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How to Calculate Dew Point: Formula and Dew Point Chart

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What happens when air cools down and moisture starts to form? This is where the dew point comes in! Understanding the dew point and using a dew point chart is essential for controlling moisture in systems like air compressors. It ensures equipment runs smoothly, prevents corrosion, and improves product quality.


Dew Point

What is Dew Point?

The dew point is the temperature at which air becomes saturated with moisture, causing water vapor to condense into liquid. It is a critical parameter in many industrial and environmental contexts, especially in air compressor systems, where it is closely monitored to ensure optimal performance and prevent issues caused by excess moisture.

Importance of Dew Point in Air Compressor Systems

In air compressor systems, compressed air is often used in manufacturing processes, pneumatic tools, and other applications requiring dry air. However, as air is compressed, its moisture content becomes more concentrated, potentially leading to condensation. Managing the dew point is crucial for several reasons:

1. Preventing Equipment Damage

  • Condensed water can cause corrosion in pipelines and damage sensitive equipment, including valves and actuators.

  • Accumulated moisture in tools can reduce their lifespan and performance.

2. Ensuring Product Quality

  • Many industrial processes, such as painting or pharmaceuticals, require extremely dry air. High dew points can lead to defects, such as bubbles in coatings or contamination in medical products.

3. Avoiding Freezing in Cold Environments

  • In systems exposed to low temperatures, moisture can freeze if the dew point is not adequately controlled, leading to blockages in pipelines.

4. Reducing Maintenance Costs

  • Controlling the dew point minimizes the need for frequent maintenance and repairs caused by moisture-induced problems.

How Dew Point is Controlled in Air Compressors

To maintain a specific dew point, air compressor systems employ various methods and equipment:

  1. Dryers

    • Refrigerated Dryers: Suitable for achieving a dew point between 2°C and 7°C. Common in general-purpose applications.

    • Desiccant Dryers: Used for achieving lower dew points (as low as -70°C), essential for processes requiring extremely dry air.

  2. Moisture Separators

    • Installed to remove liquid water from compressed air before it reaches downstream equipment.

  3. Filtration Systems

    • Filters remove fine moisture particles and oil, further drying the air.

  4. Dew Point Monitors

    • Sensors continuously measure and display the dew point, allowing for real-time adjustments.

How to Calculating the Dew Point

1. Measure the Relative Humidity and Temperature

  • Relative Humidity (RH): Measure the percentage of water vapor in the air relative to the maximum amount the air can hold at a given temperature.

  • Temperature (T): Measure the air temperature in degrees Celsius (°C) or Fahrenheit (°F).

2. Use a Dew Point Calculator or Formula

There are two common approaches:

A. Dew Point Approximation Formula

Dew Point Approximation Formula


Here is a table showing the calculated dew point temperatures (°C) for air temperatures ranging from -20°C to 30°C and relative humidity levels from 30% to 90%.

Air Temp (°C) RH 30% RH 40% RH 50% RH 60% RH 70% RH 80% RH 90%
-20 -29.4 -26.7 -24.5 -22.7 -21.2 -19.9 -18.7
-15 -23.0 -20.2 -18.0 -16.2 -14.7 -13.3 -12.1
-10 -17.5 -14.8 -12.5 -10.7 -9.2 -7.8 -6.5
-5 -12.8 -10.0 -7.7 -5.9 -4.4 -2.9 -1.6
0 -8.5 -5.7 -3.4 -1.5 0.0 1.5 2.9
5 -4.7 -2.0 0.3 2.2 3.8 5.2 6.6
10 -1.2 1.6 3.9 5.8 7.3 8.8 10.1
15 2.0 4.9 7.1 8.9 10.5 11.9 13.2
20 5.2 8.1 10.3 12.1 13.7 15.2 16.5
25 8.3 11.2 13.4 15.2 16.8 18.3 19.6
30 11.3 14.2 16.5 18.3 19.9 21.4 22.7

In similar way, we can caculate dew point when temperature is measured by Fahrenheit.

Here is the dew point chart for temperatures ranging from 20°F to 120°F (in 10°F increments) and relative humidity levels from 30% to 90% (in 5% increments):

Relative Humidity 20°F 30°F 40°F 50°F 60°F 70°F 80°F 90°F 100°F 110°F 120°F
90% 18°F 28°F 37°F 47°F 57°F 67°F 77°F 87°F 97°F 107°F 117°F
85% 17°F 26°F 36°F 45°F 55°F 65°F 75°F 84°F 95°F 105°F 115°F
80% 16°F 25°F 34°F 44°F 54°F 63°F 73°F 82°F 93°F 102°F 110°F
75% 15°F 24°F 33°F 42°F 52°F 62°F 71°F 80°F 91°F 100°F 108°F
70% 13°F 22°F 31°F 40°F 50°F 60°F 69°F 78°F 88°F 96°F 105°F
65% 12°F 20°F 29°F 38°F 47°F 57°F 66°F 76°F 85°F 93°F 103°F
60% 11°F 19°F 27°F 36°F 45°F 55°F 64°F 73°F 83°F 91°F 101°F
55% 9°F 17°F 25°F 34°F 43°F 53°F 61°F 70°F 77°F 86°F 94°F
50% 6°F 15°F 23°F 31°F 40°F 49°F 58°F 67°F 77°F 86°F 94°F
45% 4°F 13°F 21°F 29°F 37°F 47°F 56°F 64°F 73°F 82°F 91°F
40% 1°F 11°F 18°F 26°F 35°F 43°F 51°F 61°F 70°F 78°F 87°F
35% -2°F 8°F 16°F 23°F 31°F 40°F 48°F 56°F 65°F 74°F 83°F
30% -6°F 4°F 13°F 20°F 28°F 36°F 44°F 52°F 61°F 70°F 77°F

B. Use a Psychrometric Chart

  1. Locate the dry bulb temperature (ambient temperature) on the horizontal axis.

  2. Find the relative humidity curve that matches the measured RH. 3.Trace down to find the dew point temperature.

3. Adjust for Compressed Air Conditions

In an air compressor system, air is compressed, increasing the dew point due to higher pressure. Use the formula:

Compress Pressure formula

Where  is the dew point increase due to compression, often provided in system specifications.

4. Use Dew Point Sensors (Optional)

Advanced systems often include a dew point sensor or hygrometer that measures the dew point directly under specific pressure conditions.

5. Correct for System-Specific Variables

  • If your system includes a dryer, the dew point will be lower.

  • Use the corrected dew point based on the type of dryer:

    • Refrigerated dryer: Dew point is usually 2°C–10°C (35°F–50°F).

    • Desiccant dryer: Dew point can be as low as -40°C (-40°F).

By following these steps or using online calculators, you can calculate or estimate the dew point in your air compressor system for efficient operation and proper maintenance.

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