Author: Mycond Technical Department
Imagine a hot day: you take a glass of cold water out of the fridge. After a few minutes, the surface of the glass is covered with water droplets. Where did this water come from? It’s the condensation of water vapour from the air — one of the processes studied by psychrometrics.
Psychrometrics is the science of the properties and behaviour of moist air. Essentially, it’s the “user manual” for air containing water vapour. For HVAC engineers, understanding psychrometrics is as essential as maths is for an accountant.
Using psychrometric knowledge, engineers solve numerous practical tasks: they calculate how much water will condense on a supermarket’s refrigeration equipment; determine the optimal temperature and humidity for comfort in an office; explain why flats are so dry in winter that lips crack; develop strategies to prevent mould in bathrooms; calculate the energy needed to dehumidify air in pharmaceutical manufacturing.
Seven key parameters of moist air
To fully describe the state of air, you need to know several interrelated characteristics. Let’s consider a typical example: 1 kilogram of air in a living room at a temperature of 21°C and a relative humidity of 50%. We will analyse the seven most important parameters of this air.
1. Dry Bulb Temperature
This is the ordinary air temperature measured by a standard thermometer. Denoted as t or T, measured in °C. When we say “the room is +21°C”, we mean the dry-bulb temperature. On the psychrometric chart, this is the horizontal axis at the bottom, where temperature increases from left to right.
Practical significance: this is the primary parameter of thermal comfort. People feel best at 20–24°C in winter and 23–26°C in summer.
2. Relative Humidity
Relative humidity is the percentage of the maximum possible amount of water that air can hold at a given temperature. The key point: the term “relative” means this parameter depends on temperature, which often leads to misunderstandings.
Denoted as RH or φ, measured in percent (%). To explain, imagine a sponge: at 21°C the sponge can hold a maximum of 100 units of water (that’s 100% humidity), and if it currently contains 50 units — that’s 50% RH. If you heat the sponge to 30°C, it can now hold 200 units, but the water remains at 50 units, so RH is now 50/200 = 25%.
On the chart: curved lines running from the lower left to the upper right. The highest line (100% RH) is the saturation line. Comfort range for people: 40–60% RH. Below 30% — too dry (manifests as dry skin, static electricity), above 70% — too humid (promotes mould growth, feels stuffy).
3. Humidity Ratio (Specific Humidity)
The humidity ratio is the actual physical amount of water vapour in grams per kilogram of dry air. Denoted as d, w, or x, measured in g/kg. Unlike relative humidity, the humidity ratio does not depend on temperature — it is an absolute quantity.
In our example: at 21°C and 50% RH, the humidity ratio is 7.8 g/kg. This means that 1 kg of dry air contains 7.8 g of water vapour. If this air is heated to 30°C, the humidity ratio remains 7.8 g/kg, but RH decreases to approximately 27%.
4. Dew Point Temperature
The dew-point temperature is the temperature to which air must be cooled for it to become saturated (100% RH) and for moisture to start condensing. Denoted as Td, measured in °C.
Real-life example: when the surface temperature of a glass of cold water drops below the dew point of the room air, water vapour condenses on the glass. In our example, air at 21°C, 50% RH, 7.8 g/kg has a dew point of +10°C. This means condensation will form on any surface at +10°C or below.
For engineers, this is a critically important parameter. It helps solve condensation problems on windows and hidden condensation in walls, which can lead to freezing, mould, degradation of thermal insulation and corrosion of metal elements.
5. Vapour Pressure
This is the partial pressure exerted by water vapour molecules. Denoted as pv, measured in Pa or kPa. Each water molecule exerts pressure, so the more water molecules in the air, the higher the vapour pressure.
In our example: a humidity ratio of 7.8 g/kg corresponds to a vapour pressure of 1240 Pa = 1.24 kPa. This parameter is important for understanding moisture diffusion through materials: moisture moves from a region of higher vapour pressure to a region of lower pressure (similar to air escaping from a punctured tyre).
6. Enthalpy
Enthalpy is the total energy of the air, which includes both the heat of the air itself (sensible heat) and the heat used for water evaporation (latent heat). Denoted as h or i, measured in kJ/kg.
In our example: at a temperature of 21°C and a humidity ratio of 7.8 g/kg, the enthalpy is 41 kJ/kg, of which sensible heat is ~21 kJ/kg and latent heat is ~20 kJ/kg.
Practical application — calculating air-conditioner load using the formula: Cooling capacity (kW) = Airflow (kg/s) × Enthalpy difference (kJ/kg).
7. Wet Bulb Temperature
This is the temperature shown by a thermometer wrapped in a wet cloth through which air passes. Denoted as Tw, measured in °C. When water evaporates from the cloth, it absorbs heat, cooling the thermometer. The drier the air, the more intense the evaporation and the lower the wet-bulb temperature.
In our example: dry-bulb temperature is 21°C, RH 50%, wet-bulb temperature is 15°C. This parameter is used to measure humidity simply with a psychrometer and to calculate evaporative cooling systems.
The psychrometric chart — a map of moist air
The psychrometric chart (or Mollier diagram) is a graphical tool that shows the interrelationship of all moist air parameters. The main rule of use: if any two parameters are known, all the others can be determined.
Example: knowing T = 21°C and RH = 50%, on the horizontal axis find 21°C, draw a vertical line upwards, and find the intersection with the 50% RH curve. This point gives all parameters: d = 7.8 g/kg, Td = 10°C, h = 41 kJ/kg, Tw = 15°C.
Practical examples for HVAC engineers
Example 1 — Cooling and dehumidifying air with an air conditioner
Task: outdoor air with parameters 32°C and 70% RH needs to be cooled to 18°C.
Step 1: Determine the initial parameters from the chart: T₁ = 32°C, RH₁ = 70%, d₁ = 21 g/kg, h₁ = 85 kJ/kg, Td₁ = 26°C.
Step 2: Analyse the cooling process. The air passes through an evaporator with a surface temperature of +8°C. Initially, the air cools at a constant humidity ratio (movement vertically down). Upon reaching the dew point (26°C) condensation begins. Thereafter, the air cools along the 100% RH line, and at 8°C we obtain: d₂ = 6.5 g/kg, RH = 100%.
Step 3: Calculate the amount of condensate: moisture removed = d₁ − d₂ = 21 − 6.5 = 14.5 g/kg. At a flow rate of 1000 m³/h (≈ 1200 kg/h): condensate = 1200 × 14.5 / 1000 = 17.4 kg/h = 17.4 litres/h.
Step 4: Calculate the required energy: cooling capacity = 1200/3600 kg/s × (85 − 22) kJ/kg = 0.333 × 63 = 21 kW ≈ 6 tonnes of refrigeration.
Example 2 — Why indoor air is dry in winter
Situation: winter, it’s −5°C outside and 80% RH. This air enters the flat via ventilation and is heated to 21°C.
Step 1: Outdoor air parameters: T₁ = −5°C, RH₁ = 80%, from the chart d₁ = 2.2 g/kg, Td₁ = −8°C.
Step 2: When heated to room temperature, the humidity ratio remains unchanged (movement horizontally to the right): T₂ = 21°C, d₂ = 2.2 g/kg, from the chart RH₂ = 14% — very dry!
Conclusion: the problem is not low outdoor relative humidity (it’s as high as 80% RH there!), but that cold air physically contains little water. When heated, this small amount is distributed in a larger volume of warm air, yielding a low relative humidity.
Psychrometrics FAQ
What is psychrometrics in simple terms?
Psychrometrics is the science of the properties of moist air, i.e., a mixture of air and water vapour. It helps to understand how air interacts with moisture at different temperatures and pressures.
Why doesn’t relative humidity show the actual amount of water in the air?
Because relative humidity depends on temperature. The same amount of water in the air will give different relative humidity at different temperatures. To determine the actual amount of water, use the humidity ratio (g/kg).
How can I quickly determine the dew point without a chart?
You can use a simplified formula: Td ≈ T − ((100 − RH) / 5), where T is the air temperature, RH is the relative humidity. For example, at T = 21°C and RH = 50%, Td ≈ 21 − ((100 − 50) / 5) = 21 − 10 = 11°C.
What’s the difference between sensible and latent heat?
Sensible heat is the energy associated with a temperature change (heating/cooling). Latent heat is the energy associated with a phase change of water (evaporation/condensation) without a temperature change.
Conclusions — why an HVAC engineer needs psychrometrics
Understanding psychrometrics is critically important for HVAC engineers for four key reasons:
1. System design: Without psychrometrics it’s impossible to correctly calculate the cooling capacity of air conditioners, the performance of dehumidifiers, the power of humidifiers, and other ventilation system parameters.
2. Energy savings: The chart makes it possible to determine the optimal air-handling strategy, find opportunities for free cooling/dehumidification, and assess the effectiveness of recuperators.
3. Problem prevention: Understanding the dew point prevents condensation in ventilation systems, wall freezing, mould growth, and equipment corrosion.
4. Air quality control: The right combination of temperature and humidity ensures people’s comfort, preservation of materials and equipment, and compliance with technological requirements.
The basic rule: to fully determine the state of air, you need to know at least two parameters; all the others can be found from the chart.
Psychrometrics is not abstract theory but the engineer’s daily tool that helps make the right decisions, save energy, and create comfortable and safe indoor conditions.
