What’s Dew Point? (A Must-Read For Compressed Air)

Dew Point a Must-Read for Compressed Air
Post Date: December 12, 2018 Posted by: ISC
Category: Air Compressors, Air Dryers

You already know what dew point is, you just haven’t heard it called by that name yet! You’ve probably seen it almost every day.

Surprising, isn’t it?

The condensation that forms on your cold glass of water, clouds, the water that forms on grass overnight and water that collects on windows on cold winter days are all excellent examples of dew point in action.

Dew point (DP) is an important factor to consider for many professions including meteorologists and pilots, but for the purposes of this article, the main focus will be on dew point for industrial applications, specifically in compressed air systems.

1. Compressed Air Dew Point
2. What Are the Differences Between Atmospheric Dew Point, Pressure Dew Point and Frost Point?
3. What is the Effect of Pressure on Dew Point?
4. Why is Dew Point Important for Compressed Air Systems?
5. What is the Typical Range of Dew Point Temperatures Found in Compressed Air?
6. What Are the Standards for the Quality of Compressed Air?
7. How is Dew Point in Compressed Air Measured?
8. What Did We Learn?

1. Compressed Air Dew Point

Dew point is the temperature at which your compressed air becomes fully saturated, where air can’t hold any more water vapor, causing the vapor to condense. In other words, it’s the temperature when water vapors become liquid water.

When the air temperature is the same as the dew point temperature, the air is at saturation point – this is where we begin to see water droplets forming.

At the DP temperature water becomes visible. You can see the condensation when you have a cold drink, not because you spilled or because of anything else other than the dew point.

The water appears on the outside of the glass because the temperature of the glass is lower than the DP of the air.

Now, this is a little confusing but even though dew point is expressed as a temperature it is not affected by changing temperatures.

Instead, it will change based on pressure and by how much water vapor is present in the air, also humidity.

Is it starting to click? If not, don’t worry we’ve just begun!

2. What Are the Differences Between Atmospheric Dew Point, Pressure Dew Point and Frost Point?

Atmospheric Dew Point, Pressure Dew Point, and Frost Point

Atmospheric Dew Point: This is the point that occurs naturally when the temperature and saturation point/ dew point are the same and relative humidity is at 100 percent.

Pilots and meteorologists use this to predict weather conditions.

Pressure Dew Point: This is the temperature of gases at pressures higher than atmospheric pressure, like in a compressed air system.

This is important because changing the pressure of a gas changes its saturation point.

Frost Point: It is usually a temperature below 0 degrees Celsius or 32 degrees Fahrenheit, where water vapor condenses as frost instead of into liquid water.

Frost point is an important differentiation of dew point because it will be a different measurement than dew point below 0 degrees C.

For example, DP is usually about 4 degrees Celsius lower than frost point at negative 40 degrees. This is meaningful for ISO Standards- which, don’t worry, we will get into later on in the article.

3. What is the Effect of Pressure on Dew Point?

Effect of Pressure on Dew Point

Dew point is the most commonly used measurement for compressed air systems because as pressure rises the dew point temperature also rises. A fact illustrated by our handy dandy chart above.

This means that, if pressure increases, the dew point temperature will rise and approach saturation. The amount of water vapor in the air has to be reduced so that the saturation point stays the same or even lowered.

The higher the DP, the more moisture that is present in the air.

It is easier for liquid water to enter the compressed air system at this point because the gas can easily reach its saturation point. This is why air dryers are so commonly used with compressed air systems.

Temperature

°F         (°C)

Saturation Vapor Pressure (mbar)
68      (20) 23.5
32       (0) 6.1
14      (-10) 2.8
– 4     (-20) 1.3
-40   (-40) 0.2

As the pressure of a system is decreased the dew point temperature becomes lower and the gas will become relatively dryer as the difference between dew point and ambient temperature increases.

So, this means that drying the air effectively lowers DP temperatures.

Some air dryers are so powerful they can drop the dew point temperature to as low as negative 40 degrees Fahrenheit! Can you say cold!? Brrr.

4. Why is Dew Point Important for Compressed Air Systems?

Water is always present and it can be really damaging to a lot of industrial equipment and even ruin end products.

Water can cause…

  • Rust and increased wear of moving parts because it washes away important lubricants
  • Paint that is applied with compressed air systems to change colors or make it not stick to products
  • Problems with compressed air powered tools because of rust, scale and clogged openings including major damage and even costly downtime
  • Faulty operations due to frozen control lines in cold weather
  • False readings that are caused by corrosion on air or gas operated instruments which can lead to shutting down plant processes

So, short story long, water is bad news for production and manufacturing processes and if you know the saturation temperature you can effectively control the buildup of water.

Preventing water from entering your compressed air system will keep all those nasty things on that list from happening and in the words of Martha Stewart, “It’s a good thing.”

5. What is the Typical Range of Dew Point Temperatures Found in Compressed Air?

Dew points can range anywhere from ambient temperatures all the way down to 80 degrees Celsius (-112 °F) below zero and sometimes even lower in some specialized applications.

Refrigerated air dryers use a heat exchanger to condense water out the of air stream. They can usually produce air that has a dew point no lower than 5 degrees Celsius (41 °F).

Desiccant air dryers absorb water vapor from the air stream and can drop dew point significantly more. They can get dew point to as low as 40 degrees below zero.

If you’d like more information on air dryers, how they work and help on deciding which style would work best for your application check out our blog article titled “Compressed Air Dryers (The Easy Way to Eliminate Moisture!)”

6. What Are the Standards for the Quality of Compressed Air?

The International Standards Organization (ISO) published a set of standards under section 8573.1 which are air quality classes.

This standard sets the limits for three categories of air quality:

  • Maximum particle size for any remaining particles
  • Maximum allowable dew point temperature
  • Maximum remaining oil content

Classes of air are numbered one through six and you can see from the table below the specifications for each class.

Class Particles Dew point °C Dew point °F Oil mg/m^3
1 0.1 -70 -94 0.01
2 1 -40 -40 0.1
3 5 -20 -4 1
4 15 3 38 5
5 40 7 45 >5
6 10 50

7. How is Dew Point in Compressed Air Measured?

Air dryers do a good job of monitoring the air’s dew point but if you want to be 100 percent sure because you have a particularly sensitive system or production process you can also use a hygrometer.

Most air dryers have built-in transmitters that ensure that your equipment is maintaining the correct levels of moisture within your compressed air system.

You should occasionally check and calibrate your system. This should be done at least once a year or anytime you might be suspicious of the readings that you are getting for your compressed air.

8. What Did We Learn?

Ok, so that was a lot of information so let’s recap….

  • Dew point is the temperature at which your compressed air becomes fully saturated, where air can’t hold any more water vapor, causing the vapor to condense.
  • Atmospheric dew point is when the temperature and saturation point/ dew point are the same and relative humidity is at 100 percent.
  • Pressure dew point is the dew point temperature of gases at pressures higher than atmospheric pressure, like in a compressed air system. It is important for measuring air quality in those systems.
  • Frost Point is the temperature when water vapor condenses as frost instead of into liquid water.
  • Pressure changes affect dew point, not temperature changes. As pressure rises, the dew point increases and as pressure decreases dew point temperature also decreases.
  • Having liquid water in your compressed air system is bad and can cause numerous problems.
  • Dew point can range in compressed air systems from ambient temperatures all the way down to 80 degrees below zero, depending on the air dryer used.
  • Air dryers can help lower dew point temperatures and keep liquid water out of air compression systems.
  • Air dryers can monitor dew point and so can hygrometers.

Still Have Questions?

Don’t worry! The compressed air experts at ISC Sales would be happy to help! Just give us a call and we will be sure to get you the best product for your application.

We also have an entire blog article on air dryers and the effects of liquid water on compressed air systems and production processes called, “The Easiest Ways to Dry Compressed Air” Check it out!

Summary
What's Dew Point?
Article Name
What's Dew Point?
Description
All you need to know about dew point and how it affects your compressed air system is now here and easily explained.
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ISC Sales
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