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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 1: Wet Compressed Air

 

 

By Charles Henderson, Vice President

Henderson Engineering Co., Inc.

 

Compressed air must be dried. This is an undeniable statement of fact. Today's modern industry can no longer tolerate the problems of wet, dirty compressed air. Wet air causes rust, pitting, blockages, and freeze-ups, with resultant component failure and product rejection. Wet air is a major contributor of downtime, causing millions of dollars of lost production.

 

The cost of wet air varies with each and every application. However, the cost of wet air is always many times greater than the cost of a dryer. Compressed air dryers pay for themselves, many times over, by increasing production, reducing downtime and adding to your bottom line. The tough question you have to answer is which type of dryer is the best choice for your application.

 

There are three basic types of compressed air dryers: deliquescent, refrigerant, and regenerative. The dryer you should select is determined by answering the question, "How dry does my air need to be?" The Instrument Society of America has a standard for instrument air that can be applied to many non-instrument applications as well. The standard states “Air shall be dried to a dew point 10°C (18°F) below the lowest ambient temperature the air will encounter. In no event shall the dew point exceed 2°C (35°F).” Many companies have adopted this standard and even vary the dew point requirements with the season.

 

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There is no real advantage to over-specifying a dryer, because as long as the water in your compressed air lines remains in a vapor form, it will not harm your compressed air machinery. Only when water reaches its dew point and condenses from a vapor into a liquid do you have problems. Generally speaking, over-specifying the required dew point doesn’t provide tangible benefits. On the other hand, given the high cost of wet air, making sure you buy a dryer that completely eliminates all wet air problems is a pretty good idea.

 

The reason we specify a dew point 10°C (18°F) below the lowest ambient is to account for adiabatic expansion, the cooling that occurs when air is rapidly expanded to atmospheric pressure. A good example of this is a spray gun. When the air goes shooting out the spray gun, it cools; if the air has not been adequately dried, it will reach its dew point and water droplets will mix in with the paint, ruining the paint job.

 

If your air is used indoors for general shop air and does not encounter any special conditions, the dew point from a deliquescent or refrigerant dryer should be adequate. Outdoor lines, instruments, and special applications require the dew point from a regenerative dryer. See Table 1 below.

 

 

 

 

 

 

 

 

 

 

In order to properly size a dryer, you need to know three things: (1) maximum amount of air to be dried in SCFM, (2) minimum inlet air pressure in PSIG, and (3) maximum inlet air temperature in °F.

 

The reason you need these numbers is quite simple. First, you need to know specifically how much air you want dried. If you want to dry the outlet of a single compressor, size the dryer for the maximum output of that compressor. For multiple compressors, add up the total flow. All dryers are designed to operate from 0 to 100% of design flow, so we need only be concerned with the maximum air flow.

 

The pressure affects several things; the air's capacity to hold moisture varies with the pressure, as does its velocity. Most dryer manufacturers design their dryers with a standard rating of 100 PSIG. If your minimum inlet air pressure is above 100 PSIG, typically you are able to reduce the size of the dryer. The reverse of this is also true. If your minimum pressure is below 100 PSIG, you must increase the size of the dryer. The sizing factor is a pressure ratio. You can size for pressure yourself using the following formula:

 

 

 

 

 

 

 

 

 

 

The inlet temperature is perhaps the most important factor when sizing the dryer. The air's capacity to hold moisture varies directly with its temperature. In the comfort zone of temperatures, a 20°F increase in temperature will nearly double the air's moisture holding capacity. This means that if you size a dryer for 80°F and the actual operating temperature turns out to be 100°F, you've doubled the amount of water going into the dryer.

 

When sizing either refrigerant or regenerative dryers, you must account for the air temperature with a temperature modifier. Many dryer companies use 100°F as their standard. If your inlet air temperature is above 100°F, then you must increase the size of the dryer to compensate for the increased water load. Conversely, if the maximum inlet air temperature is below 100°F, the dryer size may be commensurately reduced. See Table 2 below.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

The temperature does not affect the sizing of the deliquescent dryer; however, it does have a considerable impact on performance. Before we can dry the air, we must cool it in the aftercooler. The choice of coolers is nearly as important as the dryer, since you want to reduce the air temperature as much as possible. Also, if your compressor is lubricated, we will need to filter the oil and dirt out of the lines.

 

Remember that the dryer is designed to remove water, not oil. If the oil is not filtered, it will reduce the efficiency of the dryer. Some filter salesmen try to sell their filters by saying they dry air. This is simply not true. A filter will remove liquid water that has already condensed, but it cannot remove any moisture vapor. A dryer by definition is a device which lowers the dew point of air. Filters are an essential component of the well designed compressed air system; however, they are not the solution to your wet air problems.

 

Let's assume that we want to dry 1000 SCFM at 100 PSIG and 100°F. Since temperature is not a factor in sizing deliquescent dryers, all we need to do is multiply the SCFM by our pressure modifiers; in this case, the modifier is 1.

 

046-1 Table 1 046-1 - Formula 046-1 - Table 2

The temperature does not affect the sizing of the deliquescent dryer; however, it does have a considerable impact on performance. Before we can dry the air, we must cool it in the aftercooler. The choice of coolers is nearly as important as the dryer, since you want to reduce the air temperature as much as possible. Also, if your compressor is lubricated, we will need to filter the oil and dirt out of the lines.

 

Remember that the dryer is designed to remove water, not oil. If the oil is not filtered, it will reduce the efficiency of the dryer. Some filter salesmen try to sell their filters by saying they dry air. This is simply not true. A filter will remove liquid water that has already condensed, but it cannot remove any moisture vapor. A dryer by definition is a device which lowers the dew point of air. Filters are an essential component of the well designed compressed air system; however, they are not the solution to your wet air problems.

 

Let's assume that we want to dry 1000 SCFM at 100 PSIG and 100°F. Since temperature is not a factor in sizing deliquescent dryers, all we need to do is multiply the SCFM by our pressure modifiers; in this case, the modifier is 1.