High Limit Switch

The purpose of the High Limit Switch is to provide a safety shutdown in the event that the temperature in any of the plenums has exceeded the high limit setpoint. The high limit switch senses the temperature in the plenum that it is monitoring by means of a capillary bulb temperature sensing device.

Depending on the number of burners, this will determine the number of high limit switches, with one installed per burner. The high limit switch is adjustable and should be set once at the start of a drying season or for a given grain. Adjustment of the switch is done in the ignition box for tower dryers and in the burner control cabinet for all other dryers. Note that when a high limit setpoint is exceeded, the high limit for the applicable fan/burner will become tripped as well as all other high limit switches above it because the wiring of the switches are in series. For example, if the high limit switch in plenum #1 is exceeded, the high limit for plenum 2 will no longer show to be in good status.

The safety features associated with the high limit switch(es) are for your safety and for the protection of the dryer. They should be checked for correct operation at the start of the drying season and periodically during the season as they are designed to protect the dryer from a fire that may result from fan (air flow) failure while the burner is ignited and a flame is present.

Setpoint Check and Switch Adjustment

The following procedure shall be utilized to properly check the high limit switch(es) setpoint. Note that the high limit switch for each fan/burner must be checked and set independently. Once the high limit setpoint has been reached, the switch will need to be reset by pressing the small reset button on the top of the device. After the dryer has been filled with grain and after the burner is ignited, the operation of each high limit switch must be checked.

1. Start the fan, ignite the burner and establish a stable temperature in the plenum.

2. Inside the ignition box for tower dryers and burner control cabinet(s) for all other dryers, adjust the high limit switch setpoint by turning the small dial counterclockwise to a temperature below the current temperature in the plenum.

3. Once the high limit switch determines that the temperature in the plenum has exceeded the high limit setpoint, the switch will cause the dryer to shutdown.

4. If the high limit switch does not shut down the dryer, it is defective and must be replaced. Once the switch is replaced, check its operation and make adjustments as previously outlined.

5. If the setpoint is exceeded and properly shuts down the dryer, the setpoint can be adjusted to a value that is approximately 40 deg F above the desired plenum temperature.