Welcome to Part 3 of ASI’s tutorials on industrial flotation dryers.

In this video, Rodger, our head of service, will highlight the HMI, or human machine interface, component of ASI’s drying systems. Typically, ASI provides the programming for the HMI, and the customer provides the HMI unit. Our standard controls packages utilizes Allen Bradley CompactLogix PLCs and components.


The HMI featured in this video is an Allen Bradley 19” PanelView with a 19-inch screen. The HMI shows the set points on the right side of the screen and the permissive chain on the left side of the screen. The set points for this dryer are:

• Supply air temperature

• AirBar nozzle velocity

• Box pressure—it is important to always maintain a negative box pressure to keep products of combustion and process air inside the dryer and properly exhausted

• Humidity (optional)—this can be toggled between a set point and manual


The permissive chain, on the left of the HMI screen, is the sequence of components the controller must verify are operating before moving onto the next component. This ensures that the dryer will only ignite the burners if it is safe to do so. The most important component, which is always listed first in the permissive chain, is the exhaust fan pressure switch. A properly functioning exhaust fan is required to prevent the buildup of volatiles in the dryer.


The permissive chain includes all of the fan pressure switches, gas pressure switches, and the high temperature limit switch. The permissive chain on this HMI is:

• Exhaust fan pressure switch

• Supply fan pressure switch

• Combustion fan pressure switch

• Pilot gas high pressure switch

• Main gas line low pressure switch

• Main gas line high pressure switch

• Blocking valve #1 on main gas line

• Blocking valve #2 on main gas line

• High temperature limit switch—protects the supply fan from getting too hot


As each component is verified, the chain moves to the next component until it reaches the “heat enabled” sequence which will then allow the burner to ignite. The HMI will then let the operator know when the dryer is up to temperature.


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