1. Pressure Drop Across Coils
When air passes through cooling or heating coils, resistance is created due to fins, tubes, and condensate on coil surfaces.
- Cooling Coils:
- Pressure Drop (Clean Coil): 50–100 Pa (0.2–0.4 in. w.g.)
- Higher for deep coils (6–8 rows) and high face velocity (>2.5 m/s).
- Additional drop occurs when wet (condensate increases resistance).
- Heating Coils:
- Pressure Drop (Clean Coil): 30–70 Pa (0.1–0.3 in. w.g.)
- Typically lower than cooling coils because no condensation forms.
- Impact of Fouling:
- Dust, grease, and microbial growth increase resistance dramatically.
- Results in reduced airflow, higher fan power, poor cooling/heating capacity.
2. Monitoring Coil Performance
- Differential Pressure Gauges/Sensors: Installed across coils to measure resistance.
- Temperature Sensors: Supply vs. return air temperature difference indicates coil performance.
- Signs of Problem:
- ΔP across coil rises above design by >50%.
- Supply air temperature fails to meet setpoint.
- Uneven air distribution (hot/cold spots).
3. Coil Maintenance Practices
🔹 Cooling Coils
- Cleaning Frequency: Every 6–12 months (quarterly in dusty/humid climates).
- Methods:
- Vacuuming/Brushing: For light dust.
- Low-pressure water washing from upstream side.
- Chemical Coil Cleaners (non-acidic, biodegradable) for stubborn dirt and biofilm.
- Drain Pan & Traps:
- Must be cleaned to prevent microbial growth (Legionella risk).
- Ensure slope & P-trap are functional to avoid water carryover.
🔹 Heating Coils
- Less prone to fouling (no condensation), but still collect dust.
- Clean with vacuum and soft brushing every 6–12 months.
- In cold climates:
- Inspect for freezing damage.
- Use glycol mix in hot water coils for freeze protection.
- Provide preheat coils to protect downstream filters.
4. Preventive Measures
- Ensure filters are well maintained → prevents dust reaching coils.
- Maintain face velocity within design (≤2.5 m/s) to minimize carryover.
- Apply anti-microbial coating in healthcare facilities.
- Use UV-C lamps upstream of coils to continuously disinfect and reduce biofilm.
5. Effects of Neglected Coil Maintenance
- Cooling Coil:
- Clogged fins → reduced airflow & cooling.
- High fan energy consumption.
- Increased humidity due to poor dehumidification.
- Risk of mold growth → poor IAQ.
- Heating Coil:
- Reduced heating efficiency in winter.
- Frozen coils (if not protected) → rupture of tubes.
- Higher pump energy due to scale in water circuits.
6. Typical Maintenance Schedule
| Coil Type | Check Frequency | Action |
|---|---|---|
| Cooling Coils | Quarterly (dusty climate) / Bi-annual (normal) | Clean fins, flush drain pans, check condensate traps |
| Heating Coils | Bi-annual | Clean fins, check for leaks, ensure freeze protection |
| Drain Pan | Monthly | Disinfect & clean standing water |
| UV-C Lamps (if used) | Annually | Replace lamps for effective microbial control |
7. Example: FAHU in a Hot-Humid Climate
- Outdoor air: 40°C DB / 28°C WB.
- Cooling coil ΔP design: 80 Pa.
- After 6 months of neglect, ΔP rises to 180 Pa, airflow drops by 25%.
- Result: Supply air temperature increases from 14°C to 18°C, humidity control fails.
- After coil cleaning, ΔP returns to 85 Pa, design conditions restored.
