Air Side Economizer Design Using Psychrometric Analysis - a PDH Online Course for Engineers, Architects and Surveyors

Air Side Economizer Design Using Psychrometric Analysis

Course Outline

Since the economizer mandate first appeared in ASHRAE Standard 90.1 in 2004, there has been very little guidance for designers in the professional literature. In particular, there is virtually no guidance on selecting the economizer cutoff high limits, other than a few articles in professional and commercial journals that emphasize energy saving and dismiss occupant comfort considerations. This course provides the first comprehensive design recommendations that cover damper quality control as well as methods to set high limits for energy saving without compromising occupant comfort.

This course provides air side economizer design guidance to help engineers and technicians avoid the errors that lead to economizer failure and resultant occupant discomfort and wasted energy. The most common problem is failed damper systems, which can be avoided by proper sizing and by specifying high quality components. Occupant discomfort can also result from humidity and moisture problems that occur during economizer operation if operating limits are set too aggressively. In this course, the use of psychrometric analysis to set rational operating limits will be demonstrated. Finally, the characteristics of various control systems will be discussed, with examples of how they should be specified.

1.   The ASHRAE economizer mandate
2.   Definition of important terms
3.   Basic economizer configuration
4. Sizing and quality control of louvers and dampers
5.   Air side economizer operation and control
6.   Psychrometric Fundamentals
7.   Cooling System Psychrometric State Points
8.   Economizer shutoff limits as defined and allowed by ASHRAE 90.1
9.   Psychrometric analysis to establish economizer shutoff limits
      a.         an office
      b.         an assembly occupancy
10. Economizer controls and sequences
11. Filtration considerations

This course includes a multiple-choice quiz at the end, which is designed to enhance the understanding of the course materials.

Learning Objective

At the conclusion of this course, the student will:

Understand the causes of economizer failure;
Be able to size and specify economizer system dampers to minimize the probability of failure;
Understand how air side economizers are controlled;
Be familiar with permitted economizer high limit shutoff controls and required minimum set points;
Be able to use psychrometric analysis to set economizer shutoff high limits that will both save energy and retain occupant comfort; and
Understand basic economizer control systems and control sequences.

Intended Audience

This course is intended for design professionals that plan, design, and implement HVAC systems for non-residential buildings.

Benefit to Attendees

Attendee of this course will understand the physics and psychrometrics of air side economizers and be able to apply sound design principles that will enhance reliability and avoid occupant discomfort when the economizer is operating.

Course Introduction

Air side economizers are mandated by ASHRAE 90.1-2010 for HVAC systems of five tons and greater. Most jurisdictions in the United States will adopt this mandate, and designers will need to comply. The purpose of an air-side economizer in an HVAC system t is to provide cooling energy savings by shifting cooling load from the air conditioning system to a regulated volume of outdoor air, using a combination of sensors, dampers, and actuators. Unfortunately, the reliability of economizer systems has been poor, with some estimates that only about 25% of economizers currently installed are working properly. This course provides air side economizer design guidance to help engineers and technicians avoid the errors that lead to economizer failure and resultant occupant discomfort and wasted energy.

Course Content

This course content is in the following PDF document:

Air Side Economizer Design Using Psychrometric Analysis

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Course Summary

ASHRAE Standard 90.1-2010 mandates that throughout the United States except for South Florida, all HVAC systems of five tons and larger be equipped with and air side economizer system. A basic economizer system has an outdoor air louver and damper sized to pass the full HVAC supply air flow, a return air damper, and a relief damper. When the zone requires heat, or the outdoor conditions are warm and humid, the relief damper is fully closed, the return damper is open, and the outdoor air damper is fixed in the minimum ventilation position, as established by Standard 62.1-2010 and building pressurization requirements. If the zone requires cooling and outdoor air is cool and dry, the economizer can modulate the outdoor, return, and relief dampers to either provide all of the cooling needed, or to take some of the cooling load off of the zone mechanical cooling system.

If the HVAC system is constant volume DX, when the economizer is operating the outdoor air damper will modulate to maintain zone space temperature, the return damper will modulate to maintain constant supply air flow, and the relief damper will modulate to maintain constant zone positive pressure. These dampers must be carefully sized to provide the proper pressure loss and must be specified to ensure quality, reliability, and low leakage.

The economizer high limit shutoff control is the state or states of outdoor air that define the locus of temperature and humidity below which the economizer may be activated. ASHRAE 90.1 defines six allowable high limit systems with minimum temperature, enthalpy, and dew point settings and restrictions on use in certain climate zones. These limits can be plotted on a psychrometric chart along with state points and process lines representing economizer operation at varying outdoor air conditions.

Psychrometric analysis allows the engineer to examine room or zone loads at the outdoor conditions where economizer operation is permitted, with the internal loads – people, lights, etc – expected under these ambient conditions. The room load process line can then be plotted on the psychrometric chart, as shown in the examples. Supply air conditions under economizer operation will fit one of three cases: Blended outdoor and return air, 100% outdoor air when the zone load equals the capacity of the economizer alone, and 100% outdoor air with ac unit operation to achieve room temperature set point. All of these can be plotted on the psychrometric chart, and the final room conditions can be found by assuming that the supply air will follow the slope of the room load process line as it picks up heat and moisture.

Economizer controls are complex and subject to instability, and should be furnished and installed only as factory-furnished components of the specified zone cooling system, or field furnished and installed by one of the major control system manufacturers.

Quiz

Once you finish studying the above course content, you need to take a quiz to obtain the PDH credits.

DISCLAIMER: The materials contained in the online course are not intended as a representation or warranty on the part of PDH Center or any other person/organization named herein. The materials are for general information only. They are not a substitute for competent professional advice. Application of this information to a specific project should be reviewed by a registered architect and/or professional engineer/surveyor. Anyone making use of the information set forth herein does so at their own risk and assumes any and all resulting liability arising therefrom.