Introduction
Proper fan sizing is fundamental to HVAC performance and energy efficiency. An incorrectly sized fan or blower can result in poor airflow, system imbalance, energy wastage, or even premature component failure. Whether you’re designing for a rooftop unit, an AHU, or an exhaust system, selecting the right fan requires understanding airflow requirements, pressure losses, and fan behavior. This guide will cover the complete process of fan sizing in HVAC applications — from airflow (CFM) and total static pressure (TSP) to fan curves and operating points — using ASHRAE-backed methods and real-world examples.
Selecting the correct fan or blower is vital for maintaining airflow, static pressure balance, and energy efficiency. Oversized fans lead to higher operating costs and noise, while undersized fans cause inadequate airflow and poor system performance.
Types of Fans Used in HVAC
- Centrifugal Fans: Used in AHUs, these fans handle higher static pressures. They come in subtypes like FC (Forward-Curved), BC (Backward-Curved), and Airfoil blades.
- Axial Fans: Ideal for high-volume, low-pressure applications like exhaust and ventilation.
- Mixed Flow Fans: A hybrid of centrifugal and axial, offering compact design and moderate pressure handling.
Key Fan Sizing Parameters
- Fan Efficiency: Select fans operating near peak efficiency on the performance curve.
- Airflow Quantity (CFM): Based on cooling/heating loads or ASHRAE 62.1 ventilation standards.
- Total Static Pressure (TSP): Sum of all resistances — filters, coils, ducts, diffusers.
- System Resistance: Determined through ductulator, CFD tools, or manual calculations.
- Formula:
Where,
- BHP = Brake Horsepower (hp)
- Q = Airflow rate (CFM – Cubic Feet per Minute)
- ΔP = Total pressure rise across the fan (in inches of water gauge – in. wg)
- η = Fan total efficiency (as a decimal, e.g. 65% = 0.65)
- 6356 = Conversion factor (for CFM × in.wg to horsepower)
Fan Laws and Their Applications
- CFM ∝ RPM
- Pressure ∝ RPM²
- Power ∝ RPM³
Useful for evaluating fan behavior when speed or flow requirements change.
Fan Performance Curves, System Curves, and Operating Point
- Fan Curve: Manufacturer’s performance graph of airflow vs. pressure.
- System Curve: Parabolic curve showing how pressure loss increases with airflow.
- Operating Point: Where fan curve and system curve intersect — defines actual performance.
- Design Note: A stable system operates near the fan’s peak efficiency. Adjustments in ductwork, filters, or speed will shift the system curve and operating point.
Real-Time Blower Selection and Optimization
Selecting the right blower in real-time scenarios goes beyond looking at catalog curves. It involves balancing actual site conditions, system demands, and energy efficiency. Here’s how to approach blower selection and optimization in real-world applications:
Step 1: Identify System Requirements
- Required airflow (CFM or m³/h)
- Total static pressure (TSP) including ductwork, filters, coils, etc.
- Environmental conditions (temperature, humidity, altitude)
- Application type (comfort cooling, process ventilation, exhaust, etc.)
Step 2: Choose the Right Type of Fan
Based on system resistance and flow:
| Application | Suggested Fan Type | Reason |
|---|---|---|
| High flow, low pressure | Axial | Cost-effective, compact |
| Moderate flow & pressure | Forward-curved centrifugal | Compact and quiet |
| High pressure systems | Backward-curved centrifugal | Stable & energy-efficient |
| Dirty or abrasive air | Radial or industrial blowers | Rugged design |
Step 3: Use Manufacturer Selection Software
Most blower manufacturers offer free fan selection tools (e.g., Greenheck CAPS, Twin City Fan Selector, Kruger Fan Selector). These allow:
- Input of required airflow and pressure
- Selection of multiple models
- Comparison of efficiency, power, sound levels, size, etc.
- Export of performance data and fan curves
Step 4: Optimize Based on Key Parameters
| Parameter | Optimization Goal |
|---|---|
| Efficiency | Select fan near Best Efficiency Point (BEP) |
| Sound levels | Check dB ratings—especially in AHUs, labs, or office zones |
| Power consumption | Prefer EC or VFD-controlled fans for part-load performance |
| Size and fit | Consider dimensional constraints in retrofits |
| Maintenance | Choose belt vs direct drive depending on maintenance access |
Step 5: Incorporate Flexibility
- Install Variable Frequency Drives (VFDs): Allows precise speed control, part-load efficiency, and adaptability to real-time system changes.
- Select adjustable pulleys: Useful in retrofits where flow tuning is needed.
- Account for filter loading and aging: Include margins in total static pressure.
Step 6: Validate with On-Site Testing
- Measure actual airflow using pitot tube or flow hood
- Measure static pressure at fan inlet and outlet
- Compare operating point to manufacturer’s fan curve
- Adjust speed or dampers as required
Fan Types and Operating Pressure Ranges
| Fan Type | Blade Style | Operating Pressure Range (in.w.g) | Application | Efficiency | Notes |
|---|---|---|---|---|---|
| Forward-Curved (FC) | Centrifugal | 0.5 – 3.5 | Small AHUs, fan coils | Low | Compact, can overload |
| Backward-Curved (BC) | Centrifugal | 2.0 – 5.5 | VAV, return systems | Medium | Non-overloading, stable |
| Airfoil | Centrifugal (aero) | 4.0 – 8.0+ | Cleanrooms, hospitals, high-efficiency AHUs | High | Quiet and energy-efficient |
| Axial | Propeller | 0.1 – 1.0 | Exhaust fans, cooling towers | Low | High flow, low pressure |
| Mixed Flow | Hybrid | 1.5 – 4.5 | Compact AHUs, tight spaces | Medium | Good mix of flow and pressure |
Summary – Key Takeaways
- Fan sizing depends on airflow, TSP, and duct system losses
- Match fan type to pressure and efficiency needs
- Use fan laws to adjust RPM and evaluate performance
- Ensure operating point lies near best efficiency point (BEP)
- Validate selections using fan curves and site commissioning
FAQs
1. What is TSP in fan sizing?
Total static pressure includes all resistance to airflow in the system — ducts, filters, coils.
2. Which fan type suits high-pressure systems?
Airfoil and backward-curved fans are best for medium-to-high TSP applications.
3. Can I adjust fan speed post-installation?
Yes. VFDs (Variable Frequency Drives) allow real-time speed control.
4. What if the actual system resistance is higher than expected?
Fan performance will drop unless speed is increased or fan is oversized initially.
5. Why is operating point important?
It reflects how your selected fan will behave in the actual duct system.
