Suction and compression of gas

Transcription

1 AIR JET VACUUM EJECTOR FOR LIQUID RING VACUUM PUMPS Suction and compression of gas Ejector in AISI 316 In spite of other gas ejectors, air jet vacuum ejectors for liquid ring vacuum pump use atmospheric air pressure as motive fluid. Ejectors are used to rise up the vacuum level of the liquid ring vacuum pump, preventing it from cavitation even if the suction of the ejector is closed, granting its integrity and a durable use. These ejectors are simple and sturdy and selecting adequate material are adaptable to different operating conditions. Ejectors usually are installed at the pump s suction connection; since they don t have moving parts, they don t require maintenance and work at atmospheric pressure, without energy impact. To speed up the evacuation, the air ejector should be bypassed and operated only when the required interstage pressure is achieved. Ejectors performances are strictly connected to efficiency of the liquid ring vacuum pump. The vapor pressure of the sealant fluid is the limiting factor. Usually vacuum is limited to 33 mbar ( water to 15 C) To adapt to these requirements, we offer two different series of ejectors: Ejector in acc. carb. Sealant Liquid Ring Vacuum pump Ejector suction pressure Water up to 20 C mbar Water up to 30 C mbar Ejector Series LT HT Fig.1 Necessary condition for the good working of the ejector is that the pump can suck the exit flow at a pressure equal or lower than the discharge pressure of ejector. It s recommended to choose a pump for a suction pressure 10% lower than the ejector s discharge pressure. In case it s available steam as motive fluid, it s advisable to insert a condenser between ejector and the liquid ring vacuum pump, to condense the steam without discharging it inside the pump, as it would happen in case of operating with atmospheric air pressure. Ejector in PP Page 1

2 Operating The ejector is a static pump, without moving parts. The operating, based on the Bernoulli s principle, works in two phases: in the first one the static energy of the motive fluid (atmospheric air) is converted in kinetic energy, flowing through a special designed nozzle. The sucked fluid, having absorbed part of the motive fluid kinetic energy, enters the low pressure area next the nozzle through the suction connection. In the second phase, the motive and suction gas, completely mixed, pass though the diffuser where the velocity is reconverted to pressure energy. The mixture is then inducted in to the pump at the discharge pressure of the ejector which is compressed by the liquid ring vacuum pump up to atmospheric pressure. Applications The air jet vacuum ejectors find their best application everytime is needed to extend the operational field of a liquid ring vacuum pump, limited from high temperature of the sealant liquid or where danger of running in cavitation is imminent. A typical application is for vacuum evaporators and concentration unit. Combined to the liquid ring vacuum pump they are the main element of the vacuum system circuit (Fig.2). A common application is two stage cold water degasifier. Two parallel atmospheric air ejector, discharging to the same liquid ring vacuum pump, maintain different vacuum in the two different compartments of the degasifier. Fig.2 Page 2

3 Manufacturing The air jet vacuum ejectors can be manufactured in any plastic or metallic machine tool workable material. Thanks to the many manufacturing options, they grant a high resistance to erosive/corrosive fluid and environment where they are installed. The standard manufacturing offers flanged connections at suction and discharge and threated connection at motive air access. Liquid jet connections Flanged - according to normative EN or ANSI Threaded Pipe union Butt weld Special connections on request (See Connections and dimensions scheme at page 6 ) Fig.3 Installation Modello GES Air jet vacuum ejector are usually installed at the suction pump connection. In order to get the best performance in particular situation, we advise to install a by-pass ejector pump system in parallel to the ejector (fig.4), to avoid the operating when not necessary, and prevent losses of charge due to the flowing through the diffuser. N1: motive air at atmospheric pressure N2: suction process fluid N3: ejector discharge = suction pump A: pump discharge atmosphere Fig.4 Page 3

4 PERFORMANCE CURVE (motive water at 15 C) The diagram estimates the suction flow rate at different suction pressures. Performance Coeffidient K=1 FLOW RATE [kg/h] 5,00 4,50 4,00 3,50 Motive water temperature: 15 C Discharge pressure: atmospheric Suction curve 3,00 2,50 2,00 Discharge curve 1,50 1,00 0,50 0, Fig.5 SUCTION PRESSURE AND DISCHARGE PRESSURE [mbar] Example of calculation: Suction pressure: suction flow rate : 20 mbar 1.3 kg/h As stated in fig. 5, the suction flow rate is about 0.7 kg/h of air (coefficient K=1). According with performance coefficient s chart (Fig.6), the coefficient K is obtained by the comparison between the flow rate required and the reference one (K=1) that means 1.3/0.7 = 1.71 that corresponds to the ejector s motive flow rate of 7.3 kg/h (coefficient of 1.89). Crossing the discharging line, we get the value of the delivery pressure equal to 82 mbar. The total delivery flow rate is 7.3 kg/h kg/h (0.7*1.89) = 8.62 kg/h of water that correspond at 82 mbar e 20 C a 82 m³/h. As already detailed, we recommend to choose a pump for a suction pressure 10% lower than the ejector s discharge pressure. In case of operating with a suction fluid different than air or with different temperatures, please contact our Technical Department. Motive flow rate (kg/h) Coefficient K Fig.6 Page 4

5 PERFORMANCE CURVE (motive water at 30 C) The diagram estimates the suction air flow rate at different suction pressures. Performance Coefficient K=1 PORTATE [kg/h] 5 4,5 4 3,5 Motiver water temperature: 30 C Discharge pressure: atmospheric 3 Suction curve 2,5 2 1,5 Discharge curve 1 0, Fig.7 SUCTION PRESSURE AND DISCHARGE PRESSURE [mbar] Example of calculation: Suction pressure: Suction flow rate: 35 mbar 5.5 kg/h As stated in fig. 7, the suction flow rate is about 1.0 kg/h of air (Coefficient K=1). According with the performance s coefficient chart (Fig.6), the coefficient K is obtained by the comparison between the required flow rate and the reference one (K=1) that means 5.5/1.0 = 5.5 that correspond to the ejector s motive flow rate of 20.3 kg/h. (coefficient of 5.57) Crossing the discharging line, we get the value of the delivery pressure equal to mbar. The delivery flow rate is 20.3 kg/h kg/h (1*5.57) = kg/h of air that correspond to mbar and 20 C at 188 m³/h. As already detailed, we recommend to choose a pump for a suction pressure 10% lower than the ejector s discharge pressure. In case of operating with a suction fluid different than air or with different temperatures, please contact our Technical Department. Page 5

6 Connections, dimensions and weight DN1 = motive liquid DN2 = suction fluid DN3 = discharge PVC, PP LJ FLANGES CONNECTION IN PP_V EN Connections Dimensions [mm] Weight Air motive [kg/h] DN1 DN2 DN3 L L1 L2 Kg / / / Connection air motive: threated BSP F. PVC, PP THREATED BSP CONNECTION Connections Dimensions Weigh Air motive [kg/h] DN1 DN2 DN3 L L1 L2 Kg /2 3/ /4 1 1/4 1 1/ /4 1 1/2 1 1/ /2 2 1/ Connection air motive: threated BSP F. CARBON/STEINLESS STEEL FLANGES CONNECTION EN Connections Dimensions [mm] Weight Air motive [kg/h] DN1 DN2 DN3 L L1 L2 kg / / / Connection air motive: threated BSP F CARBON/STEINLESS STEEL THREATED CONNECTIONS Connections Dimensions [mm] Weig Air motive [kg/h] DN1 DN2 DN3 L L1 L2 kg /2 3/ /4 1 1/4 1 1/ /4 1 1/2 1 1/ /2 2 1/2 2 1/ Connection air motive: threated BSP F Page 6

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