MechanicalEngineer

  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 hydrauli...

  AHU Sensors: what they do, where to put them, and how to use them 1️⃣ Outside‑air T/RH 🌤️ • Purpose: economizer enable, coil reset, frost logic. • Location: north side or shaded, away from exhausts and sun; use a shield. • Spec: accuracy ±0.3–0.5 °C and ±2–3 %RH. 2️⃣ CO₂ sensor 🫁 • Purpose: demand‑controlled ventilation. • Location: return duct or a representative zone. • Setpoint: 800–1000 ppm typical; minimum OA per ASHRAE 62.1 still enforced. 3️⃣ Mixed‑air averaging sensor 🔀 • Purpose: detect low mixed‑air temperature, protect coils. • Location: serpentine across the full plenum cross‑section upstream of the coil. • Alarm: ~7–10 °C low limit; drive OA closed and enable preheat. 4️⃣ Freeze stat 🧊 • Purpose: coil freeze protection. • Location: capillary laced on the leaving‑air face of the coil. • Trip: 3–5 °C; action = stop supply fan, close OA, open heating/preheat valve. 5️⃣ Duct smoke detector 🚨 • Purpose: life safety interlock. • Location: per NFPA 90A/72...