Weight Flow Control at Constant Speed

Motor-driven reciprocating compressors are sometimes used in tonnage oxygen plants. To maintain a uniform output, the plant must be supplied with a constant weight flow of air. As ambient conditions change weight flow, a control system as shown in Figure 8-40(f) can be used to keep the plant supplied with the proper quantity of air. The necessary steps in this system are:

1. The compensated flow transmitter senses the process weight flow. It converts this signal to one that is proportional to the process flow and sends it to the weight flow controller.

2. The weight flow controller amplifies the transmitter signal and sends a modified signal to the final element.

3. The final element is the compressor guide-vane mechanism. The variable clearance points are adjusted by means of a positioning cylinder that is operated by a servo valve in response to a signal from the flow controller.

An increase in flow over the set point causes a signal to reach the final element and will result in the closing of the guide vanes to decrease flow

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