Psychrometrics is the study of the properties and processes of dry air and water vapor. Psychrometrics shows what changes need to be made to air to make it comfortable for occupants and how much work (energy) it will take to make the changes. It is the heart of all Heating, Ventilating and Air Conditioning (HVAC) system design.
Nitrogen 78.0600%
Oxygen 20.9500%
Xenon 0.0009%
Carbon Dioxide 0.0380%
Krypton 0.0100%
Helium 0.0005%
Hydrogen 0.0001%
Neon 0.0018%
Air is a mixture of elemental gases mostly Nitrogen and Oxygen. Dry air behaves like an ideal gas following the classic ideal gas law:
pV=NRT
Where:
P= pressure
V = Volume
N = amount of gas
R = ideal gas constant
T = temperature
Adding water vapor to air has a huge impact even though the amount of water vapor (by mass) is very low. For example, air at 75°F and 50% RH has 0.923% water vapor by mass. What makes such a big difference is that water vapor can change state (condense from gas to a liquid) at typical occupant conditions and the latent energy to change state is significant.
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Humidity is one of the 5 key parameters that lead to occupant comfort. Humidity is also a property that can be calculated using psychrometrics and is discussed here. The Swegon Humidity Research paper covers the role humidity plays in the built environment.
Psychrometric Chart for sea Level [get more recent version of chart]
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.
Understanding Psychrometrics allows the designer what properties of air must be changed to achieve properties that provide acceptable thermal comfort. Psychrometrics also allows the designer to understand what processes must be applied to the air to achieve the necessary changes such as heating. Cooling etc. Understanding the required processes is the heart of HVAC system design. The following show common psychrometric processes used in HVAC system design.
Heating air is the most common thermal comfort solution. Hot water and steam coils, electric heaters, heat pump refrigerant coils, direct and indirect gas heat are all common ways heat is added to air.
Cooling air is usually accomplished with chilled water or refrigerant (DX) coils. In most locations cooling air will form condensation so the coils are placed over a condensate drain pan to collect the water and channel it to a drain.
Most dehumidification processes use a combination of cooling and then reheating. The air passes through a cooling device and then some form of heating device. Many energy codes require that the heat source utilize some form of waste heat such as hot gas reheat from a refrigeration system or condenser water reheat from a chilled water system. It is also possible to dehumidify air using chemical processes such enthalpy rotors.
Isothermal humidification requires a steam generator that boils the water prior to introduction to the air. This is an energy-intensive process that uses electricity, natural gas or steam from a boiler plant.
Adiabatic humidifiers use various technologies to expose liquid water to an airstream so the water can evaporate into it. This includes pad type residential humidifiers, evaporative media and specialized nozzles to disperse the liquid water in as small a droplet as possible (atomizing) to maximize surface area.
Direct evaporative cooling systems pour water over a media that air passes through and evaporates into the airstream. Indirect evaporative cooling uses the same module but in an exhaust airstream to cool the exhaust air. The cool exhaust air is then used to cool the primary air via an energy recovery device such as a rotor. Indirect evaporative cooling is less efficient but avoids exposing the primary air stream to a potentially dirty water source.
Mixing airstreams is commonly done in the mixing box part of an air handling unit. The two streams will blend their psychrometric properties.
Sensible energy air to air recovery devices can transfer heat from one (hotter) air stream to another (colder) airstream. They are often referred to as “heat Recovery Ventilators” or HRVs). They are most commonly used for preconditioning ventilation air. Common types include sensible rotor, plates, heatpipe and run around loops. Most require the two airstreams to be side by side but the run around loop can have the airstreams apart.
Enthalpy or Total energy air to air recovery devices can transfer both sensible and latent (moisture) energy from one air stream to another airstream. They are often referred to as “Energy Recovery ventilators” or ERV’s. They are most commonly used for preconditioning ventilation air. Common types include enthalpy rotor and enthalpy plates.
Psychrometrics can be used to calculate how much work must be done to air to change its properties to the desired condition. The energy in the air is the sum of sensible heat and latent heat.
QT = QS + QL
Where
QT = Total Energy/time in Btu/h (kW)
QS = Sensible Energy/time in Btu/h (kW)
QL = Latent Energy/time in Btu/h (kW)
In most HVAC calculations volume airflow rate (i.e. cfm or l/s) is used rather than mass flow rate. When changes temperature are small, using volume rather than mass flow rate is acceptable because the air density (0.075 lb/ft3) doesn’t change much. However, in cold climates the temperature range for outdoor air to room air can be 100°F which is a 20% change in density. A Ventilation unit that heats air 100°F has 100 cfm entering the unit and 120 cfm leaving I the winter. In the summer (when the temperature differences are small) 100 cfm enters and leaves the unit. Density correction is required.
The same is true for altitude where the density in Denver (500ft elevation) is 14% less than sea level. Again this equation must be density corrected.
The following three common equations can be used along with the psychrometric properties to calculate the loads on HVAC equipment to achieve the desired changes. By reviewing how the equations are derived, they can be properly applied.
Sensible heat transfer is based on the specific heat capacity of air and water vapor. The specific heat of air is approximately 0.24 Btu/lb°F and for water vapor is 0.45 Btu/lb°F. However there is less than 1% water vapor in air by mass so the specific heat capacity is ignored in the equation below.
QS = mcp (T2-T1)
QS = ρcp V(T2-T1)
QS = 0.075 lb/ft3 x 60min/h x 0.24 Btu/lb°F x V (T2-T1)
QS = 1.08 x cfm x (T2-T1)
Where
QS = Sensible Energy Transfer Rate in Btu/h (kW)
m = Mass flow rate in lb/h (kg/h)
V = Volume flowrate of air in cfm (l/s)
ρ = density of air in lb/ft3 (kg/m3)
cp = Specific heat capacity of air in Btu/lb°F (J/kg°C)
T2-T1 = Change in temperature in °F (°C)
Latent heat transfer is based on the latent heat of condensation of water vapor. While there is only around 1% water vapor in moist air by mass, the energy associated with change of state from liquid to gas or vice versa is significant. The latent of heat of evaporation (or condensation) changes with temperature but for HVAC temperature ranges 1065 Bth/lb is a good average.
QL = ma hv(Ɯ2-Ɯ1)
QL = ρ hv V(Ɯ2-Ɯ1)
QL = 0.075 lb/ft3 x 60min/h x 1065 Btu/lb x 1/7000 gr/lb x V(Ɯ2-Ɯ1)
QL = 0.68 x V x (Ɯ2–Ɯ1)
Where
QL = Latent Energy Transfer Rate in Btu/h (kW)
Ma = Mass flow rate of air in lb/h (kg/h)
V = Volume flowrate of air in cfm (l/s)
ρ = density of air in lb/ft3 (kg/m3)
hv = Latent heat of evaporation of water in Btu/lb (kJ/kg)
Ɯ2-Ɯ1= Change in humidity ratio in gr/lba (kgw/kga)
Total energy transfer is the sum of sensible and latent energy transfer. It can also be calculated using the change in enthalpy of air.
QT = QS + QL
QT = mV(h2-h1)
QT = ρV(T2-T1)
QT = 0.075 lb/ft3 x 60min/h x V (h2-h1)
QT = 4.5 x cfm x (h2-h1)
Where
QT = Total Energy Transfer Rate in Btu/h (kW)
m = Mass flow rate of air in lb/h (kg/h)
V = Volume flowrate of air in cfm (l/s)
ρ = density of air in lb/ft3 (kg/m3)
h2-h1 = Change in enthalpy in Btu/lb (kJ/kg)