5 strategies for handling differing system conditions.
by Ray Hardee
January 29, 2018

For example, assume there is a 40 psi pressure drop across the PIC. The temperature control valve is supplied an input signal to regulate the flow rate through the valve to the air handler. The downstream fluid pressure of the temperature control valve is measured at location P1 within the valve. The pressure at location P1 is automatically regulated by the DPR to maintain a constant differential pressure of 5 psig regardless of the flow. The position of the DPR valves inlet pressure is supplied to the DPR section of the valve. In this example the DPR positions the valve stem to provide 35 psig.

When the circuit requires reduced flow, the differential pressure across the PIC increases. The integrated pressure regulator adjusts to increase the head loss for the required flow rate, and the temperature control valves sees the same five psi differential pressure. The differential pressure regulator can absorb the excess head regardless of the flow rate required by the circuit.

Since the temperature control valve always sees a constant differential pressure of five psi, the valve the circuit is able to able to operate properly under a wider range of cooling loads and differential pressures.

PICs must be purchased and installed in each circuit for the system to operate properly. They cost more than a standard control valve, but are not affected by differential pressure across the circuit, so there is no need to install balancing valves, flow limiting valves or differential pressure controllers. In addition, by maintaining a constant differential pressure across the temperature control valve, regardless of chilled water flow rate, the system can be controlled with a varying thermal load.

Integrating the VSD into the System Control

Installing a VSD on a pump in an HVAC chilled water system allows us to vary the pump speed to regulate the flow rate through the system to meet the system thermal loads. The VSD is controlled to maintain sufficient pressure, so the pump can provide the flow to all circuits to meet the system’s thermal loads. Choosing the location of the pressure source used to supply the control input can be simplified by using simulation software to model the entire piping system so one can see how the system will operate under any expected operating conditions.

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