hvac-pressure 0.1.0

HVAC static pressure loss calculator using Darcy-Weisbach and ASHRAE fitting coefficients.

hvac_pressure

A pure Python library for calculating static pressure losses in HVAC duct systems.

hvac_pressure implements the Darcy-Weisbach equation for straight ducts and includes a built-in interpolation engine for ASHRAE fitting loss coefficients (based on the 2009 ASHRAE Handbook — Fundamentals, Chapter 21).

It is designed to be the calculation engine behind HVAC design tools, web apps, and automated sizing scripts.

Features

• Straight Ducts: Darcy-Weisbach friction loss with Colebrook-White / Swamee-Jain approximations. Supports round, rectangular, and flat oval ducts.
• ASHRAE Fittings: Built-in database of the 15 most common ASHRAE fittings (elbows, tees, transitions, dampers, entries/exits).
• Interpolation Engine: Automatically performs 1D, 2D, and 3D linear interpolation on ASHRAE tables based on your exact geometry (e.g., r/D, H/W, θ).
• Filters & Dampers: Square-law scaling for filters and blade-angle C-value lookups for dampers.
• System Chaining: A System class to chain elements together and generate pressure drop reports.
• Unit Conversion: Internal calculations are strictly SI (Pa, m³/s, m/s, m), with helper functions for Imperial units (CFM, in.w.g., fpm).
• Zero Dependencies: Pure Python, no external libraries required.

Installation

(Note: This package is not yet on PyPI. You can install it locally or drop the hvac_pressure folder into your project.)

pip install .

Quick Start

1. Single Element Calculation

import hvac_pressure as hp

# Straight duct (SI units: m³/s, m)
result = hp.duct_loss(flow_m3s=0.5, length_m=10, diameter_m=0.3)
print(f"Duct loss: {result['delta_p_pa']:.2f} Pa")

# ASHRAE Fitting (CD3-12: 3-gore round elbow)
# The library automatically interpolates the C value for r/D = 1.2
result = hp.fitting_loss('CD3-12', flow_m3s=0.5, diameter_m=0.3, r_over_D=1.2)
print(f"Fitting loss: {result['delta_p_pa']:.2f} Pa (C = {result['C']:.3f})")

2. Full System Calculation

The System class makes it easy to build a duct branch and get a total pressure drop.

import hvac_pressure as hp

# Create a system with a design flow of 0.5 m³/s (approx 1060 CFM)
sys = hp.System(flow_m3s=0.5, temp_c=20, name="AHU-1 Supply Main")

# Add elements in order
sys.add_duct(length_m=15.0, diameter_m=0.3, label="Main straight")
sys.add_fitting("CD3-12", diameter_m=0.3, r_over_D=1.5, label="90° Elbow")
sys.add_filter(rated_drop_pa=125.0, rated_flow_m3s=0.6, label="Pre-filter")
sys.add_fitting("CD9-1", diameter_m=0.3, theta_deg=20, label="Balancing Damper")

# Get total pressure drop
print(f"Total Pressure: {sys.total_pa():.1f} Pa")
print(f"Total Pressure: {sys.total_inwg():.4f} in.w.g.")

# Print a formatted report
print(sys.report(unit="IP"))  # 'IP' for Imperial, 'SI' for metric

Output:

========================================================================
  HVAC Static Pressure Report — AHU-1 Supply Main
========================================================================
  System airflow : 1059.4 CFM
  Temperature    : 20.0 °C
  Altitude       : 0 m
------------------------------------------------------------------------
  #    Label                         Type        Velocity      ΔP        Cumul.
------------------------------------------------------------------------
  1    Main straight                 duct        1393 fpm  0.1164 in.w.g.  0.1164 in.w.g.
  2    90° Elbow                     fitting     1393 fpm  0.0181 in.w.g.  0.1345 in.w.g.
  3    Pre-filter                    filter      —         0.3486 in.w.g.  0.4831 in.w.g.
  4    Balancing Damper              fitting     1393 fpm  0.1863 in.w.g.  0.6694 in.w.g.
------------------------------------------------------------------------
                               TOTAL SYSTEM PRESSURE LOSS  0.6694 in.w.g.
========================================================================

Available ASHRAE Fittings

The library currently includes the following fitting tables:

Code	Description	Required Parameters
CD3-1	Round elbow, die stamped 90°	D_mm
CD3-5	Round elbow, pleated 90°	D_mm
CD3-9	Round elbow, 5-gore 90°	D_mm
CD3-12	Round elbow, 3-gore	r_over_D
CD3-17	Round elbow, mitered 45°	D_mm
CD9-1	Round butterfly damper	theta_deg
CD9-3	Round fire damper	(none)
ED1-3	Bellmouth entry (exhaust)	r_over_D
SD1-1	Bellmouth entry (supply)	r_over_Do
ED2-1	Conical diffuser	A1_over_Ao, L_over_Do
CR3-1	Rectangular elbow, smooth	r_over_W, H_over_W, theta_deg
CR3-6	Rectangular elbow, mitered	theta_deg, H_over_W
CR3-9	Rectangular elbow, mitered w/ vanes	(none)
CR9-1	Rectangular butterfly damper	theta_deg, H_over_W
SR4-1	Rectangular transition	Ao_over_A1, theta_deg
SR5-1	Rectangular wye (tee), diverging	As_over_Ac, Ab_over_Ac, Qb_over_Qc

Note: For SR5-1 (tees), the function returns both C (branch loss) and C2 (straight path loss).

API Integration

If you are building a web app (e.g., on Replit), you can easily expose the system calculation as a JSON endpoint:

import json
import hvac_pressure as hp

sys = hp.System(flow_m3s=0.5)
sys.add_duct(length_m=10, diameter_m=0.3)
# ... add elements ...

# Returns a clean dictionary ready for JSON serialization
response_data = sys.to_dict()
print(json.dumps(response_data, indent=2))

License

MIT License.