Chilled Water Coil Selection: AHU Design Guide

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 PointCapacity Reduction

0%0°C0%
25%-12°C5-8%
35%-20°C10-15%
50%-34°C20-25%

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.