Liquid ring vacuum pump design
On the other hand, it was found from the actual applications that the minimum suction pressure of the liquid ring pump was close to the saturated vapor pressure of the operating liquid. Thus, Powle [ 11 ] developed an equation for the ratio of the actual discharge capacity to the maximum actual discharge capacity as a function of vapor pressure of the operating liquid and index of expansion process in the pump, but the maximum actual discharge capacity should be given in advance.
Concerning the power of liquid ring pump, due to the absorption and the removal of the heat by the operating liquid, the gas compression process in the liquid ring pump is generally regarded as isothermal one, so the formula for the isothermal compression power of the ideal gas is commonly used to calculate the effective power of the liquid ring pump [ 12 , 13 ].
Bodik [ 14 ] deduced an approximate relation between the shaft power and rotational speed of liquid ring pump Table 1. Recently, some investigators [ 15 — 18 ] used CFD numerical approach to simulate 3D flows and predicted performance of liquid ring pumps. Although numerical approach plays an important role for understanding internal flow field of liquid ring pump, it is still hard for practical application, due to the restriction of detailed 3D geometry of liquid ring pump, consumption of computation time, and the accuracy of gas-liquid two-phase flow model.
In this paper, therefore, a convenient and universal theoretical model is proposed for the performance prediction of liquid ring pump based on the actual operating cycle, to obtain the rules of operating cycle performance and of power consumption in liquid ring pumps, without limitation of statistical range or the detailed 3D geometry.
During the suction process Curves shown in Figure 2 a , the suction pressure keeps constant and reaches the maximum suction capacity.
In the compression process Curves , the gas volume reduces and the pressure increases to reach the discharge pressure. In the discharge process Curves , the discharge pressure keeps constant and all the gas is discharged from the pump. Like the actual operating principle of reciprocating compressor, however, some residual gas capacity is is still available in the liquid ring pump after gas discharge and returns to the suction zone; the residual gas changes to in capacity after a process of expansion Curves in Figure 2 b and goes to the next operating cycle.
Thus, the actual operating cycle is composed of four processes: The actual suction capacity in a liquid ring pump is if ,. The residual gas and its expansion process were not taken into account in the theoretical model for ideal operating cycle of liquid ring pump, resulting in deviation between theoretical and practical results.
Therefore, the theoretical model in the present work is to supplement an expansion process on the basis of the ideal operating cycle model. For the integrity of the model, the formulas in each zone are given in detail below.
The definition of geometric parameters in the liquid ring pump is shown in Figure 3. The distance from any point on the liquid ring surface in the suction zone to the center of the impeller is expressed as where Theoretical suction capacity is defined as the gas volume flow through the OA section assuming that the liquid ring pump inhales fully: The distance from any point on the liquid ring surface in the compression zone to the center of the impeller is The compression ratio of the pressure at the circumferential angle to the suction pressure needs to solve an algebraic equation: The distance from any point on the liquid ring surface in the discharge zone to the center of the impeller is expressed as 4 Expansion Zone.
The distance from any point on the liquid ring surface in the expansion zone to the center of the impeller is also calculated by 6. Therefore, the full shape of liquid ring surface in the liquid ring pump can be obtained according to 1 , 4 , and 6. The capacity of gas returning to the suction zone of liquid ring pump is calculated by where is the distance from the liquid ring surface at the circumferential angle to the center of the impeller.
The gas capacity changes to after a polytropic expansion process and then enters the next operating cycle: Therefore, the actual suction capacity of liquid ring pump is expressed as Then the actual discharge capacity can be calculated according to the isothermal compression process: The effective power of the liquid ring pump is generally calculated according to the isothermal compression power. However, accurate prediction of shaft power is essential for estimating global efficiency and studying how to reduce power consumption in a liquid ring pump.
The shaft power is mainly composed of two parts: The total power consumption of the gas is the adiabatic compression work: The empirical formula of the liquid friction loss in liquid ring pump for the turbulent flow was given by Prager [ 19 ]: By referring to the formula of disk friction power in centrifugal pump [ 20 ], the equation 12 of liquid friction loss can be corrected as Therefore, the shaft power of liquid ring pump is written as The global efficiency is finally estimated by the following expression [ 13 ]: To validate the feasibility of the proposed theoretical models on performance of liquid ring pumps, the performance tests of the liquid ring pumps were performed.
Three types of single-stage double-acting liquid ring vacuum pumps 2BE, 2BE, and 2BE were selected for performance tests. The design and operating parameters of the liquid ring vacuum pumps are listed in Table 2. The tests were performed under different rotational speed of the pumps by frequency control. A multiparameter data acquisition and processing system were used in the performance tests and the detailed measurement method was introduced in [ 21 ].
The experimental facility and the layout of measurement device are shown in Figure 4. From the performance curves, it can be seen that the results of 9 under the adiabatic expansion process are close to the test results. This indicates that the expansion process of the residue gas is near the adiabatic one due to a shorter expansion time and path.
Figure 6 shows the comparisons between the tested shaft power values and theoretical ones by 15 in liquid ring pump under different rotational speeds. From Figure 6 , it can be seen that the trends of the shaft power curves calculated by 15 agree well with the ones of the test results.
This means the effect of axial width of the impeller should be taken into account for liquid friction loss in liquid ring pump. More sophisticated models for shaft power of liquid ring pump are necessary to consider for higher predictive accuracy in further work.
Some conclusions are summarized as follows: The expansion process is likely near the adiabatic one due to a shorter expansion time and path.
Home Journals About Us. The top connection of the pump is the inlet, while the side connection is its discharge some NASH pumps do have a different orientation.
The NASH liquid ring vacuum pump uses water or any other suitable liquid, which acts as "liquid pistons", hence the name liquid ring. It's apparent that the chambers between the rotor blades, shown here in yellow, are open around the periphery. The chambers are open on the inside, as well. These inner edges of the rotor blades are machined to rotate around the cone surface, shown in red , with a close non-contact fit. An internal passage joins the openings from the pump inlet to an inlet port in the cone.
There's also a passage from the cone discharge to the discharge connection on the head. Some NASH pumps have a port plate configuration rather than conical, but the principle is the same. This diagram demonstrates what the rotor and body do while the pump is in operation. The spinning of blue liquid forms a ring due to centrifugal force. Because the rotor axis and body axis are offset from each other, the liquid ring is not concentric with the rotor. Air or gas traverses the internal passage to the cone inlet port.
As the white dots indicate, the gas is drawn into the rotor chambers by the receding liquid ring, similar to the suction stroke of a piston in a cylinder.