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Saving Energy

Question:
I can understand the variable frequency drive on the blower but how does the output of the heater vary with it?

Response:
Heat consumed is a function of the temperature rise of the air and the quantity of air heated.  This is shown in the equation:  Q (kW) = 0.0003228 * scfm * ΔTYou can reduce the heat consumed by both reducing the air heated and reducing the temperature difference of the heated air.  Typically the temperature that the air is heated to is fixed by the material, however, overcooling by using too much or too cool air in the dryer cooling system can lead to an increase in the differential temperature and higher energy consumption.  The best control system controls the air flow and the cooling system to both maintain proper air flow and prevent overcooling.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
Should we control air flow with Variable Frequency Drives or dampers?

Response:
Both of these can achieve the goal but dampers are not a form of active control.  Through changes in material temperature, material moisture and any changes in material blend ratios the amount of air required and thus the air flow change.  Variable frequency Drives can be used to make these changes automatically and don’t require the judgment of an operator.  They can be set to make the proper adjustment and have it occur whenever necessary and not leave changes to each operator’s discretion.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
How many VFD systems are in operation in the field?

Saving Energy

Response:
There are dozens of these in operation from a number of vendors.  Some of these have automatic adjustment of he process parameters and some require the operator to make the adjustments.  The use of VFD’s has increased very rapidly in the last few years as a focus on power reduction has increased and the cost and size of VFD’s has decreased

Don Rainville: Energy Consultant, Novatec, Inc.

Question:
Are the Variable frequency Drives available for all desiccant wheel applications, even if the material to be dried is not PET?

Response:
They are available in most models and for both PET and other resins.  The emphasis on PET is a function of the energy used.  The savings are highest when drying temperatures are higher.  However, large central dryers can also benefit from this technique.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

Question:
What controls the demand speed of the Variable Frequency Drive on the regen blower?

Response:
This varies among vendors but controlling by the return air temperature from the hopper provides the most responsive and immediate means of monitoring and processing changes.  When the resin temperature or moisture changes, this shows up almost immediately in the return air temperature.

Mark Haynie: Drying Systems Product Manager, Novatec, Inc.

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