Contents
Replacement air, also called makeup air, is the air admitted to a space to support the removal of smoke and hot gases. In a layered smoke-exhaust arrangement, relatively cool, uncontaminated air enters below the smoke layer while gas is removed above. The inlet and exhaust together form the ventilation path.[1] Replacement air is therefore part of smoke and heat exhaust ventilation, rather than an independent allowance added after the exhaust is selected.[2]
Its quantity, temperature, location and discharge velocity affect the smoke-control result. Too little inlet capacity can restrict natural exhaust, while poorly directed supply can disturb a plume or mix the smoke interface.[3][4] Mass and volume rates also need to be distinguished. Cooler replacement air occupies less volume than the same mass of hotter smoke, so equal numerical inlet and exhaust volume rates do not necessarily represent a balanced mass flow.[5]
The inlet as part of the smoke-exhaust system
Smoke venting includes two connected processes: gas removal and the admission of air that supports that removal. Cooper describes natural, mechanical and combined arrangements. Natural exhaust uses buoyancy, whereas mechanical exhaust uses fans to produce the required flow. In both cases, the opening and flow conditions on the supply side affect the complete arrangement.[1]
For natural venting, a small inlet can create a significant pressure loss. Cooper's initial simplified calculation assumes inlets large enough that this loss is small. When that assumption fails, the effective exhaust capacity decreases. Increasing roof-vent area without considering the inlet can therefore overstate the improvement in smoke removal.[3]
The physical size of an inlet is not its complete aerodynamic description. Aerodynamic free area accounts for the reduced effectiveness associated with edges and obstructions. Van Rossum describes it as geometrical area multiplied by a contraction factor. An area used in a flow calculation therefore needs a clear statement of whether that reduction has already been included.[6]
The opening's position also matters. Van Rossum explains that an opening can supply air, discharge smoke or contain both directions of flow. Its role depends on its position relative to the level where inside and outside pressures are equal. A facade opening cannot be classified as an inlet solely from its intended label.[7]
Why the air normally enters below the layer
A smoke reservoir receives smoke and entrained air from the plume. Supplying additional air directly into that layer increases the mass that must be removed. Milke therefore places makeup air below the smoke interface in his atrium treatment. Air supplied near the interface can also increase mixing between the upper and lower regions.[4]
This distinction concerns where the air actually goes after discharge. An inlet may be physically below the ceiling while its jet still interacts with the plume or interface. The relevant flow field includes the direction and development of that jet. Inlet elevation and the resulting local air movement are therefore related but separate considerations.[4]
For atrium smoke control, strong supply near the plume can deflect it and increase entrainment. The additional entrained air increases the quantity delivered to the upper layer. Milke also identifies the possibility that excessive supply velocity near the fire increases its burning rate. The inlet can consequently change both the smoke route and the demand placed on the exhaust.[4]
These mechanisms differ from plugholing at an extraction point. Plugholing draws lower-layer air directly into the exhaust through the smoke layer. An inlet jet can instead disturb the plume or mix the interface before gas reaches the exhaust. Both reduce the usefulness of a calculation that assumes an undisturbed upper smoke layer, but they occur in different locations.[6][4]
Interpreting velocity guidance
Milke discusses an often-recommended maximum makeup-air velocity of approximately 1 m/s near a plume. The stated reason is that plume entrainment itself induces an air velocity of about that order. A substantially stronger supply can alter the plume instead of merely replacing extracted gas. This is guidance for the cited atrium arrangement, rather than an unrestricted limit for every inlet.[4]
The source explicitly distinguishes velocity at a diffuser from velocity near the plume. Air discharged from a diffuser spreads and slows as it travels away. A diffuser-face velocity can therefore exceed the plume-vicinity value under suitable conditions. Treating the two velocities as the same quantity loses the distance and discharge-pattern information.[4]
An average opening velocity also does not describe every local velocity in a space. Van Rossum's flow equation uses the mean component normal to the opening. Milke's discussion concerns the air movement where it interacts with the plume. The two uses of velocity answer different questions and need their respective locations identified.[5][4]
An inlet assessment consequently connects the required flow to the available area and the resulting supply pattern. The area determines the mean speed for a specified volume rate. The discharge arrangement determines how that flow reaches the rest of the enclosure. The supplied sources explain these relationships without providing a universal diffuser selection or placement rule.[5][4]
Flow calculations and density conversion
Volume flow is the opening area multiplied by the average velocity normal to it. Mass flow then follows by multiplying volume flow by gas density. The equations apply to the stated conditions at the opening. They do not independently determine the pressure difference or fan performance that produces the flow.[5][8]
| Symbol | Description | Unit |
|---|---|---|
| V̇ | Volume flow rate at the stated gas conditions | m³/s |
| A | Flow opening area used with the stated mean velocity | m² |
| v | Mean velocity normal to the opening | m/s |
| ṁ | Mass flow rate | kg/s |
| ρ | Density of the flowing gas | kg/m³ |
Van Rossum gives an example with an opening of 10 m² and an average velocity of 2 m/s. Multiplication gives a volume flow of 20 m³/s. With the stated gas density of 0.7 kg/m³, the mass flow is 14 kg/s. These values illustrate the conversion and do not prescribe a replacement-air inlet velocity.[5]
For comparison, the same 14 kg/s would occupy about 12 m³/s at a density of 1.2 kg/m³. The latter density is the ambient-air value used in Cooper's venting discussion. This derived comparison holds mass constant while changing density. It illustrates why cooler supply and hotter exhaust need not have equal volume rates.[5][9]
Conversely, 20 m³/s of air at 1.2 kg/m³ carries 24 kg/s. That is a different mass flow from the 14 kg/s in the lower-density example. Matching the volume numbers would therefore fail to match those two masses. A system balance has to use the densities associated with its actual inlet and exhaust conditions.[5][9]
This comparison is a conversion exercise, not a complete building mass balance. The full balance accounts for all relevant streams across the chosen boundary and any accumulation within it. Van Rossum distinguishes steady conditions, where stored mass remains constant, from conditions changing with time. The boundary and the included flows determine what has been balanced.[10]
Pressure balance and uncontaminated supply
Milke specifies a makeup-air mass supply below the mass being exhausted in his atrium treatment. The purpose is to avoid pressurising the atrium relative to communicating spaces. Positive pressure can drive smoke through unprotected openings into those spaces. The relationship concerns mass rates, so the corresponding volume rates require density conversion.[4]
Supply air also needs to be uncontaminated. An intake close to a smoke discharge can return smoke to the space that the system is intended to protect. Milke discusses locating the intake away from that discharge. He also describes considering detection and shutdown where smoke could contaminate a mechanical supply.[4]
External conditions influence the available pressures around a building. Cooper relates wind-induced pressure to wind speed, direction, building geometry and surrounding buildings. Those pressures can alter the flow through an opening even when its area remains unchanged. An inlet description therefore includes the conditions assumed on both sides of the opening.[3]
The interaction between inlet and exhaust is also time-dependent. Cooper identifies fire growth and the operation of actuating devices among the factors determining successful venting. The required air path must exist during the period being assessed. A steady calculation with every opening already available describes a different condition from the earlier development of a system.[1]
Treatment in the supplied design guidance
The Australian Building Codes Board's 2020 data sheets describe low-level makeup air as an integral part of automatic smoke exhaust. They apply the same principle to natural smoke-and-heat vents, whose flow depends on buoyancy. The stated objective is to preserve suitable conditions for evacuation and fire-brigade intervention. This describes that Australian guidance and its cited code context.[2]
The distinction between powered and natural exhaust does not remove the need to assess supply. In the natural case, the inlet affects the pressure available to drive buoyant outflow. In the powered case, the supply location and rate affect smoke movement and relative pressures. Neither arrangement is fully described by exhaust capacity alone.[2][3][4]
Notes
- ↑ 1.0 1.1 1.2 Cooper (2002), pp. 3-219–3-220.
- ↑ 2.0 2.1 2.2 Australian Building Codes Board (2020), p. 128.
- ↑ 3.0 3.1 3.2 3.3 Cooper (2002), p. 3-225.
- ↑ 4.00 4.01 4.02 4.03 4.04 4.05 4.06 4.07 4.08 4.09 4.10 4.11 Milke (2002), p. 4-306.
- ↑ 5.0 5.1 5.2 5.3 5.4 5.5 5.6 Van Rossum (2027), §3.6.
- ↑ 6.0 6.1 Van Rossum (2027), §6.8.
- ↑ Van Rossum (2027), §9.3.
- ↑ Cooper (2002), p. 3-222.
- ↑ 9.0 9.1 Cooper (2002), pp. 3-221–3-222.
- ↑ Van Rossum (2027), §3.7.
References
- Australian Building Codes Board. (2020). Fire safety verification method data sheets: Handbook annex (Version 1.1).
- Cooper, L. Y. (2002). Smoke and heat venting. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 3-219–3-242). National Fire Protection Association.
- Milke, J. A. (2002). Smoke management in covered malls and atria. In P. J. DiNenno (Ed.), SFPE handbook of fire protection engineering (3rd ed., pp. 4-292–4-310). National Fire Protection Association.
- Van Rossum, E. (2027). Introduction to Fire Safety Engineering. Fire Safety Academy.