Coil Selection for Air Handling Units: A Step-by-Step Engineering Guide

Air handling units (AHUs) are the workhorses of commercial HVAC systems, and the heat exchanger coils within them are responsible for conditioning the air to meet building comfort and process requirements. Proper coil selection is essential for achieving design capacity, maintaining energy efficiency, and ensuring reliable long-term operation.

Understanding AHU Coil Types

An AHU may contain several types of coils, each serving a specific function:

Cooling Coils (Chilled Water): These coils use chilled water (typically 6-7°C supply, 12-13°C return) to cool and dehumidify the air. They are the most common coil type in commercial AHUs and must handle both sensible and latent heat loads.

DX Cooling Coils (Direct Expansion): These coils use refrigerant directly, with the evaporator coil inside the AHU. They are common in smaller systems and packaged units. DX coils require careful circuiting to ensure proper refrigerant distribution.

Heating Coils (Hot Water): Hot water coils (typically 80-90°C supply) provide heating capacity. They are simpler to size than cooling coils since there is no latent load component.

Steam Coils: Used in applications requiring rapid heating or where steam is readily available. Steam coils require special attention to condensate drainage and freeze protection.

Electric Heating Coils: Used where hot water or steam is not available. They offer precise temperature control but higher operating costs.

Step 1: Define the Design Conditions

Before selecting a coil, establish the following parameters:

Air-Side Conditions:

Fluid-Side Conditions:

Physical Constraints:

Step 2: Determine the Heat Load

The total cooling load consists of sensible and latent components:

Sensible Heat: Q_s = ṁ_air × c_p × (T_in - T_out)

Latent Heat: Q_l = ṁ_air × h_fg × (W_in - W_out)

Total Heat: Q_total = ṁ_air × (h_in - h_out)

Where h_in and h_out are the entering and leaving air enthalpies, and W represents humidity ratio.

The Sensible Heat Ratio (SHR) = Q_s / Q_total is a critical parameter that influences coil selection. A lower SHR indicates more dehumidification is required, which typically demands more rows and lower face velocities.

Step 3: Select Face Velocity

Face velocity is the air velocity across the coil face area and is one of the most important design parameters:

Recommended face velocities:

Lower face velocities provide better heat transfer and dehumidification but require larger coil face areas. Higher face velocities reduce coil size but increase pressure drop and may cause moisture carryover on cooling coils.

The 2.5 m/s (500 fpm) rule: For most cooling coil applications, a face velocity of 2.5 m/s provides a good balance between performance and size. Exceeding 3.0 m/s on cooling coils risks moisture carryover from the coil surface.

Step 4: Determine Coil Geometry

Tube diameter: Standard options are 3/8" (9.52 mm), 1/2" (12.7 mm), and 5/8" (15.88 mm). Smaller tubes provide more surface area per unit volume but higher pressure drop. 3/8" tubes are increasingly popular for their compact design.

Tube spacing: Transverse pitch (Pt) and longitudinal pitch (Pl) determine the tube layout. Standard configurations include 25.4 mm × 22 mm (1" × 0.866") for staggered arrangements.

Fin density: 10-14 FPI for cooling coils, up to 16 FPI for heating coils. Higher fin density increases capacity but also increases pressure drop and fouling risk.

Number of rows: Cooling coils typically use 3-8 rows depending on the required capacity and temperature difference. More rows provide more surface area but with diminishing returns due to reduced LMTD in downstream rows.

Step 5: Verify Performance

After initial selection, verify the following:

Air-side pressure drop: Should not exceed the fan's available static pressure budget for the coil. Typical values range from 100-300 Pa for cooling coils.

Fluid-side pressure drop: Must be within the pump's or system's available pressure. For chilled water coils, 20-60 kPa is typical.

Leaving air conditions: Verify that the leaving air temperature and humidity meet the design requirements.

Fluid velocity: For water coils, tube velocity should be 0.5-2.5 m/s. Below 0.5 m/s, laminar flow reduces heat transfer. Above 2.5 m/s, erosion and noise become concerns.

Condensate drainage: For cooling coils operating below the dew point, ensure adequate drainage provisions and that the coil is oriented for proper condensate removal.

Common Mistakes to Avoid

Undersizing the coil face area: This leads to excessive face velocity, high pressure drop, and moisture carryover. Always verify face velocity is within recommended limits.

Ignoring altitude effects: At higher altitudes, air density decreases, requiring larger coil face areas for the same mass flow rate. A coil sized at sea level will underperform at 1500 m altitude.

Neglecting fouling factors: Clean coil performance will degrade over time. Include appropriate fouling factors in the design calculation (typically 0.000088 m²·K/W for clean water systems).

Improper circuiting: For DX coils, improper refrigerant circuiting leads to uneven distribution, reduced capacity, and poor superheat control. Each circuit should have approximately equal length and heat load.

Using Software for Coil Selection

Modern coil selection software like ExCoil automates the complex calculations involved in coil sizing. These tools use validated heat transfer correlations (Wang-Chi-Chang for air side, Gnielinski for water side, Chen for two-phase refrigerant) to predict coil performance accurately.

The advantage of software-based selection is the ability to quickly evaluate multiple configurations and optimize the design for the specific application requirements, considering all the interrelated parameters simultaneously.

Conclusion

Coil selection for air handling units requires careful consideration of thermal, hydraulic, and physical constraints. By following a systematic approach — defining conditions, calculating loads, selecting geometry, and verifying performance — engineers can specify coils that deliver reliable, efficient performance throughout the life of the HVAC system.