The ‘United States Postal Service’s New Jersey International & Bulk Mail Centre includes three main buildings that occupy almost 170,000m² and the problem of heating the building effectively during the winter months is exacerbated by the fact that there are 72 bay doors opening directly on to the outside.
Each bay employs an air-curtain that brings down a blast of high pressure heated air that effectively isolates the interior and exterior air volume, while the bay door is open. Prior to the smv drives being fitted, the air-curtains operated at a fixed speed, with the fans pushing at 1200rpm. At this speed, and temperatures below freezing the blast of high velocity air was actually forcing cold air in at ground level, ten feet below the unit.
In dry conditions, the high pressure blasts from the air curtains also picked up unwanted. Workers felt like the unit was blowing cold air instead of warm air and so had turned the air curtains off. The onsite maintenance team carried out various tests and concluded that variable speed operation was the best method of solving the problem.
The solution was provided by the local Lenze automation office, a simple AC drive that was protected against the elements and so could be retrofitted without the cost of a complete new panel and external enclosure for each motor, the smv drive offers IP65 protection and was simple enough to be fitted and commissioned by the on-site electrical and maintenance teams.
Pleased by the fact that this automation solution could be applied by an in-house team, that would gain skills as a result, the go ahead was given for a test installation on one bay door, the drive was provided on a ‘work or return’ basis and the local crew received detailed technical support in order to select EMC filters, the frequency ranges to pick to reduce total harmonic distortion, and DC injection braking kits to keep deceleration intime with the automatic doors closing, plus advice on the wiring and set-up procedure.
The test bay was metered over a period of weeks and results showed that that the smv vector drives were very effective in improving surrounding temperature from 18°C to a toasty 25°C, especially, when outdoor temperatures were -5°C.
The ampere draw for three drives at 5.5kW each was on average 4.81amps, the total harmonic varied from 20 to 70 % and power factor improvement was approximately 34 %. Based on a US electricity tariff rate of approximately €0,08 per kWh and a 3600hour annual operating period, (the sortation centre works 24/7), the utility bills will be reduced by €23,000 in one season when all 50 remaining bays have been completed.
