Particle velocity control places salt accurately

DN Staff

March 4, 1996

3 Min Read
Particle velocity control places salt accurately

Benson, MN--Remember the last time you found yourself driving behind a truck equipped with a spinner-type salt spreader? Remember the salt rattling off your car and bouncing into the gutter or ditch? A new system ensures that most of the salt or other ice-melting chemical remains on the road.

Tyler Limited Partnership refers to this innovation as their Zero Velocity De-icing System. Engineering Manager Kevin Wald explains that it ejects salt at zero velocity relative to the roadway. Given this arrangement, de-icing chemical is placed on the roadway, not thrown at the roadway. As a result, most of the de-icing chemical remains on the roadway and forms an ice-melting brine.

To sense the velocity of the truck that carries the system, Wald and his colleagues typically monitor the signal generated by the truck's electronically controlled transmission for the vehicle's electronic speedometer. If the truck does not come with an electronic speedometer, the system installer places a pulse generator on the speedometer's cable drive system.

The system's microprocessor-based, cab-mounted Quantum(TM) controller handles three control loops and generates Pulse Width Modulation (PWM) command signals. To control the auger that delivers ice-melting chemical to the dispersal system, the Tyler engineering team uses a closed control loop. A pulse pickup on the hydraulic motor that drives the auger provides speed feedback data to the controller. Both the fan that propels salt out of the system and the liquid-injection diaphragm pump (driven by a dc motor) are controlled open-loop. "We can spread very precisely at speeds right down to zero, and 35 mph is about the upper end. After that, the pattern starts to degrrade because of vortices around the back of the truck," says Wald.

De-icing chemicals fed into the dispensing system by the auger travel through a drop tube to a venturi. An air stream generated by the fan is delivered to the venturi by an air hose. As the air moves through the venturi it pulls salt along and accelerates it. Simultaneously, liquid (usually a brine) pumped from a polyethylene tank sprays into the airstream, just ahead of where the salt falls into the airstream. "We literally spray-paint the particles; we get a very even distribution of liquid on the particles," says Wald. This process causes the particles to stick to the road surface instead of getting blown off the road by passing cars.

As the vehicle moves forward, the controller changes fan speed to govern particle velocity, auger speed to control chemical feed rate, and pump speed to vary liquid injection rate. The driver controls his truck's forward speed, while the controller ensures that chemical gets dispensed at a zero velocity relative to the road surface. Typically, the system deposits chemical in the center of the road, and a few feet on either side of the centerline. Given the crowning of the road and traffic, the chemical brines out and works to the road's edge.

Studies by the Pennsylvania Department of Transportation and Minnesota Department of Transportation indicate that zero-velocity spreading results in materials savings of 30% to 60%, Wald reports. Nationwide, that's one heckuva lot of salt.

Additional details...Contact Kevin Wald, Engineering Manager, Tyler Limited Partnership, E. Highway 12, Box 249, Benson, MN 56215.

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