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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 10: Heat-of-Compression

 

 

By Charles Henderson, Vice President

Henderson Engineering Co., Inc.

 

All of the dryers we’ve discussed so far share one common denominator; they all are designed to operate downstream of the aftercooler. But what’s happening back at the compressor? Is the air exiting the compressor saturated? The answer is no; remember the varying capacity of air to hold water at elevated temperatures. Air exiting a compressor is definitely not saturated and can in fact hold a tremendous amount of water. All of the conventional dryer designs were built to operate at the conditions following the aftercooler. Back in the 1970’s, Sahara started manufacturing compressor/dryer packages. It became obvious that a good potential source of energy was being wasted.

 

Heat is energy. During the compression process heat is essentially a by-product of compression. Air exiting a compressor is hot. Because of the high temperature the air isn’t wet. When this hot, dry air is put into an aftercooler, all of the energy is wasted. With a regenerative dryer, a new source of energy must be provided to regenerate the desiccant.

 

Back in the 70’s, Sahara developed and patented the Heat-of-Compression dryer design. We called it the Sahara-Pak, because it was a complete package; an aftercooler and dryer, specifically designed to function at peak efficiency.

 

By utilizing the normally wasted heat-of-compression to regenerate, the Sahara-Pak does not lose any compressed air and requires no electricity for either heaters or blowers. The only power required is approximately 27 watts which operates the timer and solenoid valves. See Illustration 8.

Acrobat icon Get Adobe Reader Heat-of-Compression Flow Schematic

Illustration 8: Heat-of-Compression Flow Schematic

Air enters the Sahara-Pak directly from the compressor. This hot, thirsty air is directed into the regenerating tower, where it removes the moisture from the desiccant bed. This cooler, wetter air now enters an aftercooler where it is cooled down to approximately 100°F. Moisture is condensed and removed in a coalescing separator with a dual drain trap. All the liquid water is removed through the drain traps. By using two separate traps, a mechanical and electrical trap, we are assured of continual draining. If the primary mechanical trap should fail, the liquid backs up into the electrical trap where it will be drained. Should the backup electric trap ever have to work, it will signal a primary drain failure alarm light and horn on the dryer control panel.

 

Now that the liquid has been separated and drained, we can dry the air. The air now goes into the drying tower where it reaches its final dew point. At a preset interval, the valves shift, diverting air into the opposite tower. The Sahara-Pak is one of the simplest, most trouble-free type of dryers to operate, and because of its energy-saving design, has become extremely popular. The major limitation of this dryer is that is must be located near the compressor and is, therefore, not suitable for point of use applications. Also, the compressor must be oil-free.

 

The outlet dew point is determined by three factors; ambient dew point, inlet air temperature, and cooling water temperature. The hotter the air is going into the dryer, the lower the dewpoint will be going out. The dewpoint is also affected by the temperature of the air exiting the cooler. Here we want it to be as cold as possible.

 

One other factor with the Sahara-Pak, just like other heated dryers, is temperature and dew point bump. This occurs right at the dryer, so we recommend installing a dry air receiver downstream of the dryer to allow the air a chance to mix, to assure a low dew point all of the time.

 

The Sahara Pak SP design is considered to be an instrument air dryer in that it delivers air in accordance with the requirements of the Instrument Society of America. While most customers can accept temporary dew point excursions, some process applications require a constant low dew point. For these applications Sahara developed the HC design.

 

The HC dryer operates through 3 separate cycles; heating, stripping and cooling. Illustration 9 shows the heating cycle. The heating cycle of the HC dryer is identical to the cycle of the SP; all of the hot air from the compressor enters the regenerating tower and regenerates the desiccant. From there it enters the aftercooler where it is cooled to as low as possible, at least 100°F. Condensed moisture is removed in the separator and drain traps. The cool wet air flows up through the drying tower where it is dried. The heating cycle lasts for 90 minutes.

 

Heat-of-Compression Heating Cycle

Illustration 9: Heat-of-Compression Heating Cycle

At the end of heating the HC design enters the stripping cycle. See Illustration 10. During stripping, the hot air from the compressor is directed into the aftercooler, separator, and drying tower. A small amount of dry purge air (2%) is expanded to atmospheric pressure and is used to purge the regenerating tower. Stripping lasts for 90 minutes. The amount of purge is small, only 2% and only lasts for 90 minutes every 4 hours, or longer if the dryer is equipped with the dew point demand system. The cost of the purge air is negligible when compared to other types of dryers. An additional benefit of the stripping flow is enhanced regeneration. The HC is actually capable of delivering dew points 30°F lower than the SP design.

Heat-of-Compression Stripping Cycle

Illustration 10: Heat-of-Compression Stripping Cycle

After stripping, the HC dryer enters the cooling cycle. See Illustration 11. During cooling, the hot air from the compressor enters the aftercooler, separator, and drying tower. At the outlet of the dryer, about 25% of the dry air is directed into the regenerating tower to cool the desiccant bed. This cooling air flow rejoins the process flow at the outlet of the dryer, so there is no air lost during cooling. After 60 minutes of cooling the dryer is ready to shift towers. Usually all regenerative dryers are purchased with the optional dew point demand system.  This is a direct reading dew point meter that optimizes the energy consumption of the dryer and switches towers based on outlet dew point rather than time. The dryer should remain on the drying tower for hours waiting for the dew point to increase.

Heat-of-Compression Cooling Cycle

Illustration 11: Heat-of-Compression Cooling Cycle

Table 5 shows a cost comparison of regenerative dryers. Note that all dryers require a small amount of electricity to operate the timer and solenoids.

 

 

 

 

 

 

 

 

 

 

Table 5: Cost comparison of Regenerative Dryers. (Does not include replacing desiccant.)

 

 

Since this is the same for all, we cancelled it out. In reality, this costs less than $10/year.

 

In conclusion, we believe that while any dryer will improve plant production enough to pay for itself many times over, it is the job of both the dryer manufacturer and the plant engineer to select not only the dryer that will deliver adequate dew points and assure dry air, but also have the lowest operating cost.

 

*   Costs based on 1000 SCFM dryer operating around the clock 365 days.

     Purge air at $.25/1000 SCF; electricity at $.05/KWH.

     Does not include maintenance costs.

**  Average purge loss.

*** Can use up to 5% purge during cooling, if exhaust purge cooling mode is selected.

 

 

Learn more about Sahara's Heat-of-Compression Air Dryers.

046-10 - Table 5