Rock-steady dryer performance now available at lower cost.

When product quality demands uniform process conditions, a new type of closed-loop drying may be your answer. "Pulse Cooling Technology," or PCT, is an enhancement for twin-tower dryers from Universal Dynamics, Inc., Woodbridge, Va. President Donald Rainville says PCT eliminates heat spikes and cyclic swings in dew-point and air flow that occur when the dryer switches desiccant beds.

"This development was driven by makers of PET bottle preforms who wanted better process stability," Rainville says. He sees plenty of other applications for PCT where quality depends on highly uniform process conditions.

No more spikes or swings

When a conventional twin-tower dryer changes desiccant beds, three things typically can happen:

* Air flow increases, leading to a drop in process-air temperature of as much as 10 [degrees] F, which can last from 10 to 40 min.

* Alternatively, there may be a heat spike because the newly regenerated desiccant bed may be 200 [degrees] F hotter than the bed that just went off-line. "Some materials can't take that heat shock," notes Rainville. For example, the consequence for PET preform molders can be higher acetaldehyde content in critical water-bottle applications.

* The conventional use of ambient air to cool the regenerated bed puts moisture back into the bed, typically resulting in a dewpoint drift between -112 F and -50 F when the beds are changed.

PCT, Rainville says, involves new positive-sealing changeover valves that reportedly keep air flow constant during bed changes. Also, PCT directs cooled process air over the regenerated bed to equalize its temperature with that of the in-process bed. With PCT, Una-Dyn guarantees [+ or -] 1 [degrees] C material-temperature uniformity at the feedthroat. In addition, the use of dry process air instead of moist ambient air is said to maintain dewpoint within [+ or -]15 [degrees] F instead of [+ or -]30 [degrees] F without PCT.

"No one, to our knowledge, has accomplished closed-loop cooling with process air on a twin-tower dryer before," says Rainville. "Closed-loop for twin towers has meant using a water-cooling coil and recirculating the same cooled air over and over. It involves more hardware expense and actually causes more variation in temperature and dewpoint than open-loop cooling." The PCT system lets dryers to run at temperatures as low as 150 F without the expense of a cooling coil.

Una-Dyn acknowledges that closed-loop cooling with process air is already available on some rotary-bed dryers from other suppliers. Those units have good process stability, but are more expensive, says Rainville. "Now twin-tower units can have the same or better performance as rotaries, but at lower cost," he claims.

Stability at no extra cost

PCT is now standard on the UDC Series single-blower dryers, which come in 15-cfm and 35cfm sizes. On the larger DHD Series of twin-blower models, PCT is an option that adds less than 5% cost premium.

Una-Dyn plans to extend its UDC series into larger sizes that overlap the DHD Series. Thanks to more advanced valving and internal heaters in the desiccant beds, less air flow is now required for regeneration. "Now we can go larger on single-blower units, which are more compact and less expensive than comparable twin-blower dryers," Rainville explains. Una-Dyn plans to build UDC models in sizes of 55, 100, 150, and 250 cfm. A 350-cfm or even larger unit may be possible.