The fan used on the evaporator, the key point of attention.
1. Design & Selection Phase
a. Fan Performance Curve vs. System Curve
Key Point: The fan must be selected to operate at its best efficiency point (BEP) on its performance curve, intersecting the calculated system curve (pressure drop vs. airflow).
Engineering Detail: Do not just select for CFM. Model the system pressure drop: coil pressure drop (wet and dry), filter(s) (clean and dirty), grilles, ducts, louvers, and any other components. The fan must provide the required external static pressure (ESP) at the design airflow.
Pitfall: Oversizing leads to high power draw, noise, and needing to throttle the fan (wasted energy). Undersizing results in low airflow and poor performance.
b. Fan Laws are Your Bible
Remember the affinity laws. They dictate the relationship between:
Flow (CFM) ∝ Speed (RPM)
Pressure (ESP) ∝ Speed (RPM)²
Power (BHP) ∝ Speed (RPM)³
Application: A 10% reduction in speed yields a 27% reduction in power. This is the fundamental argument for ECM/BLDC motors for energy savings in variable load conditions.
c. Motor Selection & Drive
Type: PSC (Permanent Split Capacitor) vs. ECM (Electronically Commutated Motor) / BLDC (Brushless DC).
PSC: Simpler, cheaper, fixed speed. Efficiency is lower. Power draw decreases only slightly with reduced airflow (due to pulley changes or dirty filters).
ECM/BLDC: Higher upfront cost, variable speed, dramatically higher efficiency. Power draw drops significantly with reduced airflow. Enables precise control and integration with building management systems (BMS).
Drive Method: Direct Drive vs. Belt-Driven.
Direct Drive: simpler, no maintenance, higher efficiency. Common in smaller units.
Belt-Driven: allows for flexibility in RPM adjustment via sheave changes. Requires maintenance (alignment, tension, belt replacement).
d. Material & Construction
Environment: The fan handles the final air condition. Is it a cooling-only evaporator (condensate present) or a heat pump (may see freezing conditions during defrost)?
Materials: Blower wheels and housings should be coated (epoxy, phenolic) to resist corrosion from moisture. Stainless steel components may be necessary for harsh environments (coastal, chemical exposure).
Wheel Type: Forward Curved (FC) for lower static pressure, Airfoil (AF) or Backward Inclined (BI) for higher efficiency and static pressure, non-overloading characteristics.
2. Installation & Commissioning Phase
a. Rotation and Direction
Critical: Verify fan rotation is correct. An incorrectly wired 3-phase motor will spin backwards, delivering a fraction of the designed airflow.
Check: The fan wheel should be designed for rotation in a specific direction. Ensure the installed orientation matches the design.
b. Airflow Measurement & Balancing
Mandatory: Never assume airflow. It must be measured and verified at commissioning.
Methods: Pitot tube traverse (ducts), flow hood (outlets), or manufacturer's charts for temperature rise across electric heat strips (a common field method for HVAC units).
Adjust: For belt-driven fans, adjust sheave size or motor pulley. For direct-drive ECM motors, adjust dip switches or programming to achieve design CFM.
c. Clearances and Alignment
Clearance: Ensure proper clearance between the fan wheel and the housing (scrolling). Too tight causes wear and noise; too loose reduces efficiency.
Alignment: For belt-driven systems, laser-align the motor and fan shafts. Misalignment is a primary cause of premature bearing and belt failure.
3. Operation & Maintenance Phase
a. Vibration and Bearing Health
Monitor: Listen for unusual noises. Use vibration analysis if critical.
Maintenance: Lubricate bearings per manufacturer's schedule (if not sealed-for-life). For belt-driven systems, check and adjust belt tension.
b. Cleanliness - The #1 Operational Issue
Fan Wheel: Dirt buildup on the blades changes their airfoil shape, destroying efficiency and reducing airflow. It also throws the wheel out of balance, causing vibration.
Motor: Keep cooling passages clean to prevent overheating.
Schedule: Regular inspection and cleaning of the fan assembly is non-negotiable for sustained performance.
c. System Effects
Be aware of poor inlet/outlet conditions (e.g., tight turns into the fan inlet) that create turbulent flow. This can reduce fan performance by 20-40% compared to its rated catalog performance. Use straightening vanes if necessary.