ASHRAE-90.1-2022- - Page 321

Extracted Content

G3.2.3.10 Chilled-Water Pumps (Systems 7, 8, 11, 12, and 13). Chilled-water systems shall be modeled as primary/secondary systems with constant-flow primary loop and variable-flow secondary loop. For systems with cooling capacity of 300 tons or more, the secondary pump shall be modeled with variablespeed drives and a minimum flow of 25% of the design flow rate. Chilled-water pumps shall only be enabled when a load exists on the associated chilled-water loop . For systems with less than 300 tons cooling capacity, the secondary pump shall be modeled as riding the pump curve. The baseline building constant-volume primary pump power shall be modeled as 9 W/gpm, and the variable-flow secondary pump power shall be modeled as 13 W/gpm at design conditions . For computer room systems using System 11 with an integrated fluid economizer, the baseline building design primary chilled-water pump power shall be increased by 3 W/ gpm for flow associated with the fluid economizer .

Exception to G3.2.3.10: For systems using purchased chilled water, the building distribution pump shall

be modeled with variable-speed drive, a minimum flow of 25% of the design flow rate, and a pump power of 16 W/gpm.

G3.2.3.11 Heat Rejection (Systems 7, 8, 11, 12, and 13). The heat-rejection device shall be an axialfan open-circuit cooling tower with variable-speed fan control and shall have an efficiency of 38.2 gpm/hp at the conditions specified in Table 6.8.1-7. Condenser-water design supply temperature shall be calculated using the cooling tower approach to the 0.4% evaporation design wet-bulb temperature as generated by the formula below, with a design temperature rise of 10°F:

Approach 10°F Range = 25.72 – (0.24 × WB)

where WB is the 0.4% evaporation design wet-bulb temperature (°F); valid for wet bulbs from 55°F to 90°F.

The tower shall be controlled to maintain a leaving water temperature, where weather permits, per Table G3.2.3.11, floating up to the design leaving water temperature for the cooling tower. The baseline building design condenser-water pump power shall be 19 W/gpm and modeled as constant volume. For computer room systems using System 11 with an integrated fluid economizer, the baseline building design condenserwater- pump power shall be increased by 3 W/gpm for flow associated with the fluid economizer . Each chiller shall be modeled with separate condenser-water and chilled-water pumps interlocked to operate with the associated chiller.

G3.2.3.12 Supply Air Temperature Reset (Systems 5 through 8 and 11). The air temperature for cooling shall be reset higher by 5°F under the minimum cooling load conditions.

G3.2.3.13 VAV Minimum Flow Set Points (Systems 5 and 7). Minimum volume set points for VAV reheat boxes shall be 30%of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger.

Exception to G3.2.3.13: Systems serving laboratory spaces shall reduce the exhaust and makeup air

volume during unoccupied periods to the largest of 50% of zone peak airflow, the minimum outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards.

G3.2.3.14 Fan Power and Control (Systems 6 and 8). Fans in parallel VAV fan-powered boxes shall run as the first stage of heating before the reheat coil is energized. Fans in parallel VAV fan-powered boxes shall be sized for 50% of the peak design primary air (from the VAV air-handling unit) flow rate and shall be modeled with 0.35 W/cfm fan power. Minimum volume set points for fan-powered boxes shall be equal to 30% of peak design primary airflow rate or the rate required to meet the minimum outdoor air ventilation requirement, whichever is larger. The supply air temperature set point shall be constant at the design condition .

G3.2.3.15 VAV Fan Part-Load Performance (Systems 5 through 8 and 11). VAV system supply fans shall have variable-speed drives, and their part-load performance characteristics shall be modeled using either Method 1 or Method 2 specified in Table G3.2.3.15.

G3.2.3.16 Computer Room Equipment Schedules. Computer room equipment schedules shall be modeled as a constant fraction of the peak design load per the following monthly schedule:

Month 1, 5, 9—25% Month 2, 6, 10—50% Month 3, 7, 11—75% Month 4, 8, 12—100%

G3.2.3.17 System 11 Supply Air Temperature and Fan Control. Minimum volume set point shall be 50% of the maximum design airflow rate, the minimum ventilation outdoor airflow rate, or the airflow rate required to comply with applicable codes or accreditation standards, whichever is larger.

Fan volume shall be reset from 100% airflow at 100% cooling load to minimum airflow at 50% cooling load. Supply air temperature set point shall be reset from minimum supply air temperature at 50% cooling

ANSI/ASHRAE/IES Standard 90.1-2022 (I-P) 319