Moisture in Compressed Air contd. What is Dew Point?

By Sundar Mylavarapu

This always gets me going technical, but I will try not to lose you!

In the last blog, we stated that under the laws of vapors, the maximum amount of water vapor a given space can hold is dependent only on the vapor/space temperature. When a given space is holding the maximum amount of moisture in vapor form, it is considered saturated with the vapor and the pressure exerted by this vapor alone is termed as Saturated Vapor pressure Vps. There exists a unique relationship between the temperature of air and it’s Vps. The temperature of the space when it is saturated with water vapor is called it’s Dew Point.

From the above, two important observations can be made. First, when a given space of air is not holding the maximum amount of vapor that it is capable of, it is unsaturated, and the actual temperature of the air is greater than it’s Dew Point temperature, it’s RH is less than 100%, and the actual vapor pressure Vpa is less than the saturated vapor pressure Vps (RH, by definition, is the ratio of the actual vapor pressure to the saturated vapor pressure at a given temperature). Second, when the air is holding the maximum amount of vapor that it is capable of, it is saturated, and the actual temperature of air is equal to it’s Dew Point temperature, it’s RH is 100%, and the actual vapor pressure equals the saturated vapor pressure.

  • At Dew Point, air is holding the maximum amount of vapor, RH is 100%
  • Dew Point can be lower than or equal to the actual temperature. When it is lower, air is simply unsaturated
  • When unsaturated air is cooled, there comes a point when the actual temperature equals the Dew Point, the air is saturated and moisture begins to condense. Further cooling results in a lowering of the Dew Point and more moisture condensation.

Per Dalton’s Law of Partial Pressures, in an air-vapor mixture, each component has its own partial pressure. The total pressure for the mixture is the sum of the vapor pressures of the individual components. During compression of air, the principles of partial pressure are at work. When we say atmospheric pressure is 14.7 Psia, it is made up of two main components: the pressure of dry air and that of the water vapor. As the air is compressed, its pressure goes up and so does the vapor pressure in the same proportion. That means air Dew Point goes up with pressure and comes down with pressure!

Why Dew Point Matters

As compressed air cools after leaving the compressor, it becomes saturated at some point. In the earlier blog, I calculated and stated that ambient air at 95ºF and 90% RH, after compression to 100 Psig and cooled, becomes saturated at 168ºF. The after cooler cools the air to 100ºF (and in the process knocks out about 85% of the incoming moisture). The dew point of the air leaving an after cooler is 100ºF and it is 100% saturated. Should the air temperature drop below 100ºF, some of the remaining 15% moisture will begin to condense (recall that for a 25HP compressor, that is over 6 Gals of water per day!).

So how do you prevent this moisture from condensing downstream of the after cooler? Simply by removing as much of it as possible soon after the air leaves the compressor. To remove moisture from air, its dew point needs to be lowered. By lowering the dew point to several degrees below the lowest temperature the air is likely to be exposed to downstream of the compressor, we can prevent condensation from occurring in the air distribution lines. Instrument Air Quality Standard (ANSI/ISA-S7.0.01-1996) requires, as one of the four elements of instrument air quality for use in pneumatic instruments, that the Pressure Dew point of the compressed air shall be at least 18ºF below the minimum temperature to which any apart of the instrument air system is exposed to and shall not exceed 39ºF at line pressure.

Refrigerated Air Dryers

One way to lower the dew point is to further cool the air after it leaves the after cooler. Since you cannot cool the air any further below the ambient using an air cooled after cooler, we need to resort to refrigeration (hence Refrigerated Air Dryers). Theoretically, you can cool the air to very low temperatures so as to eliminate most of the remaining moisture. However, when the temperature drops below 32ºF, the moisture that condenses will freeze. Therefore, refrigerated air dryers are used to produce dew points in the 35ºF to 40ºF range. At these dew points, there is less than 4% residual water vapor left in the air.

The CAGI (Compressed Air and Gas Institute) Standard ADF100 for Refrigerated Dryers  requires them to be rated for Compressed air at dryer Inlet Pressure of 100 Psig, Inlet Temperature of 100°F,  Saturated or 100% RH and Ambient Temperature of 100°F. A Refrigerated Air Dryers Orlando must be sized carefully, and this especially matters most here in Florida, where the ambient temperature is above 90°F for almost 6 months in an year!

We have noted earlier that a good after cooler is the essential first step in ensuring dry compressed air. When the ambient is 90°F, the compressed air temperature exiting the after cooler (and entering the dryer) is 100°F assuming the after cooler has an approach of 10°F (Approach Temperature in an after cooler is the difference between the cooling fluid temperature i.e. ambient air and the cooled fluid temperature i.e. compressed air outlet). However, in most installations, the ambient temperature in the compressor room is higher than the atmospheric ambient. This is especially true in improperly ventilated compressor rooms, since air compressors reject enormous amounts of heat to the surroundings (each HP used for compression transforms into 2,545 BTUs/Hr of heat). Improper or inadequate ventilation will result in heat build-up and it is not uncommon to see compressor room temperatures of 110°F or higher in the summer months here in Florida, resulting in an Inlet Air Temperature of 120°F or higher into the Refrigerated Air Dryer.

The amount of moisture in air at 120°F is almost double that at 100°F. This results in a doubling of the latent heat load and a 20% increase in the sensible heat load on the Refrigerated Air Dryer. Additionally, the condensing temperature is also higher. The combination of these factors will cause the dryer’s capacity to drop by a whopping 40% or more! So when sizing Refrigerated Air Dryers, allowance must be made for these factors if dew point stability is desired. Otherwise, an overloaded dryer will barely produce 70°F dew point, resulting in lots of condensation in the air lines.

Leave a Reply

Your email address will not be published. Required fields are marked *

You may use these HTML tags and attributes: <a href="" title=""> <abbr title=""> <acronym title=""> <b> <blockquote cite=""> <cite> <code> <del datetime=""> <em> <i> <q cite=""> <strike> <strong>