Economizer Psychrometrics: Dry Bulb, Enthalpy, and Differential Enthalpy Control

Author: HVACProSales.com Expert

Published: June 2024

Introduction

The integration of economizers in HVAC systems is a proven strategy to enhance energy efficiency by harnessing favorable outdoor air conditions to reduce mechanical cooling demand. To maximize these benefits, understanding the psychrometric principles behind economizer control is critical. This comprehensive guide delves into three prevalent economizer control strategies—dry bulb, enthalpy, and differential enthalpy control—providing HVAC engineers with the fundamental theory, design procedures, best practices, and troubleshooting tips necessary to optimize economizer performance.

This deep dive explores the underlying psychrometrics, including dry bulb temperature, enthalpy calculations, and their application in system control. We also cover sensor selection, system sizing, examples with full calculations, safety considerations, and return on investment (ROI) analysis. Additionally, frequent challenges and common missteps during implementation are addressed to ensure reliable, efficient economizer operation.

For foundational knowledge on psychrometrics and load calculations, visit our detailed guides at HVAC Psychrometrics Fundamentals and HVAC Load Calculations.

Technical Background: Psychrometrics & Equations

1. Dry Bulb Temperature (DBT)

Dry Bulb Temperature is the ambient air temperature measured using a standard thermometer. It's crucial in HVAC for defining the sensible heat portion of air but does not account for moisture content.

2. Enthalpy (h)

Enthalpy represents the total heat content of air per unit mass (typically BTU/lb or kJ/kg), including both sensible and latent heat. It reflects the energy needed to raise the air to a certain temperature and moisture state.

The formula for moist air enthalpy (in Imperial units) is:

h = 0.24 × T + W × (1061 + 0.444 × T)

Where:

• h = enthalpy (BTU/lb dry air)
• T = dry bulb temperature (°F)
• W = humidity ratio (lb moisture / lb dry air)

The humidity ratio (W) is computed by:

W = 0.62198 × (P_v / (P_atm - P_v))

Where:

• P_v = partial pressure of water vapor (in Hg or kPa)
• P_atm = atmospheric pressure

3. Differential Enthalpy (Δh)

Differential enthalpy control bases economizer operation on the comparison between the outdoor air enthalpy (h_o) and the return air enthalpy (h_r):

Δh = h_r - h_o

If Δh > 0, the outdoor air contains less total heat, and the economizer introduces outdoor air to reduce mechanical cooling load.

4. Psychrometric Data Table (Example)

Dry Bulb Temp (°F)	Relative Humidity (%)	Humidity Ratio (lb/lb)	Enthalpy (BTU/lb dry air)
75	50	0.0092	28.8
85	50	0.0130	38.1
55	70	0.0069	21.4
60	60	0.0076	23.5
70	40	0.0076	26.2

Economizer Control Strategies

Dry Bulb Control

Dry bulb control triggers economizer operation solely based on outdoor air temperature. For instance, the economizer damper opens when outdoor air is below a preset threshold (e.g., 60°F), assuming cooler outdoor air reduces cooling loads. This method is simple but doesn't account for humidity, potentially resulting in increased latent loads or occupant discomfort.

Enthalpy Control

This method uses the total heat content (enthalpy) of outdoor air to decide economizer activation. By measuring both temperature and humidity, enthalpy control avoids introducing high-humidity air that could increase latent cooling loads despite low temperatures.

Differential Enthalpy Control

Differential enthalpy control compares outdoor air enthalpy to return air enthalpy. The economizer operates only if outdoor air has less total heat content than return air. This strategy optimizes energy savings and ensures comfort by considering the actual internal air conditions.

Step-by-Step Design Procedure with Worked Example

Design Scenario

Assume an office building located in Atlanta, GA, with a constant air volume (CAV) air handling unit (AHU) incorporating an economizer. The system serves 10,000 CFM (cubic feet per minute) of supply air. Outdoor summer design conditions are 95°F DBT, 60% RH; indoor return air conditions are 75°F DBT, 50% RH.

Step 1: Determine Return Air Humidity Ratio (W_r)

• Use psychrometric charts or formulas. From data:
• Return air: 75°F, 50% RH → Humidity ratio, W_r ≈ 0.0092 lb moisture/lb dry air.

Step 2: Calculate Return Air Enthalpy (h_r)

h_r = 0.24 × 75 + 0.0092 × (1061 + 0.444 × 75)

h_r = 18 + 0.0092 × (1061 + 33.3) = 18 + 0.0092 × 1094.3 = 18 + 10.1 = 28.1 BTU/lb

Step 3: Calculate Outdoor Air Humidity Ratio (W_o)

• Outdoor conditions: 95°F, 60% RH.
• From psychrometric tables, W_o ≈ 0.020 lb/lb dry air (high moisture content due to heat and humidity).

Step 4: Calculate Outdoor Air Enthalpy (h_o)

h_o = 0.24 × 95 + 0.020 × (1061 + 0.444 × 95)

h_o = 22.8 + 0.020 × (1061 + 42.2) = 22.8 + 0.020 × 1103.2 = 22.8 + 22.06 = 44.86 BTU/lb

Step 5: Compare Enthalpies for Differential Enthalpy Control

Δh = h_r - h_o = 28.1 - 44.86 = -16.76 BTU/lb

Since Δh < 0, outdoor air contains greater heat content; economizer dampers remain closed, and mechanical cooling remains active.

Step 6: Dry Bulb Control

Assuming a dry bulb cut-in of 70°F, outdoor temperature (95°F) is too high; economizer remains off.

Conclusion:

Both dry bulb and enthalpy-based strategies correctly prevent economizer operation in unfavorable hot, humid outdoor conditions to maintain comfort and reduce latent loads.

Selection and Sizing Guidance

1. Sensor Selection

• Temperature Sensors: Use accurately calibrated RTDs or thermistors with ^±0.5°F accuracy.
• Enthalpy Sensors: These devices combine dry bulb temperature and humidity sensing. High-quality sensors with 2–3% accuracy and minimal drift are critical for reliable control.
• Humidity Sensors: Capacitive or chilled mirror hygrometers, considering environmental robustness.

2. Damper and Actuator Sizing

• Size economizer dampers to deliver 100% of the required outdoor air volume for ventilation and free cooling.
• Ensure actuator torque ratings exceed damper static and dynamic loads, accounting for duct pressure.
• Include failsafe damper positioning and fail-open or fail-close mechanisms as per design.

3. Controller Selection

• Use controllers capable of integrating differential enthalpy logic for superior energy savings.
• Ensure controllers support analog/digital sensor inputs and are compatible with building automation systems (BAS).
• Consider controllers with self-diagnostic and alarm features.

Best Practices

• Sensor Placement: Install outdoor air sensors in well-ventilated, shaded locations away from heat sources.
• Regular Calibration: Schedule periodic calibration of enthalpy sensors and temperature probes.
• Filter Maintenance: Keep outdoor air intakes and sensors clean from dirt and debris to prevent measurement inaccuracies.
• Control Algorithm Tuning: Adjust setpoints seasonally and tune deadbands to prevent short-cycling of dampers.
• Integration: Link economizer controls with building management systems for data logging and trend analysis.

Troubleshooting Guide

Issue	Possible Causes	Recommended Action
Economizer damper stuck open in hot weather	Damper actuator failure, sensor misreading, faulty controller output	Inspect actuator and linkages, verify sensor calibration, check controller logic
Economizer not activating when outdoor air is cool	Incorrect sensor placement, sensor contamination, improper setpoints	Relocate sensors if needed, clean or replace sensors, review and adjust setpoints
High indoor humidity after economizer activation	Dry bulb control in humid climates, enthalpy sensor failure	Switch to enthalpy or differential enthalpy control, recalibrate or replace sensors
Frequent damper oscillation (hunting)	Control deadband too narrow, faulty PID tuning	Increase control deadband, retune controller settings
Erratic enthalpy sensor readings	Sensor contamination, wiring issues	Clean sensor, verify wiring integrity, replace if necessary

Safety and Compliance

Economizer designs must comply with relevant codes and standards, such as ASHRAE Standard 90.1 for energy efficiency and local building codes. Key safety considerations include:

• Preventing outdoor air contamination: Proper placement of intakes away from exhaust fumes, standing water, or pollutant sources.
• Ensuring indoor air quality: Balancing outdoor air quantity to meet ventilation standards (ASHRAE Standard 62.1) without overcooling or excessive humidity.
• Fail-safe operation: Economizers should default to rejection of outdoor air if sensors or controls fail.
• Electrical Safety: All sensors and actuators should be installed adhering to NEC standards and manufacturer instructions.

For detailed commissioning and compliance checklists, refer to HVAC Commissioning resources.

Cost and Return on Investment (ROI)

Economizers typically increase initial capital costs due to additional sensors, dampers, and controls but offer substantial energy savings:

• Estimated Installation Cost: $2,500 to $7,000 depending on system complexity and sensor quality.
• Energy Savings: 10%–40% reduction in cooling costs depending on climate and building usage.
• Payback Period: Commonly ranges from 1.5 to 3 years, varying by utility rates and system size.

Economic benefit is maximized when:

• Site has a climate with significant hours of favorable outdoor air conditions.
• The building has substantial cooling loads.
• Proper maintenance and tuning are performed.

Common Mistakes & How to Avoid Them

• Using Dry Bulb Control in Humid Climates: Can result in latent load increase and occupant discomfort. Use enthalpy or differential enthalpy control instead.
• Ignoring Sensor Location and Maintenance: Leads to inaccurate readings and