Technical Advice: Statistical Process Control and Automatic/Engineering Process Control

An engineering or automatic process control (APC) approach recognizes that to a first-degree approximation, a change in the furnace temperature is a negative linear relationship with the ambient air temperature. If the furnace is too hot, then there is a straightforward linear relationship for increasing the flow of ambient air to the system, which brings the furnace temperature down to the set point. If the furnace is too cool, then one can use the relationship to decrease the flow of the ambient air. This simple relationship allows the engineer to construct an automatic control scheme. The key to APC is to trade variability in something of little consequence, in this case the flow rate of ambient air, for a reduction in variability in something of major consequence. The key is understanding the expected rate of disturbances to the process and the costs of process adjustments.