Psychrometrics: Understand Moist Air with Easy Chart Guide

🤔 What Is Psychrometrics?

Psychrometrics is the science of studying moist air. It helps us figure out how temperature, humidity, and air movement interact — all critical for designing HVAC systems that keep us comfy indoors! 🏠❄️🔥

It answers questions like:

• Why does a room feel “sticky” even if it’s not hot? 🥵
• How do air conditioners and dehumidifiers actually work? 🧊🌫️
• What’s happening to air as it moves through ducts and coils? 🌀

So, if you’re into air conditioning, refrigeration, or indoor air quality — this is your bread and butter 🍞🧈.

🌫️ What is Moist Air?

Moist air is simply:

💨 Dry air + 💦 Water vapor

Let’s break it down:

• Dry air = gases like nitrogen, oxygen, argon, CO₂ (basically, regular air without moisture).
• Water vapor = invisible moisture in the air — not mist or fog, but actual gaseous H₂O.

📌 In HVAC design, we treat moist air as a perfect gas mixture, which makes calculations easier while still being accurate enough!

📘 Key Terms You Need to Know (with Formulas!)

Let’s tackle the big players in the psychrometrics world:

🌡️ 1. Dry-Bulb Temperature (Tdb)

This is the regular air temperature measured by a standard thermometer.
📏 Unit: °C or K
Used to determine how “hot” or “cold” the air feels.

💧 2. Wet-Bulb Temperature (Twb)

This is the temperature of air if it were cooled by evaporation only.
Measured using a thermometer wrapped in a wet cloth and exposed to airflow.
It tells you how much moisture the air can still absorb.
📌 Always lower than or equal to Tdb.

🌫️ 3. Dew Point Temperature (Tdp)

The temperature at which air becomes saturated, and water starts to condense 🌧️.
It’s a key indicator of how “humid” the air feels.

🧪 4. Humidity Ratio (W)

Also called the mixing ratio — it’s how much water vapor is present per kg of dry air.

🧮 Formula:

📌 Unit: kg of water vapor / kg of dry air

🌈 5. Relative Humidity (φ)

This is the percentage of how “full” the air is with water vapor, compared to the max it could hold at the same temperature.

🧮 Formula (using perfect gas law):

🎈 6. Specific Volume (v)

Volume of moist air per kg of dry air.

🧮 Formula:

🔥 7. Enthalpy (h)

It represents the energy content of moist air (sensible + latent heat).

🧮 Approximate Formula:

🧭 1. What is this Chart, Really?

This chart is a visual tool that shows the thermodynamic properties of moist air. If you’re designing, analyzing, or troubleshooting HVAC systems, this is your go-to guide for understanding how air behaves under different conditions.

Imagine it like Google Maps—but for air! 🌬️

🌡️ 2. Key Terms to Know First (Jargon Buster!)

Let’s decode the HVAC alphabet soup:

• Dry-Bulb Temperature (DBT) – 📏 Measured by a regular thermometer. Shown horizontally at the bottom (x-axis). Unit: °C
• Humidity Ratio (W) – 💧 Mass of water vapor per kg of dry air. Shown vertically on the right (y-axis). Unit: g/kg
• Relative Humidity (RH) – 🌫️ % of how “full” the air is with moisture. Curved lines sweeping from top left to bottom right.
• Saturation Line – 💦 The 100% RH line at the top-left curved boundary. Air can’t hold more moisture here—condensation begins.
• Wet-Bulb Temperature (WBT) – 🌧️ Temperature sensed by a wet thermometer. Sloped diagonal lines.
• Enthalpy (h) – 🔥 Total heat content (sensible + latent). Diagonal lines running parallel to WBT. Unit: kJ/kg
• Specific Volume (v) – 🎈 Volume occupied per kg of dry air. Curved lines sweeping downward. Unit: m³/kg
• Dew Point – ❄️ Temp at which air becomes saturated (100% RH) and water condenses.

🧱 3. Axes and Their Meaning

➤ X-Axis: Dry-Bulb Temperature (°C)

• From 0°C to 50°C
• Tells you how "hot" or "cold" the air feels

➤ Y-Axis (Right Side): Humidity Ratio (g/kg)

• From 0 to ~30 g/kg
• Tells you how “moist” the air is (how much water vapor is in it)

🧮 4. How to Read a Point on the Chart

Let’s say you're told:

• DBT = 25°C
• RH = 50%

🎯 Steps:

1. Start at DBT = 25°C on the x-axis.
2. Move up vertically until you hit the 50% RH curve.
3. That intersection point is your state point.

Now at that point, you can read:

• Wet-Bulb Temp (follow sloped lines)
• Enthalpy (read diagonal lines in kJ/kg)
• Humidity Ratio (read across to y-axis)
• Dew Point Temp (move horizontally left to saturation curve)
• Specific Volume (curved lines labeled 0.83, 0.85, etc.)

🔴 Red Line – Dry-Bulb Temperature (DBT)

• This is the vertical reference line starting from 25°C on the x-axis.
• Represents the actual air temperature (sensed by a regular thermometer).
  📍 You always start your readings vertically from this line.

🔵 Blue Line – Relative Humidity (RH) Curve (50%)

• The blue curve intersecting the red line represents 50% RH.
• It’s where your air state lies if the air holds half the moisture it can at 25°C.
  📌 RH lines curve from bottom right toward the upper left saturation line.

🟢 Green Line – Wet-Bulb Temperature (WBT)

• This sloped line cutting diagonally upward left represents the wet-bulb temperature, which is around 17.5°C here.
• Tells how much cooling is possible through evaporation only, without adding/reducing moisture.

🌸 Pink Line – Dew Point Temperature (DPT)

• Horizontal line drawn from the state point (intersection of red and blue) to the saturation curve.
• Tells you at which temperature condensation would begin.
  📌 In this case, DPT ≈ 15°C.

⚫Black Line – Enthalpy Line (Total Heat, h)

• A diagonal line running almost parallel to the wet-bulb line, intersecting the enthalpy scale on the left.
• Enthalpy is total heat content of the air (sensible + latent).
  📍 From this point:

🟡 Cyan Line – Specific Volume Line (v)

• This curved line moves downward from the right.
• Represents air volume per kg of dry air.
  📍 At 25°C, 50% RH, specific volume ≈ 0.86 m³/kg