34 Process Control Case Histories FURNACE CONTROL IN THE NETHERWORLD I was invited to a plant that had an electrical furnace with pressure control problems caused by large variations in the rate of internal gas generation. To hold the furnace pressure within safe limits, the unit had been designed with a positive displacement compressor, which pulled out gas and directed it to a flare. A pressure con- trol loop was supposed to keep this system operating properly. Unfortunately, changes in gas generation rate created disturbances that were so large compared to the volume, it was like trying to regulate an explosion. The unit was an ominous black structure, like what you'd imag- ine provides heat for the netherworld. Everybody at the plant held this unit in great awe. I understood why as soon as I saw it. I understood even better when we climbed to the top for an inspec- tion, because the monster spewed sparks and fires broke out where we had just stepped. I noticed what appeared to be a ceremonial ring of filled bathtubs around the circumference of the furnace.