Call toll free: 800-544-4379

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 3: Refrigerant

 

 

By Charles Henderson, Vice President

Henderson Engineering Co., Inc.

 

The refrigerant dryer reduces the dew point of the air, by reducing the temperature of the air, then separates out the condensed water. The refrigerant dryer is sized using both the pressure and temperature modifiers. Using our example of 1000 SCFM, 100 PSIG, 100°F, and the following formula, we determine the correct dryer size:

 

 

 

 

 

 

 

 

 

 

 

 

Thus, the refrigerant dryer should be sized for 1000 SCFM.

 

The refrigerant dryer is considerably more complex than the deliquescent and is, therefore, prone to mechanical failure. Wet air enters the refrigerant dryer through an air to air heat exchanger, where the temperature of the inlet air is reduced. This cooler air now goes into a freon to air heat exchanger where the temperature is reduced even further. At this point, the temperature of the air can be either +50°F or +35°F, depending on the dew point capabilities of the dryer. The moisture in the air has been condensed into liquid and is removed in a separator. The dry air now goes out of the dryer through the air to air heat exchanger where the temperature of the air is increased, typically to 70°F. See Illustration 2.

 

046-3 - Formula Acrobat icon Get Adobe Reader Refrigerant - Flow

Illustration 2 Legend

1. Air to air heat exchanger

2. Separator

3. Air to refrigerant heat exchanger

4. Hot gas bypass valve

5. Liquid receiving tank

6. Hand shut-off valve

7. Air-cooled condenser

8. Refrigerant compressor

9. Expansion valve

10. Suction line heat exchanger

11. Filter

Illustration 2: Refrigerant Dryer

On large refrigerant dryers, an air to water heat exchanger may be used instead of the air to air exchanger. The refrigerant dryer consumes electricity to operate the freon compressor and fans on the heat exchanger and also consumes water where a water to air exchanger is used.

 

Our example of a dryer rated for 1000 SCFM would typically have a 5 HP motor on the compressor and two 1/4 HP motors on the fans. If we assume that electricity costs .05/KWH, then the annual operating cost for this dryer used around the clock would be $2,409.00 (5.5 KW x 8760 hours x .05/KWH).

 

There are a few drawbacks with the refrigerant dryer. It's common for a leak to develop in the freon system and once the freon leaks out of the dryer, it no longer functions. In addition, we have found that the actual performance is not as good as most manufacturers claim. The separators are not 100% efficient; they are, in fact, maybe 70-75% efficient, especially with varying flow rates, which means that the condensed water is not being removed, and is re-entrained back in the air stream. We have found that the actual dew points out of refrigerant dryer are approximately +40°F or +60°F.

 

If these dryers are delivering the specified dew points then they are acceptable; the problem is what happens if they don’t perform up to spec. Then you may have very little margin for error.

 

Another problem is the choice of Freon. Several refrigerants have been banned and their replacements don’t perform as well and are much more costly.

 

One of the biggest failures of refrigerant dryers is as simple as the drain trap. Drain traps don’t work very well. Manufacturers constantly try to reduce cost to be more competitive. Where do they skimp? On the drain trap. If a chain is as strong as it’s weakest link, then the poor drain trap is certainly the biggest problem with refrigerant dryers. If the trap doesn’t work, the entire dryer doesn’t work. Water exits the dryer at only one place; the drain trap. If you don’t see water coming out of the trap you might as well unplug the dryer.

 

Another limitation of this dryer is that the dew point cannot go below 32°F under any conditions, so that there is no way this dryer can prevent freeze-ups of outdoor lines. We would recommend the refrigerant dryer only be used indoors to protect against water getting in the plant air system.

 

While we have spent several pages discussing the operation and relative merits of the deliquescent and refrigerant dryer, the truth of the matter is that both of them are really obsolete. If all air lines are indoors and you don’t have a specific application that requires lower dew points, then the performance of a deliquescent or refrigerant dryer should be acceptable. But what happens if the dew point goes up? Even a little bit?

 

Table 3 illustrates the amount of water present in compressed air at varying dew points, both in terms of gallons of water and percentage remaining.

046-3 - Table

As you can see, the dew point performance from both a deliquescent and refrigerant dryer still leaves a tremendous amount of water in the compressed air. The absolute lowest attainable dew point from a refrigerant dryer means that over 10% of the water stays in the air; with a typical 1,000 SCFM system, operating around the clock, this means that there is 2,608 gallons of water still present in the air. If the ambient never drops below 55 F then this should be acceptable, but what happens in you have a problem. If there’s a Freon leak or, a trap gets plugged or, the dryer hiccups or you’re having a bad hair day, the dew point can climb up. If it climbs up 25 degrees, up to 60°F, then you now have over 6,000 gallons of water flooding your air lines. Open a valve and your shoes get wet. Your instruments freeze, your tools rust. Production grinds to a halt.

 

There are different ways of looking at this situation. While we don’t want to over-specify, we also don’t want to get a phone call in the middle of the night because the plant is shut down. Even operating at peak efficiency, the refrigerant dryer puts more than 50 times more water in your air line than a regenerative dryer does. That really doesn’t leave you any margin for error. Everything has to be perfect or you’ve got real problems. Perhaps the best solution is to use a dryer that gives you more; more breathing room, an opportunity to recover if there’s a problem.

 

The ideal solution is to provide a dryer with a dew point low enough that if it hiccups you’re still in operation. The solution is a regenerative dryer.