Experimental and numerical investigations on stair pressurization systems in very high rise buildings

Pressurized stairwells are the primary vertical evacuation route from the building. The practice of the pressurized stairwells is defined in the fire safety codes and standards of many countries. The fresh air is supplied by fan or fans into stairwells by this means pressurization is provided and smoke spread into stairwell is prevented. The limits of the pressurization are defined in many codes and standards. The minimum pressurization limit prevents the stairwell to be smoke filled and the maximum pressurization limit restricts the door blocking forces exerted on the stairwell door. The specified limits should be provided along the height of the stairwell under any circumstances. The stairwell pressurization systems are affected by the temperature difference between the stairwell and ambient (stack effect), wind effect, excessive temperatures of the fire, HVAC systems of the building and elevator piston effect. Also some doors of the pressurized stairwell is opened and closed in compliance with the fire floor and building evacuation plan. Under these dynamic circumstances, some precautions are taken to fulfill the requirements of pressurization limit along the height of the stairwell. One of the precautions is altering the flow rate of the air into the stairwell by modulating frequency converters of the fans, which are controlled by one or more static pressure sensors that sense the pressure difference between the stairwell and the building. The other one is discharging pressure buildup in the stairwell directly from stairwell to the outside via relief vents. Pressurization system of each stairwell are individually designed and two scenarios are taken into the consideration for each design. All of the doors of the stairwell are closed in one scenario and the doors of the selected floors are opened for the other scenario. The air flow rate of the supply system depends on the all of the doors closed scenario and the size of the relief vent depends on the doors of the selected floors open scenario. Over the past few decades, very high rise buildings have been constructed widely and the number of buildings with a height more than 100 m is increasing. The fire safety measures in such high rise buildings are still investigated. Thus stairwell pressurization system performance, design and system components in high rise buildings are still exanimated. In practice, additional measures are taken to fulfill the requirements such as dividing a stairwell into a number of pressurization zones, supplying pressurization air into stairwell by multiple air supply points such as an air supply point for each floor and providing a protected lobby in front of the stairwell. In this study, parameters that affect the stairwell pressurization system and the system design are investigated via executing field tests and numerical studies. The stairwell in which the field tests were executed is 163 m height and serves 40 floors of the building. The stairwell divided vertically into three pressurization zones by use of wall and door assembly at two intermediate landings of the staircase and a protected lobby is constructed in front of the stairwell at each floor. Three identical pressurization fans serve to the stairwell and untreated pressurized air is introduced into the staircase through grilles at each floor. A pressurization shaft is utilized for the transfer of pressurization air into the stairwell. The tests are conducted after working hours of the office building in order to prevent the factors disturbing the accuracy of the measurements, such as an uncontrolled opening of a stair door by an occupant. Set of the tests which are compared with each other is completed on the same night, to minimize the effect of outside weather conditions, e.g. wind and temperature. Three sets of tests is performed and each test set consist four test. Each test set investigate an individual affected parameter of the stairwell pressurization system. Two separate conditions, which are based on the system design are taken into account for the test sets. These are referred as "all doors closed" and "critical doors open" conditions. By this means the effect of the investigated parameters are examined under two different conditions. Frequency converters of the pressurization fans are adjusted to a fixed value for each condition in order to supply the same amount of air into the stairwell, i.e. 20 Hz for all doors closed condition and 40 Hz for critical doors open condition. For each test case, two different pressure difference values are measured for all floors, one of the pressure difference value is taken in between the stairwell and the fire protection lobby and the other one is taken in between the stairwell and the corridor. Air velocity at the door openings are measured as an average of 15 readings for the tests of critical doors open condition. Also the ambient temperature and wind flow rate is measured at the beginning of each set of the test. For the tests of all doors closed condition, the pressurization air flow rate is obtained by calculating the leakage air flow rate through the closed doors by using pressure difference measurements in between the stairwell and the corridor. The air flow rate of the critical doors open condition tests are calculated by obtaining the leakages through the closed doors and adding the air flow rates measured across open doors. The supply air flow rates of each test are calculated individually by this method which based on conservation of mass principle. After completion of the tests, it was later decided that the flow rate values obtained by the above method be compared with those obtained by using another method. As the alternative method, velocity is measured at the fresh air inlet connection of each fan, which is a more straight forward approach than using the fan characteristic curve to estimate the flow rate. Each fan inlet is divided into 21 equally sized meshes and velocity of each mesh is measured approximately at the mesh center. One measurement are conducted for each condition by this method. First set of test is comprised of the critical doors open condition tests and investigates which floor chosen as design fire floor is more critical for the stairwell pressurization system. The staircase doors at discharge floor are opened in each of four tests of this test set . The staircase doors at 33th and 22th floors are held in open position for first and second tests respectively, these floors are located in the top pressurization zone of the stairwell. The 6th floor of the stairwell which coincide the middle pressurization zone is chosen for the third test and the doors in between the staircase and the corridor are held in open position for this test. For the fourth test, all of the stairwell doors are closed except the doors of discharge floor. Pressure difference curves with respect to height of building of first and second test are the same, thus it is concluded that opening stairwell doors at any floor of a specific zone does not have a significant impact on the system. The comparison of the opening stairwell doors at different zones is done by using the results of first (or second) test with the third test. A slight difference between the pressure difference curves of this two test is observed and it is considered that the cause of this difference is due to the floors of the zones which has the connection of the pressurization fans and the pressurization shaft. The presence of fire protection lobby is investigated in the second set of tests. The fire protection lobbies locate in front of the stair at each floor, i.e. when a person on a floor needs to use the stair, he/she has to open the fire protection lobby door and the stair door in sequence. The fire protection lobbies are not pressurized with another pressurization system indeed these spaced are pressured indirectly with stairwell pressurization. The building without fire protection lobby is simulated by keeping only one of the two doors (of the stair and fire protection lobby) open at each floor. Two different condition are taken into account, where the first one (all doors closed) analyses the condition that all stair doors are kept closed and the second one (critical doors are open) is conducted when the stairwell is opened to the floor on a given level (fire floor) and the stair termination door at the discharge floor is kept open. The comparison of the results for test cases with and without fire protection lobby revealed that, at constant pressurized air flow rate, the presence of fire protection lobby increases the pressure difference between the staircase and the corridor. The ratio of this increment depends on whether or not an open door is available in the corresponding vertical pressurization zone. This result is proved mathematically. In a zone where an open door is available, fire protection lobby introduces a relatively low increment of the pressure difference between the stair and the corridor. In the third set of tests, the effect of dividing stairwell into vertical pressurization zones is investigated. The main advantage of this practice is that it allows acceptable pressurization of stairwells of high rise buildings despite the negative effect of stack effect. Two tests are executed for each condition; the stairwell is comprised of three zones in one of the tests and the stairwell consists of one pressurization zone by opening the doors which locates in between the pressurization zones. It was obtained that the dividing stairwell into zones has positive effect on the pressurization of stairwells. The CFD modeling of stairwell pressurization system in a very high rise building is executed in this study. One test of the second set of test is modeled by using Fire Dynamics Simulator (FDS) which is extensively used for fire applications. The model consists of the volumes of stairwell, pressurization shaft and fire protection lobbies. After the modeling studies including grid dependency analysis, the pressure difference curves of the test and the model are compared. It is concluded that the modeling a stairwell pressurization system in a very high rise building with FDS gives acceptable results. Time dependent model results are examined in order to investigate the rate of change of stack effect. Although the difference between the stairwell and ambient temperatures is 7°C, the thermodynamic equilibrium is ensured after 10 minutes in the modeling studies.