1. What is a positive-pressure air supply valve? It works like a bicycle pump and like a check valve. Suppose the valve divides a space into A and B. When A and B are pressurised at different times, gas can pass in only one direction; gas from the other side cannot return. The releasing side is A. When A is pressurised, positive pressure is supplied; when B gains gas it is negative relative to A, but the valve forward supply keeps B gas from returning to A.
2. What is a positive-pressure supply fan? A fan that pushes air into the escape stairwell during an emergency, aiding escape; the stairwell stays positively pressurised so smoke cannot seep in and suffocate people.
3. What is a smoke exhaust fan? A fan that removes smoke from a building during an emergency, improving visibility and aiding firefighting.
4. Function of the positive-pressure air supply outlet. On a fire, its motor opens the outlet; a heat or smoke detector or manual alarm triggers it, the roof positive-pressure fan opens and pressurises the supply shaft, and the front room receives a steady supply that keeps it positive and stops smoke spreading, giving an escape space. Above 280°C a person cannot escape; the internal fuse melts and the outlet closes to stop fire spread.
Must the positive-pressure outlet be in the stair front room? By code, supply outlets go in stairwells, front rooms and sealed refuge floors. A front room of a fire lift without natural smoke exhaust must have supply. If a project puts the first-floor supply inside a room because the front room is misaligned, that is not allowed; you must add a duct to the front room. The rules are in the Code for Fire Protection Design of Buildings and the Code for Fire Protection Design of High-Rise Civil Buildings.
5. Difference between make-up fan and positive-pressure fan. The make-up fan feeds outside air into deep basements (-2, -3) to cut the negative pressure that a single exhaust fan would create, so smoke leaves faster. The pressurisation fan serves stairwells and lift front rooms so toxic smoke cannot enter; because stairwells connect vertically, supply outlets can be self-closing louvres every 2 or 3 floors, while front rooms are separate, so electric outlets are used and opened on the fire floor and adjacent floors, normally with a 280°C fire damper that fuses shut.
Positive-pressure supply vs fresh air
1. Positive-pressure supply pushes air into escape routes during an emergency, keeping them positive so smoke cannot enter; it is used only in a fire. Fresh air is part of the comfort system, exchanges indoor and outdoor air to keep it clean, and is shut on a fire so it does not feed the flames.
2. Can the central fresh-air system replace fire positive-pressure supply? No. On a fire the controller closes the fresh-air fan and outlets. Positive-pressure supply serves escape routes where velocity and pressure must far exceed fresh air, keeping the refuge area pressure absolutely above other areas.
Positive-pressure system
1. A positive-pressure system supplies enough fresh air to certain building parts to keep their pressure above other parts, blocking smoke from the pressurised parts. It comprises fan, ducts, valves, inlets or outlets and the controlled zone.
2. Its role: by fire code, any fire compartment without direct ventilation needs positive-pressure supply, so occupants get fresh air and do not breathe harmful smoke.
High-rise positive-pressure systems
Positive-pressure supply is a widely used smoke-control method for stairs and front rooms. In design you must set the number of doors open during evacuation, the pressure to hold in stairs and front rooms, and the outlet form.
1. Pressure. All codes require stair pressure greater than front-room pressure greater than corridor or room pressure. The pressure difference is across the fire doors. The maximum allowable difference is what a person can push open, often taken as 50 Pa. The minimum difference keeps a reverse flow at the door opening when doors open; values differ by country.
2. Outlet form. Stair outlets are open louvres every 2 to 3 floors, single or double; double aids balancing. Front-room outlets are normally closed electric louvres; on a fire only the fire floor and adjacent floor or floors open, concentrating supply on those floors. This needs a relief valve per floor and, if relieving indoors, a fire damper to keep the fire separation. Closed valves are harder to fit, need more control and can seize if poorly maintained. Open louvres give a small, even supply to all front rooms, about 600 to 1000 m³/h each, when all doors are shut, avoiding over-pressure.
3. Supply calculation. Treat stairs and front rooms as one smoke-control whole. When shut, keep stair at 50 Pa and front room at 25 Pa. When doors open, usually 3 floors, keep at least 0.7 m/s at the opening. For a normal front room, stair leakage through the door gap may suffice, so a separate front-room supply is optional. For a shared front room, leakage is insufficient, so a front-room supply is required; its form changes the needed airflow. With open louvres all floors get supply and the design holds; with closed louvres only 3 floors get supply and the others rely on stair leakage, often failing to reach 25 Pa. Compute the open-door airflow to hold the minimum difference, then add the extra airflow so every open door keeps 0.7 m/s or more.