Page 36 - Practical Control Engineering a Guide for Engineers, Managers, and Practitioners

Qualitative Concepts in Control Engineering 11

FtouRE 1-14 A molten glass forehearth.

explanation of the process depicted in Fig. 1-14 is necessary. The fore-
hearth is a rectangular duct made of refractory material about 1 ft
wide, about 16ft long, and about 6 in deep. Molten glass at a relatively
high temperature, here 1163°C, enters the forehearth from a so-called
refiner. The forehearth is designed to cool the glass down to a suitable
forming temperature, in this case 838°C. There is a gas combustion
zone above the glass where the energy loss from the glass is con-
trolled by maintaining the gas (not tlte glass) temperatures at desired
values via controllers, the details of which we will gloss over for the
time being.
There are three zones: the rear, mid, and bowl. In each zone, the
gas combustion zone temperature above the glass is controlled by
manipulating the flow of the air that is mixed with the natural gas
before combustion. The amount of gas drawn into the combustion
zone depends on the amount of air flow via a ventura valve. In the
rear zone, a master control loop measures the TG(l) glass tempera-
ture (as measured by a thermocouple inserted into the molten glass)
and adjusts the set point for a second loop, called a slave loop, which
controls the gas combustion zone temperature TG(2) by in tum
manipulating the flow of combustion air. There is a similar pair of
control loops in the midzone and the bowl zone. In Chap. 11, we will
treat this combination of two control loops, called a cascade control
structure, in detail.
Therefore, in each zone the control challenge is to adjust the com-
bustion zone temperature set point so as to keep the bowl tempera-
ture TG(3) sufficiently close to 838°C. It is a tough task. The incoming
glass varies in temperature, the manufacturing environment ambient