By Ron Marshall for the Compressed Air Challenge
A mineral processor purchased a VSD controlled compressor for their plant, to save energy. This unit very nicely supplied the variable compressed air load, automatically ramping up to full speed when peak demands required full flow, but efficiently turning down to low speed when normal demands consumed low flow.
When deciding to install this compressor, the plant personnel chose the existing compressor room, which was a modular skid package connected to the bagging area of the plant. Next door to this location was the truck loading area, where the powdered product was loaded onto large trucks. Very often, significant spills occurred, which are typically cleaned using compressed air—sending large clouds of fine powder into the air near the compressor area.
The modular skid package used for a compressor room has very poor ventilation. In order to keep the compressors from overheating, the doors of the area must be kept open all summer long. In winter, the environment is so cold the compressor ventilation air must be drawn from a heated—but dusty—bagging area.
When new, the VSD air compressor and desiccant dryer operated very efficiently. Over time, however, the dust being drawn into the compressor area started to build up on the fins of the compressor oil cooler and aftercooler, causing overheating. The cooling fins inside the VSD module were also affected, which caused frequent nuisance trips of the compressor. The compressor ingested the dust, which clogged the inlet filters and formed a sludge in the compressor oil and condensate drains. The failure of the drains allowed free water to travel to the desiccant dryer, which contaminated the desiccant and fouled the dew point control system that is intended to reduce dryer purge flow.
Needless to say, the VSD module finally failed a few years after installation. Rather than repairing it, it was simply removed and the compressor was switched to fixed speed operation. Unfortunately, this reduced the compressor capacity by 25% as the compressor design requires the motor to over speed to 80 Hz at full load. The lower of capacity caused the need for a second compressor to run at light loading, greatly reducing system efficiency. Estimated system power consumption due to these conditions is 60 kW per 100 cfm of compressed air consumed, more than twice what is considered normal!
Repairs and redesign are ongoing, but plans are to relocate the air compressors to a location away from dusty conditions inside a well-ventilated area where the heat of compression can be used to supplement building heat. Secondary filtration of the compressor room ventilation air is being added to provide a more suitable environment for air cooled air compressors. The VSD module will be replaced and located in a clean and cool electrical room. With relocation and good design practices, the system operation will return to normal in the coming year.
Learn more about compressor control in our next Compressed Air Challenge seminar in your area. Visit www.compressedairchallenge.org for more information.
Filed Under: Pneumatic Tips