Psychrometrics and Thermal Comfort
A clear introduction to psychrometrics — the science of moist air — explaining the key properties, the psychrometric chart, the comfort zone, and how cooling coils both cool and dehumidify air in the UAE climate.
Psychrometrics is the study of moist air: how temperature and moisture combine to define the air we condition. It is the language HVAC engineers use to describe what air conditioning actually does, because comfort depends not only on temperature but also on humidity. In a humid coastal climate like the UAE, managing moisture is as important as managing heat.
Thermal comfort is the condition in which most occupants feel neither too warm nor too cold and humidity feels acceptable. Reaching it reliably requires understanding the properties of air and the processes — cooling, dehumidifying, sometimes reheating — that move air into the comfort zone.
How it works
A few key properties describe any state of moist air. Dry-bulb temperature is what an ordinary thermometer reads. Wet-bulb temperature reflects the cooling effect of evaporation and depends on humidity. Relative humidity (RH) is how much moisture the air holds compared with the maximum it could hold at that temperature. Humidity ratio is the actual mass of water vapour per kilogram of dry air, and dew point is the temperature at which that moisture would start to condense.
The psychrometric chart plots these properties together so any air state is a single point. Dry-bulb runs along the bottom and humidity ratio up the side; curved lines show constant relative humidity, with the top curve being 100% RH (saturation). Engineers draw processes as lines on the chart to see exactly how the air changes.
The comfort zone is a region on the chart — broadly around 22–26 °C with relative humidity roughly between 40% and 60%, adjusted for clothing, activity and air movement. Standards such as ASHRAE 55 define comfort more precisely, but the principle is that both temperature and humidity must sit in an acceptable band.
Cooling and dehumidification is the main process in the Gulf. When humid air is blown across a cooling coil below its dew point, the air both cools (sensible) and loses moisture as water condenses on the coil (latent). On the chart this appears as a line moving down and to the left — lower temperature and lower humidity — which is why air conditioning produces condensate that must be drained away.
Sometimes air is reheated or humidified. After deep dehumidification, supply air may be slightly reheated to avoid overcooling a space, and in dry climates or specialised rooms humidity may be added. Each of these is another process line on the chart, and good design uses the minimum energy to land the air in the comfort zone.
Main types
In the UAE
- High coastal humidity in the UAE makes dehumidification a major part of air conditioning, so cooling coils are selected to handle a significant latent load, not just to lower temperature.
- Maintaining humidity in a comfortable, healthy band also limits condensation and mould risk, which matters in the humid climate and supports indoor-air-quality goals under Estidama.
- Designing to recognised comfort criteria (e.g. ASHRAE 55) and minimising reheat energy aligns with UAE energy efficiency and sustainability objectives.
How GPR applies this
GPR uses psychrometric analysis to design air conditioning that delivers genuine comfort in the UAE climate, not just a cold air temperature. We size cooling coils for both the sensible and latent (moisture) load, set supply conditions that land each space in the comfort zone, and control humidity to limit condensation and mould — coordinating coil selection, airflow and controls so systems meet comfort criteria efficiently across Abu Dhabi projects.
Frequently asked questions
What is psychrometrics in simple terms?
It is the science of moist air — how temperature and moisture together define the air we condition. HVAC engineers use it to describe and design what air conditioning does to the air.
What is the difference between relative humidity and humidity ratio?
Relative humidity is the percentage of moisture present versus the maximum the air could hold at that temperature; humidity ratio is the actual mass of water vapour per kilogram of dry air.
What is the comfort zone?
A range of temperature and humidity in which most people feel comfortable — broadly around 22–26 °C and 40–60% RH, adjusted for clothing, activity and air movement, per standards like ASHRAE 55.
Why does air conditioning remove moisture, not just heat?
When humid air is cooled below its dew point on the coil, water condenses out. In the humid UAE this dehumidification is essential for comfort, which is why AC units produce condensate.
Why is humidity control important in the UAE?
High humidity feels uncomfortable and raises condensation and mould risk. Controlling it within a healthy band is a core part of comfort and indoor air quality in the Gulf climate.