A linear valve is a control valve where equal increments of stem movement produce equal changes in flow capacity. The flow increases proportionally with valve opening, providing straightforward but less stable control. Linear valves work well in systems with constant resistance but struggle in variable load applications.
Technical Details
Linear valves have constant slope when plotting flow against percentage open, with rangeability typically between 10:1 and 15:1. They exhibit poor control authority in systems where downstream resistance varies, often producing hunting and oscillation. The plug design features a uniform orifice area that increases linearly with stem travel.
Applications in HVAC
Linear valves are suitable for simple two-way isolation in constant resistance circuits and low-cost applications. They work acceptably when valve authority remains above 0.5, such as small branch circuits with high permanent pressure drop.
Practical Significance
While less expensive than equal percentage valves, linear valves require larger permanent pressure drops to maintain adequate authority. Modern variable speed pump systems have reduced linear valve popularity due to authority problems at reduced loads. Characterized or equal percentage valves are preferred for most HVAC installations.