The Economics of Air Drying-Part 7, Presented by Sahara Air Products

The exhaust purge dryers are similar to the heatless in that there is a purge air loss; however, because we are heating the purge air and desiccant, we can reduce the purge air to 7% of the dryer flow rate By reducing the purge loss by half, we greatly reduce the operating cost of the dryer. See Illustration 5 below.

Wet air enters the dryer and is diverted into the drying tower where it is dried to its -40°F dewpoint. At the outlet of the dryer, 7% of this dried air is diverted into the regenerating tower where it is heated and expanded to atmospheric pressure. This hot dry air comes into equilibrium with the desiccant and regenerates it back to full capacity. The wet air is then exhausted to atmosphere. We heat the desiccant for 3 hours to fully regenerate it, then turn off the heater for the last hour prior to tower shift to cool the bed down. The reason for this is that the desiccant is not effective if its temperature is above 140°F. So we cool the bed down with cool, dry air. This minimizes any temperature or dewpoint spikes at tower shift.

The operating cost of the exhaust purge dryer is determined by the purge air loss and the electricity used by the heaters. You can calculate your purge loss by multiplying the modified flow rate (1000) by .07 to arrive at a purge loss of 70 SCFM.

An exhaust purge dryer rated for 1000 SCFM would typically have a 12 KW heater. The heaters used are normally incoloy sheathed and derated to 14 watts per square inch. These heaters are capable of our 1000°F; however, we only need heat the purge air to 375°F to assure good regeneration. Usually, there will be some excess capacity in the heater KW, and this can be calculated using the following formula:

KW = (PR x 1.08 x TD) / 3412

Where: KW = actual KW required

PR = purge rate in SCFM

TD = temperature differential between
375°F and inlet air temperature

We see that we only need 6 KW to heat 70 SCFM from 100°F to 375°F. We can now calculate the operating cost of the dryer with the following formula:

C = ([PR x 525600] / 1000) x CA + (KW x 6570 x E)

Where: C = annual cost

PR = purge rate in SCFM

CA = cost of compressed air per 1000 cu. ft.

KW = actual KW required

E = cost of electricity

The total cost to operate the exhaust purge dryer for one year is $11,169, using .05/kw for electricity.