Hot-Aisle Containment Bypass Airflow
Balances IT airflow against cooling-unit supply, calculates bypass fraction and mixed return temperature.
Example
You enter
- IT equipment load (kW) 500
- Temperature rise across equipment (deg F) 20
- Running cooling units 8
- Airflow per cooling unit (cfm) 12000
- Supply air temperature (deg F) 65
- Server exhaust temperature (deg F) 85
- Fan power per unit (kW) 7.5
- Electricity tariff ($/kWh) 0.1
You get
- IT airflow demand 78,981 cfm
- Bypass fraction (%) 17.7276
- Mixed return temperature 81.5 deg F
- Annual fan savings 23290.4
- Two failed margin (CFM) -6981.48
Checked against published worked examples from: Upsite Technologies (Lars Strong).
Details, formula, and sources
ScopeBalances IT airflow against cooling-unit supply, calculates bypass fraction and mixed return temperature, and tests one- and two-unit failure cases. It also estimates fan-energy savings from eliminating bypass; commissioning measurements govern.
Formula or tableIT airflow = IT kW x 3,412/(1.08 x equipment delta-T); bypass = supply - IT airflow; mixed return follows the sensible heat balance; fan power varies with airflow cubed.
Source and editionFirst-principles airflow mixing and the fan affinity laws, with ASHRAE TC 9.9 thermal guidance. Rack-level inlet surveys and commissioning measurements govern.
Free accessMass/heat balance plus the public cube-law fan relation.
What governsEstimate. AHJ and licensed professional govern.
API field namesit_load_kw, equipment_delta_t_f, unit_count, airflow_per_unit_cfm, supply_temp_f, exhaust_temp_f, fan_power_per_unit_kw, tariff_per_kwh, it_airflow_cfm, bypass_fraction_pct, return_temp_f, annual_fan_savings, two_failed_margin_cfm
- Uniform equipment rise all IT airflow is represented by one entered delta-Tplanning model
- Fan affinity variable-speed fan power scales with the cube of airflowfan laws