Psychrometry in HVAC: Moist Air Properties, Humidity, Temperature, Enthalpy, and Coil Performance
Introduction to Psychrometry
Psychrometry is the study of air and its properties. More specifically, it is the study of the thermodynamic properties of moist air.
Psychrometry is important in HVAC systems because heating, ventilation, air-conditioning, and refrigeration equipment depend on the condition of the air. Properties such as temperature, humidity, heat, pressure, density, specific volume, enthalpy, entropy, and vapor pressure are used to describe air and its behavior. Psychrometric charts are also used to read and determine these values.
Two important terms are involved in the definition of psychrometry: thermodynamic properties and moist air. Thermodynamic properties of air include temperature, humidity, pressure, density, and enthalpy. Moist air is a combination of dry air and water vapor. Water vapor is also known as moisture. Dry air is air that does not contain moisture. When dry air is mixed with water vapor, it becomes moist air.
Ideal Gas Equation
The ideal gas equation is used because air and water vapor behave like gases in psychrometric analysis.
In this equation, P is pressure, V is volume, n is the number of moles, R is the universal gas constant, and T is absolute temperature. Absolute temperature may be measured in Kelvin or Rankine. Volume may be measured in cubic feet or cubic meters, while pressure may be measured in pounds per square foot or pascals. The volume of a gas is equal to the volume of the container in which the gas is present.
Moist Air and Moisture-Holding Capacity
Before studying humidity, it is necessary to understand that air can hold water vapor.
Consider air contained inside a bottle. If the air contains no water vapor, it is dry. Now imagine that holes are made on the surface of the bottle. These holes represent the maximum amount of water vapor the air can hold.
If water is poured into the bottle, it can be filled only up to the level of the holes. After that limit, water will come out or precipitate. In the same way, air can hold only a certain amount of water vapor. If the moisture content in the air exceeds this limit, the excess moisture is precipitated out as water.
At a higher temperature, air can hold more water vapor. In the bottle example, this means the holes are positioned higher.
The capacity of air to hold water vapor and the actual amount of water vapor present in the air are not always the same. The capacity shown in the bottle analogy represents the maximum strength or capacity of air to hold water vapor. It does not necessarily mean that water vapor is actually present.
If some actual amount of water vapor is present in the air, but the air is not holding moisture up to its maximum capacity, the air is called unsaturated air. If a sample of air contains water vapor at its 100% capacity, it is called saturated air.
Humidity Properties
Humidity Ratio
Humidity ratio is the ratio of the mass of water vapor to the mass of dry air. It has nothing to do with the maximum amount of water vapor the air can hold. It depends only on the actual amount of water vapor present and the mass of dry air.
In the bottle example, the water-vapor region represents the actual water vapor present in the air. The dry-air region represents the mass of dry air. Humidity ratio considers these actual quantities, not the maximum amount of moisture the air could hold.
Relative Humidity
Relative humidity is the ratio of the actual moisture in the air to the maximum moisture the air can hold, expressed as a percentage. Unlike the humidity ratio, relative humidity is related to the maximum amount of water vapor that air can hold.
Example: Relative Humidity
At 25°C, consider a sample of air. Suppose the maximum moisture-holding capacity of the air is 60 grams. The actual moisture present in the air is 30 grams.
RH = (30 / 60) × 100% = 50%
Now increase the temperature of the same air sample. At the higher temperature, suppose the maximum moisture the air can hold becomes 80 grams. The actual moisture in the air remains at 30 grams.
RH = (30 / 80) × 100% = 37.5%
This example shows that relative humidity depends on the maximum moisture-holding capacity of air, which in turn depends on temperature. When temperature changes, the actual moisture content may remain the same, but the relative humidity changes.
Degree of Saturation
The degree of saturation compares the humidity ratio of a sample of air with the humidity ratio of the same air when it is saturated. This comparison must be made at the same temperature and pressure.
In the bottle example, the holes in two bottles may be at the same height. This means their maximum moisture-holding capacity is the same. However, the actual amount of water vapor in the two samples may be different. The degree of saturation indicates how close the air is to saturation.
Absolute Humidity
Absolute humidity is the total mass of water vapor present in a unit volume of moist air. To find absolute humidity, a unit volume of moist air is considered. This volume may be one cubic centimeter, one cubic meter, or one liter. The unit of absolute humidity depends on the unit of volume used.
Suppose the total volume of moist air is 2 m³. In this 2 m³ volume, the mass of moisture or water vapor present is 60 grams.
AH = 60 g / 2 m³ = 30 g/m³
This means that in one cubic meter of moist air, the mass of water vapor present is 30 grams.
Temperature Properties
Temperature is the measure of the hotness or coldness of a body. It gives an idea of how hot or how cold a body is. Temperature does not directly indicate the total heat content of a body. Two bodies may be at the same temperature but may contain different amounts of heat.
If two bodies are at different temperatures, heat flows from the higher-temperature body to the lower-temperature body. As heat flows, the temperature of the hotter body decreases, and the temperature of the colder body increases. After some time, both bodies reach the same temperature, and no heat flows between them. This condition is called thermal equilibrium.
Dry-Bulb Temperature
Dry-bulb temperature, also known as DBT, is the temperature of air measured by a thermometer freely exposed to air. The thermometer must be isolated from radiation and moisture. Otherwise, the reading will not accurately represent the dry-bulb temperature.
Dry-bulb temperature indicates the heat content of the air, although temperature itself is not equal to heat content. If a thermometer is placed near a hot body, the reading will be higher. If it is placed near a cold body, the reading will be lower.
Wet-Bulb Temperature
Wet-bulb temperature, also known as WBT, is measured using a thermometer whose bulb is covered with a wet cloth or wick.
First, consider a thermometer placed in the air. The reading shown is the dry-bulb temperature. Now place a wet cloth or a wick on the thermometer’s bulb. If unsaturated air passes over the thermometer, water from the wet wick evaporates into the air.
The reading obtained after this evaporation process is called wet-bulb temperature. Wet-bulb temperature is the lowest temperature to which air can be cooled by the evaporation of water into the air at constant pressure.
Wet-bulb depression = DBT – WBT
It is important that the air passing over the thermometer is unsaturated. If saturated air passes over the wet wick, water will not evaporate. In that case, the thermometer reading will not fall, and the wet-bulb temperature will be equal to the dry-bulb temperature.
When RH = 100%, DBT = WBT
Dew-Point Temperature
Dew-point temperature is the temperature to which air must be cooled to become saturated with the water vapor it already contains.
Consider a sample of unsaturated air. At a given temperature, the air has a certain maximum capacity to hold moisture. As the temperature increases, the air’s ability to hold moisture increases. When the air is cooled, its ability to hold moisture decreases.
As cooling continues, the air eventually reaches a temperature at which it becomes saturated. If the temperature decreases further, water vapor begins to condense into dew. The temperature at which this begins to happen is called the dew-point temperature.
Volume, Energy, and Entropy Properties
Specific volume is the volume occupied per unit mass of dry air.
Enthalpy and Specific Enthalpy
Enthalpy is the sum of internal energy and pressure-volume energy of air. It represents the total heat content of air.
H = U + PV
Consider ice and boiling water. Ice is at a lower temperature, while boiling water is at a higher temperature. The enthalpy of boiling water is higher than that of ice because its temperature is higher. This analogy does not give the exact calculation of enthalpy, but it gives a general idea of how enthalpy can be understood.
ΔH = m Cp ΔT
For a process at constant pressure, the enthalpy change can be expressed in terms of mass, specific heat at constant pressure, and the change in temperature. From this relation, it is clear that enthalpy changes when temperature changes.
h = H / mda
Specific enthalpy is the sum of internal energy and pressure-volume energy of air per unit mass of dry air. Specific enthalpy is denoted by small h, while total enthalpy is denoted by capital H. The SI unit of specific enthalpy is kilojoules per kilogram. The imperial unit is BTU per pound.
h = hda + W.hv
h = 0.240t + W(1061 + 0.444t) BTU/lb
In the imperial formula, t is the temperature in degrees Fahrenheit, and W is the humidity ratio. The constants shown are the approximate specific heat and latent heat values used in the original calculation.
Entropy and Specific Entropy
Entropy is the measure of molecular disorder or randomness of a system. If the molecules of a system are highly disordered and random, the entropy is high. If the arrangement becomes more ordered, entropy decreases.
For example, when water is heated and boils, its randomness increases. Therefore, the water has a higher level of entropy.
Entropy is also described as the measure of a system’s thermal energy per unit temperature that is unavailable for doing useful work. The thermal energy referred to here is not the total thermal energy. It is the part of the system’s thermal energy that cannot be converted into useful work.
s = S / m
Specific entropy is the total entropy of a system divided by the mass of the system. In imperial units, it may be expressed as BTU/(lb·°F).
Vapor Pressure and Density
Vapor Pressure
When water is boiled, it converts into steam and rises. If the top of the container is sealed, the space above the water is filled with dry air and water vapor.
Water vapor is formed because of the evaporation of liquid water due to the heat supplied to it. When the air in this region is unsaturated, the rate of evaporation is greater than the rate of condensation. Unsaturated air can still hold more water vapor, so the water vapor produced by boiling is accommodated in the air above the water.
Unsaturated condition: Pv = partial pressure of water vapor
Saturated condition: Pv = Pvs = maximum vapor pressure
When the air becomes saturated, the rates of evaporation and condensation are equal. At saturation, the air contains the maximum amount of water vapor it can hold. The pressure exerted by the water vapor is then maximum.
Density
Density is the mass per unit volume of air.
ρ = m / V
At standard temperature and pressure, the density of air is approximately 1.225 kg/m³. In imperial units, it is approximately 0.0765 lb/ft³.
7. Heat Effects in Moist Air
Heat can affect a substance in two ways. First, heat may increase a substance’s temperature. For example, consider water at any temperature between 0°C and 100°C. When this water is heated, its temperature increases.
Second, heat may change a substance’s phase while the temperature remains constant. For example, ice at 0°C changes into water at 0°C when heat is supplied. In this case, the phase changes, but the temperature remains constant.
When heat increases the temperature, it is called sensible heat, and the process is called sensible heating. When heat causes a phase change, it is called latent heat, and the process is called latent heating.
When a sample of moist air is heated, part of the heat increases the temperature of the dry air. This part is called sensible heat. The remaining part is used to convert liquid water into water vapor. This part is called latent heat.
Coil Performance: Bypass Factor and Contact Factor
Bypass Factor of Cooling and Heating Coils
Consider a cooling coil through which air is passed. Let Ti be the temperature of air entering the coil, To be the temperature of air leaving the coil, and Tc be the temperature of the coil. For cooling to occur, the coil temperature must be lower than the inlet air temperature.
In an ideal situation, the outlet air temperature should equal the coil temperature. This is possible only if all the air comes in contact with the coil. In reality, some amount of air always passes through the coil without contacting its surface. Therefore, the outlet air temperature is not equal to the coil temperature.
Cooling coil: BPF = (To – Tc) / (Ti – Tc)
Heating coil: BPF = (Tc – To) / (Tc – Ti)
The value of the bypass factor ranges from 0 to 1. If the bypass factor is 0, all the air has come into contact with the coil, and the outlet air temperature equals the coil temperature. This is an ideal condition and cannot be achieved in practice. If the bypass factor is 1, none of the air has contacted the coil, which is highly undesirable.
Contact Factor
Bypass factor gives the fraction of air that has not come in contact with the coil. Once the bypass factor is known, the contact factor can be found.
Contact factor = 1 – Bypass factor
The value of the contact factor lies between 0 and 1. If the contact factor is 0, none of the air has come in contact with the coil, and no cooling or heating has taken place. If the contact factor is 1, 100 percent of the air has come in contact with the coil. This is a highly efficient condition, but it is an ideal case and cannot be achieved in practice.
A lower bypass factor means a higher coil efficiency. A higher contact factor means a higher efficiency of the cooling or heating coil.
Formula Review Sheet
The formulas below are repeated in clean text form so that all subscripts, superscripts, and variable references remain readable outside the figures.
| Concept | Formula |
|---|---|
| Ideal gas equation | PV = nRT |
| Humidity ratio | W = mv / mda |
| Humidity ratio using vapor pressure | W = 0.6219 Pv / (PT - Pv) |
| Relative humidity | RH = (mv / mvs) × 100% |
| Relative humidity using pressure | RH = (Pv / Pvs) × 100% |
| Degree of saturation | μ = W / Ws |
| Absolute humidity | AH = mv / V |
| Wet-bulb depression | DBT - WBT |
| Specific volume | v = V / mda |
| Enthalpy | H = U + PV |
| Enthalpy change at constant pressure | ΔH = m Cp ΔT |
| Specific enthalpy | h = H / mda |
| Specific enthalpy of moist air | h = hda + W hv |
| Moist-air enthalpy in imperial units | h = 0.240t + W(1061 + 0.444t) BTU/lb |
| Specific entropy | s = S / m |
| Density | ρ = m / V |
| Cooling-coil bypass factor | BPF = (To - Tc) / (Ti - Tc) |
| Heating-coil bypass factor | BPF = (Tc - To) / (Tc - Ti) |
| Contact factor | CF = 1 - BPF |
Summary
Psychrometry explains the behavior of moist air by studying properties such as temperature, humidity, specific volume, enthalpy, entropy, vapor pressure, density, sensible heat, latent heat, bypass factor, and contact factor.
Dry air becomes moist air when water vapor is added. Air may be unsaturated or saturated depending on whether it contains less than or equal to the maximum moisture it can hold. Humidity ratio, relative humidity, degree of saturation, and absolute humidity describe the moisture content of air in different ways.
Temperature concepts such as dry-bulb temperature, wet-bulb temperature, wet-bulb depression, and dew-point temperature are essential for understanding air-conditioning and refrigeration processes.
Specific volume, enthalpy, entropy, vapor pressure, density, sensible heat, and latent heat explain how air stores energy, occupies volume, and changes condition. Bypass factor and contact factor describe how effectively air interacts with cooling or heating coils.
Together, these concepts form the foundation for understanding HVAC psychrometric processes.
Authored By:
Ramesh Bhandari
Mechanical Engineer
More FAQs About Psychrometry in HVAC
Why is psychrometry important in HVAC systems?
Psychrometry is important in HVAC systems because it helps describe the condition of air using properties such as dry-bulb temperature, wet-bulb temperature, relative humidity, humidity ratio, enthalpy, specific volume, and dew-point temperature.
What are the main psychrometric properties of moist air?
The main psychrometric properties of moist air include dry-bulb temperature, wet-bulb temperature, dew-point temperature, relative humidity, humidity ratio, specific volume, enthalpy, vapor pressure, density, and degree of saturation.
What is the difference between dry air and moist air?
Dry air is air without water vapor. Moist air is a mixture of dry air and water vapor. In HVAC psychrometry, moist air is studied because real atmospheric air usually contains some amount of moisture.
What is the formula for humidity ratio in psychrometry?
The basic humidity ratio formula is W = mv / mda, where mv is the mass of water vapor and mda is the mass of dry air.
How is humidity ratio different from absolute humidity?
Humidity ratio compares the mass of water vapor to the mass of dry air. Absolute humidity compares the mass of water vapor to the total volume of moist air. Humidity ratio is usually written as W = mv / mda, while absolute humidity is written as AH = mv / V.
Why does relative humidity decrease when temperature increases?
Relative humidity decreases when temperature increases because warmer air can hold more water vapor. If the actual moisture content remains the same but the air temperature rises, the maximum moisture-holding capacity increases, so the relative humidity becomes lower.
What happens when air becomes saturated?
Air becomes saturated when it contains the maximum amount of water vapor it can hold at a given temperature and pressure. At saturation, relative humidity is 100%, and if cooling continues, excess water vapor begins to condense.
What is the difference between wet-bulb temperature and dew-point temperature?
Wet-bulb temperature is the lowest temperature air can reach by evaporative cooling at constant pressure. Dew-point temperature is the temperature at which air becomes saturated and water vapor starts to condense into liquid water.
Why are dry-bulb temperature and wet-bulb temperature equal at 100% relative humidity?
Dry-bulb temperature and wet-bulb temperature are equal at 100% relative humidity because saturated air cannot accept more water vapor. Since evaporation from the wet wick does not occur, there is no evaporative cooling, and both temperatures become the same.
What is specific volume of moist air in HVAC?
Specific volume of moist air is the volume occupied by moist air per unit mass of dry air. It is commonly written as v = V / mda, where V is volume and mda is the mass of dry air.
What is enthalpy of moist air?
Enthalpy of moist air is the total heat content of air, including the heat content of dry air and the heat content of water vapor. In psychrometry, moist-air enthalpy is commonly represented as h = hda + W hv.
What is sensible heat in psychrometry?
Sensible heat is the heat that changes the temperature of air without changing its phase. In HVAC, sensible heating or cooling changes dry-bulb temperature.
What is latent heat in psychrometry?
Latent heat is the heat involved in a phase change, such as water evaporating into water vapor or water vapor condensing into liquid water. In HVAC, latent heat is directly related to moisture removal or moisture addition.
What is vapor pressure in moist air?
Vapor pressure is the pressure exerted by water vapor in moist air. In unsaturated air, water vapor exerts partial pressure. At saturation, the vapor pressure reaches its maximum value for that temperature.
What is bypass factor of a cooling coil?
Bypass factor of a cooling coil is the fraction of air that passes through the coil without effectively contacting the coil surface. For a cooling coil, it may be written as BPF = (To - Tc) / (Ti - Tc).
What is bypass factor of a heating coil?
Bypass factor of a heating coil represents the fraction of air that does not effectively contact the heating coil surface. For a heating coil, it may be written as BPF = (Tc - To) / (Tc - Ti).
How are bypass factor and contact factor related?
Bypass factor and contact factor are related by the equation CF = 1 - BPF. A lower bypass factor means a higher contact factor and better coil performance.
How does psychrometry help in air-conditioning design?
Psychrometry helps in air-conditioning design by showing how air temperature, humidity, enthalpy, and moisture content change during heating, cooling, humidification, dehumidification, and coil operation.
What is a psychrometric chart used for?
A psychrometric chart is used to find and compare air properties such as dry-bulb temperature, wet-bulb temperature, relative humidity, humidity ratio, specific volume, dew-point temperature, and enthalpy.