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Ali Uday Harobey , profile picture
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Comprehensive HVAC Air-Side Design: ESP, Pressure Loss & Fan Selection

From Duct Design to Fan Selection Fan selection is not simply about choosing a fan that can deliver the required airflow. A proper HVAC design must consider the complete air-distribution system and the resistance created by every component. In this presentation, I explain the relationship between: Airflow → Pressure Loss → ESP → Fan Curve → System Curve → Operating Point → Fan Selection The presentation covers the major sources of pressure loss, including filters, coils, ducts, fittings, dampers, flexible connections, and air terminals. It also introduces the concept of the critical path and explains why it governs the required system pressure. A practical illustrative example of 7,500 CFM at approximately 500 Pa is used to demonstrate how pressure losses can be combined to determine the required fan duty point. I also explain fan curves, system curves, operating points, fan laws, efficiency, motor power, and common fan-selection mistakes. The key takeaway: > A fan is not selected by airflow alone.** The final selection should satisfy the required airflow and system resistance while also considering efficiency, motor power, noise, operating range, and verified manufacturer performance data. Eng. Ali Uday Harobey | HVAC Design Engineer

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ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION H V A C A I R - S I D E D E S I G N From Duct Design to Fan Selection Understanding ESP, Pressure Loss & Fan Curves PRACTICAL HVAC ENGINEERING AHU FAN FILTER 120 Pa COIL 150 Pa DUCT 70 Pa FITTINGS 55 Pa TERMINAL 70 Pa Design duty • 7,500 CFM @ ≈ 500 Pa 1 ENG. ALI UDAY HAROBEY – HVAC DESIGN ENGINEER
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 01 — WHY FAN SELECTI O N MATTERS A fan is not selected by CFM alone. The system resistance is part of the duty point. AIRFLOW 7,500 CFM SYSTEM RESISTANCE ≈ 500 Pa DUTY POINT Q + ΔP required VERIFY η + motor + sound after selection UNDERSIZED Airflow falls below design • poor space delivery • comfort complaints CORRECT Meets duty at a stable operating region • efficient • controllable OVERSIZED High noise / throttling • poor efficiency • unnecessary cost Engineering idea: the fan must overcome the resistance of the actual air path at the required airflow. 2
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 02 — THE AIR PATH Every component in the air path consumes pressure. Follow the actual flow path before you calculate ESP. OUTDOOR / AHU FILTER COIL FAN SUPPLY DUCT FITTINGS DAMPER TERMINAL SPACE Pressure demand builds as the air moves through the system. Component data: filters, coils, dampers, terminals Duct data: length, size, velocity, roughness Fittings: local losses and transitions Critical path: the most resistant required branch 3
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 03 — PRESSURE BASI C S Static pressure, velocity pressure and total pressure Use the right pressure term for the job. AIRFLOW Pᵗ = Pˢ + Pᵛ Pˢ = Static pressure Pᵛ = Velocity pressure Pᵗ = Total pressure 4
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 04 — WHAT IS ESP? External Static Pressure (ESP) The definition must match the equipment rating convention. ESP is the pressure capability required to overcome the external system resistance connected to the fan. EXTERNAL SYSTEM Supply / return ductwork Fittings and dampers Terminal pressure drop Other connected components FAN DUCT FITTINGS DAMPER TERMINAL Design rule: align your system-pressure calculation with the manufacturer’s fan/AHU pressure-rating convention. 5
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 05 — WHERE DOES THE PRESSURE GO? Illustrative pressure-loss budget Example values below are teaching values, not universal requirements. Filter 120 Pa Cooling coil Supply duct 70 Pa Fittings 55 Pa Damper 25 Pa Flex connection 10 Pa Terminal 70 Pa TOTAL ≈ 500 Pa The fan duty is governed by the required airflow and the total resistance along the governing path. 6
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 06 — DUCT FRICTI O N Straight-duct friction is only one part of the system resistance. A simple design relationship is ΔP = R × L. ΔP = R × L ΔP = pressure loss R = friction rate L = duct length Fittings are additional local losses. STRAIGHT DUCT ELBOW → LOCAL LOSS 7
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 07 — FITTINGS & DYNAM I C LOSSES Pressure loss also comes from changes in flow direction and area. For local losses, a common model is ΔP = K · (ρV² / 2). ELBOW local loss coefficient BRANCH local loss coefficient TRANSITION local loss coefficient DAMPER local loss coefficient ENTRY / EXIT local loss coefficient ΔP = K × (ρV² / 2) Velocity matters: because the term contains V², higher velocity can increase dynamic pressure quickly. 8
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 08 — THE AIR TERMIN A L ALSO MATTERS Terminal pressure drop is part of the air-delivery design. Pressure is not “free” at the diffuser, grille, or jet nozzle. DIFFUSER Pressure drop Throw / spread NC / noise Design target ✓ verified from selection data GRILLE Effective area Face velocity Pressure drop Design target ✓ verified from selection data JET NOZZLE Throw / penetration Angle Pressure drop Design target ✓ verified from selection data 9
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 09 — BUI LD THE PRESSURE BUDGET Think of fan pressure as a finite budget. Every component spends part of it. FILTER COIL DUCT FITTINGS DAMPER FLE X TERMINAL 0 Pa ≈ 500 Pa Available fan pressure A good pressure budget makes it obvious where the system is expensive — and where redesign may pay off. 10
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 10 — ESP CALCU L AT I O N WORKFLO W From airflow requirement to fan duty in eight checks. Use the actual connected system, not a guess. 01 Determine airflow 02 Lay out ducts 03 Size ducts 04 Calculate friction 05 Calculate fittings 06 Add component drops 07 Find critical path 08 Set fan duty Governing path → ΣΔP → required fan duty 11
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 11 — FIND THE CRITIC A L PATH The fan must satisfy the governing branch. A shorter branch can still be the winner if it has less resistance. FAN BRANCH A 250 Pa BRANCH B 480 Pa GOVERNING PATH Branch B The required fan duty is driven by the required airflow at the highest resistance that must be served. 12
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 12 — FROM ESP TO FAN DUTY Turn the pressure budget into a clear selection point. This is the moment where the system calculation becomes an equipment requirement. REQUIRED AIRFLOW 7,500 CFM SYSTEM RESISTANCE ≈ 500 Pa FAN DUTY Q @ ΔP select here 7,500 CFM @ ≈ 500 Pa FAN OPERATING POINT 13
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 13 — WHAT IS A FAN CURVE? The fan curve shows pressure capability versus airflow. Illustrative curve — use certified manufacturer data for final selection. Airflow (CFM) Pressure (Pa) FAN CURVE 0 2,000 4,000 6,000 8,000 0 250 500 750 1,000 For a fixed fan configuration and speed, pressure capability typically decreases as airflow increases. 14
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 14 — SYSTEM CURVE System resistance rises approximately with Q² for a fixed system. That makes airflow increases progressively more expensive in pressure terms. Airflow (CFM) System pressure (Pa) ΔP ∝ Q² FIXED SYSTEM If the system configuration changes, the system curve changes too — for example through dirty filters or damper position. 15
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 15 — FAN CURVE × SYSTEM CURVE Operating point = where the two curves intersect. This is the key visual for practical fan selection. Airflow (CFM) Pressure (Pa) OPERATING POINT Fan curve System curve Design intent: hit the required airflow at the required resistance. 16
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 16 — WHEN THE SYSTEM CHANGES Dirty filters, damper position and added resistance move the operating point. Controls and maintenance are part of the performance story. Airflow (CFM) Pressure (Pa) Normal system Higher resistance DIRTY FILTER / EXTRA RESISTANCE 17
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 17 — FAN LAWS Speed changes can strongly change system performance. Idealized relationships — real equipment still needs manufacturer verification. FLOW Q ∝ N speed PRESSURE ΔP ∝ N² speed² POWER P ∝ N³ speed³ Example: speed ↑ 10% Flow 1.10× Pressure 1.21× Power 1.33× The cubic power relationship is why speed control can have a major energy impact when operating conditions permit. 18
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 18 — FAN EFFICI E N CY Meeting duty is not enough — look at where the fan operates. Efficiency, stability and motor sizing matter. Airflow (CFM) Pressure / Efficiency PREFERRED REGION Pressure curve Efficiency region Do not force a fan to operate near an unstable or highly inefficient part of its map just to “hit the number.” 19
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 19 — MOTOR POWER A quick power estimate ties airflow, pressure and efficiency together. Use consistent SI units in the equation. P ≈ Q × ΔP / η Illustrative example AIRFLOW 3.54 m³/s PRESSURE 500 Pa ASSUMED η 0.62 illustrative P ≈ 2.9 kW This is an estimate for teaching — final motor selection includes manufacturer data, service conditions, drive losses and sizing practice. 20
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 20 — WORKED EXAMPLE 7,500 CFM @ ≈ 500 Pa Illustrative system: add the pressure drops along the governing path. Filter 120 Pa Cooling coil 150 Pa Supply duct 70 Pa Fittings 55 Pa Damper 25 Pa Flex connection 10 Pa Terminal 70 Pa Σ ΔP ≈ 500 Pa 7,500 CFM FAN DUTY POINT Final selection must be verified against certified fan performance at the specified operating conditions. 21
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 21 — FAN SELECT IO N A practical selection sequence Choose the fan to satisfy the duty, then verify the whole equipment package. 01 REQUIRED DUTY Q + ΔP 02 CANDIDATE FAN catalogue / software 03 OPERATING POINT curve intersection 04 EFFICIENCY preferred region 05 MOTOR power + margin 06 SOUND NC / dBA 07 CONTROL VFD / speed 08 VERIFY manufacturer data Selection ≠“capacity check” → it is a duty-point + performance + application decision. 22
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 22 — FAN SELECT IO N CHECKLIS T Before you sign the selection, verify the full duty. Use this as a design QA gate. ✓ Required airflow ✓ Required pressure / rating convention ✓ Critical path verified ✓ Filter & coil drops checked ✓ Terminal pressure drop checked ✓ Efficiency checked ✓ Motor power checked ✓ Noise checked ✓ Operating point checked ✓ Control method checked ✓ Manufacturer data verified Quality control principle: every fan selection should be reproducible from the design calculations and the manufacturer performance data. 23
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 23 — COMM O N MISTAK ES Seven ways fan selection goes wrong The goal is not just a fan that runs — it is a fan that performs the design. 01 SELECTING FROM CFM ONLY 02 IGNORING THE CRITICAL PATH 03 FORGETTING FITTING LOSSES 04 IGNORING DIRTY-FILTER DROP 05 CONFUSING STATIC & TOTAL PRESSURE 06 SELECTING AN INEFFICIENT DUTY POINT 07 IGNORING NOISE / MOTOR POWER Most fan problems are not caused by the fan alone — they begin in the system definition. 24
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 24 — THE COMPLE TE ENGINEER I NG WORKFLO W From load calculation to selected fan This is where the separate HVAC calculations become one system. COOLING LOAD AIRFLOW DUCT DESIGN PRESSURE LOSS ESP FAN CURVE SYSTEM CURVE OPERATING POINT FAN SELECTION VERIFICATION Design intent The fan is the endpoint of the system calculation — not the starting point. ENGINEERING TRACEABILITY 25
ENG. ALI UDAY HAROBEY | HVAC DESIGN ENGINEER ESP • PRESSURE LOSS • FAN SELECTION 25 — FINAL TAKEAW AY A fan is not selected by airflow alone. Select the fan at the required airflow and system resistance — then verify performance. Q + ΔP → Operating Point → Efficiency → Motor → Sound → Verification References / further reading ASHRAE Handbook — Fundamentals: air-system pressure and fan/system fundamentals. AMCA fan performance resources: fan curves, pressure definitions and testing concepts. SMACNA duct design guidance: friction, fittings and air-system design practice. Manufacturer-certified fan performance data for final equipment selection. HVAC Air-Side Design | Eng. Ali Uday Harobey 26