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Gul Bahar Shah
Gul Bahar Shah
External Static Pressure (ESP) — zero ➜ hero
Goal
📌 ESP = fan pressure needed to overcome all losses outside the unit casing.
If the AHU has a return fan, compute Supply ESP and Return ESP separately.
1️⃣ Define the system
• Airflow (Q) (m³/s).
• Paths: supply to most remote diffuser, and return from farthest grille to unit.
• What’s outside the unit: ducts, fittings, terminals, silencers, dampers, coils/filters in the duct, louvers.
2️⃣ Sketch the critical path
🧭 One‑line from fan discharge → last diffuser.
Do the same back to the unit for return. The longest sum is the critical path.
3️⃣ Collect data
• Duct sizes, lengths, number of fittings.
• Device drops at design flow (Pa): filters, coils, VAVs, attenuators, grilles/diffusers, louvers.
• Air properties: use ρ ≈ 1.2 kg/m³.
• Conversions: 1 in.wg = 249 Pa.
4️⃣ Equations (plain text)
• Area A = W × H (for round: A = πD²/4).
• Velocity v = Q / A.
• Velocity pressure q = 0.5 × ρ × v² (Pa).
• Rectangular hydraulic dia: Dh = 2ab/(a+b).
Two ways to get losses:
a) Friction‑rate method: use tables/ductulator to get R (Pa/m) at your v → ΔP_straight = R × L.
b) Darcy/K method: ΔP_straight = f × (L/Dh) × q; fittings: ΔP_fitting = K × q.
5️⃣ Typical values (starting points)
• Low‑pressure mains: pick R ≈ 0.6–1.0 Pa/m.
• Clean filter 75–125 Pa, dirty 150–250 Pa.
• Cooling coil 100–200 Pa.
• Diffuser 30–70 Pa.
• Fire/smoke damper 10–30 Pa open.
• Louver 40–80 Pa at rated face velocity.
Always replace with manufacturer data when available.
6️⃣ Worked micro‑example (supply path)
Q = 4.8 m³/s. Main duct 1200×650 → A=0.78 m² → v≈6.15 m/s → q≈22.6 Pa.
Straight duct: L=45 m, R=0.9 Pa/m → 40 Pa.
Fittings (sum K ≈ 5.0 for elbows/tees/transitions) → ΔP= K×q ≈ 113 Pa.
Devices: filter 120 Pa, coil 140 Pa, attenuator 60 Pa, damper 10 Pa, diffuser 50 Pa, flex 10 Pa.
Supply ESP ≈ 40+113+120+140+60+10+50+10 = 543 Pa.
Add 10–15% unknowns → ~600 Pa fan selection.
Do the same for return if a return fan exists.
7️⃣ How to build this in Excel (fast and accurate)
🧱 Sheet 1 – Inputs: project air density, Q, branch IDs.
📐 Sheet 2 – Duct segments: columns = Run, W, H, L, A, v, q, R, ΔP_straight.
🔧 Sheet 3 – Fittings: drop‑down type → lookup K or Leq from a library (SMACNA/ASHRAE) → ΔP_fitting = K×q (or R×Leq).
📦 Sheet 4 – Devices: filter/coil/damper data at design flow.
🧮 Sheet 5 – Totals: sum per path, show critical path with MAX().
🎯 Checks:
• Velocity limits (mains 4–7 m/s; branches 2–5 m/s; grilles per spec).
• Voltage drop… for fans? not here—keep electrical separate.
• Use conditional formatting to flag v or ΔP out of range.
8️⃣ Software helpers
• Duct sizing/pressure: Revit MEP, Lindab/AS Duct, McGill Airflow, Elite, Carrier Duct Designer.
• Ductulators (paper/app) give R quickly.
• Keep Excel as the master because it documents assumptions transparently.
📌 ESP = fan pressure needed to overcome all losses outside the unit casing.
If the AHU has a return fan, compute Supply ESP and Return ESP separately.
1️⃣ Define the system
• Airflow (Q) (m³/s).
• Paths: supply to most remote diffuser, and return from farthest grille to unit.
• What’s outside the unit: ducts, fittings, terminals, silencers, dampers, coils/filters in the duct, louvers.
2️⃣ Sketch the critical path
🧭 One‑line from fan discharge → last diffuser.
Do the same back to the unit for return. The longest sum is the critical path.
3️⃣ Collect data
• Duct sizes, lengths, number of fittings.
• Device drops at design flow (Pa): filters, coils, VAVs, attenuators, grilles/diffusers, louvers.
• Air properties: use ρ ≈ 1.2 kg/m³.
• Conversions: 1 in.wg = 249 Pa.
4️⃣ Equations (plain text)
• Area A = W × H (for round: A = πD²/4).
• Velocity v = Q / A.
• Velocity pressure q = 0.5 × ρ × v² (Pa).
• Rectangular hydraulic dia: Dh = 2ab/(a+b).
Two ways to get losses:
a) Friction‑rate method: use tables/ductulator to get R (Pa/m) at your v → ΔP_straight = R × L.
b) Darcy/K method: ΔP_straight = f × (L/Dh) × q; fittings: ΔP_fitting = K × q.
5️⃣ Typical values (starting points)
• Low‑pressure mains: pick R ≈ 0.6–1.0 Pa/m.
• Clean filter 75–125 Pa, dirty 150–250 Pa.
• Cooling coil 100–200 Pa.
• Diffuser 30–70 Pa.
• Fire/smoke damper 10–30 Pa open.
• Louver 40–80 Pa at rated face velocity.
Always replace with manufacturer data when available.
6️⃣ Worked micro‑example (supply path)
Q = 4.8 m³/s. Main duct 1200×650 → A=0.78 m² → v≈6.15 m/s → q≈22.6 Pa.
Straight duct: L=45 m, R=0.9 Pa/m → 40 Pa.
Fittings (sum K ≈ 5.0 for elbows/tees/transitions) → ΔP= K×q ≈ 113 Pa.
Devices: filter 120 Pa, coil 140 Pa, attenuator 60 Pa, damper 10 Pa, diffuser 50 Pa, flex 10 Pa.
Supply ESP ≈ 40+113+120+140+60+10+50+10 = 543 Pa.
Add 10–15% unknowns → ~600 Pa fan selection.
Do the same for return if a return fan exists.
7️⃣ How to build this in Excel (fast and accurate)
🧱 Sheet 1 – Inputs: project air density, Q, branch IDs.
📐 Sheet 2 – Duct segments: columns = Run, W, H, L, A, v, q, R, ΔP_straight.
🔧 Sheet 3 – Fittings: drop‑down type → lookup K or Leq from a library (SMACNA/ASHRAE) → ΔP_fitting = K×q (or R×Leq).
📦 Sheet 4 – Devices: filter/coil/damper data at design flow.
🧮 Sheet 5 – Totals: sum per path, show critical path with MAX().
🎯 Checks:
• Velocity limits (mains 4–7 m/s; branches 2–5 m/s; grilles per spec).
• Voltage drop… for fans? not here—keep electrical separate.
• Use conditional formatting to flag v or ΔP out of range.
8️⃣ Software helpers
• Duct sizing/pressure: Revit MEP, Lindab/AS Duct, McGill Airflow, Elite, Carrier Duct Designer.
• Ductulators (paper/app) give R quickly.
• Keep Excel as the master because it documents assumptions transparently.
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- X
- Other Apps
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