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Cooling Coil Selection: ADP, Bypass Factor, and Psychrometric Design Procedure

Cooling Coil Selection: ADP, Bypass Factor, and Psychrometric Design Procedure

In HVAC system design, cooling coils are crucial components responsible for removing sensible and latent heat from conditioned air streams. The selection and sizing of these coils directly affect system performance, comfort, and energy efficiency. This comprehensive guide dives deep into key cooling coil design concepts such as Apparatus Dew Point (ADP), Bypass Factor (BF), and the psychrometric design procedure, supplemented with detailed equations, data tables, worked examples, and best practices.

Introduction

Cooling coil selection is an art and science grounded in thermodynamics and psychrometrics. Correct coil sizing ensures indoor comfort conditions, controls humidity levels, and optimizes energy consumption. Missteps in coil selection can lead to poor dehumidification, increased energy costs, and mechanical issues.

This article explores:

  • The physical and thermodynamic basis for cooling coil operation
  • The definition and significance of Apparatus Dew Point (ADP)
  • The bypass factor and its relationship to coil performance
  • Step-by-step psychrometric design procedure with worked-out examples
  • Selection and sizing guidelines
  • Best practices and troubleshooting tips
  • Safety, compliance, cost, and ROI considerations

For foundational knowledge related to air properties, please refer to our HVAC Psychrometrics Fundamentals page. For load calculation methodologies, visit our HVAC Load Calculations resource. For commissioning insights tailored to cooling coils, see HVAC Commissioning. For terminology clarification, the HVAC Glossary is highly recommended.

Technical Background

Cooling Coil Operation Fundamentals

The cooling coil removes heat from the incoming air by transferring it to a chilled water or refrigerant medium. The coil surface temperature is generally below the dew point of the incoming air, causing moisture to condense and thus providing latent cooling. Sensible cooling lowers the air dry-bulb temperature.

Apparatus Dew Point (ADP)

The Apparatus Dew Point (ADP) is the theoretical temperature at which the coil surface is saturated with moisture. It is the effective coil surface temperature where air leaving the coil reaches saturation in contact with the coil’s surface. The ADP provides a basis to determine latent and sensible heat removal capacities independently from the bypass factor.

Graphically, on the psychrometric chart, the ADP lies on the saturation curve, and the line connecting the coil entering air conditions to the apparatus dew point defines the coil’s cooling line.

Physical Meaning:

  • ADP represents the point where air temperature and humidity are in equilibrium with the coil surface.
  • Helps in determining latent loads without explicit knowledge of bypass factor.
  • Used for calculating coil capacity and coil surface temperature.

Bypass Factor (BF)

The bypass factor is a dimensionless ratio that indicates the portion of air stream that passes through the coil without effective heat exchange with the coil surface. It accounts for air that bypasses the coil surface due to airflow channeling or insufficient coil face velocity.

Mathematically,

BF = T_{\text{out}} - T_{\text{ADP}} T_{\text{in}} - T_{\text{ADP}}

Where:

  • T_in = Coil entering air dry-bulb temperature (°F or °C)
  • T_out = Coil leaving air dry-bulb temperature (°F or °C)
  • T_ADP = Apparatus dew point temperature (°F or °C)

Values of BF typically range from 0.01 (tight coil with no bypass) to 0.3 or more in poorly designed or installed coils.

Basic Equations for Cooling Coil Calculations

The total cooling load (Q) is the sum of sensible and latent loads:

Q = Q_sensible + Q_latent

Sensible heat removed from the air (dry coil load):

Q_sensible = 1.08 ⁣ CFM ⁣ ( T_{in} - T_{out} )

Where:

  • Q_sensible in BTU/hr
  • CFM = airflow rate in cubic feet per minute
  • T in °F

Latent heat load (moisture removal):

Q_latent = 4840 ⁣ CFM ⁣ ( W_{in} - W_{out} )

Where:

  • Q_latent in BTU/hr
  • W = humidity ratio (lb moisture/lb dry air)

Useful Constants & Units Table

Parameter Symbol Value Units
Specific heat of air Cp 0.24 BTU/lb·°F
Density of air ρ 0.075 lb/ft³
Latent heat of vaporization (approx.) h_fg 970 BTU/lb
Conversion constant 1.08 (for sensible heat Q=1.08xCFMxΔT)

Step-by-Step Psychrometric Design Procedure

This section details a methodical procedure to select and size a cooling coil based on psychrometric principles, airflow, and load requirements. Calculations are demonstrated using a typical design example.

Step 1: Define Design Conditions

Determine:

  • Entering air conditions: dry bulb, wet bulb, RH, humidity ratio (e.g., 85°F DB, 67°F WB)
  • Required leaving air conditions: typically 55°F DB or set by comfort/humidity requirements
  • Supply airflow rate (CFM)

Example:

  • Entering air: 85°F DB / 67°F WB
  • Leaving air: 55°F DB (RH not given, will be calculated)
  • Airflow: 4,000 CFM

Step 2: Find the Coil Leaving Air Humidity Ratio

On a psychrometric chart or using equations, find the saturation humidity ratio at leaving dry-bulb temperature 55°F:

  • At 55°F DB and assuming saturated air (RH=100%), saturated humidity ratio, W_s ≈ 0.0085 lb/lb (refer to psychrometric tables)

This assumes coil leaves the air saturated (100% RH) as is typical for cooling coils before mixing/distribution.

Step 3: Calculate Entering Air Humidity Ratio

Using the psychrometric chart or equations, find humidity ratio at entering conditions:

  • W_in at 85°F DB / 67°F WB ≈ 0.0150 lb/lb

Step 4: Calculate Sensible Cooling Load

Use the sensible heat formula:

Q_sensible = 1.08 \times CFM \times (T_{in} - T_{out})

Substitute:

  • CFM = 4,000
  • T_in = 85°F, T_out = 55°F

Calculate:

Q_sensible = 1.08 \times 4000 \times (85 - 55) = 1.08 \times 4000 \times 30 = 129,600 \text{ BTU/hr}

Step 5: Calculate Latent Cooling Load

Use latent heat formula:

Q_latent = 4840 \times CFM \times (W_{in} - W_{out})

Substitute values:

  • W_in = 0.0150 lb/lb
  • W_out = 0.0085 lb/lb

Calculation:

Q_latent = 4840 \times 4000 \times (0.0150 - 0.0085) = 4840 \times 4000 \times 0.0065 = 125,840 \text{ BTU/hr}

Step 6: Calculate Total Cooling Load

Sum both latent and sensible cooling:

Q_total = Q_sensible + Q_latent = 129,600 + 125,840 = 255,440 \text{ BTU/hr}

Step 7: Estimate Apparatus Dew Point (ADP)

Use energy and humidity balances, or graphically find the point on saturation curve where line joining entering and leaving air condition intersects saturation line.

Pragmatically:

T_{ADP} = \frac{T_{out} - BF \times T_{in}}{1 - BF}

However, since BF is not yet known, this needs iterative or graphical methods or use of coil manufacturer data.

Step 8: Calculate Bypass Factor (BF)

If coil surface temperature (ADP) is approximated or known:

BF = \frac{T_{out} - T_{ADP}}{T_{in} - T_{ADP}}

Step 9: Determine Coil Surface Temperature

Use ADP or calculate from BF and coil entering/leaving air temperature.

Step 10: Verify Coil Face Velocity and Pressure Drop

Ensure coil face velocity is within recommended ranges (typically 400-600 feet per minute) for optimal performance. Pressure drop should also be checked with manufacturer curves.

Worked Example Summary

Parameter Value Units
Entering Air Temperature (T_in) 85 °F
Leaving Air Temperature (T_out) 55 °F
Entering Air Humidity Ratio (W_in) 0.0150 lb moisture/lb dry air
Leaving Air Humidity Ratio (W_out) 0.0085 lb moisture/lb dry air
Airflow (CFM) 4000 cfm