HVAC Air Cycle: Complete Airflow Reference

Outdoor Air (OA) Stage 1

Outdoor air is brought into the building from outside, and it does more than one job: it dilutes indoor contaminants, provides ventilation, controls CO₂ levels, replaces some exhausted air, and maintains acceptable indoor air quality overall. You'll also see it called "fresh air" (less precise) or "ventilation air."

Key distinction: outdoor air and make-up air are related but are not always the same thing — more on that below.

Outdoor Air Intake, Louver & Damper

The outdoor air intake is the opening where outside air enters the system, typically built from an intake louver, weather hood, bird screen, and outdoor-air damper. Intake location matters — it should sit away from vehicle exhaust, building exhaust outlets, loading areas, or industrial process sources, so the system isn't drawing in the very contaminants it's meant to dilute.

The intake louver is the set of angled blades at the opening: it lets air in while reducing rain and snow penetration and limiting entry of birds and debris.

The Outdoor Air Damper (OAD) regulates how much outdoor air (Q_OA) actually enters — open, partially open, or closed. Modern systems modulate this automatically based on occupancy, pressure, temperature, or air-quality sensors.

Return Air (RA) Stage 2

Return air is air that has circulated through the occupied space and is heading back toward the AHU. By the time it returns, it may carry CO₂, heat, moisture, dust, fibres, odours, and other indoor contaminants picked up in the room. From here it's either recirculated back through the system or exhausted outdoors.

A return air grille is the opening that collects this air — in ceilings, walls, or corridors. Simple distinction worth remembering: a diffuser distributes supply air, a return grille collects return air.

Ceiling Return-Air Plenum

The space above a suspended ceiling can double as an air pathway. When it's intentionally used to carry return air, it's called a return-air plenum, and the typical sequence is Room → Return Grille → Ceiling Return-Air Plenum → Return Duct/Damper → AHU. Not every building uses its ceiling this way — some route return air through dedicated ductwork instead.

Mixing Box & Mixed Air (MA) Stage 3

The mixing box is the section of the AHU where outdoor air and recirculated return air are combined into mixed air, using a set of outdoor-air, return-air, and exhaust-air dampers plus sensors and controls.

For example, a system running 20% outdoor air and 80% return air produces a mixed-air stream in that proportion. Some of the return air can also be diverted straight to exhaust instead of being recirculated back into the mix — that split is what the exhaust and make-up air dampers manage.

Heating Coil & Cooling Coil

After mixing, air passes through conditioning components. A heating coil raises air temperature when needed, using hot water, steam, electric resistance, gas, or a heat pump as the source. A cooling coil lowers air temperature using chilled water, direct expansion (DX) refrigerant, or a heat pump.

Dehumidification side effect: when warm, humid air is cooled below its dew point, water condenses on the coil and drains away — so a cooling coil often removes moisture as a byproduct of cooling, not just heat.

Humidification & Dehumidification

A humidifier adds moisture when indoor humidity is too low — common in offices, hospitals, laboratories, museums, and manufacturing facilities where dry air causes problems. Dehumidification removes moisture, most commonly by cooling air below its dew point so the excess water condenses and can be drained off.

Supply Fan / Blower Stage 4

The supply fan (or blower) moves conditioned air through the supply-duct system, providing the pressure needed to overcome resistance from filters, coils, ducts, dampers, diffusers, grilles, and every other fitting along the way.

The Fundamental Airflow Equation

Rearranged for velocity: V = Q ÷ A. This one relationship explains a lot of duct design intuition:

• If duct area increases while airflow stays constant, velocity decreases.
• If duct area increases while velocity stays constant, airflow increases.

Cross-sectional area itself depends on duct shape: round duct uses A = πD²/4 (D = inside diameter), square duct uses A = S × S (S = side length), and rectangular duct uses A = W × H (width × height).

Supply Air (SA) & Supply Duct Stage 5

Supply air is the conditioned air delivered to occupied spaces, typically described by its Supply Air Temperature (SAT) — for example, "SAT = 13°C." The supply duct carries it from the AHU through the building: AHU → Main Supply Duct → Branch Duct → Terminal/Diffuser → Room.

Dampers: Volume Control, Balancing, Fire & Smoke

A Volume Control Damper (VCD) adjusts airflow through a duct, manually or mechanically. A balancing damper is used during air balancing to bring actual airflow at each branch closer to the design target — e.g. Room A designed for 500 cfm but measured at 495 cfm, Room B designed for 400 but measured at 405.

Three dampers exist purely for life safety: a fire damper (FD) at duct penetrations through fire-rated walls or floors, limiting fire spread through the ductwork; a smoke damper (SD), which controls smoke movement through the system; and a fire/smoke damper (FSD), which combines both functions in one device.

Diffusers & Grilles

A supply air diffuser distributes conditioned air into the room, and doing that well means controlling throw, spread, air mixing, induction, occupant comfort, and draft risk — not just dumping air into the space. A return air grille collects air from the room and sends it into the return-air system. An exhaust grille collects air being intentionally removed — washrooms, kitchens, laboratories, chemical rooms, and other contaminated-area applications.

Exhaust Air (EA) Stage 6

Exhaust air is air intentionally removed from the building rather than recirculated — bathroom exhaust, kitchen exhaust, laboratory exhaust, industrial process exhaust, or local exhaust ventilation (LEV). The typical sequence is Room → Exhaust Grille/Hood → Exhaust Duct → Exhaust Fan → Outside, and the Exhaust Air Damper (EAD) controls how much air is discharged. For industrial hygiene, exhaust systems matter because they can remove contaminants before they ever reach a worker's breathing zone.

Make-Up Air (MUA) Stage 7

Make-up air is replacement air brought in specifically to compensate for air that was intentionally exhausted — if a process exhausts 10,000 cfm, roughly 10,000 cfm of make-up air needs to come back in from outside, or the building runs into trouble.

Why it matters: insufficient make-up air causes negative building pressure, difficulty opening doors, backdrafting of combustion appliances, reduced exhaust-system performance, uncontrolled infiltration, comfort problems, and potential issues with combustion equipment.

A Make-Up Air Unit (MAU) is dedicated equipment built to introduce this replacement air — and depending on the application, it may be filtered, heated, cooled, humidified, or dehumidified before it enters the space.

Outdoor Air vs. Make-Up Air — Not Automatically the Same

These two terms get used interchangeably, but the design intent behind them is different:

Outdoor air is introduced primarily for ventilation, indoor air quality control, and occupancy-based ventilation needs.
Make-up air is introduced specifically to replace air that has been intentionally removed.

A system can use outdoor air to serve both purposes at once — but on the exam, the distinction to hold onto is why the air is being brought in, not just where it comes from.

Why This Matters Beyond Comfort

HVAC systems directly influence contaminant transport and worker exposure. A well-designed system can dilute contaminants, remove them, control airflow direction, maintain pressure relationships between spaces, provide required outdoor air, prevent contaminated air from migrating into cleaner areas, support local exhaust ventilation, and improve thermal comfort.

The one thing general HVAC doesn't replace: when a contaminant is generated at a specific source, general ventilation shouldn't be treated as a substitute for local exhaust ventilation (LEV). The preferred sequence for source control stays the same regardless of how good the building's general HVAC is:

Dilution rate and the mixing factor

Dilution airflow is sized from the evaporation rate and the acceptable concentration, then multiplied by a mixing factor K for imperfect mixing (OSHA’s manual uses K = 2 to 5, with 2 as the best case). The acceptable concentration is often taken as half the exposure limit. Put exhausts near sources, keep workers upwind, and supply make-up air where it helps.

ASHRAE 62.1 vs ASHRAE 55

ASHRAE 62.1 (Ventilation for Acceptable Indoor Air Quality) sets minimum outdoor air, adding a per-person rate and a per-floor-area rate. ASHRAE 55 (Thermal Environmental Conditions for Human Occupancy) sets comfortable temperature and humidity using the PMV/PPD comfort model. 62.1 answers how much outdoor air; 55 answers what condition that air should be in.

0.075 lb/ft³

Dry standard air has a density of 0.075 lb/ft³ (OSHA’s manual defines it at 68 °F and 29.92 in Hg; some texts use 70 °F). Hot air, high altitude or heavy moisture lowers the density, so readings and fan ratings made for standard air need a density correction factor before they are compared.

Measuring it: pitot tube and traverse

A pitot tube reads total and static pressure; the difference is velocity pressure. In a straight run of round duct, 4–6 diameters downstream and 2–3 diameters upstream of bends or obstructions, the average velocity is about 90% of the centerline velocity. A traverse is more accurate: 6 or 10 readings along each of 2 or 3 passes across the duct. Flow in industrial ducts is almost always turbulent (Reynolds number well above the laminar range of about 2,000).

Capture velocity, hood static pressure and entry loss

Capture velocity is the air speed the hood creates at the source to pull the contaminant in; OSHA’s manual sets a floor of about 50 fpm. Hood static pressure SP_h = VP + h_e: suction must both accelerate the air into the duct (VP) and pay the hood entry loss (h_e = K × VP). The coefficient of entry C_e = √(VP / |SP_h|) compares actual with ideal flow; a C_e that drops over time means something has changed. Measure SP_h about 4–6 duct diameters down the take-off duct.

Keep the source close and enclosed

Put capture hoods as close to the source as possible, add flanges, baffles or side curtains, and reduce what the process releases. Every one of these lowers the airflow needed.

Minimum transport velocity

The duct velocity that keeps particles airborne with little settling. It depends on particle density, loading and the air. Too slow and dust drops out and plugs the duct; condensing vapor can wet particles and cause build-up. Industrial duct velocities typically run about 2,000–6,000 fpm.

Where energy is lost

Friction along straight duct, plus fittings: elbows (short-radius elbows cost more), branch entries at sharp angles, contractions and expansions, the air cleaner, and the stack. A weather cap too close to a stack also restricts discharge; OSHA’s manual advises against weather caps.

Two classes, three laws

Fans are axial (air moves along the shaft: propeller and tube-axial, high flow at low pressure) or centrifugal (air turns 90°: the usual choice for industrial exhaust against high pressure). For a given fan and system, the fan laws say flow varies with rpm, pressure with rpm², and power with rpm³. Doubling the speed doubles the flow, quadruples the pressure and needs eight times the power.

The six-and-three rule

Give the fan at least six diameters of straight duct at its inlet and three at its outlet to keep losses at the fan to a minimum. An elbow right at the inlet can cost about 20% of flow for every 2½ diameters of straight duct that is missing.