The problem With Using 2-way Chilled Water Control Valves

 The problem with using 2-way Chilled Water control valves.

As a system with two-way control valves becomes satisfied, it begins to close its two-way valve.  The greater the number of two way valves closed, the greater the head against the chilled water flow to the building and the greater the head against the chilled water pumps.

In a two-way installation, the temperature difference between chilled water supply and chilled water return is not an adequate indication of the building's load.

Since the overall building chilled water flow decreases as the load decreases, the temperature difference does not always change, especially if there are leaky bypass differential valves.  Water flow downstream of the bypass differential valves is the beat indication of true building load.

In situations where many similar zones are drawing from a common supply, the problem of selecting the highest multitude of the valve positions may be circumvented.  Determining the highest multitude of valve positions is the best way to control the entire system.

If all the valves are the two-way type, flow will vary directly with heat load under conditions of constant supply temperature.

A valve position control system would attempt to maximize the valve positions, and therefore flow, by raising supply temperature to its highest acceptable level.

A flow controller setting supply temperature can reach the same objective.  As the terminal units reduce their individual flow rates due to a decrease in cooling load, the flow controller will respond by gradually raising the overall supply temperature. 

In the steady state condition, total flow will have to be  restored, with the decrease in cooling load being reflected by a temperature change, rather than the expected flow change.  Like a valve position controller, this flow controller requires integral action with a time constant  long enough for the zones to respond.

There is never a guarantee that controlling the total flow will satisfy all the zones.  It is actually only capable of satisfying the average zone.  Reducing the flow set point somewhat below the maximum attainable value can provide some measure of protection.

This might be acceptable for most air-conditioning systems where cooling loads are generally evenly distributed and being affected by similarly ambient conditions and occupancy.

In cases where one or two critical zones may periodically require more cooling than the average, a valve position controller can be utilized to operate those particular zones.  The valve position controller could then override the flow controller to insure the selected critical zone is satisfied.  During the override situation, total flow would fall below its set point as the other zones reduce their demand for cooling.

In order for the building's two-way valves to inter-relate properly with the control system and any building interface, it is advisable for the two-way valves to be the equal percentage type, selected on an adequate pressure differential.

In general, equal percentage valves selected in such a way that their overall pressure drop is 10% of the circuit pressure drop, provides efficient control.  
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