Subject:- Refrigeration and Air Conditioning

Topic:- Psychrometry

Difficulty:- HIgh

Fog is a visible mass consisting of cloud water droplets or ice crystals suspended in the air. In the atmosphere, fog can be considered a type of low-lying cloud and is heavily influenced by nearby bodies of water, topography, and wind conditions.

When hot humid air is mixed with cold air the result may be fog. When very cold and dry air mixes with warm air at high relative humidity, the resulting mixture condition may lie in the two-phase region, as a result there will be condensation of water vapor and some amount of water will leave the system as liquid water.

Due to this, the humidity ratio of the resulting mixture (point 3) will be less than that at point 4. Corresponding to this will be an increase in temperature of air due to the release of latent heat of condensation.

This process rarely occurs in an air conditioning system, but this is the phenomenon which results in the formation of fog or frost (if the mixture temperature is below 0°C). This happens in winter when the cold air near the earth mixes with the humid and warm air, which develops towards the evening or after rains.

The mixing point is below the saturation line, and the moisture in the air condensates as small droplets floating in the air. The fog process can be expressed in the Mollier diagram as:

For temperatures below 0oC the water droplets freezes to ice. If the mixing point is below the saturation line - water is condensed as droplets and fog is created.

The amount of condensate water can be found by following the constant enthalpy line from B to the saturation line. The condensate water is the difference between the specific humidity in point B and in the point where the enthalpy line crosses the saturation line.

The mixed temperature is where the enthalpy line crosses the saturation line. As long as the mixed air is above the saturation line no fog will be formed in the mix.