In a psychometric chart, what does a vertical downward line represent

Explanation:

Basic Processes in Conditioning of Air:

Sensible heating: 

The moisture content of air remains constant so specific humidity is constant, temperature increases as it flows over a heating coil.

Sensible cooling: 

The moisture content of air remains constant so specific humidity is constant, but its temperature decreases as it flows over a cooling coil.

Dehumidification:

• When the temperature remains constant but specific humidity decreases.
• It is represented by a vertical line.

Humidification:

• When in a process, the temperature remains constant but specific humidity increases.
• It is represented by a vertical line.

Cooling and dehumidification: 

• This process involves lowering both the air temperature and specific humidity. 
• This process is commonly used in summer air conditioning in which air passes over a cooling coil. 
• When moist air is cooled below its dew point, the vapor is condensed from the air resulting in simultaneous cooling and dehumidification.

Heating and Humidification: 

• During winter it is essential to heat and humidify the room air for comfort.
• It is done by first sensibly heating the air and then adding water vapor to the air stream through steam nozzles, as a result, both the temperature and humidity ratio of air increases.

Cooling & humidification: 

Air temperature drops and its humidity increases. 

Heating and de-humidification: 

This process can be achieved by using a hygroscopic material, which absorbs the water vapor from the moisture. If this process is thermally isolated, then the enthalpy of air remains constant, as a result, the temperature of air increases.

Additional Information

Dry Bulb Temperature: Actual temperature of gas or mixture of gases.

Wet Bulb temperature: Temperature obtained by an accurate thermometer having a wick moistened with distilled water.

Dew point temperature: Temperature at which the liquid droplets just appear when the moist air is cooled continuously.

Relative humidity along the saturation line is 100%.