Recycle Control

Recycle control uses feedback from an absolute pressure transmitter to control the valve position on a recycle line. By changing the position of the recycle valve, this control arrangement is able to control the load to the ejectors suction, which allows for control of the ejector’s pressure. The load is recycled back to the suction of the ejector because of the pressure differential between the ejector’s discharge and suction. The benefit to this control arrangement is that the load used to control the vacuum is not from an external source so there is no operating cost. At low loads or no load, the ejector’s motive steam is always available to be recycled, meaning the control functions over the entire load range. The draw back to this type of control is that it often requires a sizable line and control valve. Recycle control is the preferred method of vacuum control.

!!! WARNING !!! When utilizing recycle control in a system with multiple stages of ejectors, the recycle line must be piped from the first stage ejector discharge back to the first stage ejector suction. If the recycle line is piped around multiple stages of ejectors, the recycle load will have a tendency to be made up mostly of noncondensables. When recycled, that load then reloads all of the ejectors that are upstream of where the recycled load is being drawing from. This can overload the downstream ejectors and cause vacuum instability issues.

Throttle Control

Throttle control uses feedback from an absolute pressure transmitter to control the throttling valve’s position. By changing the position, the pressure drop across that valve is controllable. This allows the user to vary the pressure drop across the throttle valve which in turn controls the vacuum in the process vessel. The benefit to a throttle control system is that all it requires is an actuated valve with no external source of load. The downside to a throttle control system is that it’s expensive to implement on a bigger vacuum system with a large line size, making it better suited for smaller systems. A throttle control valve also losses sensitivity at lower loads because flow is needed to generate pressure drop.

Bleed Control

Bleed control utilizes feedback from an absolute pressure transmitter to control the position of a bleed valve. This allows the user to add load to the ejector suction and by controlling that load the ejector suction pressure can be controlled. The benefit to a bleed control arrangement is that they are fairly easy to install and the bleed line and control valve are normally quite small. The downside to a bleed control arrangement is that the steam or medium you are bleeding into a process normally has an associated cost and thus there is an operating cost to most bleed control arrangements.

!!! WARNING !!! Noncondensable loads like air or nitrogen should never be used in a bleed control if the ejector system has multiple stages of ejectors. Noncondensables will load the entire ejector system. This can overload the last stage ejector in the system and cause vacuum instability issues. In a multiple stage ejector system, a condensable load should be added if a bleed control is being used. A noncondensable load can only be added in a single stage system.

In systems were ejectors are operated in parallel, one can utilize turndown to help control vacuum. For example, if a system has three (1/3) ejector elements but the loading is below 66% of its design, one ejector can be turned off to achieve the design suction pressure. Care should be taken when shutting down a parallel ejector that the ejector is isolated correctly. If the ejector being taken out of service is not isolated correctly, load can recycle back though it and degrade the suction pressure of the still operating parallel ejectors. When used correctly, this arrangement can also reduce a system’s steam consumption when operating at loads that are lower than a system’s design.

These control arrangements (bleed control, throttle control, and recycle control) are applicable to systems where the process vessel is having its vacuum controlled by an ejector. Vacuum systems that have a precondenser must be thoroughly analyzed to determine the best method of control. In a precondenser system, the precondenser size, its load make up and flow, and the cooling water flow and temperature will set the process vessel’s operating pressure.