This is one of the application of digital omputer application of in process control. In such application the computer receives the directly the measurements from the process and based on the control law which is already programmed and resides in it's memory calculates the value of the manipulated variables, These,decisions, are,now implemented directly on the process by the computer through the proper adjustment of the final control elements (valves, pumps, compressors, switches, etc.). This dfrect implementation of the control decisions gave rise to .the name direct digital control, or simply DDC.
The process can be any of the units we,have already considered such as; heaters, reactors, separators, etc. The two interfaces before and after the computer are hardware elements and they are used to create the interface between the computer and the process.
Today the chemical industry is moving more and more towards the DDC of the plants. A typical system of 'DDC's for an ethylene plant can include, between 300 and 400 control loops. Al1 the companies which furnish the control systems for the chemical industry: rely more and more on DDC.
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