Moisture in Compressed Air

By Sundar Mylavarapu

We are in the middle of a hot and humid summer here in Florida, and so I thought this would an appropriate topic of discussion to start our blog!

graphMoisture in the form of Water Vapor is inherently present in atmospheric air. We in Florida know that more than most others! Under the Laws of Vapors, the maximum amount of Water Vapor the atmosphere can hold is dependent only on the ambient temperature.  It increases with the temperature almost exponentially, as illustrated in the adjacent chart!

An air compressor’s source of air is the atmosphere, and the amount of water vapor ingested by it is dependent on the ambient temperature (T), relative humidity (RH), and the capacity (Cfm) or size (HP) of the air compressor.

As air is compressed, it’s volume reduces. At 100 psig pressure, the compression ratio is 7.8 and volume reduction is 87%. Although it may feel obvious that with compression and consequent reduction in volume, some moisture would be squeezed out of the air, it is seldom the case. This is because, with compression, there is a substantial increase in the air temperature. All of the Horsepower used in compression heats the air. The temperature rise will more than compensate for the volume reduction and will result in the compressed air leaving the compressor unsaturated, meaning the RH is still less than 100%.

sundarAs an example, 100 Cubic Feet of ambient air at 95ºF and 90% RH (common in Florida in the summer months) holds about 3.8 Ozs of water vapor. When compressed to 100 Psig, air temperature may rise to almost 500ºF, depending on the type of compressor. The compressed air will not saturate unless it has cooled to 168ºF! Once the air temperature starts to drop below the saturation temperature, water vapor will start to condense as liquid water. At 100ºF, the compressed air holds about 0.6 Ozs of water vapor, meaning 3.2 Ozs or 85% of the incoming water will drop off as condensate! Note that this is per each 100 Cft of ambient air. A 25HP air compressor that has a capacity of 100 Cfm is ingesting 3.8 Ozs x 1440 mins/Day (about 42 Gals) of water a day!

Therefore, the easiest and most economical way to remove moisture from compressed air is by using the ambient air to cool it. An air cooled After Cooler (a radiator) can go a long way in condensing a bulk of the water vapor in the compressed air.

water contentThe adjacent Table 1 illustrates the amount of moisture an air compressor ingests at various ambient conditions and how much of that moisture is condensed and removed by a good After Cooler.

A good after cooler is the essential first step in ensuring dry compressed air. It is therefore imperative that an after cooler, if already present, be kept clean, especially where the environment is dirty. Regular cleaning and occasional pressure washing (using an industrial detergent/degreaser like Simple Green) of the cooler will go a long way in ensuring moisture-free air in the summer months. If an after cooler is not present (as is the case in most small HP Piston or Reciprocating units), it must be added to the system before any dryers are added.

We will discuss more about the need for air dryers Orlando, the importance of proper sizing and application of them, specially here in hot & humid Florida, in our next blog.

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