Characterization of Vacuum Pumps and Engneering Analysis of Vacuum Units for Automated Pumping Plants

Abstract

This work is concerned with examining the operating conditions of a large-scale vacuum unit composed of two vacuum pumps, a vacum boiler of capacity 900 liters, and three feed pipes of different length and diameter. Other components of the vacuum unit were an ejector equipped with a tray flowmeter and a feeding centrifugal pump (Fig. 1). To begin with, we give a brief characterization of vacuum pumps conventionally used in laboratory research. In [1], two types of operating characteristics for vacuum pumps have been given: (i) for RMK-type vacuum pumps, it was a Q–V diagram; (ii) for KVN and VVN-type vacuum pumps, it was a Q–V diagram (see Fig. 2). Here Q is the air flow rate, m3 sec, reduced to a constant density n0 = 1.2928 kg m3; Q is the same flow rate, m3 sec, but with the variable density n = n0(1 – 0.1V), where V is the vacuum, m, at the vacuum pump inlet. As is seen in Fig. 1, the Q–V characteristic is simpler than the Q–V characteristic for use in analysis. The former is described by a linear equation Q = a – bV, whereas the latter — by a polynomial of order 3 to 5, Q = A + BV + CV2 + ... Still, the Q–V characteristic is not always a convenient tool either: each time, knowledge of which air — with a constant or a variable density — was used in the analysis or calculations is required. Therefore in our study of the operational regime of a USUWNM vacuum unit, we used the G–V characteristic, where G is the mass flow rate, kg sec, related to the Q flow rate as G = n0Q = 1.2928Q. Schematically, this characteristic is shown in Fig. 2 (for convenience of viewing, the dimensions m3 sec and kg sec are given in a single scale). The G–V characteristic is described by a linear equation G = a – âV, where a is the x-coordinate for the point A in Fig. 2, and â is the ratio of the x-coordinate (point A) to the y-coordinate (point B). In [2], coefficients a and â are given in a tabular form for the vacuum pumps reported in [1] and for two other vacuum pumps that were used in laboratory research. Under laboratory conditions, the vacuum unit operates in a cyclic mode: the vacuum pump switches on and off periodically. Schematically, the operating cycle of a vacuum unit is shown in Fig. 3, where time is measured on the x-axis, and vacuum V in the air cushion of the vacuum boiler and water level Z in the boiler are plotted on the y-axis. The operating cycle involves, as a rule, three periods [2]: — a period of duration t1 during the course of which the vacuum increases from VA to VB, and the vacuum-driven water level in the vacuum boiler rises from ZA to ZB; — a period during which the water level in the boiler reaches a maximum (with vacuum pump switched off); because of the difference between vacuums V B and VB = ZB, the water level continues rising from ZB to ZC, whereas the vacuum in the boiler’s air cushion decreases from V B to VC. This process continues until the two vacuums become equal, that is, VC = ZC. The duration of this period is t2; — a drawdown period during which, because of the ambient air in-leakage and gas release inside the unit, both vacuum and water level simultaneously decrease from VC = ZC to the initial value V A = VA = ZA. The duration of this period is t3. Thus, the total operating cycle time is tu = t1 + t2 + t3. It is seen from Fig. 1 (with reference to Fig. 1) that all points, except for point B , indicate simultaneously the water level and the vacuum in the water medium of the boiler.