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Evgenii Konkin
Evgenii Konkin

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The Four Psychrometric Processes Every HVAC Engineer Should Be Able to Plot

#hvac #engineering #psychrometrics #tutorial

The psychrometric chart plots six air properties on two axes. Every point on the chart represents a unique air condition. If you know any two properties, you can find the other four.

The two most useful starting inputs:

Dry-bulb temperature (horizontal axis)
Humidity ratio (vertical axis, right side)
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From these two, you derive:

Relative humidity (curved lines)
Wet-bulb temperature (diagonal lines)
Dew point temperature (horizontal projection to saturation curve)
Enthalpy (diagonal lines, left edge)
Specific volume (diagonal lines, less commonly used)
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The four basic HVAC processes

Every air handling operation maps to one of four movements on the chart.

1. Sensible heating 

Direction: horizontal, left to right
What changes: dry-bulb temperature increases
What stays: humidity ratio (no moisture added)
RH effect: decreases (warmer air, same moisture)
Example: duct heater, reheat coil
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2. Sensible cooling 

Direction: horizontal, right to left
What changes: dry-bulb temperature decreases
What stays: humidity ratio (no moisture removed yet)
RH effect: increases (cooler air, same moisture)
Limit: stops when air reaches saturation curve (100% RH)
Example: sensible cooling above dew point
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3. Cooling with dehumidification 

Direction: follows saturation curve downward
What changes: both temperature and humidity ratio decrease
What happens: moisture condenses on the coil surface
RH effect: stays near 100% along the saturation curve
Example: cooling coil operating below entering air dew point
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This is the most important process for comfort cooling. The coil must operate below the dew point of the entering air to remove moisture. If it only operates above the dew point, it provides sensible cooling only and the space humidity rises.

4. Humidification 

Direction: upward from any point
What changes: humidity ratio increases
Temperature effect: depends on humidification method
  Steam: nearly vertical (temperature stays almost constant)
  Adiabatic (spray): follows wet-bulb line (temperature drops)
Example: steam humidifier, spray chamber
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Why this matters for coil sizing

The sensible heat formula:

Q_sensible = 1.08 × CFM × ΔT_dry-bulb
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Only captures the horizontal movement on the chart.

The latent heat formula:

Q_latent = 0.68 × CFM × ΔW
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Where ΔW is the change in humidity ratio (grains/lb).

The total cooling load is:

Q_total = Q_sensible + Q_latent
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If you only calculate sensible load, you undersize the coil for any space with significant moisture removal needs.

Example: office cooling with dehumidification 

Entering air:  80°F dry-bulb, 67°F wet-bulb
                → humidity ratio: 78 gr/lb
                → RH: 51%

Leaving air:   55°F dry-bulb, 54°F wet-bulb
                → humidity ratio: 61 gr/lb
                → RH: 92%

CFM: 5,000
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Sensible component:

Q_s = 1.08 × 5,000 × (80 − 55)
Q_s = 135,000 BTU/hr
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Latent component:

Q_l = 0.68 × 5,000 × (78 − 61)
Q_l = 57,800 BTU/hr
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Total:

Q_total = 135,000 + 57,800 = 192,800 BTU/hr = 16.1 tons
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The latent load is 30% of the total. Sizing by sensible only would select a 11.25-ton coil instead of a 16.1-ton coil. The space would stay cool but humid.

Sensible Heat Ratio 

SHR = Q_sensible / Q_total
SHR = 135,000 / 192,800
SHR = 0.70
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An SHR of 0.70 means 70% of the cooling is sensible and 30% is latent. This ratio drives coil selection — manufacturers publish performance data at specific SHR values.


text
SHR > 0.90: mostly sensible (dry climates, server rooms)
SHR 0.70–0.85: mixed (typical offices, commercial)
SHR < 0.65: moisture-dominant (pools, kitchens, humid climates)
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