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Economics of Air Drying

10-Part Series

- Part 1: Wet Compressed Air
- Part 2: Deliquescent
- Part 3: Refrigerant
- Part 4: Regenerative
- Part 5: Heatless
- Part 6: Heat Reactivated
- Part 7: Exhaust Purge
- Part 8: Blower Purge
- Part 9: Closed System
- Part 10: Heat-of-Compression

*Complete article available in PDF format:* Ref. 046 - 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.

**By Charles Henderson, Vice President**

**Henderson Engineering Co., Inc.**

Because of the large purge loss of the heatless, many customers look at the heat reactivated type of regenerative dryers. These are the exhaust purge, blower purge, closed system, and the Sahara-Pak™ heat-of-compression.Heat reactivated dryers regenerate by passing hot air over the saturated desiccant bed. Air has a varying capacity to hold moisture; the hotter the air is, the more water it can hold. So by heating the regeneration air, we are able to either reduce or eliminate completely the need for purge air loss. This is significant when dealing with large volumes of compressed air.

The sizing of a heat reactivated dryer is the same as a refrigerant; you multiply the flow rate by a pressure and temperature modifier to arrive at a corrected flow rate.

If you go back to Table 2 for just a second, take a look at the difference between an inlet temperature of 120°F, 100°F and 60°F. Big difference. Remember that the air's capacity to hold water doubles with every 20°F increase. Size a dryer for 100°F and give it 120°F and you need twice as much desiccant to hold that water. When designing a compressed air system, you must do everything possible to reduce the inlet air temperature entering the drying tower. Using chilled water has a tremendous impact on dew point performance, life of components, and initial cost; you save more money than you spend. Conversely, skimping on your cooling system and having hot water with a high approach aftercooler leads you down the road to ruin. The single most important factor in a drying system is the temperature of the air.

When sizing any regenerative dryer, there are several factors that should also be mentioned; velocity, contact time, desiccant capacity, and pressure drop. These items should be considered by the manufacturer. However, a prudent engineer should double check the manufacturer's figures to guarantee proper sizing of the dryer. This becomes particularly important if the pressure or temperature modifiers vary greatly from the standards. For example, if our inlet temperature was 60°F, we could reduce the size of the heat reactivated dryer by nearly 2/3, due to the reduced water load. However, the velocity through the bed would be too high and would fluidize the desiccant, the contact time would be too low to obtain a good dew point, and the pressure drop would be excessive.

You can calculate the flow velocity with the following formula:

You can calculate the tower area, if you know the tower diameter with the following formula:

A heat reactivated dryer rated for 1000 SCFM would typically have a 24 inch diameter tower. So, using our tower area formula, we arrive with a tower area of 3.14 sq. ft. We can now calculate the velocity to see if the desiccant will be fluidized. We see that the flow velocity would be 33.5 feet per minute. This is an acceptable flow velocity. Generally, velocity shall not exceed 60 feet per minute.

Most dryer manufacturers use 3/16 inch spherical activated alumina desiccant. Experience has shown this to be the best choice of regenerative desiccants. It has a dynamic design capacity of 24%; that is, it will hold 24% of its weight in water.

If the velocity of the air is high, it's a pretty sure bet that the contact time will be too low. The air must be in contact with the desiccant for at least 3 seconds in order for the desiccant to pull the water out of the air in sufficient quantity to give a good dew point. You can calculate the contact time with the following formula:

A heat reactivated dryer rated for 1000 SCFM would hold approximately 530 pounds of activated alumina per tower. The contact time would be 5.9 seconds; which is acceptable.

Most dryers are designed with a 3-5 pound pressure drop. You can calculate the approximate pressure drop using the following formula:

The line size on a dryer rated for 1000 SCFM would be 3 inches. The pressure drop would be .96 PSIG.

By going through these calculations, you can guarantee that the dryer you purchase will be adequate for your current needs and will even handle future expansion.

There are two basic methods of regenerating the desiccant with a heated dryer; they are convection and conduction.

A convection system heats a stream of air and allows the hot air to heat and regenerate the desiccant.

A conduction system typically has multiple heaters embedded throughout the desiccant bed and conducts heat through the desiccant. A small amount of air is required to pick up the moisture that is being regenerated off the desiccant.

There are several serious mechanical problems that can occur with strictly conduction heating. The desiccant itself is a very poor conductor of heat. In order to distribute heat throughout the desiccant bed, you generate very high temperatures right at the heater. This drastically reduces desiccant life. Heater tubes expand and contract and can crush desiccant, the tubes can break and hot spots develop on the heater element. Typically this design has very high discharge temperatures at tower shift as well as elevated dew points. A much better approach is convection; using the heater to increase the temperature of the air and then allowing the hot air to regenerate the desiccant.

Heat reactivated dryers operate on a much longer time cycle than heatless dryers do. Typically, the heated dryer uses an 8 hour cycle, versus a 4 or 8 minute cycle for the heatless. This is necessary because of the time involved in heating and cooling the desiccant during regeneration.