Chilled Water Coil Calculator
Complete guide to designing and sizing chilled water coils for air handling units, fan coil units, and central cooling systems.
1. Overview
The Chilled Water Coil Calculator performs a rigorous thermal analysis of finned-tube heat exchangers used in HVAC cooling applications. It supports both rating mode (evaluating a known coil geometry) and sizing mode (determining the required geometry for a target capacity). The engine uses the NTU-effectiveness method with corrections for wet coil conditions, fin efficiency, and fouling factors.
Key Capabilities
- Sensible and latent capacity calculation with SHR (Sensible Heat Ratio)
- Wet coil analysis with condensate rate estimation
- Water-side pressure drop with glycol correction
- Air-side pressure drop across the coil
- Leaving air conditions (temperature and humidity)
2. Air-Side Inputs
The air-side parameters define the incoming air conditions and flow rate through the coil.
| Parameter | Unit | Description | Typical Range |
|---|---|---|---|
| Dry Bulb Temperature | °C | Entering air dry bulb temperature | 24–35°C |
| Wet Bulb Temperature | °C | Entering air wet bulb temperature (determines humidity) | 17–27°C |
| Air Flow Rate | m³/h | Volumetric air flow through the coil face | 500–100,000 m³/h |
| Altitude | m | Installation altitude (affects air density) | 0–3,000 m |
Important: The wet bulb temperature must always be lower than the dry bulb temperature. If wet bulb equals dry bulb, the air is at 100% relative humidity (fog conditions).
3. Water-Side Inputs
Configure the chilled water supply conditions and flow characteristics.
| Parameter | Unit | Description |
|---|---|---|
| Inlet Temperature | °C | Chilled water supply temperature (typically 5–10°C) |
| Outlet Temperature | °C | Return water temperature (typically 10–15°C) |
| Flow Rate | L/s | Water volumetric flow rate |
| Glycol Type | — | None, Ethylene Glycol, or Propylene Glycol |
| Glycol Concentration | % | Glycol percentage by volume (0–60%) |
| Fouling Factor | m²·K/W | Internal fouling resistance (default: 0.000176) |
Tip: A typical chilled water system operates at 7°C supply / 12°C return (ΔT = 5°C). For low-temperature applications (e.g., dehumidification), use 5°C supply / 10°C return.
4. Coil Geometry
Define the physical construction of the heat exchanger.
| Parameter | Description |
|---|---|
| Tube OD | Outer diameter of tubes (7.94mm, 9.52mm, 12.7mm, 15.88mm) |
| Fin Spacing (FPI) | Fins per inch — higher FPI = more surface area but higher air pressure drop |
| Number of Rows | Tube rows deep in the air flow direction (1–16) |
| Tubes Per Row | Number of tubes in each row (determines coil height) |
| Fin Length | Length of the finned section (determines coil width) |
| Tube Material | Copper (standard) or Copper-Nickel (corrosive environments) |
| Fin Material | Aluminum (standard), Copper, or Pre-coated Aluminum |
5. Understanding Results
After calculation, the following key results are displayed:
| Result | Description |
|---|---|
| Total Capacity | Total heat transfer rate (sensible + latent) in kW |
| Sensible Capacity | Heat removed by lowering air temperature (dry cooling) |
| Latent Capacity | Heat removed by condensing moisture from the air |
| SHR | Sensible Heat Ratio = Sensible / Total (1.0 = dry coil) |
| Water Pressure Drop | Pressure loss through the water circuit in kPa |
| Air Pressure Drop | Pressure loss across the coil face in Pa |
| Leaving Air DB/WB | Leaving air dry bulb and wet bulb temperatures |
| Condensate Rate | Rate of moisture removal in kg/h (wet coil only) |
6. Wet vs Dry Coil Analysis
A coil operates in 'wet' mode when the coil surface temperature drops below the dew point of the entering air. In this condition, moisture condenses on the fins and tubes, providing both sensible cooling (temperature reduction) and latent cooling (dehumidification).
The calculator automatically detects wet coil conditions and applies the appropriate heat transfer correlations. When a wet coil is detected, you will see:
- SHR < 1.0 (indicating latent capacity)
- Condensate rate in kg/h
- A note indicating 'Wet coil' with the SHR value
Design Tip: For dehumidification applications, target an SHR of 0.6–0.75. Use lower chilled water temperatures (5–6°C) and higher fin density (10–14 FPI) to maximize latent capacity.
7. Rating vs Sizing Modes
The calculator supports two operating modes:
Rating Mode: You define the complete coil geometry and operating conditions. The calculator determines the resulting capacity, pressure drops, and leaving conditions. Use this when you have a specific coil and want to know its performance.
Sizing Mode: You define the target capacity and operating conditions. The calculator determines the required coil geometry (number of rows, tubes per row, fin length) to meet the target. Use this when starting a new design.
8. Warnings & Limits
The calculator generates contextual warnings based on the results:
- Water velocity > 3.0 m/s: May cause erosion of copper tubes and excessive pressure drop. Consider increasing the number of circuits or tube diameter.
- Water velocity < 0.5 m/s: Laminar flow regime reduces heat transfer significantly. Consider reducing the number of circuits.
- Number of rows > 10: Deep coils have diminishing returns on heat transfer. The last rows contribute very little to total capacity.
- Air pressure drop > 150 Pa: High air-side pressure drop. Verify that the fan can handle this additional static pressure.
- Face velocity > 3.0 m/s: May cause moisture carryover from wet coils. Keep face velocity below 2.5 m/s for wet coil applications.
9. Best Practices
- Start with standard ΔT of 5°C (7/12°C) for comfort cooling applications
- Use 8–12 FPI for standard applications; 14+ FPI only for clean air environments
- Keep face velocity between 2.0–2.5 m/s for optimal heat transfer and moisture control
- For glycol systems, always account for the reduced heat transfer coefficient (10–30% penalty)
- Verify that water velocity stays between 0.5–3.0 m/s for turbulent flow without erosion
- Consider fouling factors for open-loop or untreated water systems (increase fouling factor to 0.00035)
- Always check the air pressure drop against the available fan static pressure
10. Common Mistakes
- Entering wet bulb temperature higher than dry bulb temperature (physically impossible)
- Ignoring glycol concentration when the system uses antifreeze (significantly affects capacity)
- Using too many rows (>8) when fewer rows with a larger face area would be more efficient
- Not accounting for altitude effects on air density at high-elevation installations
- Setting water flow rate without checking the resulting water velocity in the tubes