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Sahara Air Products

Economics of Air Drying

Presented in a 10-part series, this informative article

takes a look at wet compressed air and

how various types of dryers function to dry the air.

 

 

Part 8: Blower Purge

 

 

By Charles Henderson, Vice President

Henderson Engineering Co., Inc.

 

The blower purge is another type of heat reactivated dryer. This dryer is used when you do not want to lose any compressed air. Regeneration is accomplished by blowing ambient air into a heater, heating the air to 375°F, then passing this hot air into the regenerating tower. Because there is no purge air loss, the blower purge dryer is used to dry large volumes of compressed air. See Illustration 6.

Blower Purge Flow Schematic Acrobat icon Get Adobe Reader

Illustration 6: Blower Purge Flow Schematic

The Blower Purge dryer also operates on an 8 hour time cycle; 4 hours of drying, 3 hours of heating, and 1 hour of cooling. Normally an electric heater is used; however, if waste steam is available then this is usually more energy efficient. A source of 125 PSIG steam is the minimum requirement. When a steam heat exchanger is selected, it is important to bypass the heater during cooling. Simply shutting off the steam flow results in shortened life of the exchanger.

 

The Sahara Blower Purge dryer design incorporates a very unique feature to provide constant low dew points. Conventional Blower Purge dryers use the ambient air blower during both heating and cooling. Unfortunately, if the dryer is located in a hot, humid environment, during cooling the moisture present in the ambient air is deposited on the bottom of the desiccant bed. When the dryer shifts towers and that bed is used for drying, the moisture on the bottom of the bed is picked up by the air; thus there may be a fairly substantial dew point spike at tower shift. To prevent this, Sahara provides a selector switch on the control panel, allowing each user to choose either blower cooling or exhaust cooling. If exhaust cooling is selected then the blower and heater are both turned off after 3 hours of heating. A small (5%) purge of dry air then enters the regenerating tower and cools the desiccant bed. This exhaust cooling has the additional benefit of providing lower dew points than conventional blower cooling.

 

The operating cost of the blower purge dryer is determined by the blower HP and the heater KW. A blower purge dryer rated for 1000 SCFM would have a 3 HP blower and a 24 KW heater. The blower motor would consume approximately 3 KW and generate 180 SCFM at 32 oz. Of pressure. We can calculate the actual heater requirement for the blower purge just as we did for the exhaust purge. We see that the actual requirement is 15.6 KW.

 

The annual operating cost can be determined with the following:

 

 

 

 

 

 

 

 

 

 

 

The annual operating cost of the blower purge dryer is $6,438.60, based on .05/kw for electricity.

 

Learn more about Sahara's Blower Purge Air Dryer.

 

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