Importance of Control Curves in PICVs
By Christina Cohr, CCO, M.Eng (Export) and Torben Ibsen, President, M.Sc (Mech.Eng), FlowCon International ApS, Denmark - 08.2019
A pressure Independent Control Valve or PICV, combines 3 functions into one unit, (1) Modulating Control, (2) Dynamic Balancing and (3) Differential Pressure Control.
Control Curve Theory
According to ASHRAE’s “Fundamentals of HVAC control”, The desired heat transfer output of a coil should be linear. The heat transfer output is provided as the correlation between the coil characteristic and control valve characteristic. ASHRAE provides a theoretical terminal characteristic, equivalent to a square root curve, meaning that a linear heat output will require an identical curve, except with its structure reversed, as provided by an equal percentage control valve.
Theoretical Coil Characteristic: Linear Heat Output with Equal% Control
Please note that the coil performance curve provided by ASHRAE should be considered more a theoretical representation or symbol, rather than an actual curve. By examining the coil curves published in the manufacturers’ literature it can be seen that the actual coil performance is not a mirroring of an equal percentage control curve and a PICV equal% control will therefore not per definition provide the preferred linear heat transfer.
Actual McQuay MCM-DW Coil Characteristic: Non-linear Heat Output with Equal% Control
As a rule of thumb, equal percentage control valves are significantly more accurate in low load situations i.e. 0-50% of rated valve capacity or predominantly used in the 0-5VDC control range, whereas linear valves are generally preferred for higher loads and control ranges.
When discussing heat transfer output and control curves, it is – besides choosing the applicable characteristic to secure optimum heat output control – very important to discuss accuracy, turn down ratio and repeatability.
- Accuracy identifies the tolerance between the manufacturer-provided flow values, control curves and the actual values provided. The accuracy generally covers the mechanical tolerance – i.e. variations between different production series – and the pressure accuracy over pressure fluctuations.
- Turn down ratio identifies the lowest flow rate the valve can control to and can be used to identify the “opening jump” of a valve. Equal percentage controls generally provide smaller “opening jumps” than linear valves, as they open slower.
- Repeatability identifies the tolerance seen in the same valve due to hysteresis, pressure fluctuations etc. Valves with poor repeatability will essentially be capable of offering different flow output at the same control signals, meaning that a control curve discussion becomes less relevant, as it becomes a variable.
In other words, poor flow control provides inconsistent control curves, regardless of the control curve characteristics. Please see below curves illustrating each of the above points.
Pressure Independency Curves: Competition (poor flow control)
Control Curves: Competition, Linear Control Mode (poor control curves)
FlowCon PICV Solutions
The FlowCon range of PICVs includes FlowCon UniQ®, FlowCon Green and FlowCon SM, which cover sizes from DN15-250. Key characteristics for all the FlowCon PICVs are a wide range of max. flow settings, external flow adjust-ability, 100% authority, automatic balancing and pressure independency. Furthermore, an important detail involving the entire range is the fact that actuator and valve are developed as a pair and not as two individual components, allowing focus purely on the flow output of the PICV (combination of actuator and valve). This results in best curves and accuracies seen in the market.
Operating pressure and flow range would vary as per valve type and size, but pressure independency will be achieved as illustrated below regardless of valve choice or the valve’s chosen max. flow pre-setting.....
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