Is the outlet section behind the steam ejector nozzle under positive pressure or negative pressure?
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Working principle of steam jet vacuum pumps: Steam jet vacuum pumps create a vacuum by utilizing the principles of gas dynamics, namely the conversion between static pressure energy and kinetic energy as fluid flows. Water vapor under certain pressure reaches the speed of sound as it passes through the throat diameter of the Laval nozzle. Upon reaching the diffuser section of the nozzle, all of the static pressure energy is converted into kinetic energy, resulting in supersonic flow. At the same time, a vacuum is created at the nozzle exit; the gas to be drawn in is pulled into the suction chamber due to the pressure difference, and it mixes with the supersonic steam as it enters the venturi tube. From there, it exits the diffuser section of the venturi at subsonic speeds, during which the velocity of the mixed gas gradually decreases while the pressure increases, before being discharged from the outlet. If several jet pumps are used in series, with a condenser placed between each pump to condense the steam, a higher vacuum level can be achieved. The entire steam jet vacuum pump consists of two main parts: several stages of pump bodies and a condenser. Pump bodies at all levels consist of nozzles, suction chambers, and diffusers. There can be one or multiple nozzles, and these nozzles are typically made of stainless steel. Other components such as the suction chamber and diffusers can be fabricated from materials like stainless steel, cast iron, and carbon steel. Is the outlet section behind the steam ejector nozzle under positive pressure or negative pressure? That is, at the injection point.
If it’s only level 1, then it’s at normal pressure; all lower levels are at negative pressure!
If there is only one stage, then it is at normal pressure; in the case of multiple stages, the stages before the last one are at negative pressure, while the last stage is at normal pressure or above atmospheric pressure……
At the outlet of the cooler, and at the inlet of the next stage of ejectors, there is definitely negative pressure. But is there negative pressure at the diffuser of the first-stage ejector? Steam at 1.0, going through diameter changes, diameter changes. . . . . Does it become negative pressure? It seems incredible; it feels like it is only after passing through the cooler that negative pressure is gradually established.
Negative pressure at the diffuser of the first-stage nozzle exists only in one situation: that is, when the outlet itself is under negative pressure... otherwise, it is the backpressure at the outlet； To maintain a negative pressure inside the condenser, it is necessary for the back pressure at the outlet of the condenser to also be negative… The condensers in power plants are a great example of this!
The pressure at the outlet of the ejector nozzle is generally slightly lower than the pressure at the inlet, so that the gas at the inlet can flow to the nozzle outlet, where it mixes with the other gas to create a pumping effect. It is not certain whether there is negative pressure at the nozzle outlet; it mainly depends on the level of inlet pressure. When the ejector is used as a compressor, positive pressure exists ； When performing vacuuming, it is definitely under negative pressure.
When a ejector is used as a compressor, what is the maximum positive pressure it can achieve in kilograms?
When a ejector is used as a hot press, the compression ratio = outlet pressure/inlet pressure; a value of around 2 is generally suitable. Additionally, the pressure of the driving steam also plays a role; typically, the expansion ratio = driving steam pressure/inlet pressure, and a value of around 10 is acceptable. For example, with a compression ratio of 2 and an expansion ratio of 10, 0.67 tons of low-pressure steam can be obtained from 1 ton of high-pressure steam ； While with a compression ratio of 2 and an expansion ratio of 5, only 0.36 tons can be extracted from one ton of high pressure. Temperature also has a certain impact; the influence of temperature was ignored above.
If the inlet gas pressure is at atmospheric pressure, and the outlet gas pressure needs to reach 4 kilograms, with the working gas being 8 kilograms of compressed air, is this achievable?
That’s more difficult! 1 ton extracts 50 kg/h, so the output should be 1050 kg/h! Injectors are rarely used in applications involving non-condensable gases! !