Our topic for this month is ejector system vacuum control. There are multiple ways to control an ejector’s suction pressure. This article will aim to cover all of the primary ways to control an ejector’s suction pressure and provide some insight into the pros and cons of each configuration.
Most ejector vacuum systems are comprised of “critical compression” ejectors, which are ejectors that operate with a supersonic shockwave. If an ejector’s compression ratio, which is the difference between its suction and discharge pressures, is greater than 1.8, then the ejector will operate with a shockwave. These “critical” ejectors achieve a suction pressure that is directly related to the ejector loading. As the load to the ejector increases the ejector’s suction pressure degrades and as the load to the ejector decreases the ejector’s suction pressure improves.
By controlling the load to an ejector, control of the ejector’s suction pressure is achieved. It should be noted that the motive steam to a “critical” ejector shouldn’t be throttled. Increasing or decreasing the motive pressure or motive steam flow to the ejector will not impact the ejector’s suction pressure or capacity. If the motive steam is throttled too low, the ejector’s internal shockwave will dissipate and the vacuum will degrade in a rapid and uncontrollable manner. Throttling the motive steam is not a good method of vacuum control. Acceptable forms of vacuum control are shown below.