CONTROL RELAYS 0 15 30 5 10 20 25 MINIMUM MIXED AIR MINIMUM POSITIONER SIGNAL POSITIONER HS P IN OUT EP AIR TO EP AIR TO DAMPERS M B DAMPERS MIXED IN AIR PRV MPR SIGNAL SIEMENS MULTIPURPOSE RELAY MINIMUM POSITIONER REVERSING RELAY BIASING RELAY (-1-) MIXED AIR (-2-) (-3-) SIGNAL SIGNAL TD TR TD TR SIGNAL TD TR EP AIR S R TO EP AIR S R TO EP AIR S R TO DAMPERS DEVICE DEVICE MPR RR BR Minimum ventilation is some times achieved via a pressure reducing valve (PRV) and a high selector (HS), as illustrated above. If the controller signal drops below the pressure required keeping the dampers at or above the minimum ventilation rate, the HS passes the PRV signal to the dampers, maintaining minimum ventilation. Minimum ventilation is sometimes achieved via a minimum positioning relay (MPR). The MPR is set at a pressure which holds the fresh air damper open enough to assure at least minimum ventilation when the controller signal is below that pressure. SIEMENS multipurpose relay may used as: -1- A minimum positioning relay (MPR), as per the top left hand MPR in this illustration. -2- A reversing relay (RR), which reverses the input signal (TR) and allows biasing of the signal. Example: If the signal increases by 2 PSIG, the output (R) decreases by 2 PSIG. The output may also be biased relative to the signal. Example: The relay may be set for 10 PSIG signal provides 10 PSIG output: you may adjust the relay to 10 PSIG provides an out put pressure other than 10 PSIG suiting the sequence of the circuit. -3- A direct acting biasing relay (BR), which offsets the signal (TD) in the output signal (R), and remains a direct acting signal. This is used to sequence devices, eliminating function over-laps (heat/cool, preheat/reheat, face & bypass damper/valve, etc.) NOTE: The TAC AKR-40605 is also a multi use relay. The AKR-40605 may be used as a minimum or maximum positioning relay, a 1:1 ratio relay (as #3 SIEMENS illustrated)), a two signal low sector or a manual positioner. 2.12
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