PsychroLab — Technical Thermodynamics Analysis and HVAC Engineering Tools for Engineers Technicians and Students

Interactive Psychrometric Chart

Sat Curve (100% RH) RH % Lines Enthalpy (h) Lines Process Path

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Custom Pair State Point Setup

Property 1

°C

Property 2

Property 1

°C

Property 2

Property 1

°C

Property 2

Point Delta Process Analyzer

Cooling & Dehumidification

Start Point (Point A)

End Point (Point B)

Δ Dry-Bulb Temp

-23 °C

Δ Humidity Ratio

-0.01315 kg/kg

Δ Enthalpy

-57.24 kJ/kg

Δ Rel. Humidity

+35 %

Thermodynamic State Data Summary

Property Name	Symbol	Unit	State 1 (Outdoor Air)	State 2 (Coil Leaving)	State 3 (Supply / Mixed)
Dry-Bulb Temperature	T_db	°C	35	12	22
Wet-Bulb Temperature	T_wb	°C	28.17	11.55	15.43
Dew Point Temperature	T_dp	°C	26.07	11.22	11.11
Relative Humidity	RH	%	60 %	95 %	50 %
Humidity Ratio	W	kg/kg	0.02144	0.00829	0.00822
Specific Enthalpy	h	kJ/kg	90.23	32.99	43.04
Specific Volume	v	m³/kg	0.903	0.819	0.847
Vapor Partial Pressure	p_v	kPa	3.377	1.332	1.322

Engineering Guide to Psychrometric Analysis

ASHRAE Fundamentals, Sensible Heat Ratio (SHR) & Real-World HVAC Equipment

Open Mollier Diagram

1. Fundamentals of Moist Air & Governing Variables

Psychrometrics is the branch of thermodynamics evaluating moist air—a binary mixture of dry air and water vapor. Standard sea-level psychrometric charts and Mollier diagrams represent state points governed by Dalton's law of partial pressures at 101.325 kPa (14.696 psi).

Dry-Bulb Temp (T_db)

True thermodynamic ambient air temperature.

Wet-Bulb Temp (T_wb)

Dynamic equilibrium temperature reached by adiabatic evaporation.

Dew Point Temp (T_dp)

Saturation temperature where vapor condenses into liquid water.

Relative Humidity (RH)

Ratio of partial vapor pressure p_v to saturation pressure p_ws.

Humidity Ratio (W)

Mass of moisture per unit mass dry air (kg/kg or grains/lb).

Enthalpy (h)

Total internal energy content (h = c_paT + W·h_g).

2. Sensible Heat Ratio (SHR) & Coil Load Vector

The Sensible Heat Ratio (SHR) defines the proportion of sensible cooling capacity to total cooling capacity (sensible + latent) required by a conditioned zone or cooling coil:

SHR = Q_sensible / Q_total = Q_sensible / (Q_sensible + Q_latent)

SHR = (c_pa · ΔT_db) / Δh_total

High SHR (0.85 – 1.0)

High sensible heat load with negligible moisture addition (e.g. Data Centers, Server Rooms, Dry Climates).

Standard Comfort SHR (0.70 – 0.80)

Standard human occupancy with combined sensible heat (lighting, people, envelope) and latent sweat/breathing loads (e.g. Commercial Offices, Classrooms).

Low SHR (0.50 – 0.65)

Heavy latent moisture loads requiring intensive dehumidification (e.g. Indoor Swimming Pools, Hot Yoga Studios, Nightclubs, Kitchens).

3. Thermodynamic Air Conditioning Processes & Real Equipment Examples

Sensible Heating (Horizontal Right: ΔT_db > 0, ΔW = 0)

Heat added to moist air with zero moisture addition. Relative humidity drops while dry-bulb temperature increases at constant absolute humidity.

Real Devices: Electric resistance duct heaters, hot water heating coils (hydronic boiler loops), gas-fired furnace heat exchangers, hydronic perimeter baseboards.

Sensible Cooling (Horizontal Left: ΔT_db < 0, ΔW = 0)

Heat removed from air without moisture condensation. The cooling coil surface temperature remains strictly above the entering air dew point temperature (T_coil > T_dp).

Real Devices: Chilled water dry coils in high-temperature chilled water systems (radiant chilled ceiling panels), air-to-air plate heat recovery exchangers.

Cooling & Dehumidification (Down & Left: ΔT_db < 0, ΔW < 0)

Moist air passes across a coil operating below its entering dew point temperature (T_coil < T_dp). Water vapor condenses out as condensate while air temperature drops.

Real Devices: DX (Direct Expansion) evaporator coils in rooftop units (RTU), standard chilled water AHU cooling coils (6°C/12°C chilled water), computer room air conditioners (CRAC).

Heating & Humidification (Up & Right: ΔT_db > 0, ΔW > 0)

Simultaneous heat and water vapor addition. Often implemented in cold winter climates to prevent excessively dry indoor environments and static shocks.

Real Devices: Isothermal steam injection humidifiers (electrode boiler steam generators, gas-fired steam humidifiers) injected into warm AHU supply airstreams.

Evaporative Cooling / Adiabatic Saturation (Δh ≈ 0)

Liquid water evaporates into the air without external heat addition. Sensible heat is converted into latent heat: dry-bulb temperature drops while humidity ratio rises along constant wet-bulb lines.

Real Devices: Direct evaporative media pads (swamp coolers), open cooling towers, high-pressure ultrasonic atomizing fogging nozzles, adiabatic indirect coolers.

Heating & Dehumidification / Desiccant Drying (ΔT_db > 0, ΔW < 0)

Moisture is absorbed by a hygroscopic desiccant medium. Releasing the latent heat of adsorption warms the leaving air while driving humidity ratio to ultra-low dew points (-40°C).

Real Devices: Rotary desiccant dehumidifier wheels (silica gel / molecular sieve rotors in EV lithium battery dry rooms, pharmaceutical cleanrooms).

4. Carrier Psychrometric Chart vs. Mollier Diagram

Switch to Mollier View

While the American Carrier-style Psychrometric Chart plots dry-bulb temperature on the horizontal axis and humidity ratio on the vertical axis, European mechanical engineering standards (DIN 4107, VDI 4672) utilize the Mollier Diagram (h-x). In the Mollier Diagram, enthalpy (h) lines are tilted obliquely so that the 0°C isotherm lies perfectly horizontal, providing distinct clarity for process direction vectors (Δh/Δx) and steam injection trajectories.