Energy Choices for Plastic Processing
from Natural Gas Applications in Business & Industry, Spring 2003 (continued)



Process air passes through about 70% of the wheel's face. The wheel, made of titanium enhanced silica gel, adsorbs moisture from the air and delivers it both dryer and warmer.

"To get rid of that moisture," Siemasko continues, "a separate scavenger air stream that is typically taken from the outside is heated to between 250 and 300 degrees. This airstream is passed over a separate quadrant of the wheel to drive out the water vapor. This warm, wet airstream is then ducted to the outside. Simplistically, moisture is being transferred from one air stream to another."

"The process air stream would be the air stream that we're dehumidifying," explains Todd Bradley, President of dehumidifier manufacturer Controlled Dehumidification IMS "The reactivation air is a separate air stream that's heated to continuously dry the water vapor off of the desiccant wheel.

"We use a desiccant dehumidifier coupled to a pre-cooling coil. The cooling coil cools the ambient or plant air then delivers it to the desiccant dehumidifier, which takes water out of the air, then delivers the air at the proper dew point directly into the mold."
The reactivation air has to be hot. "We take plant air or outside air, heat it up to about 280°F, and send it through about 30% of the desiccant wheel," Bradley continues, "then discharge it to the outdoors."

How do they heat the reactivation air to that high temperature? "The most cost-effective way is with a direct fired natural gas burner," says Bradley."The burner is 99% efficient, so you get the most cost-effective heat that way. Other options are electricity, steam or thermal fluids.

"Natural gas is typically the least costly avenue," Bradley reports. "Electricity is almost always more costly. Steam would be another choice if it is available, but generally these facilities don't have a highpressure steam boiler."

Thermal fluids, such as thermal oils, have a high specific heat capacity and don't vaporize at the temperatures involved. Since they don't vaporize, they can carry heat through pipes to a heat exchanger without building pressure.

"Typically," says Bradley, "you have thermal fluids operating at 350° to 400°F without generating pressure."

The heat for the thermal fluid, of course, has to come from somewhere. In the end, you go back to the choice between natural gas and electricity, although burning fuel oil might be another option. Direct-fired gas technology is very simple, easy to maintain and very efficient in the transfer of gas to heat. A gas burner upstream of the desiccant wheel produces the heat. Makeup air then mixes with the burner's exhaust gas to cool the stream to the proper temperature. The diluted exhaust then goes directly through the desiccant wheel, pulling out the moisture before exiting through a stack.

Natural gas' clean-burning characteristic makes it possible to use direct-fired technology without either fouling the desiccant wheel or carrying noxious combustion products into the mold enclosure.

Plastic-product manufacturers like Graham have found that surrounding their blow-molding machines with dry air speeds up production while improving yields. Dry air's low dew point allows them to cool their molds to a lower temperature so that the soft plastic hardens faster, while preventing condensation droplets from forming inside the mold. No condensation means no surface-degrading ripples and pits marring the finish, higher yields of good product and less scrap.

Using gas technology to dry the air means low pollution, low energy costs and high reliability. Ultimately, they can produce a better product faster and at a lower cost, all of which means more profit.