11.5.4 Pneumatic feedback
In Fig. 11.18b pneumatic control is used in a local closed loop system for maintaining
water at a set temperature. Cold water and steam are mixed in a heat
exchanger; the temperature of the exiting hot water is monitored by a pressure–
spring thermometer. The pressure from the thermometer is used to operate
and control a linear globe valve in the incoming steam pipe. If the temperature
of the hot water increases above a set temperature, the pressure from the thermometer
increases and starts to close the valve in the steam line, keeping the
hot water at the set temperature. If the flow of hot water increases, the temperature
of the water will start to lower and this will reduce the pressure from
the thermometer to the valve increasing the steam flow, bringing the temperature
back to its set point.