B M 1 2 3 B M 1 B M T VAV BOX PNEUMATIC TRANSMITTERS AVERAGING BULB ROD Pneumatic transmitters sense a medium with their element and only report the condition to a gauge, receiver controller or both: they do not control. All commercial transmitters have a 3 PSIG to 15 PSIG range. A few are two pipe with relay volume capabilities, but most are low volume, one pipe using a .005”, .007” or a .0075” restrictor. The reporting characteristics may be temperature, relative humidity, pressure, enthalpy, etc., with multiple ranges in each. The indication gauges seen on the control panels are actually pressure gauges with a 3 PSIG to 15 PSIG range calibrated to indicate temperature, humidity, etc. relative to their associated transmitter. DUMP BOX (BYPASS BOX) CEILING SPACE RETURN AIR SUPPLY DUCT OCCUPIED SPACE Dump (bypass) boxes are the mechanical means of regulating the quantity of cooling air allowed into the occupied space based on the thermostat’s control of the damper actuator in the box. The fan provides a constant volume, as the supply duct always passes the same quantity of air. RECEIVER CONTROLLERS SINGLE INPUT MULTIPLE INPUT Receiver controllers receive a pneumatic signal or signals allowing the receiver controller to determine what position it should put the valve or damper to attempting to maintain the receiver controller’s set point. The proportional band determines the throttling range of the controlled valve or damper actuator. The authority determines the relationship of temperature change on the primary and secondary ports to rebalance the instrument. The control point adjustment (CPA) allows remote adjustment of the set point regarding the instrument. VAV BOX (VARIABLE AIR VOLUME) SUPPLY DUCT OCCUPIED SPACE AIRFLOW CONTROLLER VAV boxes are the mechanical means of regulating the quantity of cooling air allowed into the occupied space based on the thermostat’s control of the damper actuator in the box. The fan requires inlet vanes or variable frequency drive control (VFD) controlling the supply duct pressure as the total quantity of air required by the VAV boxes varies on thermal load changes. Systems with the airflow controller are called pressure independent and systems where the thermostat goes directly to the damper actuator are called pressure dependent. 2.19
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