Dew point

This is defined as the temperature of saturated air which has the same vapour pressure as the moist air under consideration.

It is not possible to express this definition in the form of a simple equation by means of the ideal gas laws. This is evident if we refer to equation (2.10). It is more convenient to refer to tabulated values when saturated vapour pressure is required. However, provided such tables are available, we can determine the dew point of air at a given psychrometric state and barometric pressure.

EXAMPLE 2.7

Calculate the dew point of moist air at a dry-bulb temperature of 20°C and a moisture content of 0.007 34 kg per kg dry air at a barometric pressure of 95 kPa.

Answer

In Example 2.6 the partial pressure of the superheated steam mixed with 1 kg of dry air at a barometric pressure of 95 kPa was calculated by equation (2.13) as 1.1080 kPa. At its dew point temperature, the steam present in the moist air will be saturated and will have this value as its saturated vapour pressure. Reference to steam tables or to the CIBSE psychrometric tables shows, by interpolation, that this corresponds to a saturated temperature of 8.49°C. This is the dew point temperature.

Posted in Air Conditioning Engineering


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