Psychrometrics

Drybulb (db) temperature is the temperature measured with a thermometer and given in a weather report. It is a key parameter in thermal comfort.  Temperatures are measured in  °F (°C).  It represents how fast the air and water vapor molecules are moving.  Adding sensible heat to the air will raise the drybulb temperature while removing sensible heat will lower the drybulb temperature.

On the psychrometric chart, drybulb temperatures are shown as vertical lines.  Moving to the right on the chart shows an increase in drybulb temperature while moving to the left shows a decrease in drybulb temperature.

Relative Humidity (RH) is another common property of air given in the weather report and is also a key parameter in thermal comfort.  It is the ratio of how much water vapor is in the air vs. how much water vapor the air can hold at a given drybulb temperature.  It is given as a percent.  To be given air properties of 75°F and 50% RH means the air is 75°F drybulb and has 50% of the total water vapor air it can hold at 75°F.

Warm air can hold much more water vapor than cold air.  For example air at 95°F and 50% RH has twice as  much water vapor as 75°F and 50% RH air by mass.

On the psychrometric chart, RH lines are shown as curved lines.  Moving to the right on the chart shows an decrease in RH while moving to the left shows increase in RH with the curved line on the left hand edge of the chart representing 100% RH or saturated air (the air cannot hold any more moisture).

Humidity Ratio (HR) is the ratio of mass of water vapor over the mass of dry air and is given in grwater/lbdry air  or lbwater/lbdry air ( kgwater/kgdry air).  Note grains is a unit of mass and there are 7000 grains in a pound.    Humidity ratio shows how many water molecules there are relative to the number of air molecules.  Changing the air temperature will not impact the humidity ratio.

75°F and 50% RH air has 64.6 gr of water per pound of dry air.  If this air is cooled to 55.5°F,  the humidity ratio will still be 64.6  gr/lb (it has the same amount of water molecules as before the cooling) but the relative humidity has climbed to 98.6% – it is almost saturated.

On the psychrometric chart, humidity ratio lines are horizontal .  Moving up the chart shows an increase in humidity ratio while moving down shows a decrease in humidity ratio.

Dewpoint is another property closely associated with weather.  It is given as a temperature in °F (°C).    It is the coldest temperature air can be for a specific amount of water vapor before condensation will occur.

The dewpoint for 75°F air with  64.6 gr of water per lb of air is 55.2°F.  If this air is cooled from 75 °F to 55.2°F, it will be 100% saturated and have 100% relative humidity.  To cool the air any further will require some water vapor to condense which in turn means the air must give up the latent heat of evaporation for water which is 1065 Btu/lb of water (2256 kJ/kg).

For weather, air that is warmed during the day gains water vapor.  As the air cools at night it can reach its dew point temperature.  As the temperature continues to drop the water condenses out in the form of dew on plants and grass or as fog (minute water droplets floating in air – a cloud at ground level).

In HVAC systems, cooling the air below its dewpoint temperature is how most systems  dehumidify air.  The cooling coil cools the air until condensate forms on the coil and is carried away in the condensate pan.

Dewpoint lines are not typically shown on a psychrometric chart.  If there were added , they would be horizontal like humidity ratio lines but they are not evenly spaced.  Moving up the chart shows an increase in dewpoint while moving down shows a decrease in dewpoint.

It is hard to explain what wetbulb temperature is in terms of human experience.  Technically it is the temperature of water vapor at adiabatic saturation and is given in °F (°C).  It is easier to appreciate when considering how it was traditionally measured.  A thermometer has its bulb inserted in a wad of cotton which was soaked with water.  It is spun around for a few seconds.  As the water evaporates from the cotton it lowers its temperature to the “wetbulb” temperature.

Once wetbulb and drybulb temperatures were measured, the rest of the air’s properties could be calculated (it is actually not easy to measure RH, humidity ratio, dew point etc in the field).

On the psychrometric chart, wetbulb lines are diagonal .  Moving up the chart shows an increase in wetbulb temperature while moving down shows a decrease in wetbulb temperature.

Enthalpy is the sum of the internal energy (u) + the workflow energy (P*v)  for both water vapor and dry air.  It is measured in Btu/lb (kJ/kg)

H = u+P x v

Where

h = enthalpy

U = internal energy

P = pressure

v = specific volume

To change the properties of air you must do work on it (heat it, cool it etc).  This will change its enthalpy.  The change in enthalpy is work load on the HVAC heating and cooling equipment so while enthalpy level is not part of the human experience it is critical to designing the HVAC system.

The enthalpy of 95°F and 50% RH air is 42.4 Btu/lb.  If the air is cooled to 75°F and 50% RH the enthalpy drops to 28.1 Btu/lb.  An HVAC system must be designed to remove 14.3 Btu from each pound of air that needs to be cooled.

On the psychrometric chart, enthalpy lines are diagonal but not exactly parallel to wetbulb lines.  Moving up the chart shows an increase in enthalpy while moving down shows a decrease in enthalpy.

Specific volume is the amount of volume of dry air per unit mass. It is measured as ft³/lb dryair (m³/kg dryair). As air is warmed  it expands and its specific volume increases.  Specific volume is the inverse of density.

On the psychrometric chart, specific volume lines are diagonal but not parallel to wetbulb or enthalpy lines.  Moving up the chart shows an increase in specific while moving down shows a decrease in specific volume.

If the air pressure changes, then all the air properties change.  An increase in altitude will lower the air pressure and change the properties.  Psychrometric charts are created for a specific barometric pressure.  Most are based  on sea level but charts for 5000 ft are common.  When Apps or software are used, it is critical to adjust the altitude to match the location.