Dry Pipe Sprinkler System – Working Principle A Dry Pipe Sprinkler System is an automatic fire protection system designed primarily for areas where low or freezing temperatures could cause water-filled sprinkler pipes to freeze and become damaged. It is commonly used in locations such as cold storage areas, unheated spaces, parking areas, warehouses, and other temperature-sensitive environments. How the System Works Under normal conditions, the sprinkler piping is not filled with water . Instead, the pipes are maintained with pressurized air or nitrogen . Water is held on the supply side of a dry pipe valve , which prevents water from entering the sprinkler piping until the system is activated. When a fire occurs, the heat generated by the fire causes the nearest sprinkler head to reach its operating temperature. The sprinkler head then opens, releasing the pressurized air or nitrogen from the piping. As the compressed air pressure decreases, the pressure difference across t...
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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