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Keeping Cool with Outdoor Air: Airside Economizers

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Keeping Cool with Outdoor Air: Airside Economizers engineers newsletter volume 35–2 ● providing insights for today’s hvac system designer handler with economizer controls has keeping cool with outdoor air … four operating modes. Airside Economizers Heating mode. During very cold weather, the air handler brings in minimum outdoor airflow (for many jurisdictions, by the building ventilation) and mixes it with return from the editor … code. For the denizens of commercial air. The mixed air is then heated as buildings, comfort cooling is one of life’s Let’s take a high-level look at airside necessary to maintain the desired basic necessities. Without it, we quickly supply-air (or space) temperature. become irritable, lethargic, and unable economizing: what it is, what it to concentrate. But the cost of keeping requires, and how it’s done. ASHRAE Standard 62.1–2004[2] us comfortably cool rises with each passing day … and that cost isn’t (Standard 62) or building code confined to the person or organization requirements determine the minimum who pays the building utility bill. intake of outdoor air. This minimum How does economizer may be reset downward from the Using outdoor air to help cool a building isn’t a new concept. Thanks to energy cooling work? design value (to save heating energy) standards and local building codes, it’s using various dynamic reset no longer optional in many areas, either. Most commercial buildings have some approaches. But when it’s cold outside, spaces that need cooling all year long. no more than minimum outdoor air In this EN, Dennis Stanke (Trane staff If it’s colder outdoors than indoors, it enters the building, whether or not the engineer and chair of ASHRAE SSPC often makes energy-sense to 62.1) reviews ASHRAE Standard 90.1’s system includes airside economizer requirements for airside economizers “economize” by bringing in more-than- controls. and discusses the underlying design minimum outdoor air to reduce the decisions and benefits of effective airside hours of mechanical cooling system Modulated economizer mode. economizer systems. operation. During “cool” weather (30°F to 55°F A typical “mixed-air” air handler [1°C to 13°C], for example), the You’re in your kitchen on a beautiful, includes dampers for outdoor air, return required space temperature can be breezy fall day, with a pie baking in the air, and relief air (Figure 1). These maintained without any mechanical oven. Would you rather cool off the dampers can be controlled to provide cooling or heating by simply adjusting kitchen by opening your windows or by airside economizing. Usually, an air the mix of outdoor air and return air. turning on your central air conditioner? By opening the windows, of course! Why? If the outdoor air is cool and dry, Figure 1. Typical air handler it can cool the space nicely without using electricity for compressors and fans, and it ventilates the space with fresh outdoor air in the bargain. The same logic holds in non-residential buildings. In fact, it not only makes sense to introduce outdoor air when conditions are “right” but it’s also required by ASHRAE Standard 90.1–2004 [1] (Standard 90) and, in © 2006 Trane All rights reserved ● 1 In this mode, the economizer system Figure 2. U.S. climate zones adjusts both the outdoor- and return-air dampers, modulating these airflows to match cooling capacity with cooling load—without mechanical cooling. Intake airflow varies between the minimum setting and a maximum value (100% of supply airflow) to maintain the supply-air (or space) temperature at setpoint. Integrated economizer mode. During mild weather (55°F to 75°F [13°C to 24°C], for example), outdoor air can provide some cooling capacity, but not enough to satisfy the load, so mechanical cooling supplements the economizer cooling provided by the wide-open outdoor-air damper. We refer to this mode as integrated economizer because it combines “free” cooling (100% outdoor air) downward from its design value to Standard. Figure 3 shows regional with mechanical cooling to meet the save cooling energy. But, when it’s hot economizer requirements, which aim required cooling capacity. The system outside, no more than minimum to minimize the ratio of economizer- stays in integrated economizer mode outdoor air enters the building related costs to energy-related savings. until outdoor conditions reach the high- whether the system includes an These requirements are based on limit shutoff setting (discussed in more airside economizer or not. cooling system capacity and the detail on p. 5), or until the outdoor expected number of hours with conditions fall to the point where Note: Specific control sequences for outdoor conditions that are appropriate modulated economizer operation the preceding operating modes may for economizing. can handle the cooling load. vary with equipment configuration, type of economizer control, and high- Potential energy savings are highest limit settings. where the weather is dry or marine, so Mechanical cooling mode. systems with capacities larger than Economizer operation is disabled ≈5 tons (65,000 Btu/h [19 kW]) in during warm weather, when outdoor these locations must include conditions exceed the high-limit economizer cooling. Moist, cool shutoff setting. Minimum outdoor air What does climates provide fewer opportunities for ventilation (determined by Standard Standard 90 require? for “free” cooling, so the economizer 62 or the local building code) mixes requirement only applies to systems with return air. The mixed air then is According to Standard 90, with capacities larger than ≈11 tons mechanically cooled as needed to Section 6.5.1, economizer cooling (135,000 Btu/h [40 kW]). maintain the supply-air (or space) systems (either airside or waterside) temperature at setpoint. must be used in all cooling systems with fans. There’s also a long list of Potential benefits are lowest where the weather is moist and warm. As in the heating mode, the exceptions to this requirement (see Economizer cooling is not required in minimum outdoor airflow may be reset inset, p. 3) because the value of economizer cooling depends on these climates (although it is allowed) climate, building type, system type, because ASHRAE studies indicate that and control settings. Nevertheless, the potential savings in mechanical Standard 90 requires economizer cooling in many locations for many * buildings and many different systems. Appendix B of Standard 90 also identifies climate zones for various cities elsewhere in North America and around the world. Figure 2 shows the U.S. climate zones* defined in Appendix B of the 2 ● Trane Engineers Newsletter volume 35–2 providing insights for today’s HVAC system designer Figure 3. Standard 90’s regional economizer requirements based on cooling system capacity Figure 4a and Figure 4b (p. 4) show typical economizer sequencing for constant- and variable-volume systems. Standard 90 does not specifically require these operating modes, but they result from logical sequencing of mechanical and economizer cooling. These diagrams expand on the single diagram presented in the Standard 90.1–2004 User’s Manual.[3] Figure 4a: Constant-volume systems. In heating mode, minimum outdoor-air intake flow enters the system. Heating capacity decreases as the outdoor-air heating load decreases. As the weather warms, when the cooling energy may not be sufficient to mechanical cooling to maximize energy system no longer needs heating, it justify the additional cost of savings while avoiding wasteful enters the modulated economizer implementing it. simultaneous cooling and heating. mode. Outdoor air and recirculated To help assure proper sequencing, return air modulate to maintain space Section 6.5.1.1.1: Design capacity. control of the economizer dampers (or supply-air) temperature at setpoint. Airside-economizer systems must shouldn’t be based on mixed-air Outdoor air provides the needed include outdoor- and return-air conditions alone. cooling capacity without any dampers that are sized and modulated mechanical cooling. The black area in so that up to 100% of design supply Figure 4a represents the mechanical airflow can be outdoor air. In other † Warmer supply air increases the delivered airflow cooling energy that’s saved during words, the system must be designed (therefore fan energy) and space humidity levels modulated economizer operation or while reducing both mechanical cooling and local to allow outdoor airflow ranging from reheat energy. Thoroughly analyze the effects of “free cooling.” the minimum required for ventilation to a supply-air-temperature reset strategy before the maximum delivered by the making it part of the system design. As the cooling load increases, the supply fan. outdoor-air damper eventually opens For VAV systems, the supply fan usually delivers less than cooling- Exceptions to Standard 90’s economizer requirement design airflow during the integrated economizer mode—even though the Section 6.5.1 requires economizers for (d) Systems with condenser heat all systems in all locations. But it also recovery. outdoor-air damper is wide open. Of identifies the nine exceptions listed below. course, supply fan airflow (and (e) Any residential space system with a therefore, intake airflow) increases if Note: Economizer systems may be capacity that’s less than five times the the control resets the supply-air used even if not required, provided that applicable limit listed in Exception (a). temperature upward at part load.† the economizer system conforms to the requirements in Sections 6.5.1.1 through (f) Systems with space sensible cooling 6.5.1.4. loads (excluding transmission and Section 6.5.1.1.2: Control signal. infiltration loads) equal to or less than Airside economizer operation must (a) Systems using fan-cooling units transmission and infiltration loads at 60°F. with individual capacities less than be appropriately sequenced with 65,000 Btu/h (19 kW) in dry climates, less (g) Systems that are expected to operate than 135,000 Btu/h (40 kW) in cool-moist less than 20 hours per week.
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