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By: Brad Taylor

Relative Humidity vs. Dew Point in Compressed Air SystemsDo you know how to calculate dew point and relative humidity for your compressed air system? Understanding these measures and how they are related will help you maintain your compressed air system at optimal performance. To minimize condensation problems in your system, it is important to know how much moisture is in the air vs. how much moisture the air is capable of holding. Measuring or calculating your dew point and relative humidity (RH) can help you avoid problems with your compressed air system and production applications. Here’s how to know if your pressure dew point is too high—and what you can do to fix it.

What is the Relationship Between Relative Humidity and Dew Point?

Dew point and relative humidity are related, but they are not the same thing. It helps to have an understanding of what each term means and how they are interrelated.

What Is Dew Point?

The dew point is the temperature at which water vapor in the air starts to condense into a liquid. The dew point is a measure of how much moisture is in the air. A higher dew point means there is more moisture in the air.

The amount of water that can be held in a volume of air is directly related to its temperature. Warmer air can hold more water vapor than cooler air. As air is cooled at a constant pressure, it becomes more and more saturated. As the temperature continues to drop, excess water condenses as a liquid. The temperature at which this happens is the dew point.

We see this in action in nature at atmospheric pressure. Warm daytime air holds more moisture (or humidity). As temperatures cool overnight, dew forms on grass and other surfaces. The more humidity in the air, the higher the temperature at which condensation starts to form. In very low humidity, condensation may not form until temperatures are below the freezing point of water. We call this the frost point.

Relative Humidity vs. Dew Point in Compressed Air Systems

What Is Relative Humidity?

Relative humidity (RH) is a measure of how saturated the air is—in other words, how much moisture is in the air relative to the total capacity of the air to hold moisture. Remember, this is a relative measure. RH alone does not tell you how much total moisture is in the air. It simply tells you how close you are to saturation for a given temperature and pressure. At 100% RH, the air is completely saturated. Any additional moisture entering the air at this point will cause condensation to start to form. At 50% RH, the air can continue to absorb additional moisture without causing condensation.

Like dew point, relative humidity is directly related to temperature. A 40°F day with 100% humidity has much less moisture in the air in absolute terms than an 85°F day at 50% humidity. That’s why cooler days feel drier, even if the reported RH is the same. As air cools, RH rises, even though no additional moisture has entered the air. When we reach the temperature where RH is 100%, we have hit the dew point.

Relative Humidity vs. Dew Point in Compressed Air Systems
Calculating Pressure Dew point and Relative Humidity for Compressed Air

The amount of moisture in the air of your compressed air system depends in large part on the humidity of the air outside. The more moisture present in the intake air, the more water vapor (and potentially condensation) will end up in your compressed air system.

The dew point in a compressed air system is known as the pressure dew point (PDP), e.g., the dew point at the PSI at which the system is operating. (This is contrasted to atmospheric dew point, or the dew point of air at atmospheric pressure.) RH and dew point are specific to the pressure the air is under. As air is pressurized, excess moisture is squeezed from the air like water from a sponge.

To visualize this, remember that the water holding capacity of air depends on its temperature. That means that 1 m3 of intake air and 1 m3 of compressed air at the same temperature can hold the same amount of water. But, during compression, we’re squeezing a lot more air (in terms of actual air and water vapor molecules) into our 1 m3. If we raise the pressure from 1 bar (atmospheric pressure) to 8 bar (roughly 116 PSI), we are squeezing that 1 m3 of intake air into 1/8 m3 of space, so our 1 m3 of compressed air now contains eight times the water vapor as 1 m3 under atmospheric pressure. If the air was already saturated going in (100% RH), all of that excess water will fall out as condensation.

It is important for manufacturers to know the dew point and relative humidity for the pressures and operating temperatures at which their system is running. In other words, how much moisture is in the air in the system (absolute humidity) and how close they are to the dew point of their system (relative humidity).

How to Measure Pressure Dew Point for Compressed Air Systems

The easiest way to determine the pressure dew point for your compressed air system is to measure it directly. You can measure the PDP for your compressed air system using a dew point sensor. The sensor measures temperature and trace moisture to determine the point at which condensation starts to occur.

Aire Tip: Most air dryers have integrated dew point sensors to continually monitor the dew point of your system.

How to Calculate Dew Point and Relative Humidity for Compressed Air

It is also possible to calculate the dew point and relative humidity for your compressed air system. To do this, you have to know how much moisture is in the air coming into the compressor. We will first need to calculate the absolute humidity of intake air—that is, the total amount of water vapor present in a given volume of air. We can figure out absolute humidity in grams per cubic meter if we know the relative humidity and temperature of the intake air. To calculate absolute humidity in g/m3, you will need the intake air temperature in Celsius and the RH (expressed as a %). You will also use the natural log (e). The formula is:

Or, you can use one of many handy online RH to AH calculators to do this for you.

Now, we need to determine the moisture carrying capacity (also known as the saturation vapor density) of the compressed air. Again, this depends on its temperature. This is easiest to do using a saturation curve or table, like this one:

Relative Humidity vs. Dew Point in Compressed Air Systems

 

The dew point of your system is the point at which the RH of your compressed air is 100%, or when the absolute humidity of air coming in equals the moisture carrying capacity of the compressed air. RH is calculated as:

Relative Humidity vs. Dew Point in Compressed Air Systems

 

Or, you can use one of many handy online RH to AH calculators to do this for you.

Now, we need to determine the moisture carrying capacity (also known as the saturation vapor density) of the compressed air. Again, this depends on its temperature. This is easiest to do using a saturation curve or table, like this one: 

The dew point of your system is the point at which the RH of your compressed air is 100%, or when the absolute humidity of air coming in equals the moisture carrying capacity of the compressed air. RH is calculated as: 

RH = (actual vapor density/saturation vapor density) *100

If RH is greater than 100, you will have condensation. A number lower than 100 gives you the RH of your system and tells you how close you are to the dew point.

Let’s look at an example:

  • Intake air temperature is 20°C (68°F) with RH of 60%, for absolute humidity (actual vapor density) of 10.3 g/m3 (from NOAA calculator). 
  • Temperature of compressed air is 40°C (104°F). Saturation vapor density = 51 g/m3 (from chart). 
  •  Air is compressed from 1 bar to 8 bar. New absolute humidity of the air is 10.3 g/m3 x 8 = 82.4 g/m3.
  • RH = (82.4/51) * 100 = 162%

Since 100% is the maximum RH, we know that we will have condensation in this example.

Now, let’s lower the incoming RH to 20%:

  • Actual vapor density = 3.4 g/m3 (from NOAA calculator)
  • With the same compression, new absolute humidity = 3.4 g/m3 x 8 = 27.2 g/m3  
  • RH = (27.2/51) * 100 = 53%

In this case, RH is lower than the saturation vapor density inside the system, so condensation will not occur.

Compressed Air Temperature and Dew Point

Remember that the saturation vapor density for air inside your compressed air system is dependent on the temperature—and the temperature of that air is changing as it goes through the system. Air coming directly out of the compressor will be very hot, so it is able to hold more moisture than cooler air. That’s why condensation tends to fall out of compressed air as it sits in your air receiver or moves through the air lines. Just like dew forms overnight as air cools, condensation will start to form when air in your compressed air system cools to the dew point.

Aire Tip: Drain excess water daily from air receiver tanks and other places water is likely to condense. Programmable or zero-loss drain valves will do this for you automatically.
Why Dew Point Matters for Compressed Air Systems

Why does dew point matter for compressed air?

If your dew point is too high—that is, if you have a lot of moisture in the air in your system—you will have more condensation and more water vapor in the air coming out of the lines. If you can lower the dew point of your air, you can avoid problems associated with excess liquid in your compressed air system.

Common Problems with Water in Compressed Air Lines

Water in compressed air systems can cause a number of problems. The most serious of these is corrosion in the compressor itself, air lines and other components of the system. In addition, excess liquid in the air lines can lead to:

  • Corrosion of tools and equipment powered by compressed air
  • Product quality issues (especially for paint booth applications, pharmaceuticals, food processing, and other applications that are sensitive to moisture)
  • Risk of freezing in the control lines and damage to equipment
  • False readings on air compressor instruments 
  • Microbial growth in control lines and air lines 

What Should the Dew Point Be for a Compressed Air System?

A continuous dew point monitor will tell you the dew point at which your system is currently running. Typical dew points for compressed air systems are between 50°F and 94°F.

Some applications—such as paint booths, printing, food processing and pharmaceuticals—require super-dry air. For these applications, you will want to lower the dew point of your system by removing moisture from the air.

Aire Tip: For most applications, maintaining an RH of 75% or lower relative to the dew point of your system will be adequate to keep air lines liquid-free.

How to Lower Dew Point in Compressed Air Systems

You can lower the dew point of your compressed air system by removing excess moisture from the air. This can be done by installing a water separator filter, an air dryer, or both. There are two types of dryers:

  • A refrigerated air dryer works by chilling air to 33-40°F, allowing excess water to condense, and then bringing air back up to ambient temperatures before adding it to the system. This lowers the dew point to 33-40°F (the temperature to which the air was chilled). Refrigerated dryers lower the dew point adequately for most applications. 
  • Desiccant air dryers remove water from the air through a chemical process. They can reduce the dew point to -40 to -100°F, creating ultra-dry air for sensitive applications. 

Read more: Getting Rid of Moisture in Your Compressed Air System.

Need help diagnosing moisture problems in your compressed air system? Fluid-Aire Dynamics can help you design an air drying system to reduce your dew point to the levels needed by your application. Contact us for an evaluation.

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