World Leaders in Ultra-Pure Heating
Revision 10
www.heateflex.com
01/17/13
405 E. Santa Clara St.
Arcadia, CA 91006-7218
Tel: (626) 599-8566 Fax: (626) 599-9567
3.0 System Control
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3.1 Temperature-Control Overview
The temperature and flow of the D.I. water is monitored by a thermocouple and shedding
vortex flow meter, respectively. These signals are inputted to the programmable logic
controller (PLC), the values of which are indicated on the touch-screen display.
The flow signal, along with the set point temperature are used as inputs to a look-up table
which outputs the proper actuator position for the hot and cold mixing valves to yield the
desired set point temperature.
At any given flow rate, there is a specific mix-ratio of hot D.I. water exiting the heat
exchanger, blended with ambient-temperature water from a bypass line, which will yield the
proper set point temperature. These values are programmed into the PLC‟s look-up table.
When there is a sufficient change in flow rate, as sensed by the flow meter, the PLC
immediately causes the actuator, which positions the mixing valves, to move to the new value
listed in the look-up table. This response to the change in flow rate is immediate without
waiting until there is a detected deviation from set point for the process temperature as with
conventional temperature control systems.
Additionally, the PLC has a trimmer function, which is based on the temperature sensor at the
outlet of the unit to make small adjustments in the actuator position to fine tune the unit.
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3.1.1 Temperature-Control Specifics
Temperature control is a function of two inputs, D.I. Flow Rate and D.I. Temperature,
compared with the set point temperature. If the actual D.I. temperature is within ±1°C of set
point, the actuator will maintain its current position and the look-up table will not be consulted,
nor will the trimmer function make further adjustments. If the actual temperature is not within
that range, then one of two things will happen.
First, the PLC does a comparison on flow rates. If the flow rate has change by a specified
value of ±1 GPM (Factory Default) or more, then the look-up table is consulted for the proper
actuator position corresponding to the new flow rate. (This 1 GPM value can be field adjusted
during start-up as necessary depending on the actual process flows.)
If the flow rate has not changed more than ±1 GPM, and the D.I. temperature is not within
±1°C, then a trimmer function takes over. This function checks to see if the temperature is
within the desired range, and if not, the actuator is adjusted slightly to a new position. After a
specified time interval, the temperature is checked again and the actuator moves another small
