How CFD Eases Calculation of Key Thermal Comfort and Ventilation Parameters in Indoor Environments

Introduction

Designing HVAC systems that deliver comfortable and healthy conditions in indoor environments is a complex challenge. Architects, engineers, and ventilation product manufacturers must ensure their designs comply with international and Australian standards while maintaining energy efficiency and occupant satisfaction.

To achieve this, several thermal comfort and ventilation performance parameters are used, including ADPI, PVM, PPD, Throw, DR, EDT, and ACE. Each parameter has defined criteria in standards such as ASHRAE 55, ASHRAE 113, ASHRAE 129, AIRAH design guides, and AS 1668.2. Traditionally, measuring these metrics has required extensive experimental setups or simplified assumptions, which can be time-consuming and costly.

Computational Fluid Dynamics (CFD) now enables these calculations to be performed faster, more accurately, and with greater visual clarity, providing engineers and designers with deeper insights into how air moves and how occupants perceive indoor spaces.

3D model for occupational thermal comfort

Objectives

• Explain the key thermal comfort and ventilation parameters used in designing indoor environments.

• Describe how each parameter is measured or calculated and the relevant criteria from ASHRAE, AIRAH, and Australian standards.

• Demonstrate how CFD simplifies these calculations and enhances design accuracy.

• Showcase how Deratec’s CFD expertise helps clients optimise ventilation products and systems.

Key Parameters, Measurement Methods, and Standards

1. Air Diffusion Performance Index (ADPI)

• Definition: Percentage of points in the occupied zone with acceptable velocity and temperature, representing good air mixing without drafts.

• Measurement: Velocity and temperature are measured or simulated at multiple points 0.1–1.1 m above the floor.

• Standards: ASHRAE Standard 113, AIRAH HVAC Design Guide, AS 1668.2.

• Criteria: ADPI ≥ 80% is typically considered excellent.

2. Predicted Velocity Magnitude (PVM)

• Definition: Expected air velocity at occupant level, used to limit draft risk.

• Measurement: Measured by anemometers or calculated in CFD at head height.

• Standards: ASHRAE 55, AIRAH Thermal Comfort Handbook.

• Criteria: Indoor air velocities should generally be 0.15–0.25 m/s.

PVM index definition (ASHRAE 55)

3. Predicted Percentage of Dissatisfied (PPD)

• Definition: The estimated percentage of occupants likely to feel thermally uncomfortable.

• Measurement: Calculated using Fanger’s model with temperature, humidity, and velocity inputs from CFD or field data.

• Standards: ASHRAE 55, ISO 7730.

• Criteria: PPD ≤ 10% indicates high comfort.

PVM vs PPD (ASHRAE 55)

4. Throw

• Definition: The distance air travels from a diffuser before slowing to a target velocity (commonly 0.15 m/s).

• Measurement: Velocity decay along diffuser jet paths from CFD simulations or lab measurements.

• Standards: ASHRAE HVAC Applications Handbook, AS 1668.2.

• Criteria: Diffusers must be spaced to achieve overlap without drafts.

Throw definition

5. Draft Rate (DR)

• Definition: The percentage of occupants likely to feel uncomfortable from excessive local air movement.

• Measurement: Derived from CFD velocity fields or measured velocities with temperature conditions.

• Standards: ASHRAE 55, AIRAH guidelines.

• Criteria: DR < 15% is acceptable.

6. Effective Draft Temperature (EDT)

• Definition: Combines air speed and air temperature to quantify perceived draft sensation.

• Measurement: Calculated from CFD data or field measurements using ASHRAE equations.

• Standards: ASHRAE 55, ISO 7730.

• Criteria: EDT values near zero indicate no discomfort.

7. Air Change Effectiveness (ACE)

• Definition: A measure of how efficiently supplied air is used in a space to remove heat or contaminants.

• Measurement: Based on air age, tracer gas, or CFD analysis of air mixing patterns.

• Standards: ASHRAE 129 and ASHRAE 25F.

• Criteria: ACE ≈ 1.0 indicates ideal mixing; values > 1.0 indicate displacement ventilation with even better performance.

How CFD Simplifies and Improves These Calculations

CFD allows engineers to visualise and quantify air movement, temperature fields, and ventilation performance before systems are built. Instead of relying solely on physical testing, CFD provides:

• 3D visualisation: Full velocity and temperature fields throughout the occupied zone.

• Rapid iteration: Test multiple diffuser types or layouts virtually.

• Direct metric calculation: Derive ADPI, PVM, PPD, Throw, DR, EDT, and ACE directly from simulation results.

• Standards compliance: Validate designs against ASHRAE, AIRAH, and Australian criteria early in the design phase.

Visualisation of indoor air movement showing distinct velocity streamlines, highlighting airflow patterns.

Ceiling diffusers efficiently cool an indoor space, as visualised by the temperature gradient in this simulation, showing cooler blue areas spreading downward from the ceiling.

Example summary CFD results for indoor thermal comfort metrics:

Metrics	Values
Inlet temperature (°C)	12
Avg room temperature (°C)	23
Inlet static pressure (Pa)	10
Throw (m)	3.5
PMV (-)	0.65
PPD (%)	12
APDI (%)	87
Max DR (%)	11

Deratec’s Capability

At Deratec, we combine deep knowledge of building physics with advanced CFD technology to provide:

✅ Detailed analysis of airflow and thermal comfort parameters for any indoor environment.

✅ Standard‑aligned reporting following ASHRAE 55, 113, 129, 25F, AIRAH guidelines, and AS 1668.2.

✅ Optimisation of ventilation products, including diffuser design, placement, and system settings.

✅ Clear visual outputs, such as contour maps and vector plots, to support decision‑making.

✅ Integrated experimental validation when required.

With Deratec’s CFD expertise, you can confidently design and assess ventilation systems that deliver superior thermal comfort, comply with all relevant standards, and enhance occupant well-being.

Contact Deratec today to see how our CFD services can elevate your ventilation product design and help you achieve optimal indoor thermal environments.

📚 References

1. ASHRAE Standard 55 – Thermal Environmental Conditions for Human Occupancy, American Society of Heating, Refrigerating and Air‑Conditioning Engineers (ASHRAE).

2. ASHRAE Standard 113 – Method of Testing for Room Air Diffusion defines the method for calculating the Air Diffusion Performance Index (ADPI).

3. ASHRAE Standard 129 – Measurement of Air Change Effectiveness Methodology for determining Air Change Effectiveness (ACE).

4. ASHRAE Standard 70 – Method of Testing the Performance of Air Outlets and Inlets. Standard test procedures for diffuser throw and related performance.

5. ASHRAE 25F – Air Change Effectiveness Research Supplemental research guidance on ACE evaluation.

6. ISO 7730 – Ergonomics of the Thermal Environment: International method for PMV and PPD calculations.

7. AIRAH Design Application (DA) Manuals

   • DA09 – Air Distribution Design

   • AIRAH Technical Handbook (Thermal comfort and ventilation design guidance).

8. AS 1668.2:2024– The Use of Ventilation and Air Conditioning in Buildings, Part 2: Ventilation Design for Acceptable Indoor Air Quality. Australian Standard for ventilation system design and indoor air quality.

9. Price Industries – Air Distribution Engineering Guide. A practical engineering guide for diffuser selection, throw performance, and air distribution strategies.

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