Sonic boost

When an airplane exceeds the speed of sound, we say that it breaks the sound barrier. In so doing, it generates a sonic wave or pressure wave-front. When steam and gas flow into the converging section of the jet diffuser shown in Fig. 16.1, the same thing happens. The gradually converging sides of the diffuser increase the velocity of the steam and gas, as the vapor enters the diffuser throat up to, and even above, the speed of sound. This creates a pressure wavefront, or sonic boost. This sonic boost, will multiply the pressure of the flowing steam and gas by a factor of perhaps 3 or 4. 

While smaller than the sonic boost, the velocity boost is more reliable. Even though the velocity in the diffuser throat in Fig. 16.1 falls well below the speed of sound, the increase in pressure in the diverging portion of the diffuser is only slightly reduced. 

The overall pressure boost of a steam jet is obtained by multiplying the sonic boost effect times the velocity boost effect. The overall boost is called the jet s compression ratio. 

Let s further assume that the pressure drop from the jet discharge, through the condenser discharge, is 10 mm Hg. Then, the jet discharge pressure would be 90 mm Hg. Let s also say that the sonic boost is equal to 3.60. The velocity boost is assumed to be equal to 2.5. The overall compression ratio is then... 

The lower velocity in the throat does not affect the jet s performance, as long as the velocity remains above the speed of sound. If the velocity in the throat falls below the speed of sound, we say that the jet has been forced out of critical flow. The sonic pressure boost is lost. As soon as the sonic boost is lost, the pressure in the vacuum tower suddenly increases. This partly suppresses vapor flow from the vacuum tower. The reduced vapor flow slightly unloads condenser 1 and jet 2 shown in Fig. 16.2. This briefly draws down the discharge pressure from jet 1. The pressure in the diffuser throat declines. The diffuser throat velocity increases back to, or above, sonic velocity. Critical flow is restored, and so is the sonic boost. The compression ratio of the jet is restored, and the vacuum tower pressure is pulled down. This sucks more vapor out of the vacuum tower, and increases the loads on condenser 1 and... 

Let s say that a jet is already in its critical-flow mode. It is already benefiting from both the sonic boost and the velocity boost. What, then, will be the effect of a reduction in the jet s discharge pressure on the jet s suction pressure Answer—not very much. If a reduction in discharge pressure is made on a jet, which is not working in its critical mode, there will always be some benefit. 

Recently, on a job in Arkansas City, I was able to force a jet to surge and lose its sonic boost, simply by raising the condensate level in its downstream condenser by just 6 in. Lowering the level drew down the jet s discharge pressure by a few millimeters of mercury and restored it to critical flow. 

As the cooling-water temperature rises, the sonic boost is lost more easily and more rapidly. The surging cycles increase in frequency... 

The critical discharge pressure for each jet is determined experimentally by the manufacturer. It is usually noted on the jet specification sheet. My experience indicates that exceeding this critical jet discharge pressure by the smallest amount will force the jet out of critical flow. Or, the way 1 see it, will cause the jet to suddenly surge a few times and then lose its sonic boost. 

The inverse is not always true. A jet with less than its critical discharge pressure may not pick up its sonic boost. Operationally, this business of gaining or losing sonic boost is a rather dramatic effect. 

Second— In the converging section, the velocity of the steam is used to compress the off-gas, air leaks, and the motive steam itself, due to the combined vapor flow exceeding critical velocity (the sonic boost). 

Second—If the inlet temperature at the converging section of the ejector is 100°F, then the temperature halfway along the ejector (at its throat or narrowest portion) will be about 200°F. The ejector has been heated by 100°F, due to the heat of compression of the flowing vapor provided by the sonic boost. 

Summary A small (10°F to 20°F) temperature rise across the converging section of a steam jet is a definite indication of the loss on the sonic boost and of a low ejector compression ratio. The overall temperature rise across the ejector s diffuser section should be 150°F to 200°F. 1 have written an entire book on this complex subject. Troubleshooting Vacuum Systems (Wiley, 2012). 

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