Don't Get Caught Short

June 1, 2011
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PLC-based temperature control works where single loop temperature controllers fall short.

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Figure 1. The meltblown machine is used to manufacture nonwoven materials by extruding, heating and stretching fibers into a web.

In many applications, single-loop temperature controllers are sufficient to provide control of heating and cooling. But in some applications, including the one detailed in this article, closely coordinated control among many temperature control loops is needed. In those cases and others, PLC-based temperature control can be the best solution.

Meltblown Machine

A meltblown machine (figure 1) used to manufacture nonwoven materials consisted of four main components:

  • An extrusion system provided a steady supply of molten polymer.
  • A heated spinbeam distributed the polymer into an even sheet or web.
  • A heated process air system was used to attenuate, or stretch, the fibers.
  • A vacuum system removed the process air as the nonwoven web was formed.

The spinbeam consisted of a large 304 stainless steel die body block (figure 2) weighing approximately 8 tons; it was heated by 72 electric cartridge heaters rated at 1.1 kW. The heaters were paired into 36 zones spaced on 4" intervals down the length of the block.

Each of the 36 zones had an RTD temperature sensor for feedback to that zone’s single-loop temperature controller. An output from each controller modulated a pair of cartridge heaters to maintain an average block temperature around 425°F (218°C). Biasing setpoints for each of the 36 individual zone temperature controllers could be entered by the machine operators to adjust the temperature profile across the length of the spinbeam.

Process air heated by a 320 kW electric circulation heater was delivered by a low-pressure blower through large air manifold assemblies. These assemblies were constructed of 0.75" thick 304 stainless steel plate attached to the die body using 28 ~0.47" (12 mm) grade-8 bolts.

Varying Rates of Thermal Expansion

Normally, thermal expansion of metal as it is heated is not a problem if all assembled parts expand at the same rate, or if clearances with tolerances down to the thousandths of an inch are not required. But in this case, there were significantly different rates of thermal expansion for the die body and the air manifold. This was causing these two components to separate to the extent that the bolts connecting the assemblies were shearing.

When the meltblown machine was started, the solid die body was heated with the cartridge heaters at a rate of 2°F per minute. The air manifold assemblies were heated only by the preheated process air; thus, they were increasing in temperature at a rate of about 1°F per minute. With the two different heating rates, it did not take long to develop a significant temperature differential between the die body and the air manifold assemblies.

Because the solid stainless steel die body was more efficient at conducting heat than the hollow air manifolds, it could expand in length by as much as 1 percent - or 1.4" - before the air manifolds could expand. Something had to give and, in this case, it was often one or more of the 28 bolts attaching the air manifold assemblies to the sides of the spinbeam.

The workaround solution was continuous intervention from the machine operators to monitor the die body and air manifold temperatures and to adjust the temperature setpoints on the 36 individual controllers as required. A solution that replaced constant operator attention with automation control was needed.

The Power of the PLC

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Figure 2. Significantly different rates of thermal expansion for the die body and the air manifolds were causing these two components to separate and the connecting bolts to shear.  

After analysis and process trials, it was determined that the system could handle a maximum temperature differential of about 50°F (10°C) between the die body and the air manifolds without shearing the bolts.

One possible solution to maintain a maximum temperature differential was to install heaters and additional controls on the air manifold assemblies. However, due to the complex shape and space limitations, this solution was cost prohibitive.

A better solution was an improved automation system in the form of a PLC and a human machine interface (HMI). On the meltblown machine, the individual single-loop controllers were eliminated and the 36 RTD zone temperature sensors were wired to individual analog inputs at the PLC. Also, temperature sensors were added to monitor the incoming temperature of the preheated process air entering the air manifolds.

Using the internal math functions of the PLC, the 36 die-body-zone temperatures were averaged and compared to the incoming air temperature in the air manifolds. Based on the measured temperature differential, the PLC automatically ramped the setpoints of the die body zones to individually control each zone’s pair of cartridge heaters.

Specifically, the PLC determined the correct setpoint for each of 36 PID control loops. Each loop read an RTD input as the process variable and used the internally derived setpoint to modulate a control variable analog output to control the heaters.

This PLC-based control solution allowed the machine to automatically heat at a rate that was compatible with the thermal expansion of the different components. The bolt shearing problem was solved and the need for constant operator intervention was eliminated.

In addition, preprogrammed biased setpoint temperature profiles in the PLC now can be used automatically as required, eliminating the need for machine operators to enter separate biasing setpoints in the individual single-loop controllers.

And, the PLC continuously provides and trends valuable data to the HMI, including the process variable, the setpoint (including any biasing) and the loop output control variable. Alarm points and conditions, either globally or for each loop, also can be viewed at the HMI. Touchscreen HMI inputs allow changes depending on different conditions such as the polymer type.

Prior to the use of PLC-based control, constant operator intervention was required to provide safeguards against damage and to manually track various process conditions. By introducing PLC-based control, possible machine damage due to varying rates of thermal expansion was eliminated, and insight was provided by tracking and trending data that was not readily available from the single-loop controllers.

For complex temperature control applications with different heating or cooling profiles, PLC-based control offers a versatile, powerful solution. When paired with a touch screen HMI, additional process data can be viewed and analyzed, with adjustments to improve operations easily entered through the touchscreen.

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