What are the respective features of water ring vacuum pumps and steam-based vacuum systems, and in what environments are they suitable?

The biggest advantage of the water ring vacuum pump is that it does not consume steam! The biggest drawback of steam injection is its high consumption of steam and low thermal efficiency!

Advantages of water ring pumps: simple structure, low requirements for manufacturing precision, and easy to manufacture. It has a compact structure and a high pump speed; it can generally be connected directly to the motor without the need for a reduction gear. Therefore, a small structural size can yield 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, there is no need for lubrication within 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. Disadvantage: Low efficiency, generally around 30%, with better cases reaching 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 pump is isothermal, it is possible to pump out 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. Before a steam turbine is started at cold conditions, there is a large amount of air inside it; failing to evacuate this air can lead to the following problems: (1) During startup, a large volume of steam is required to overcome the frictional forces in the bearings as well as the inertial forces of the rotor, which results in increased stress on the blades due to the steam impact. (2) The presence of air in the cylinder intensifies the aerodynamic friction on the long blades of the last stage, resulting in an increase in exhaust temperature. (3) The presence of air in the condenser reduces the heat exchange between steam and water within it, which leads to an increase in the exhaust steam temperature and causes deformation of the cylinder metal; moreover, the expansion joints of the copper tubes in the condenser become loose, resulting in leaks. (4) The non-condensation of air causes the exhaust pressure of the turbine to rise, triggering the operation of the condenser safety valve. For the above reasons, a vacuum must be created before starting a condensing steam turbine. That’s right; during a cold start, it is necessary to evacuate the vacuum first, and then apply the shaft seal. If the shaft seal is installed first and then vacuum is created, the steam supplied to the shaft seal will enter the cylinder directly, especially in the low-pressure cylinders. This will lead to uneven heating of the cylinder and rotor, as well as a large temperature difference between the upper and lower cylinders; it may cause thermal bending of the rotor and deformation of the cylinder in the form of a \"bowed\" shape. Furthermore, if the shaft seal is installed first and then vacuum is created, it will cause the rotor at the shaft seal area to heat up and expand. The longer this process takes, the more severe the heating will be, which may lead to an excessive positive expansion difference during startup. Even if such an excess does not occur, it will still limit the speed at which load can be applied and prolong the warm-up time; in the case of coal-fired units, it will increase the consumption of auxiliary fuel. So generally, the shaft seal is installed half an hour before starting up. It also serves to prevent the shaft seal from activating first, which could cause the low-pressure cylinder explosion vent to open.