What is the difference between water ring vacuum pumps and oil-based vacuum pumps?

Detailed information on this water ring vacuum pump can be found at http://bbs.hcbbs.com/forum.php?mod=viewthread&tid=832304. Water ring vacuum pumps and ordinary vacuum pumps are not considered to be the same type of pump; rather, there is a hierarchical relationship between them. You can find plenty of information on this topic in the forum

Thank you to the friend upstairs for the answer. I see. In fact, both water ring vacuum pumps and oil-based vacuum pumps belong to the category of liquid ring vacuum pumps; the difference is that the former uses water as the working fluid, while the latter uses oil as the working fluid. Using water as the working fluid, the ultimate pressure can only reach 2000~4000 Pa. Using oil as the working fluid, a pressure of 130 Pa can be achieved. Why can oil create a lower vacuum than water?

Working principle of water ring vacuum pump/liquid ring vacuum pump: The water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump; the ultimate vacuum it can achieve is 2000–4000 Pa, and with an atmospheric ejector in series, this value can reach 270–670 Pa. Water ring pumps can also be used as compressors, known as water ring compressors; they are low-pressure compressors with a pressure range of 1~2×105 Pa gauge pressure. Water ring pumps were initially used as self-priming pumps, and later came to be employed in many industrial sectors such as petroleum, chemicals, machinery, mining, light industry, pharmaceuticals, and food processing. Water ring pumps are widely used in many industrial production processes, such as vacuum filtration, vacuum water lifting, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum rehumidification, and vacuum degassing. Due to the rapid advancement of vacuum application technology, water ring pumps have always been highly regarded for achieving rough vacuums. Since the gas compression in a water ring pump is isothermal, it can be used to extract flammable and explosive gases, as well as gases containing dust or moisture; therefore, water ring pumps are being used more and more often. An appropriate amount of water is installed in the pump body as the working fluid. As the impeller rotates clockwise as shown in the diagram, water is thrown out in all directions by the impeller; due to centrifugal force, the water forms a closed ring of approximately uniform thickness, whose shape is determined by that of the pump chamber. The inner surface of the lower part of the water ring is in exact tangency with the impeller hub, while the inner surface of the upper part of the water ring is in contact with the tips of the blades (in fact, the blades extend to a certain depth within the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the water ring, and this space is further divided by the impeller into several small chambers, one for each blade. If 0° at the lower part of the impeller is taken as the starting point, then when the impeller has rotated 180°, the volume of the small chamber increases and it becomes connected to the suction port on the end face; at this point, gas is drawn in. Once suction is complete, the small chamber becomes isolated from the suction port ； As the impeller continues to rotate, the small chamber becomes smaller, causing the gas to be compressed ； When the small chamber is connected to the exhaust port, the gas is expelled outside the pump. Original article: http://www.yzj.cc/contents.asp?tid=173. Please indicate the source of the article and its completeness when reproducing it

The water ring vacuum pump (abbreviated as water ring pump) is a type of rough vacuum pump; the ultimate vacuum it can achieve is 2000–4000 Pa, and with an atmospheric ejector in series, this value can reach 270–670 Pa. The water ring vacuum pump can also be used as a compressor, known as a water ring compressor; it is a low-pressure compressor with a pressure range of 1~2×105 Pa gauge pressure. 　　The water ring vacuum pump was initially used as a self-priming water pump, and later came to be employed in many industrial sectors such as petroleum, chemicals, machinery, mining, light industry, pharmaceuticals, and food processing. Water ring vacuum pumps are widely used in many industrial processing steps, such as vacuum filtration, vacuum water pumping, vacuum feeding, vacuum evaporation, vacuum concentration, vacuum rehumidification, and vacuum degassing. Due to the rapid advancement of vacuum application technology, water ring vacuum pumps have always been highly regarded for achieving rough vacuums. Since the gas compression in water ring vacuum pumps is isothermal, they can be used to extract flammable and explosive gases, as well as gases containing dust or moisture; therefore, water ring pumps are being used more and more frequently. 　　As shown in the figure: an appropriate amount of water is placed inside the pump body as the working fluid. When the impeller rotates clockwise in the direction indicated in the diagram, water is thrown out in all directions by the impeller; due to centrifugal force, the water forms a closed ring of approximately uniform thickness, shaped according to the geometry of the pump chamber. The inner surface of the upper part of the water ring is in exact tangency with the impeller hub, while the inner surface of the lower part of the water ring is in contact with the tips of the blades (in fact, the blades extend to a certain depth within the water ring). At this point, a crescent-shaped space is formed between the impeller hub and the water ring, and this space is further divided by the impeller into several small chambers, each containing an equal number of blades. If 0° at the upper part of the impeller is taken as the starting point, then 180° before the impeller starts rotating, the volume of the small chamber increases and it becomes connected to the suction port on the end face; at this point, gas is drawn in. Once suction is complete, the small chamber becomes separated from the suction port ； As the impeller continues to rotate, the small chamber becomes smaller, causing the gas to be compressed ； When the small chamber is connected to the exhaust port, the gas is expelled outside the pump. 　　In summary, a water ring vacuum pump relies on changes in the volume of its pumping chamber to achieve suction, compression, and exhaust, and therefore it belongs to the category of variable-volume vacuum pumps. Compared to other types of mechanical vacuum pumps, water ring pumps have the following advantages: they have a simple structure, do not require high manufacturing precision, and are easy to manufacture. It has a compact structure, high pump speed, and can generally be connected directly to the motor without the need for a reduction gear. Therefore, a small structural size can result in a large displacement, while also requiring less floor space. Compressed gas is essentially isothermal, meaning that the temperature changes very little during the compression process. Since there are no metal friction surfaces inside the pump chamber, no lubrication is required for the pump, and wear is minimal. The sealing between the rotating part and the fixed part can be achieved directly by a water seal. It features even air intake, stable and reliable operation, simple handling, and easy maintenance. Water ring vacuum pumps also have their disadvantages: they have low efficiency, generally around 30%, with better models reaching up to 50%. The low vacuum level is due not only to structural limitations but, more importantly, to the saturated vapor pressure of the working fluid. Using water as the working fluid, the ultimate pressure can only reach 2000~4000 Pa. Using oil as the working fluid, a pressure of 130 Pa can be achieved. 　　In short, since the gas compression in a water ring vacuum pump is isothermal, it is possible to evacuate flammable and explosive gases. Due to the absence of exhaust valves and friction surfaces, it is possible to extract gas containing dust, condensable gases, and gas-water mixtures. Thanks to these prominent features, it is still widely used despite its low efficiency. Original article: http://www.yzj.cc/contents.asp?tid=20 Please indicate the source of the article and its completeness when reproducing it

Conversions for vacuum pump units and commonly used formulas
Vacuum unit conversions:
Pascal (Pa), Torr, micrometers of mercury (μmHg), millibar (mbar), atmosphere (atm), engineering atmosphere (am)
1 Pa = 1
1 Torr = 133.32
1 μmHg = 0.13332
1 mbar = 102

Relationship between the pressure and temperature of saturated water vapor:
Temperature (°C) | Water vapor pressure (MPa) | Corresponding vacuum level (MPa)
22 | 0.00264 | 0.09869
24 | 0.00298 | 0.09835
26 | 0.00336 | 0.09797
28 | 0.00378 | 0.09755
30 | 0.00424 | 0.09709
32 | 0.00475 | 0.09658
34 | 0.00532 | 0.09601
36 | 0.00594 | 0.09539
38 | 0.00662 | 0.09471
40 | 0.00738 | 0.09395

Temperature (°C) | Water vapor pressure (MPa) | Corresponding vacuum level (MPa)
42 | 0.00820 | 0.09313
44 | 0.00910 | 0.09223
46 | 0.01009 | 0.09124
48 | 0.01116 | 0.09017
50 | 0.01234 | 0.08899
52 | 0.01361 | 0.08772
54 | 0.01500 | 0.08633
56 | 0.01651 | 0.08482
58 | 0.01815 | 0.08318
60 | 0.01992 | 0.08141

Temperature (°C) | Water vapor pressure (MPa) | Corresponding vacuum level (MPa)
62 | 0.02184 | 0.07949
64 | 0.02391 | 0.07742
66 | 0.02615 | 0.07518
68 | 0.02856 | 0.07277
70 | 0.03116 | 0.07017
72 | 0.03396 | 0.06737
74 | 0.03696 | 0.06437
76 | 0.04019 | 0.06114
78 | 0.04365 | 0.05768
80 | 0.04736 | 0.05397

Temperature (°C) | Water vapor pressure (MPa) | Corresponding vacuum level (MPa)
82 | 0.05133 | 0.05000
84 | 0.05557 | 0.04576
86 | 0.06011 | 0.04122
88 | 0.06495 | 0.03638
90 | 0.07011 | 0.03122
92 | 0.07561 | 0.02572
94 | 0.08146 | 0.01987
96 | 0.08769 | 0.01364
98 | 0.09430 | 0.00703
100 | 0.10133 | 0

Temperature (°C) | Water vapor pressure (MPa)
102 | 0.10878
104 | 0.11668
106 | 0.12504
108 | 0.13390
110 | 0.14327
112 | 0.15316
114 | 0.16362
116 | 0.17465
118 | 0.18628
120 | 0.19854

Temperature (°C) | Water vapor pressure (MPa)
122 | 0.21145
124 | 0.22504
126 | 0.23933
128 | 0.25435
130 | 0.27013
132 | 0.27831
134 | 0.30407
136 | 0.32229
138 | 0.34138
140 | 0.36138

Commonly used formulas for vacuum calculations:
1. Boyle’s Law: For a given mass of gas at constant temperature, volume V and pressure P are inversely proportional; i.e., P·V = constant. That is, P1/P2 = V2/V1). 2. Gay-Lussac’s law: When the pressure P remains constant, for a given mass of gas, its volume V is directly proportional to the absolute temperature T: (V1/V2 = T1/T2 = constant). When the pressure stays constant, for a given mass of gas, an increase (or decrease) in temperature of 1°C results in a corresponding increase (or decrease) in volume of 1/273. 3. Charles’s Law: When the volume V of a gas remains constant, for a given mass of gas, the pressure P is directly proportional to its absolute temperature T; that is, P1/P2 = T1/T2. At a constant volume, for a given mass of gas, every increase (or decrease) of 1°C in temperature results in an increase (or decrease) in pressure of 1/273 relative to the original value. 4. Mean free path: λ = (5×10^-3)/P (cm)
5. Pumping speed: S = dv/dt (liters/second), or S = Q/P. Where Q is the flow rate (torr·liters/second), P is the pressure (torr), V is the volume (liters), and t is the time (seconds).
6. Conductance: C = Q/(P2 – P1) (liters/second)
7. Vacuum pumping time: For pumping from atmospheric pressure to 1 torr, the formula for calculating the time is t = 8V/S (an empirical formula). Here, V is the volume and S is the pumping rate; typically, the value of t ranges between 5 and 10 minutes. 8. Selection of the maintenance pump: S維 = S前/10
9. Estimation of the pumping speed of a diffusion pump: S = 3D² (D = diameter in cm)
10. Pumping speed of the pre-stage pump for a Roots pump: S = (0.1–0.2)S罗 (l/s)
11. Leakage rate: Q漏 = V(P2 – P1)/(t2 – t1); Q漏 represents the system’s leakage rate in mmHg·l/s, where V is the volume of the system in liters, P1 is the pressure in the system when the vacuum pump is turned off in mmHg, P2 is the pressure reached in the vacuum chamber after time t in mmHg, and t is the time it takes for the pressure to rise from P1 to P2 in seconds.
12. Selection of the pumping speed for a roughing pump: S = Q1/P预 (l/s); alternatively, S = 2.3V·lg(Pa/P预)/t. Here, S is the effective pumping speed of the mechanical pump, Q1 is the leakage rate of the vacuum system in torr·liter/second, P预 is the desired pre-vacuum level in torr, V is the volume of the vacuum system in liters, and t is the time required to reach P预. Pa represents the atmospheric pressure in torr.
13. Selection of the pumping speed for the pre-stage pump: For transfer pumps such as diffusion pumps, oil-enhanced pumps, Roots pumps, and turbomolecular pumps, whose exhaust pressure is below one atmosphere, a pre-stage pump is needed to keep the pressure before them below a critical value. The pre-stage pump must be capable of removing the maximum amount of gas produced by the main pump. Based on the principle that the flow rate at each section of the pipeline remains constant, we have: PnSg ≥ PgS or Sg ≥ Pgs/Pn. Here, Sg is the effective pumping speed of the pre-stage pump in l/s, Pn is the critical pre-stage pressure of the main pump (the maximum exhaust pressure) also in l/s, Pg is the highest operating pressure in the vacuum chamber in torr, and S is the effective pumping speed of the main pump at pressure Pg. (l/s) 14. Formula for calculating the pumping speed of a diffusion pump: S = Q/P = (K·n) / (P·t) (liters/second). Where: S – the pumping speed of the pump under test (l/s); n – the number of scale marks by which the oil column rises in the dropper (marks); t – the time required for the oil column to rise by n marks (seconds); P – the pressure measured near the pump outlet (torr); K – the dropper coefficient (torr·liters/second). K = V0·(L/n)·(Υ0/Υm) + Pa△Vt. Where V0 – the original volume of the dropper and vacuum tubing (liters); L – the length of the scaled portion of the dropper (mm); n – the number of scale marks on that portion (marks); Υ0 – the specific gravity of the oil (grams/cm3); Υm – the specific gravity of mercury (grams/cm3); Pa – the local atmospheric pressure (torr); △Vt – the volume corresponding to one scale mark on the dropper (liters/mark). 15. Formula for calculating the geometric pumping speed of a rotary vane vacuum pump: S = πZnLKv(D2–d2) / (24×104) (l/s). Where: Z is the number of rotary vanes; n is the rotational speed (revolutions per minute); L is the length of the pump chamber; D is the diameter of the pump chamber; d is the diameter of the rotor (cm); Kv is the volume utilization factor (usually 95%). Original article: http://www.yzj.cc/contents.asp?tid=668. Please indicate the source of the article and its completeness when reproducing it: http://www.yzj.cc/contents.asp?tid=668

Because the saturated vapor pressures of oil and water are different.

Thank you, Haiyou, for your enthusiastic reply. I think it must be that the saturated vapor pressure of oil is lower than that of water?