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.

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.

Introduction to Florida Air Technologies, LLC

By Sundar Mylavarapu

Hi fellow bloggers,

This is our first blog post and we are excited about sharing our expertise with other bloggers.

Florida Air Technologies is an authorized Distributor of Gardner Denver Compressor parts in Western, Central, North Florida & Southeast Georgia.

We offer Gardner Denver’s complete line of air compressors, air dryers, vacuum pumps, air treatment products, lubricants and repair parts.

In addition, we are also an authorized distributor for On Site Gas Systems‘ Nitrogen & Oxygen Generators, Parker Legris-Transair® Advanced Air Piping Systems, Squire-Cogswell Vacuum Systems and Vortron Industrial Blowers.

We have vast and varied qualifications and experience in the compressed air industry with our combined industry experience exceeding 100 years in areas such as Applications Engineering, Business Development, Customer Service, Distributor Relations & Management, Field Service Support, Manufacturing, New Business Development, Product Development, Product Engineering & Sales & Marketing.

With such a unique industry experience, we can now deliver a full array of end-to-end solutions to all our customers in the compressed air industry, including Total System Design, Specification development, Sizing, Equipment Sales, Layout, Maintenance and Rentals.

Your business is our priority. Our outstanding Sales and Service staff can help you decide on how we could deliver solutions that best meet your needs.

About The author:

Sundar Mylavarapu has over 34 years experience in the Compressed Air Industry.

For the past 13 years, he has owned and operated Florida Air Technologies.

Prior to that, Sundar was associated with Flair Corporation (now SPX) for about 10 years as a Senior Manager responsible for Product & Business Development.

At Flair, he was inventor of US Patent No. 5,350,442 in September ‘94 and Canadian Patent No. 2,128,194 in December ’96 (both related to Compressed Natural Gas drying).

Prior to Flair, Sundar was associated with Ingersoll-Rand for over 10 years in Sales & Marketing.

Sundar holds an MS in Mechanical & Aerospace Engineering from Rutgers University.

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