Chapter 4: Psychrometrics
📍 The Science of Moist Air Properties in HVAC
🔹 4.1 What is Psychrometrics?
Psychrometrics is the study of moist air—specifically, the physical and thermal properties of air mixed with water vapor.
In HVAC, it helps to:
Analyze indoor comfort
Design air conditioning systemsCalculate humidity, dew point, enthalpy, and more
📌 Why Important?
Human comfort depends not just on temperature but also on humidity. Psychrometrics helps balance both.
🔹 4.2 Key Properties of Air (Psychrometric Terms)
| Term | Definition |
|---|---|
| Dry Bulb Temperature (DBT) | The temperature of air measured by a thermometer without moisture effect |
| Wet Bulb Temperature (WBT) | Temperature measured with a moist thermometer — shows evaporative cooling effect |
| Dew Point Temperature (DPT) | Temp at which water vapor condenses into liquid (air becomes saturated) |
| Relative Humidity (RH) | % of moisture in the air compared to its maximum capacity at a given temp |
| Humidity Ratio (W) | Amount of water vapor in kg per kg of dry air |
| Enthalpy (h) | Total heat content in the air (includes sensible + latent heat) |
| Specific Volume (v) | Volume of air per kg of dry air |
🔹 4.3 Psychrometric Chart
A Psychrometric Chart is a graphical representation of all the air properties. It helps in:
Visualizing how air changes during cooling/heating
Plotting HVAC processes like dehumidification or humidification🧩 Main Processes Represented on the Chart:
Sensible Heating/Cooling → Horizontal lines
Latent Heating/Cooling → Vertical or curved linesDehumidification → Downward and to the left
Humidification → Upward and to the right
📌 Real-World Use: Engineers use this chart to:
Design AC units
Select dehumidifiersControl moisture in buildings
🔹 4.4 HVAC Processes Using Psychrometrics
| Process | What Happens? | Example |
|---|---|---|
| Sensible Cooling | Lowering dry bulb temp only | Room air cooled without moisture change |
| Latent Cooling | Removing moisture (dehumidification) | AC removes humidity in humid climates |
| Sensible Heating | Raising dry bulb temp | Room heater in winter |
| Humidification | Adding moisture to air | Steam humidifiers in dry areas |
| Adiabatic Cooling | Cooling using evaporation | Desert coolers (No change in enthalpy) |
🔹 4.5 Real-Life Examples of Psychrometrics in HVAC
In AC Design: Engineers calculate how much heat to remove (sensible + latent).
In Hospitals: Maintain strict humidity levels (40–60%) to avoid infections.In Supermarkets: Use dehumidifiers to avoid fog on freezers.
In Data Centers: Precise RH control to prevent static or condensation.
🔹 4.6 Tools for Psychrometric Analysis
Psychrometric Chart
Digital Psychrometer – measures DBT, WBT, and RHSoftware Tools – Carrier HAP, Trane Trace, ISHRAE software
🛠️ Practical Tips
Use a sling psychrometer to measure DBT and WBT.
Try plotting a cooling process (from 32°C DBT, 60% RH to 24°C DBT, 50% RH) on a psychrometric chart.Observe the condensate from your AC — that’s latent heat removal!
📄 Assignment Questions
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Define Dry Bulb and Wet Bulb Temperature.
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What is Relative Humidity? Why is it important in HVAC?
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List 3 HVAC processes shown on the psychrometric chart.
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What happens to air when it is cooled below its dew point?
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How does a dehumidifier work?
💼 Interview Questions
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What is the role of psychrometrics in HVAC design?
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Explain the difference between sensible and latent heat.
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Why is Dew Point temperature important in AC systems?
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What does the psychrometric chart show?
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How does latent cooling occur in an AC?