 603.1
38.5 1613 615.0
39.0 1651 627.1
39.5 1691 639.2
40.0 1730 651.8
40.5 1770 664.2
41.0 1810 676.6
41.5 1851 689.1
42.0 1892 701.9
42.5 1935 714.9
43.0 1978 728.2
43.5 2021 741.3
44.0 2064 754.4
44.5 2108 767.6
45.0 2152 780.9
45.5 2197 794.5
46.0 2243 808.2
46.5 2288 821.9
47.0 2333 835.5
47.5 2380 849.2
48.0 2426 863.0
48.5 2473 876.9
49.0 2521 890.9
49.5 2570 905.3
50.0 2618 919.6
50.5 2667 933.6
51.0 2717 948.2
51.5 2767 962.9
52.0 2817 977.5
52.5 2867 992.1
53.0 2918 1007
53.5 2969 1021
54.0 3020 1036
54.5 3073 1051
55.0 3126 1066
55.5 3180 1082
56.0 3233 1097
56.5 3286 1112
57.0 3340 1127
57.5 3396 1143
58.0 3452 1159
58.5 3507 1175
59.0 3563 1190
59.5 3619 1206
60.0 3676 1222
60.5 3734 1238
61.0 3792 1254
61.5 3850 1270
62.0 3908 1286
62.5 3966 1303
63.0 4026 1319
63.5 4087 1336
64.0 4147 1352
64.5 4207 1396
65.0 4268 1386
65.5 4329 1402
66.0 4392 1419
66.5 4455 1436
67.0 4517 1454
67.5 4580 1471
68.0 4645 1488
68.5 4709 1506
69.0 4773 1523
69.5 4839 1541
70.0 4905 1558

To determine the viscosity index, we can use the following formula:

Viszkozitási index képlete

The symbols that are used in the above formula are the following:

  • VI: viscosity index
  • U: the kinematic viscosity of the wanted oil sample at 40°C (mm2/s)
  • L: the viscosity index of that base oil at 40°C (mm2/s), which has a viscosity index of 0, and its viscosity at 100°C is the same as the viscosity of the wanted oil
  • H: the viscosity index of that base oil at 40°C (mm2/s), which has a viscosity index of 100, and its viscosity at 100°C is the same as the viscosity of the wanted oil

The following graph shows the determining of viscosity index:

Viszkozitási index kiszámítása grafikon