ASHRAE-90.1-2022- - Page 088
Extracted Content
Table 6.5.1.1.3 High-Limit Shutoff Control Settings for Air Economizers [ a]
| Control Type | Allowed Only in Climate Zone at Listed Set Point | Required High-Limit Set Points (Economizer Off when): | Col4 |
|---|---|---|---|
| Control Type | Allowed Only in Climate Zone at Listed Set Point | Equation | Description |
| Fixed dry-bulb temperature | 0B, 1B, 2B, 3B, 3C, 4B, 4C, 5B, 5C, 6B, 7, 8 | TOA > 75°F | Outdoor air temperature exceeds 75°F |
| Fixed dry-bulb temperature | 5A, 6A | TOA > 70°F | Outdoor air temperature exceeds 70°F |
| Fixed dry-bulb temperature | 0A, 1A, 2A, 3A, 4A, | TOA > 65°F | Outdoor air temperature exceeds 65°F |
| Differential dry-bulb temperature | 0B, 1B, 2B, 3B, 3C, 4B, 4C, 5A, 5B, 5C, 6A, 6B, 7, 8 | TOA >TRA | Outdoor air temperature exceeds return air temperature |
| Fixed enthalpy with fixed dry-bulb temperature | All | hOA > 28 Btu/lb b or_ TOA >_75°F | Outdoor air enthalpy exceeds 28 Btu/lb b of dry air b or_outdoor air_ temperature exceeds 75°F |
| Differential enthalpy with fixed dry-bulb temperature | All | hOA >hRA or_ TOA >_75°F | Outdoor air enthalpy exceeds return air enthalpy or_outdoor air_ temperature exceeds 75°F |
a. Devices with selectable rather than adjustable set points shall be capable of being set to within 2°F and 2 Btu/lb of the set point listed. b. At altitudes substantially different than sea level, the fixed enthalpy limit shall be set to the enthalpy value at 75°F and 50% rh. As an example, at approximately 6000 ft elevation,
the fixed enthalpy limit is approximately 30.7 Btu/lb.
- Where the use of outdoor air for cooling will affect supermarket open refrigerated casework systems .
- For comfort cooling, where the cooling efficiency meets or exceeds the efficiency improvement requirements in Table 6.5.1-2.
- Systems primarily serving computer rooms where a. the total design cooling load of all computer rooms in the building is less than 3,000,000 Btu/h
and the building in which they are located is not served by a centralized chilled-water plant; b. the room total design cooling load is less than 600,000 Btu/h and the building in which they are
located is served by a centralized chilled-water plant; c. the local water authority does not allow cooling towers; or d. less than 600,000 Btu/h of computer-room cooling equipment capacity is being added to an exist-
ing building . 12. Dedicated systems for computer rooms, where a minimum of 75% of the design load serves a. those spaces classified as an essential facility, b. those spaces having a design of Tier IV as defined by ANSI/TIA-942, c. those spaces classified under NFPA 70 Article 708— Critical Operations Power Systems (COPS),
or d. those spaces where core clearing and settlement services are performed such that their failure to
settle pending financial transactions could present systemic risk as described in “The Interagency Paper on Sound Practices to Strengthen the Resilience of the U.S. Financial System” (April 7, 2003).
6.5.1.1 Air Economizers 6.5.1.1.1 Design Capacity. Air economizer systems shall be capable of and configured to modulate outdoor air and return air dampers to provide up to 100% of the design supply air quantity as outdoor air for cooling.
6.5.1.1.2 Control Signal. Economizer controls shall be capable of and configured to sequence the dampers with the mechanical cooling equipment and shall not be controlled by only mixed-air temperature.
Exception to 6.5.1.1.2: The use of mixed-air temperature limit control shall be permitted for systems
controlled from space temperature (such as single-zone systems ).
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for sequencing air economizers with mechanical cooling using supply air (not mixed air) control loops.
6.5.1.1.3 High-Limit Shutoff. All air economizers shall be capable of and configured to automati- cally reduce outdoor air intake to the design minimum outdoor air quantity when outdoor air intake will no longer reduce cooling energy use. High-limit shutoff control types and associated set points for specific climate zones shall be chosen from Table 6.5.1.1.3.
Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for disabling air econ- omizers using all options of high-limit shutoff options with compliant set points .
86 ANSI/ASHRAE/IES Standard 90.1-2022 (I-P)