Delta T (ΔT) in HVAC Systems – Complete Explanation In HVAC, Delta T (ΔT) means: Difference between two temperatures Formula: ΔT=T1−T2 Where: ΔT = Temperature difference T₁ = First temperature T₂ = Second temperature Why Delta T is Important in HVAC Delta T tells us: • How much cooling or heating is happening • Whether equipment is working properly • Energy efficiency of system • Water flow and air flow condition • Coil performance • Chiller efficiency In HVAC mainly two Delta T are used: • Water Side Delta T • Air Side Delta T 1. Water Side Delta T (Chilled Water / Hot Water) This is the temperature difference between: • Supply water temperature • Return water temperature Usually used in: • Chillers • FCU • AHU • Heat exchangers Chilled Water Delta T In cooling system: • Chiller sends cold water → called CHWS (Chilled Water Supply) • Water absorbs room heat • Warm water returns → called CHWR (Chilled Water Return) Formula: ΔTwater = Treturn−Tsupply Example 1 – Normal ...
Gul Bahar Shah
Gul Bahar Shah
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 and OEM spacing; avoid turbulent elbows; sample straight runs.
• Action: stop fans and close smoke/fire dampers.
6️⃣ Filter differential pressure 🧱
• Purpose: filter loading and alarms.
• Location: high side upstream, low side downstream; use proper pitot rings or static tips.
• Numbers: note clean DP (e.g., 90 Pa). Alarm at ≈2× clean DP or manufacturer limit.
7️⃣ Supply duct static pressure 📈
• Purpose: VAV static reset and fan control.
• Location: 2/3 of the main supply trunk; avoid takeoffs and elbows.
• Setpoint: 250–500 Pa typical VAV; reset to keep the “most‑open” VAV at ~90%.
• Fan law (plain text): ΔP ∝ Q^2, Power ∝ ΔP × Q.
8️⃣ Airflow/pressure switch (AFS) 💨
• Purpose: proof of airflow for safety sequences.
• Location: straight duct runs with correct pickup orientation.
• Setpoint: verify against design velocity; avoid nuisance trips at low flow.
9️⃣ Motor current switch ⚡
• Purpose: fan run status feedback to BMS.
• Setting: trip just below normal running current; avoids false “off” during soft‑start.
🔟 Duct temperature sensors 🌡️
• Purpose: supply, return, and discharge control and trending.
• Location: ≥5 duct diameters downstream of fittings; average or multi‑point in large ducts.
• Spec: ±0.3–0.5 °C; immersion wells for coils, strap‑on for casings.
1️⃣1️⃣ Space/Return RH sensor 💧
• Purpose: dehumidification, reheat, and IAQ alarms.
• Setpoints: comfort 40–60 %RH; critical spaces per ASHRAE 55 and project specs.
1️⃣2️⃣ Pressure transmitters (coils/fans) 🧪
• Use across coil to trend fouling; across fans to validate curves.
• Equation (plain text): Q = k × √(ΔP/ρ) for calibrated flow rings; document k.
Eg:
🏢 VAV AHU 40,000 m³/h, clean filter DP 90 Pa, design supply static 350 Pa.
• Static reset band: 250→350 Pa; trend fan kW before/after reset.
• Freeze stat trip: 4 °C; mixed‑air low limit: 8 °C.
• CO₂ DCV: setpoint 900 ppm; minimum OA = code value even at low occupancy.
• Filter alarm: 180 Pa; change‑out target 200 Pa.
• 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 and OEM spacing; avoid turbulent elbows; sample straight runs.
• Action: stop fans and close smoke/fire dampers.
6️⃣ Filter differential pressure 🧱
• Purpose: filter loading and alarms.
• Location: high side upstream, low side downstream; use proper pitot rings or static tips.
• Numbers: note clean DP (e.g., 90 Pa). Alarm at ≈2× clean DP or manufacturer limit.
7️⃣ Supply duct static pressure 📈
• Purpose: VAV static reset and fan control.
• Location: 2/3 of the main supply trunk; avoid takeoffs and elbows.
• Setpoint: 250–500 Pa typical VAV; reset to keep the “most‑open” VAV at ~90%.
• Fan law (plain text): ΔP ∝ Q^2, Power ∝ ΔP × Q.
8️⃣ Airflow/pressure switch (AFS) 💨
• Purpose: proof of airflow for safety sequences.
• Location: straight duct runs with correct pickup orientation.
• Setpoint: verify against design velocity; avoid nuisance trips at low flow.
9️⃣ Motor current switch ⚡
• Purpose: fan run status feedback to BMS.
• Setting: trip just below normal running current; avoids false “off” during soft‑start.
🔟 Duct temperature sensors 🌡️
• Purpose: supply, return, and discharge control and trending.
• Location: ≥5 duct diameters downstream of fittings; average or multi‑point in large ducts.
• Spec: ±0.3–0.5 °C; immersion wells for coils, strap‑on for casings.
1️⃣1️⃣ Space/Return RH sensor 💧
• Purpose: dehumidification, reheat, and IAQ alarms.
• Setpoints: comfort 40–60 %RH; critical spaces per ASHRAE 55 and project specs.
1️⃣2️⃣ Pressure transmitters (coils/fans) 🧪
• Use across coil to trend fouling; across fans to validate curves.
• Equation (plain text): Q = k × √(ΔP/ρ) for calibrated flow rings; document k.
Eg:
🏢 VAV AHU 40,000 m³/h, clean filter DP 90 Pa, design supply static 350 Pa.
• Static reset band: 250→350 Pa; trend fan kW before/after reset.
• Freeze stat trip: 4 °C; mixed‑air low limit: 8 °C.
• CO₂ DCV: setpoint 900 ppm; minimum OA = code value even at low occupancy.
• Filter alarm: 180 Pa; change‑out target 200 Pa.
- Get link
- X
- Other Apps
- Get link
- X
- Other Apps
Comments
Post a Comment