Fan and Pump Horsepower Formulas for HVAC Systems

 

Introduction

Fans and pumps are critical for moving air and water efficiently in HVAC systems. Oversized equipment wastes energy, while undersized equipment fails to meet design requirements. Correct horsepower calculations ensure efficiency and reliability.

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Key Formulas

1. Fan Horsepower:

$$FHP = \frac{CFM \times SP}{6356 \times \eta}$$

• SP: Static Pressure (in.wg)

• η: Fan efficiency

2. Pump Horsepower:

$$PHP = \frac{Flow \times Head \times SG}{3960 \times Pump Efficiency}$$

• Flow: gpm

• Head: ft of water

• SG: Specific Gravity

3. Fan Laws:

• Flow ∝ RPM

• Pressure ∝ (RPM)²

• Power ∝ (RPM)³

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Example Calculation

• Fan: 5,000 CFM, SP = 2.5 in.wg, efficiency = 70%

• Horsepower: $FHP = (5000 × 2.5) / (6356 × 0.7) ≈ 2.8 HP$

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Practical Tips

• Always round up to nearest standard motor size.

• Consider variable speed drives for energy savings.

• Check manufacturer charts for real-world corrections.

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Suggested Images

• Fan and pump schematic with labels

• Fan law diagram showing flow, pressure, and power relationships

Internal Linking:

• Link back to Duct Design page: Duct Design and Airflow Formulas

• Link back to Master Page: HVAC Formulas Master Guide

Tags:
fan horsepower formula, pump horsepower formula, HVAC fan calculation, MEP pump formulas, fan laws HVAC