Introduction to Chilled Water Coil Selection
Chilled water coil selection is a critical step in the design of Air Handling Units (AHUs). A properly sized coil ensures that the AHU can meet the sensible and latent cooling loads of the conditioned space while maintaining acceptable pressure drops on both the air and water sides. For HVAC and refrigeration engineers, mastering the engineering calculations behind chilled water coil selection is essential for optimizing system performance and energy efficiency.
In this comprehensive guide, we will explore the fundamental principles of chilled water coil selection, including water flow rate calculations, entering and leaving water temperatures, air-side analysis, and the impact of glycol mixtures. We will also demonstrate how modern coil selection software like ExCoil can streamline these complex calculations, allowing engineers to design with confidence and precision.
Water-Side Calculations: Flow Rate and Temperatures
The first step in chilled water coil selection is determining the required water flow rate based on the cooling capacity and the design temperature difference (Delta-T). The fundamental heat transfer equation for the water side is:
Q = m_dot × C_p × ΔT
Where:
- Q = Cooling capacity (Btu/hr or kW)
- m_dot = Mass flow rate of water (lb/hr or kg/s)
- C_p = Specific heat of water (1.0 Btu/lb·°F or 4.18 kJ/kg·°C)
- ΔT = Temperature difference between entering and leaving water (°F or °C)
In practical HVAC applications, this equation is often simplified to calculate the volumetric flow rate in Gallons Per Minute (GPM):
GPM = Q_total / (500 × ΔT)
Where Q_total is the total cooling capacity in Btu/hr, and ΔT is the water temperature difference in °F. The constant 500 is derived from the density of water (8.33 lb/gal) multiplied by 60 minutes per hour.
Typical Entering and Leaving Water Temperatures
The selection of entering water temperature (EWT) and leaving water temperature (LWT) significantly impacts the coil's performance and the overall chiller plant efficiency. Typical design values are:
- Entering Water Temperature (EWT): 44°F to 45°F (6.7°C to 7.2°C)
- Leaving Water Temperature (LWT): 54°F to 60°F (12.2°C to 15.6°C)
- Typical Delta-T: 10°F to 16°F (5.5°C to 8.9°C)
A higher Delta-T reduces the required water flow rate, which lowers pumping energy and allows for smaller pipe sizes. However, it also reduces the Log Mean Temperature Difference (LMTD), requiring a larger coil surface area to achieve the same cooling capacity. Balancing these factors is a key aspect of chilled water coil selection.
If you are tired of manually iterating through these calculations, try ExCoil free at excoil.net. Our coil selection software instantly calculates water flow rates and optimizes coil geometry based on your specific design conditions.
Air-Side Analysis: Sensible and Latent Cooling
The air-side performance of a chilled water coil involves both sensible cooling (temperature reduction) and latent cooling (moisture removal). The total cooling capacity (Q_total) is the sum of the sensible capacity (Q_sensible) and the latent capacity (Q_latent).
The sensible cooling capacity can be calculated using the following equation:
Q_sensible = 1.08 × CFM × (EAT_db - LAT_db)
Where:
- CFM = Airflow rate in Cubic Feet per Minute
- EAT_db = Entering Air Temperature, dry bulb (°F)
- LAT_db = Leaving Air Temperature, dry bulb (°F)
- 1.08 = Constant derived from the density and specific heat of standard air
The total cooling capacity, accounting for both sensible and latent loads, is calculated using enthalpy:
Q_total = 4.5 × CFM × (h_entering - h_leaving)
Where:
- h_entering = Enthalpy of entering air (Btu/lb)
- h_leaving = Enthalpy of leaving air (Btu/lb)
- 4.5 = Constant derived from the density of standard air and 60 minutes per hour
Wet Coil vs. Dry Coil Detection
A critical aspect of chilled water coil selection is determining whether the coil will operate under dry or wet conditions. This depends on the Apparatus Dew Point (ADP) and the entering air conditions.
If the coil surface temperature (specifically, the tube and fin surface) drops below the dew point temperature of the entering air, condensation will occur, and the coil operates as a "wet coil." This significantly increases the overall heat transfer coefficient (U-value) due to the presence of a water film, but it also increases the air-side pressure drop.
Engineers must accurately predict the wet/dry boundary to calculate the correct heat transfer area. The Wang-Chi-Chang correlation is widely used to model the air-side heat transfer and friction characteristics of wet fin-and-tube heat exchangers.
With ExCoil's advanced calculation engine, wet and dry coil zones are automatically detected and modeled with high precision. Run this calculation instantly with ExCoil to ensure accurate performance predictions for your AHU designs.
The Impact of Face Velocity and Coil Rows
Face velocity is the speed at which air approaches the coil face, calculated as:
Face Velocity (FPM) = CFM / Coil Face Area (sq ft)
Typical face velocities for chilled water coils range from 400 to 550 FPM. Higher face velocities increase the air-side heat transfer coefficient but also result in higher air pressure drops and the risk of moisture carryover (water droplets being blown off the coil into the ductwork). To prevent moisture carryover, face velocities are generally limited to a maximum of 500-550 FPM for wet coils.
The number of coil rows determines the depth of the heat exchanger and the total surface area available for heat transfer. More rows provide greater cooling capacity and a closer approach temperature between the leaving air and entering water. However, adding rows increases both air-side and water-side pressure drops.
Typical Heat Transfer Coefficients (U-Values)
The overall heat transfer coefficient (U) is a measure of the coil's ability to transfer heat between the water and the air. It is calculated based on the thermal resistances of the inside water film, the tube wall, and the outside air film (including fins).
| Application | Typical U-Value (Btu/hr·ft²·°F) | Typical U-Value (W/m²·K) |
|---|---|---|
| Chilled Water (Dry Coil) | 8 to 12 | 45 to 68 |
| Chilled Water (Wet Coil) | 12 to 18 | 68 to 102 |
| Hot Water | 10 to 15 | 57 to 85 |
Note: These are approximate values. Actual U-values depend on tube velocity, face velocity, fin geometry, and fluid properties.
Glycol Correction Factors
In applications where the AHU is exposed to freezing temperatures, or in process cooling systems, ethylene glycol or propylene glycol is often added to the chilled water loop. While glycol prevents freezing, it negatively impacts the heat transfer and fluid flow characteristics of the mixture.
Compared to pure water, glycol mixtures have:
- Lower specific heat (reducing cooling capacity for a given flow rate)
- Lower thermal conductivity (reducing the inside heat transfer coefficient)
- Higher viscosity (increasing water-side pressure drop and reducing turbulence)
When performing chilled water coil selection with glycol, engineers must apply correction factors to account for these changes. The Dittus-Boelter or Gnielinski correlations are typically used to calculate the inside heat transfer coefficient for the specific fluid mixture.
Ethylene Glycol Correction Factors (at 45°F / 7.2°C)
| Glycol Concentration (%) | Capacity Multiplier | GPM Multiplier (for same capacity) | Pressure Drop Multiplier |
|---|---|---|---|
| 0% (Pure Water) | 1.00 | 1.00 | 1.00 |
| 10% | 0.98 | 1.02 | 1.05 |
| 20% | 0.96 | 1.04 | 1.12 |
| 30% | 0.93 | 1.08 | 1.22 |
| 40% | 0.89 | 1.13 | 1.35 |
| 50% | 0.85 | 1.18 | 1.50 |
Table 1: Approximate correction factors for ethylene glycol mixtures compared to pure water.
As shown in the table, a 30% ethylene glycol mixture requires an 8% increase in flow rate to achieve the same cooling capacity, while the pressure drop increases by 22%. ExCoil's multi-refrigerant and fluid support automatically handles these complex property calculations, ensuring accurate coil sizing for any glycol concentration.
Chilled Water Coil Selection Example: 20 TR AHU
Let's walk through a practical chilled water coil selection example for a 20-ton (240,000 Btu/hr) Air Handling Unit.
Design Conditions:
- Total Cooling Load: 240,000 Btu/hr (20 TR)
- Sensible Cooling Load: 180,000 Btu/hr
- Airflow: 8,000 CFM
- Entering Air: 80°F DB / 67°F WB
- Entering Water Temperature (EWT): 45°F
- Leaving Water Temperature (LWT): 55°F
- Fluid: 100% Water
Step 1: Calculate Water Flow Rate (GPM) ΔT = 55°F - 45°F = 10°F GPM = 240,000 / (500 × 10) = 48.0 GPM
Step 2: Determine Coil Face Area and Velocity Assuming a maximum face velocity of 500 FPM to prevent moisture carryover: Face Area = 8,000 CFM / 500 FPM = 16.0 sq ft A typical coil dimension could be 48 inches finned length by 48 inches finned height (4 ft × 4 ft = 16 sq ft).
Step 3: Select Tube and Fin Geometry Using 5/8" OD copper tubes with aluminum corrugated fins at 10 Fins Per Inch (FPI).
Step 4: Determine Rows and Circuits Through iterative calculations (or using coil selection software), we determine that a 4-row coil with a half-circuit arrangement provides the required capacity while maintaining a water velocity of approximately 4.5 ft/s, which yields a reasonable water pressure drop of 12 ft H2O.
Step 5: Verify Air-Side Performance The 4-row, 10 FPI coil yields an air-side pressure drop of 0.65 in. w.g., which is acceptable for typical AHU fans.
While this manual example illustrates the process, real-world design requires evaluating hundreds of combinations of rows, FPI, circuiting, and fin types to find the optimal balance of cost, capacity, and pressure drop.
Streamline Your Coil Design with ExCoil
Chilled water coil selection involves complex, iterative calculations that balance thermodynamics, fluid dynamics, and psychrometrics. Relying on traditional tools or spreadsheets can be time-consuming and prone to errors, especially when dealing with wet coil conditions or glycol mixtures.
ExCoil is a modern, cloud-based heat exchanger design software built specifically for HVAC and refrigeration engineers. With ExCoil, you can:
- Instantly calculate water flow rates, capacities, and pressure drops
- Automatically detect wet and dry coil zones for accurate performance modeling
- Evaluate the impact of various glycol concentrations with built-in fluid properties
- Generate professional PDF reports and utilize 3D visualization for your designs
- Manage multiple designs efficiently with the integrated project manager
Stop wasting time on manual iterations. Start your free trial at excoil.net today and experience the future of chilled water coil selection.