What is the different between normal atmosphere dewpoint and working dewpoint?

When discussing “dew point,” it’s crucial to understand the distinction between atmospheric dew point and pressure dew point (often referred to as “working dew point” in industrial contexts, especially with compressed air). While both describe the temperature at which water vapor in the air will condense into liquid, the difference lies in the pressure at which this condensation occurs.

Atmospheric Dew Point

The atmospheric dew point is the temperature to which ambient air, at its normal atmospheric pressure, must be cooled for its water vapor to reach saturation and begin to condense. This is the dew point you hear about in weather forecasts. When the air temperature drops to the atmospheric dew point, dew forms on surfaces, or fog/clouds can form in the air. It’s a key indicator of how much moisture is in the air and how “muggy” it feels. A higher atmospheric dew point means more moisture in the air.

Pressure Dew Point (Working Dew Point)

The pressure dew point is the temperature to which compressed air (or any gas under pressure) must be cooled for its water vapor to condense into liquid water. This is the “working dew point” relevant in industrial applications, particularly for compressed air systems. When air is compressed, its pressure increases, and this significantly impacts its ability to hold water vapor. Specifically, increasing the pressure raises the dew point temperature. This means that compressed air will reach its saturation point and condense water at a higher temperature than the same air would at atmospheric pressure, even if the absolute amount of water vapor remains the same.

Why is this important for compressed air? Air compressors draw in ambient air, which always contains some moisture. When this air is compressed, the water vapor becomes more concentrated. If this hot, compressed air isn’t dried, as it cools down in the system, the water vapor will condense into liquid water. This can cause significant problems:

• Corrosion and Rust: Water in air lines leads to rust and corrosion of pipes, valves, and pneumatic tools.
• Equipment Damage: Moisture can wash away lubricants, damage sensitive instruments, and cause malfunctions in air-driven machinery.
• Contamination: In industries like food and beverage, pharmaceuticals, or electronics, water in compressed air can contaminate products.
• Freezing: In cold environments, condensed water can freeze within the lines, causing blockages and damaging equipment.

Therefore, for compressed air applications, a lower pressure dew point is desirable, indicating drier air. Air dryers are used to remove moisture from compressed air to achieve the necessary pressure dew point for a given application. Different types of dryers (e.g., refrigerated, desiccant) achieve different pressure dew point levels.

Key Differences Summarized