Calculators › Operations, maintenance and commissioning Flow-normalized filter resistance calculator
Flow-normalized filter resistance
For the same filter grade and air density, a quadratic pressure-flow model separates changing flow from changing resistance. Treat the ratio as a diagnostic screening model.
Units IP (US) SI (metric)
R rel = Δ P Δ P clean × ( Q Q clean ) 2 R_{\text{rel}} = \frac{\Delta P}{{\Delta P_{\text{clean}}} \times \left(\frac{Q}{Q_{\text{clean}}}\right)^{2}} R rel = Δ P clean × ( Q clean Q ) 2 Δ P R rel = Δ P Δ P clean × ( Q Q clean ) 2 R_{\text{rel}} = \frac{\Delta P}{{\Delta P_{\text{clean}}} \times \left(\frac{Q}{Q_{\text{clean}}}\right)^{2}} R rel = Δ P clean × ( Q clean Q ) 2 Δ P
Calculator
Enter the values you know and leave one box empty. Calculate finds the empty one, the result or any input.
Worked example
Given
Δ P \Delta P Δ P 1.7 in water Δ P clean {\Delta P_{\text{clean}}} Δ P clean 0.9 in water Q Q Q 1,300 cfm Q clean Q_{\text{clean}} Q clean 1,100 cfm R rel = 1.7 0.9 × ( 1300 1100 ) 2 ≈ 1.352 R_{\text{rel}} = \frac{1.7}{0.9 \times \left(\frac{1300}{1100}\right)^{2}} \approx 1.352 R rel = 0.9 × ( 1100 1300 ) 2 1.7 ≈ 1.352
Given
Δ P \Delta P Δ P 480 Pa Δ P clean {\Delta P_{\text{clean}}} Δ P clean 200 Pa Q Q Q 620 L/s Q clean Q_{\text{clean}} Q clean 500 L/s R rel = 480 200 × ( 620 500 ) 2 ≈ 1.561 R_{\text{rel}} = \frac{480}{200 \times \left(\frac{620}{500}\right)^{2}} \approx 1.561 R rel = 200 × ( 500 620 ) 2 480 ≈ 1.561
Symbols and units
Symbol Meaning Unit
R rel R_{\text{rel}} R rel relative filter resistance ratio Δ P \Delta P Δ P current differential pressure in water Δ P clean {\Delta P_{\text{clean}}} Δ P clean clean-reference differential pressure in water Q Q Q current volumetric airflow cfm Q clean Q_{\text{clean}} Q clean clean-reference volumetric airflow cfm
Symbol Meaning Unit
R rel R_{\text{rel}} R rel relative filter resistance ratio Δ P \Delta P Δ P current differential pressure Pa Δ P clean {\Delta P_{\text{clean}}} Δ P clean clean-reference differential pressure Pa Q Q Q current volumetric airflow L/s Q clean Q_{\text{clean}} Q clean clean-reference volumetric airflow L/s
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