A building automation system or BAS is a network of controlling and monitoring devices mainly responsible for the heating, ventilation, and air conditioning (HVAC) control system in large facilities and commercial buildings. Building automation systems are most commonly implemented in large facilities such as healthcare facilities, schools, universities, hotels, residential buildings, and data centers. A BAS network allows building operators to supervise and control the HVAC systems from a centralized location known as the Building Management System or BMS. VAV system, the air handling unit (AHU) and the VAV boxes are physically separated from each other, and each one has its own controller, also distanced from one another. However, for a VAV system to operate effectively, the VAV Boxes and the AHU have to talk to each other; in other words, they need to share information to properly adjust their setpoints based on zone heating or cooling demand.
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.
- Get link
- X
- Other Apps
- Get link
- X
- Other Apps
Comments
Post a Comment