12.1 Sensors, Actuators & Dampers: Selection, Placement, Fail-Safe Position & Damper Authority

12.1 Sensors, Actuators & Dampers: Selection, Placement, Fail-Safe Position & Damper Authority

NCEES sub-topic 2D reads "Control Concepts (e.g., valves, dampers, sensors, actuators, temperature reset, PID)." Section 11.3 handled control valves in depth. This section covers the rest of the physical layer, and Section 12.2 covers the loop logic that drives it.

The organizing idea: a control loop can be no better than the measurement that feeds it or the final element that executes it. Most HVAC control failures diagnosed as "bad tuning" are actually a sensor in the wrong place or a device that is hydraulically or aerodynamically oversized.

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1. Sensors

Placement Determines Whether the Reading Means Anything

• Space temperature sensors go on interior walls at roughly 48 to 60 in. above the floor, away from direct sun, supply diffusers, exterior walls, and heat-producing equipment. A sensor above a copier reads the copier.
• Duct static pressure sensors for fan speed control are placed two-thirds to three-quarters down the longest duct run. Placed at the fan discharge, the sensor holds the fan at constant discharge pressure, defeating the entire purpose of the variable-speed drive and starving the far end of the system.
• Mixed air temperature sensors must be averaging elements serpentined across the duct cross section, because outdoor and return air do not mix instantly. A single-point probe in a mixing plenum reads whichever stream happens to strike it.
• Outdoor air sensors belong on a shaded north-facing wall, away from exhaust discharges and roof surfaces.
• Discharge air sensors must be far enough downstream of the coil that the reading is not affected by radiant view of the coil surface.

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2. Actuators

Fail-Safe Position Is a Design Decision

On loss of power or control signal, a spring-return actuator drives to a defined position. This is achieved mechanically, not by logic, so it survives a controller failure:

Actuator selection also requires a torque calculation: damper torque scales with blade area and differential pressure, typically 3 to 8 in.-lb per square foot depending on seal type and pressure. Under-torqued actuators stall against the seals of a low-leakage damper, and the resulting partially closed damper is invisible to the control system unless position feedback is provided.

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3. Dampers

Blade Arrangement

Leakage Class

AMCA 511 rates damper leakage at a stated differential pressure. Low-leakage classes matter directly to energy code compliance, because an outdoor air damper that leaks when closed imposes a heating and cooling load whenever the system is off. Specify a low-leakage, gasketed, blade-and-jamb-sealed damper for every outdoor air and relief application; standard industrial-grade dampers are acceptable only for internal balancing service.

Damper Sizing and Authority

Damper authority is the same concept developed for control valves in Section 11.3:

$\alpha = \frac{\Delta P_{damper,\ open}}{\Delta P_{damper,\ open} + \Delta P_{rest\ of\ branch}}$

When authority is high, closing the damper changes the branch resistance substantially and flow responds smoothly across the stroke. When authority is low - below roughly 0.10 - the branch resistance is dominated by everything except the damper, so the first 70% of damper closure changes almost nothing and the last 20% slams the flow shut. The loop then hunts, because the controller sees a huge process gain in a narrow band of stroke.

The practical rule that follows: size a control damper for a face velocity of roughly 1,500 to 2,000 fpm at full flow, which usually makes it smaller than the duct, with a transition on each side. A damper built to match the duct size is aerodynamically oversized, its wide-open pressure drop is negligible, and its authority is destroyed.

Where Dampers Appear in the System

  OUTDOOR AIR ---->[ low-leakage OA damper ]----+
                                                |
                                                v
  RETURN AIR ----->[ return air damper ]----> MIXING ----> [filters] -> [coils] -> FAN -> SUPPLY
                                                ^
  RELIEF/EXHAUST <-[ relief damper ]------------+

  Interlocks: OA and relief dampers open together; return damper closes as they open.
              All three are driven from one economizer signal with staggered ranges.