The recirculation of exhaust air or generator gases by your external units (IEC, Chiller, etc.) not only wastes energy but also causes capacity derating and PUE degradation.
kW
$
15%
Ideal (0-5%)
Risky Field Avg.(25%)
Critical Max Case Data (57%)
Annual Financial Loss
$0
Monthly: $0
ESG: Excess Carbon Emissions
0Tons CO₂
0 kWh wasted energy
Hidden Engineering Costs
Degraded PUE
1.35
+0.00 penalty
Intake Temp. Rise
+0°C
Thermal Stress
Capacity Loss
0 kW
Derating Effect
Recirculation of hot air lowers thermodynamic capacity. Equipment is forced to work harder.
ASHRAE TC 9.9 & Alkazar Case Study
Real Field Finding: In a real scenario study where rooftop generators are active, wind effect directed 400°C exhaust gas to the cooling units, increasing the intake temperatures up to 49.4°C and the bypass ratio to 57%. This leads to capacity failure and hardware malfunction.
The External Flow & Bypass Loss Analysis tool estimates the annual hidden cost of data-centre outdoor units (CRAC/chiller) recirculating their own exhaust air or generator exhaust — in wasted energy, PUE degradation and carbon footprint. It is free to use.
Effective bypass ratio: the entered bypass percentage is scaled by a generator-location/wind-exposure multiplier (1.0 / 1.3 / 1.8)
Extra load:Pextra = Pcooling × (Effective bypass / 100)
Annual energy loss:E = Pextra × 8760 h × 70% capacity utilisation factor
Annual cost:E × unit electricity price; carbon:E × 0.43 kg CO₂/kWh (Türkiye grid average)
Temperature rise and capacity loss: scaled linearly from a real field case (57% bypass → +7.5 °C intake temperature rise); capacity loss assumes roughly 1.5% derating per 1 °C intake temperature rise.
Worked example
Opening the tool with its unchanged defaults (2,000 kW cooling capacity, 3.5 currency units/kWh, current PUE 1.35, 15% estimated bypass, generators on the roof/near the units → 1.8× multiplier) shows:
Quantity
Value
Effective bypass ratio (15% × 1.8)
27%
Wasted annual energy
≈1,159,000 kWh
Annual financial loss
≈4,056,000 (in the entered currency)
Added carbon footprint
≈498 t CO₂/yr
Degraded PUE
1.35 → 1.44 (+0.09 penalty)
Intake temperature rise / capacity loss
+3.6 °C / −107 kW (5.3%)
When this tool is not enough
The bypass ratio is a user-supplied estimate based on site observation or assumption; the tool itself is not a CFD solver and does not measure actual recirculation.
The 70% capacity utilisation factor and the grid carbon factor (0.43 kg CO₂/kWh) are fixed assumptions and may differ by site.
The temperature-rise/bypass relationship is scaled linearly from a single reference field case (wind directing rooftop generator exhaust into the units) and may not hold at the same ratio for every site.
Determining the actual bypass ratio and its source (generator, neighbouring unit, building facade) requires a site-specific CFD external-flow analysis.
To measure your actual bypass/recirculation ratio and evaluate deflector/barrier optimisation, see our Data Centre CFD service, or contact us.
Frequently asked questions
Is this tool free?
Yes, the External Flow & Bypass Loss Analysis tool is free.
How do I estimate the bypass/recirculation ratio?
Use site observation, thermal-camera measurement, or reference values from comparable facilities; the slider marks typical "ideal", "risky site average" and "critical case" bands from 0–60%.
How does generator location affect the result?
Enclosed/isolated generators have no effect (1.0× multiplier); open generators near the cooling units, especially on the roof, significantly raise the effective bypass ratio (1.8× multiplier).
Does this replace an actual PUE measurement?
No, this is an estimation/awareness tool. Actual PUE and recirculation ratio require field measurement or CFD analysis.
What currency and grid factor are used?
The default uses Turkish lira and the Türkiye grid carbon factor; you can substitute your own facility's figures.
The explanation on this page is for general information and does not replace project-specific engineering services. Final values should be determined under the currently applicable edition of the relevant standard.