AERCO SWDW68 U-Tube Double-Wall Heaters w/ECS - Manual (Page 25)

HE–
111 − FUNCTIONAL DESCRIPTION
Rev B Mar 2013 3-5
Figure 3-5. ECS Simplified Block Diagram
3.3.2 ECS Operational Summary
The ECS adjusts the flow rate of steam into the heat exchanger in response to:
• The water temperature at the Hot Water Output (at A)
• The water temperature of the sampled Hot Water Output mixed with the Cold Water Input (at B)
Changes in hot water demand vary the flow rate of service water through the heat exchanger, requiring
compensating adjustments of the steam flow rate to maintain the service water temperature. To
determine what steam flow rate adjustments are required, the Temperature Controller monitors (via the
feed-forward signal) the temperature of a continuous sample of output hot water mixed (at B) with the
cold water input. The mixture temperature changes according to the flow rate of the incoming cold water,
which in turn depends on the hot water demand.
The Temperature Controller calculation of the optimum steam flow rate into the heat exchanger is based
on a mathematical process known as the Proportional Integral Derivative (PID) algorithm, which accepts
as inputs the measured outlet and inlet temperatures of the system.
3.3.3 Over-temperature Condition
At startup, the operator programs the maximum safe water temperature into the Control Box. During
operation, the Temperature Controller monitors the Hot Water Output temperature to ensure that the
programmed maximum safe temperature is not exceeded. If it is, the Temperature Controller operates the
Over-temperature Solenoid Valve (see 3.3.4.4) to relieve excessive temperature buildup in the heat
exchanger.