Staircase Pressurization system Design & Calculations
Staircase Pressurization system Design & Calculations
In a high-rise building, the stairs typically represent the sole means of egress during a fire. It is imperative for the exit stairs to be free of smoke and to incorporate design features that improve the speed of occupant egress. Most building codes require the fire stairwells in a high-rise building to be pressurized to keep smoke out.
The stairwell pressurization serves several purposes:
- Inhibit migration of smoke to stairwells, areas of refuge, elevator shafts, or similar areas.
- Maintain a tenable environment in areas of refuge and means of egress during the time required for evacuation.
- Facilitate the fire and rescue operation by improving visibility in the building for the firefighting crew.
- Protect life and reduce damage to property.
PRESSURIZATION METHOD ( IBC SECTION 909.6 )
Pressurization systems use mechanical fans to produce a positive pressure on the stairwells. The two key pressurization principles are maintaining:
- A pressure difference across a barrier
- Average velocity of sufficient magnitude.
Consider an example of a stairwell “A” maintained at a positive pressure relative to lift lobby “B” which is at a higher pressure relative to the fire zone corridor space “C”.
Each of the three zones A, B and C are separated by the partition barriers and the doors. The lobby and stairwell (marked green) can be classified as escape routes, refuge area and tenable environment. These invariably repeat themselves at the same position at each floor level for at least a number of successive floors.
We learned that air flows from the higher pressure area to the lower pressure area. The principle of maintaining a tenable environment is to prevent migration of smoke laden air from zone “C” to zone “B” and to zone “A”. This is achieved by pressurizing the stairwell and lobby such that the pressure in zone A is greater than the pressure in zone B, which in turn is maintained greater than zone C. Two scenarios exist:
- Door closed - When the lobby doors are closed, an over pressure of the elevator lobby with respect to the building will prevent smoke infiltration from the building spaces into the lobby. An excess pressure must be maintained in zones A and B so that flow is always outwards, preventing the entry of smoke from zone C.
- Caution - The pressure should not be so high that it prevents the door opening. A right balance of minimum and maximum pressure difference must be maintained.
- Door open - When the doors are opened during evacuation, the gaps get much wider and the pressure tends to equalize between two zones. A good pressurization system design must maintain minimum pressures and respond quickly to stair door openings. Studies indicate that the smoke tends to be held back by the outflow of air if the egress velocity from pressurized spaces is sufficiently high. An egress velocity of 200 fpm (~1 m/s) is recommended.
SYSTEM DESIGN
The stairwell pressurization system is a mechanical ventilation system. In order to pressurize the stairs of vertical buildings, it is necessary to install sets of fans that suck air into the stairwell, keeping pressure of 0.10 – 0.45 inch water gauge. The main purpose is to prevent infiltration of smoke in the event of a fire.
The system consists of the installation of a fan with an electric motor mounted in an isolated compartment. The outside air is captured through a shutter that has a particle filter. The excess air is relieved through manual and automatic dampers properly calculated and installed at the suitable locations.
COMPONENTS
A pressurization system consists of two main components:
- Supply air (where air is injected into the area that is to be protected)
- Relief air (to avoid overpressure when doors are closed)
The System comprises:
- Supply fans for introducing air.
- Distribution systems comprising ducting, terminal diffusers and venting arrangements.
- Automatic air release vents/dampers to release excess air and avoid over pressurization when doors are closed
- Automatic control system comprising fire alarms, smoke detectors, safety switches and devices at locations to suit fire service.
- Duct work : There
is no real difference between supply air ductwork used for comfort HVAC
and the pressurization air systems. The pressurization air can be
delivered either from a
dedicated galvanized duct work or a masonry shaft. Experience indicates that
without sealing and internal lining of the surfaces, the masonry ducts
have excessive friction loss and 20 to 50% air leakage.
- Motorized Smoke & Fire Dampers (MSFD) : Dampers for smoke control systems are required to meet UL 555S. This is consistent with those requirements found in NFPA 92. The IBC further regulates the type of smoke damper to be used.
PRESSURE RELIEF VENTING
Stairwell pressurization systems are accompanied with a vent system. The purpose of the vent is to relieve excess pressures in the stair when doors are opened and closed.
When a door is opened, the pressure in the stair is reduced and the relief vent closes, thereby diverting the excess air to the open door. When the door closes, the vent serves as a relief for excess pressures in the stair to reduce door-opening forces. One of the following methods may be adopted to relieve excess air:
- Automatic Opening Door : This is the simplest form of control. It uses electric unlocking of door in the event of high pressure in the stairwell.
- Barometric Dampers : Barometric dampers use adjustable counterweights which are adjusted so that the damper opens when a particular (excessive) pressure is reached. The location of dampers needs to be carefully chosen since dampers located too close to the supply openings can operate too quickly.
- Motor-Operated Dampers : Motor-operated dampers use signals from a differential pressure sensor to actuate.
- Exhaust Fan : An exhaust fan can be used to relieve excess pressure. The operation of fan is triggered by signals from a differential pressure sensor. The fan will shut off when the pressure difference falls below a specified level.
DESIGN CALCULATION
This section deals with the design criteria and methodology for calculating the air flow rates and venting requirements for pressurization systems.
The Staircase Pressurization calculation is usually based on the following design criteria:
- Δp min. = min. pressure difference = 0.18 in wg between staircase and adjacent accommodation space (NFPA 92A, Table 5.2.1.1).
- Δp max. = max. pressure difference = 0.37 in wg between staircase and adjacent accommodation space (IBC 2006, Section 909.7.2).
- Airflow velocity of not greater than 200 fpm (~1 m/s) through doors when 3 doors are open (IBC 909. 7.2).
- Maximum Force required to open any door at the door handle shall not exceed 30 lbs (133 N).
DESIGN METHODOLOGY
In the straightforward case of a lobby or staircase, the air flow rate is simply determined by the sum of leakage allowances at the desired pressurization.
- Identify all the airflow paths with doors closed and calculate the leakage rate via these airflow paths (construction cracks in walls and floors).
- Identify all the airflow paths with doors closed and calculate the leakage rate via the gaps around the doors.
- Calculate the air supply required with all the doors closed with a 50% increase for unknown.
- Identify all the different doors and calculate the air supply required through these doors with v= 200 fpm and an allowance of 15% for duct work losses.
- Calculate the total airflow (all doors closed + 3 doors open) and determine the fan duty point.
- Check that the door opening forces are below 30 lbs
- Calculate the size of the pressure relief damper
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