The graphs show the opening characteristics of a valve set at typical values using various refrigerants to drive the pressure actuator. The characteristics of the valve can be designated by temperature or pressure.
Using the graphs, the pressures at which the valves start to open or the “crack” pressure is noted. At pressures below the crack pressure the valve remains closed.
The crack pressure is adjustable as shown in the table by changing the force of the spring with an adjustment screw. Adjusting the force shifts the entire curve to the right (increasing spring tension) or to the left (decreasing spring tension).
Full open represents the position of the valve at “full rated capacity” or rated Cv.
The increased pressure required to bring the valve to full open or “span” is shown. This value is calculated by subtracting the cracking pressure from the full open pressure. A typical span for a pilot operated valve is 25 PSI, for a direct acting valve it is 40-80 PSI (depending on the refrigerant used).
When the actuating pressure is between crack and full open, the valve is open in proportion to pressure, to modulate the flow of water required for a given condition.
The span allows for operation of the system within a desirable range as system conditions vary. The refrigerant head pressure will vary as conditions change, but only within a narrow range in a manner desirable from a control and energy efficiency standpoint.
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