Constant-volume air-conditioning systems control space air temperature directly and humidity indirectly by modulating or cycling the cooling apparatus. This article examines the effect of economizer operation on the comfort of spaces served by constant-volume air-conditioning systems.

As they relate to integrated operation, the ideas presented here are applicable primarily to direct-expansion (DX) systems with limited unloading capability; however, many also apply to chilled-water systems. Two hypothetical cases are considered: an office building and a church sanctuary, the latter of which will be discussed next month, in Part 2 of this article.

Background
ANSI/ASHRAE/IESNA Standard 90.1-2010, Energy Standard for Buildings Except Low-Rise Residential Buildings, establishes eight climate zones, numbered 1 through 8, and a number of sub-zones designated with appended letters, such as 2B, 4C, etc. The 2007 version of the standard mandates economizers in all zones except 1A, 2A, 3A, and 4A, which basically are all of the counties east of the Mississippi River and south of the Mason-Dixon line plus western Louisiana and southeastern Texas. (Appendix B of the standard lists the climate zones for every state and county in the United States.) The recently released 2010 version of the standard removes zones 2A, 3A, and 4A from the exemption, leaving only Zone 1A. In all zones, systems under 58,000-Btuh capacity are exempt.

Figure 1 is a schematic representation of an air-side economizer. The outdoor-air and relief dampers are sized to pass the full required cooling supply airflow, while the return damper is sized to provide the supply airflow less the airflow required for space pressurization. When the zone requires heat or 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 ANSI/ASHRAE Standard 62.1-2010, Ventilation for Acceptable Indoor Air Quality, 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 provide all of the cooling needed or to lessen the cooling load when the mechanical cooling system and the economizer operate concurrently. The circled numbers in Figure 1 relate to state points plotted on the psychrometric charts presented in this article.

Operation and Control
Standard 90.1 prescribes a limited number of high-limit controls: fixed dry bulb, differential dry bulb, fixed enthalpy, electronic enthalpy, differential enthalpy, and dew point/dry bulb. Each defines an outdoor temperature/humidity condition below which "free" cooling using outdoor air is enabled.

Figure 2 is a graphic representation of an economizer cooling cycle. At some low cooling loads, when outdoor conditions permit economizer operation, all cooling can be provided with the outdoor-air damper in the minimum-ventilation position, the return damper fully open, and the relief damper closed. As cooling load increases, the outdoor-air damper modulates open to maintain the space-temperature cooling set point, the relief damper modulates open to maintain constant zone positive pressure, and the return damper modulates to maintain constant supply airflow. Eventually, cooling load increases to the point the return damper closes, and all zone airflow is supplied through the outdoor-air damper.

As cooling load continues to increase, Standard 90.1-2010 mandates "integrated" economizer operation, with the outdoor-air damper fully open (providing all supply air) and the zone cooling system modulating or cycling to maintain the space-temperature set point. This mode of operation continues until outdoor conditions exceed the high-limit set point, when dampers return to "normal" cooling operation: outdoor-air damper in minimum-ventilation position, relief damper closed, and return damper open. (Note that Standard 90.1-2007 mandates integrated economizer operation only in zones 2B, 3B, 3C, 4B, and 4C, and some jurisdictions may continue to allow the 2007 exemptions.) There are problems with integrated operation when cooling is provided by systems with limited unloading capability, as will be discussed later in this article.

The object of the mandatory economizer requirement is, of course, to save energy without causing discomfort in occupied spaces. All allowed high-limit-control methods are assumed capable of achieving space design temperature. However, as will be shown, certain allowed high-limit-control methods can result in space relative humidity exceeding base design, which usually is between 50 and 60 percent. Thus, psychrometric analysis of a chosen economizer high limit is essential.

The control schemes examined for the office and church-sanctuary examples are fixed dry bulb, electronic enthalpy, and dew point/dry bulb. Both buildings are located in Minneapolis. Minneapolis was chosen because it is in a climate zone (6A) with relatively moist conditions, but no extremely hot days.

Office Example
Figure 3 is a psychrometric chart of the office operating at its air-conditioning design point. A 5-ton air-conditioning unit is operating with a supply airflow of 2,000 cfm and an outdoor airflow of 330 cfm at design outdoor conditions of 91°F DB and 73.5°F WB—Point 2 on the chart. The design room condition is Point 1: 76°F DB and 50-percent RH. The coil entering conditions are Point 3. The process line, Point 1 to Point 1A, represents the room sensible and total air-conditioning load at the design condition and airflow. The process line from Point 3 to Point 4 represents the cooling and dehumidification of supply air as it passes through the evaporator coil. Point 4 is the state point of the cooling/dehumidifying air being delivered to the room air diffusers. Because supply air follows the slope of the room process line (Point 1A to Point 1) as it picks up heat and moisture, Point 4 must be at a lower dew point and dry-bulb temperature than Point 1A. If Point 4 is at a higher dew point and dry-bulb temperature than Point 1A, the system will not hold the desired design condition, and the room will be warmer and more humid than Point 1.

The points plotted in Figure 3 correspond to the circled numbers in Figure 1. Table 1 lists the important psychrometric parameters of each state point. Point 1, the return, has the same state conditions as the occupied spaces, except for return airflow, which is lower than supply airflow (Point 4) because a portion is exhausted or leaks from the building.