RES. Low limits are sometimes called Freeze Stats. The coil in a fan can still freeze even after the low limit has shut down the fan in some situations. They do not have averaging elements. The element will cause the low limit to trip at its set point (40°F) if any one foot of the element senses a temperature below set point. They can be electric or pneumatic with a PE being controlled by the pneumatic low limit. They can be manual reset or automatic. High limit circuits are sometimes installed using sprinkler heads, with the correct temperature rating for the locations and a pressure switch fed by restricted main air. If any sprinkler head melts, the air is bled from the common line down-stream of the restrictor and the pressure switch opens its contacts shutting down the fan system. If the compressed air supply fails, the fan system will shut down, as a high limit situation will be simulated. High limits shut down the fan system if they sense a temperature above their set point. The set point should be no more than 50F° above the normal highest temperature in the duct. They usually have to be manually reset after they have shut down the fan system. They can be electric or pneumatic. If they are pneumatic, they cycle a pressure electric switch attaining the same result as the electric units. Some have adjustable set points, while some have fixed set points. Differential pressure switches open and close their electrical contacts based on pressure variation sensed by the switch. They normally have low and high pressure connections. Often they are used providing status on fans or used as safeties on electric heating coils preventing the coil from heating when the air flow stops. They are normally electric devices, but pneumatic units exist, which must cycle a pressure electric switch, attaining the same result. If a diffuser is blocked the switch will sense pressure, but no air flow is allowed by the blockage; therefore, the electric coil may still be damaged. SAFETY CIRCUIT COMPONENTS DIFFERENTIAL HIGH LIMIT PRESSURE SWITCH LOW HIGH LIMIT LIMIT SPRINKLER CIRCUIT HEADS RETURN N.O. AIR PE SUPPLY MAIN AIR AIR FRESH AIR INTAKE 2.16
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