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Latest Energy-Saving Technologies

Some dryer manufacturers are touting energy savings while others, who really don't have testing facilities nor qualified personnel to conduct testing, try to convince processors that energy savings in dryers is not worth talking about. A look at the chart, "Energy Costs of Old vs. New Dryers" will quickly convince you otherwise.and this chart is based on current average electric costs. We all know that the trend of electricity cost is UP so planning ahead to reduce your energy costs is the sensible thing to do. The larger the dryer - the greater the savings and the better the ROI.

The performance of any drying system is based on using the minimum heat to raise the temperature to the appropriate drying temperature while maintaining a constant material temperature at the throat of the injection molding machine or extruder. Also the regeneration system should minimize the energy used to heat the desiccant material while dedicating most of the energy to removing the moisture gained during the drying process.

Reducing Process Heat Loss

You have to be cautious about how dryer manufacturers claim to attain energy savings. Temperature Setback or Second Set Point are sometimes sold as energy-saving features, but they are not. Most rely on "Temperature Setback". Let's take a look at what that means.

Temperature Setback lowers the process temperature in the dryer and thus the resin temperature entering the processing equipment. The result is that the processing machine must make up for the lower heat in the resin by increasing the heat from shear and the band heaters. The processing machine ends up using more energy to overcome the shortcomings of the dryer, resulting in no energy savings, and contributing to product inconsistency.

Reducing process heat loss to increase energy savings can also be accomplished through the use of VFD's (variable frequency drives) to control process blower speed.

If the air flow rate and temperature rise are minimized, the process heating energy can be as low as necessary to heat the resin. The idea is to minimize the air required so that the minimum energy is used for process heating. Yet, the temperature of the resin is raised to the appropriate temperature for the operation of the process machine but all of the energy remains in the resin, in the drying hopper, and a minimum of the generated heat is returned to the dryer.

To accomplish this, the temperature of the resin and the temperature leaving the drying hopper are measured and the air flow rate is adjusted so that the return air temperature (the temperature returning to the dryer from the drying hopper) is only slightly higher than the temperature of the resin entering the drying hopper.

This process is controlled by varying the speed of the blower with a Variable Frequency Drive (VFD) that changes the speed of the blower and thus the flow rate of the air. By minimizing the air flow rate, while maintaining the temperature of the resin, the process heat is maintained at the lowest possible level. This allows the required power to adapt to any changes in material rate, resin moisture and temperature of the resin.

The result is shown in Figure 1 above

Reducing Regeneration Heat Loss

To maximize the energy savings, the power used in regeneration of the desiccant must be minimized - further reducing heat loss. Regeneration can account for as much as 35% of the total power usage. Regeneration consists of heating the desiccant to a temperature at which it releases the moisture gained during the resin drying process. This involves raising the temperature of the desiccant to a point where the moisture retained in the desiccant is "boiled off".

There are two parts to this system and each has a specific task. First, the speed (RPM) of the desiccant wheel is minimized which reduces pounds per minute of desiccant media to be heated. This is important because heating of the desiccant media detracts from the primary goal of vaporizing moisture and heat is lost without accomplishing the primary goal of water removal. The wheel speed is controlled by a variable frequency drive (VFD) to no more than that required to adsorb the moisture from the return air.

By controlling the speed of the wheel, the desiccant media is loaded to its maximum while maintaining a consistent dew point of less than - 40°F/C.

The second part of reducing regeneration heat loss is accomplished with the use of a VFD on the regeneration air blower. The VFD minimizes the air flow to a point that water is desorbed from the molecular sieve but only minimal heat leaves the wheel during the regeneration process.

As the ambient air is heated and passes through the wheel, the discharge temperature is constantly monitored and the VFD adjusts the air flow ensuring that minimal air flow is used to remove all of the moisture gained in drying but no excess air is used.

The temperature of the air exiting the top of the wheel is just enough to remove the moisture and carry it away but is kept to a constant temperature. This ensures that the wheel will remove all of the moisture from the resin regardless of the resin moisture level and will automatically adjust as the moisture changes due to seasonal variations or changes in the virgin/flake ratios.

See Figure 2 above