ASHRAE-90.1-2022- - Page 089
Extracted Content
6.5.1.1.4 Dampers. Exhaust/relief, and outdoor air dampers shall meet the requirements of Table 6.4.3.4.3. Return dampers shall meet the requirements of motorized exhaust/relief dampers in Table 6.4.3.4.3.
Exception to 6.5.1.1.4: Exhaust/relief and outdoor air intake dampers on systems intended to operate
continuously.
6.5.1.1.5 Relief of Excess Outdoor Air
a. Systems shall provide one of the following means to relieve excess outdoor air during air economizer
operation to prevent overpressurizing the building :
- Return or relief fan(s) meeting the requirements of Section 6.5.3.2.4.
- Barometric or motorized damper relief path with a total pressure drop at design relief airflow rate less than 0.10 in. of water from the occupied space to outdoors. Design relief airflow rate shall be the design supply airflow rate minus any continuous exhaust flows, such as toilet exhaust fans, whose makeup is provided by the economizer system . b. The relief air outlet shall be located so as to avoid recirculation into the building .
6.5.1.1.6 Sensor Accuracy. Outdoor air, return air, mixed air, and supply air sensors shall be calibrated within the following accuracies:
a. Dry-bulb and wet-bulb temperatures shall be accurate to ±2°F over the range of 40°F to 80°F. b. Enthalpy and the value of a differential enthalpy sensor shall be accurate to ±3 Btu/lb over the range of
20 to 36 Btu/lb. c. Relative humidity shall be accurate to ±5% over the range of 20% to 80% rh.
6.5.1.2 Fluid Economizers Informative Note: ASHRAE Guideline 36 includes detailed sequences of control for enabling and controlling fluid economizers in water-cooled chilled-water plants.
6.5.1.2.1 Design Capacity. Fluid economizer systems shall be capable of providing up to 100% of the expected system cooling load at outdoor air temperatures of 50°F dry bulb/45°F wet bulb and below.
Exceptions to 6.5.1.2.1:
- Systems primarily serving computer rooms in which 100% of the expected system cooling load at the dry-bulb and wet-bulb temperatures listed in Table 6.5.1.2.1 is met with water-cooled fluid economizers .
- Systems primarily serving computer rooms in which 100% of the expected system cooling load at the dry-bulb temperatures listed in Table 6.5.1.2.1 is met with air-cooled fluid economizers .
- Systems where dehumidification requirements cannot be met using outdoor air temperatures of 50°F dry-bulb/45°F wet-bulb and where 100% of the expected system cooling load at 45°F drybulb/40°F wet-bulb is met with water-cooled fluid economizers .
6.5.1.2.2 Maximum Hydronic Pressure Drop. Precooling coils and fluid-to-water heat exchangers used as part of a fluid economizer system shall either have a water-side pressure drop of less than 15 ft of water, or a secondary loop shall be created so that the coil or heat exchanger pressure drop is not seen by the circulating pumps when the system is in the normal cooling (noneconomizer) mode.
6.5.1.3 Integrated Economizer Control. Economizer systems shall be integrated with the mechanical cooling system and be capable of and configured to provide partial cooling even when additional mechanical cooling is required to meet the remainder of the cooling load. Controls shall not false load the mechanical cooling systems by limiting or disabling the economizer or by any other means, such as hot-gas bypass, except at the lowest stage of mechanical cooling .
Units that include an air economizer shall comply with the following:
a. Unit controls shall have the mechanical cooling capacity control interlocked with the air economizer
controls such that the outdoor air damper is at the 100% open position when mechanical cooling is on, and the outdoor air damper does not begin to close to prevent coil freezing due to minimum compressor run time until the leaving air temperature is less than 45°F. b. DX units with a rated capacity no less than 65,000 Btu/h that control the capacity of the mechanical
cooling directly based on occupied space temperature shall have a minimum of two stages of mechanical cooling capacity. c. All other DX units, including those that control space temperature by modulating the airflow to the
space, shall comply with the requirements of Table 6.5.1.3.
ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 87