Chilled water coils are essential components in central air conditioning systems. This guide covers the selection and sizing process for optimal performance.
Chilled Water System Basics
Typical Operating Conditions
- Supply water temperature: 6-7°C (42-45°F)
- Return water temperature: 12-14°C (54-57°F)
- Temperature difference (ΔT): 5-7 K
Water Flow Calculation
ṁ = Q / (c_p × ΔT)
Where:
- ṁ = mass flow rate (kg/s)
- Q = cooling capacity (kW)
- c_p = specific heat (4.18 kJ/kg·K for water)
- ΔT = temperature difference (K)
Coil Selection Process
Step 1: Define Air-Side Requirements
- Air flow rate (m³/h or CFM)
- Entering air conditions (DB/WB or DB/RH)
- Leaving air conditions (target)
Step 2: Calculate Cooling Loads
- Sensible cooling: Temperature reduction
- Latent cooling: Moisture removal
- Total cooling: Sum of both
Step 3: Determine Water-Side Parameters
- Available supply temperature
- Allowable pressure drop
- Flow rate constraints
Step 4: Select Coil Geometry
- Face area (based on velocity)
- Number of rows (based on capacity)
- Fin spacing (based on application)
Sensible Heat Ratio (SHR)
SHR = Q_sensible / Q_total
Typical SHR Values
- Office spaces: 0.75-0.85
- Retail: 0.70-0.80
- Hospitals: 0.65-0.75
- Data centers: 0.95-1.0
Coil Design Impact
- Low SHR = more rows needed
- High SHR = fewer rows, higher velocity OK
Dehumidification Considerations
Apparatus Dew Point (ADP)
The effective surface temperature of the coil:
ADP = T_wb,in - BF × (T_wb,in - T_water,avg)
Where BF is the bypass factor.
Bypass Factor
Fraction of air that doesn't contact the coil surface:
- 4-row coil: BF ≈ 0.15-0.25
- 6-row coil: BF ≈ 0.05-0.15
- 8-row coil: BF ≈ 0.02-0.08
Ensuring Dehumidification
- Coil surface must be below dew point
- Lower water temperature = better dehumidification
- More rows = lower bypass factor
Glycol Effects
Why Use Glycol?
- Freeze protection
- Lower operating temperatures
- Required for some applications
Performance Impact
| Glycol % | Freeze Point | Capacity Reduction |
|---|---|---|
| 0% | 0°C | 0% |
| 25% | -12°C | 5-8% |
| 35% | -20°C | 10-15% |
| 50% | -34°C | 20-25% |
Compensation Strategies
- Increase coil size
- Lower water temperature
- Increase flow rate
Water-Side Design
Velocity Limits
- Minimum: 0.5 m/s (avoid stratification)
- Maximum: 2.5 m/s (erosion, noise)
- Optimal: 1.0-2.0 m/s
Pressure Drop
- Typical: 20-50 kPa
- Higher ΔP = more pump energy
- Consider system curve
Circuiting Options
- Series: Higher velocity, more ΔP
- Parallel: Lower velocity, better distribution
- Counter-cross flow: Best thermal performance
Control Strategies
Two-Way Valve
- Variable flow
- Better for variable load
- Requires variable speed pumping
Three-Way Valve
- Constant flow
- Simpler control
- Higher pump energy
Face and Bypass
- Constant water flow
- Air bypasses coil
- Good humidity control
Common Selection Mistakes
Undersizing for dehumidification
- Check ADP vs. required dew point
- Add rows if needed
Ignoring glycol effects
- Always account for capacity reduction
- Verify freeze protection adequate
Excessive water velocity
- Causes erosion and noise
- Increases pump energy
Insufficient air-side clearance
- Affects air distribution
- Reduces effective capacity
Conclusion
Proper chilled water coil selection requires balancing air-side and water-side parameters while considering dehumidification requirements and system constraints. Using professional selection software ensures accurate sizing and optimal performance.