Stair pressurization systems for smoke control

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Stair pressurization systems for smoke control

Stair pressurization systems for

smoke control

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Stair pressurization systems

for smoke control

Test results of critical air flows needed to prevent

smoke backflow at open stair doors are described

By

George

T.

Tamura,

P.E.

Fellow ASH RAE

V

arious methods for protecting stairwells from smoke contamination during a fire have evolved over the past several years. The one used most often in North America is the stair pressurization system.

Designing these systems is complicated because an intermit-tent loss of effective pressurization occurs when occupants enter and leave the stairs during evacuation. Therefore, the pressuriza-tion system should have a supply air fan with sufficient capacity to provide effective pressurization to prevent smoke entry when doors are open.

It should also have means of preventing overpressurization, which can make door opening difficult when all doors are closed. To prevent such overpressures, the design concepts of baromet-ric damper relief, feedback control with fan bypass, variable-speed or variable-pitch fan, and exit door relief have been deve-loped. Although niany such systems have been built, it is not known at present how effective they are.

In this article, the results of preliminary studies conducted during the first phase of ASH RAE Research Project 559RP are presented. The studies examined the critical air velocities needed to prevent smoke backftow at an open stair door.

Stair pressurization systems

. The stair pressurization systems reviewed include systems with and without lobbies. The former provides an additional door to restrict loss of pressurization air while the lobby serves as a stag-ing area for firefighters or as a temporary holdstag-ing area for occupants. The lobby, stairshaft or both can be pressurized or, in some instances, they can also be exhausted. Design guidelines for stairshafts with lobbies have been published by Hobson and Stewart' and for stairshafts without lobbies by Klote and Fothergill' and Thornberry."

In North America, pressurization systems for stairshafts without lobbies are more prevalent than systems for stairshafts with lobbies. Therefore, this article is concerned with the former. The early stair pressurization systems were the single-injection type with a fan usually located at the top of the build-ing. Tests of these systems with the fan sized to pressurize a stair-shaft with the exit door open revealed that pressure differences across the stair doors near the injection point can be excessive, making them difficult to open. Pressure differences far from the injection point can be minimal and may fail to prevent smoke infiltration. This variation in pressurization caused by the flow resistance in the stairwell led to the design of a stairwell pressu-rization system with multiple injection points.

The pressures inside the stairshaft should be controlled

to prevent under- or overpressurization of the stairshaft when

stair doors are used during a fire. Some methods being used to achieve pressure control are: a supply air fan and relief vents in the stairshaft walls; a supply air fan with speed, variable-pitch blades; and a supply air fan with supply air bypass damp-ers, all controlled by a static pressure sensor in the stairshaft. Evacuation

A means of egress is designed to evacuate occupants from endangered areas as quickly and efficiently as possible. The design is based on factors such as number of occupants, occupant densities and occupant characteristics (such as physical size, need for space between people, and walking speed) to meet the desired flow rates of people for efficient evacuation.'

Evacuation drills have been conducted in multi-story build-ings to develop models for predicting egress times and to assess the problems encountered during evacuation. The two methods of planned evacuation are uncontrolled total evacuation (build-ing occupants attempt to evacuate at the same time) and con-trolled selective evacuation (building occupants evacuate under instruction from a public address system).

The operation of stair doors during evacuation is particu-larly important for the design of stairshaft pressurization and for code requirements. Open stair doors can cause loss of pressuri-zation and thereby reduce the system's capability to prevent smoke from infiltrating the stairshaft. Operation of stair doors can vary with the method of evacuation, occupant density, type of build· · ing' occupancy, firefighting and other factors ..

Under uncontrolled total evacuation, all stair doors can be open for a short time soon after an alarm is sounded, except for the doors on the fire and exit floors, which can be open for a prolonged period. During controlled selective evacuation, a few doors other than those on the fire and exit floors may be open for a short period at any given time. ,

Evacuation in a residential occupancy building can be prolonged, as reported by Bryan' on the MOM Grand Hotel fire. Because of low occupant density, doors are likely to be open for considerably shorter periods in hotels and apartments compared to those in office buildings.

About the author

George T. Tamura is a senior research officer with the National

Fire Laboratory, Institute for Research in Construction,

National Research Council of Canada, in Ottawa, Ontario. He received a B.Sc.(Mech. Eng.) from the University of Manitoba. Tamura is a member of ASHRAE Technical Committee 5.6

(Control of Fire and Smoke) and GPC 5P (Guideline for Com-missioning Smoke Management Systems). He is also a member of the National Fire Protection Association (NFPA 92, Smoke Management Systems) and the Society of Fire Protection Engineers.

Stair pressurization systems

Continued from page 14 Codes

The requirements in the building codes for stairshaft pres-surization systems include supply air rates, required minimum -and allowable maximum pressurization, and minimum air

veloc-ity through open stair doors.

In the Australian Standard 1668, pressure differences with all doors closed are not to exceed 0.20 in. of water (50 Pa), or the force required to open the door at the door knob is not to exceed 25lb (110 N). With three doors open, the airflow velocity from the stair-shaft to the adjacent floor space should not be less than 200 fpm (I m/s), averaged over the full area of the door opening. The pres-surization system is to be automatically controlled so that, when doors or other factors cause significant variations in airflow and pressure differences, the above conditions can be restored as soon as practicable.'

The British Standards Institution recommends a simple lobby to reduce the effect of an open door to the pressurized stair-shaft. The required pressurization is 0.20 in. of water (50 Pa). 4

New York City requires a supply air rate of at least 24,000 cfm (11.3 m3_{/s) plus 200 cfm (0.09 m}3_{/s) per floor. The maximum}

velocity of air supplied at the openings into the stairs is 3,000 fpm (15 m/s) at its discharge pointwithin the stairshaft. The maximum permissible pressure difference between the stair and the floor space is 0.40 in. of water (100 Pa). The minimum permissible pres-sure difference is 0.10 in. of water (25 Pa) when all stair doors are closed or not less than 0.05 in. of water (0.13 Pa) when any three doors are open. 5

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As an alternative to maintaining 0.05 in. of water (0.13 Pa), a minimum average velocity of 400 fpm (2 m/s) through any stair door.with any three doors open is to be maintained. The maxi-mum velocity permitted through a single open door with all other doors closed is 2,000 fpm (10 m/s). The door opening force at the door knob is limited to 25lb (110 N) using mechanical assistance as required. s

The requirements in the Standard Building Co dell and the Uniform Building Code" are similar. Both specify S)110keproof enclosures. They may be omitted for buildings with a complete sprinkler system provided that all required stairways are equipped with a dampered relief opening at the top and supplied mechan-ically with sufficient air to discharge a minimum of2,500 cfm (1.2 m/s) through the relief opening while maintaining a minimum positive pressure of 0.15 in. of water (37.3 Pa) relative to atmospheric pressure with all stair doors closed. 2

For buildings with a fire suppression system throughout, Building Officials and Code Administrators allows elimination of smokeproof enclosures provided that all interior stairshafts are pressurized to a minimum of0.15 in. of water (37.3 Pa) and a max-imum of 0.35 in. of water (87 Pa) in the shaft relative to the build-ing with all stair doors closed. 2

The Supplement to the National Building Code of Canada recommends a supply air rate of 10,000 cfm (4.7 m3_{/s) plus 200}

cfm (0.09 m3_{/s) for every door opening into the stairshaft. The}

exit door to outdoors in each stairshaft is to be.held open when the supply air fan is initiated."

Critical air velocities

The critical velocities required to prevent smoke backflow in a,corridor have been developed by Thomas" in terms of energy

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Stair pressurization systems

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release rate into the corridor. Also, Shaw and Whyte 10 _{dealt with}

the velocity required to prevent contaminated air from moving through an open doorway in the presence of small temperature differences. Klote and Fothergill' discussed these references in the ASHRAE smoke control design manual. More recently, Heskestad and Spaulding' reported on their studies on air flow required to prevent smoke flow through openings in walls and ceilings.

In conducting the first phase of ASH RAE Research Project 559RP, preliminary tests on critical air velocity were conducted in the 10-story experimental fire tower of the National Fire Laboratory of the National Research Council of Canada, located near Ottawa, Ontario. 13

Figure I shows the pressure difference across the wall of the

staii·shaft at the fire floor (stairshaft pressure burn area pressure) without stair shaft pressurization. That is, the pressure difference caused only by the buoyancy force for 570'F (300'C) and 1,300 'F (700 'C) fire temperatures. The pressure differences are about the same, whether the stair door at the fire floor is closed or open. The neutral pressure level is located 4.80 ft (1.46 m) above floor level.

Figure 2 shows the centerline velocity pressure profiles at the

stair door opening on the fire floor without stairshaft pressuri-zation for fire temperatures of 570'F (300'C) and 1,300'F (700'C). The velocity pressures referenced to the burn area pres-sure at 6.66 ft (2.03 m) were -0.014 in. of water (- 3.5 Pa) for a 570'F (300'C) fire temperature and -0.019 in. of water ( -4.7 Pa) for a 1,300'F (700'C) fire temperature. These values compare with pressure differences measured across the stairshaft wall at the 7-ft (2.13 m) level of -0.014 in. of water (- 3.5 Pa) and -0.021 in. of water ( -5.2 Pa), respectively (Figure I).

With the stairshaft pressurized and with fire, the flow rate was increased until no back flow was observed. At a 90' stair door opening, when the velocity pressure was balanced at the top of the door opening, the flow direction was from the stairs haft into the burn area for the full height of the stair door (Figure 3). Hence, smoke backflow was prevented.

The flow rates required to prevent smoke backflow were 7,380 cfm (3.5 m3_{/s) for a 570'F (300'C) fire temperature and}

PRESSURE DIFFERENCE ACROSS • STAIRSHAFT WALL, Pa -15 -10 ·5 0 5 10 12

570F 3.5

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\(1300'GI

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_{0 STAIR DOOR CLOSED 3.0}

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\

\

0

-0.06 -0.04 -0.02 0.00 0.02 0.04 0.06 PRESSURE DIFFERENCE ACROSS STAIRS HAFT WALL, INCHES OF WATER

Figure 1. Pressure difference across stairs haft wall with stair door open and closed on fire floor.

9,200 cfm (4.3 m' /s) for a 1,076 °F (580 'C) fire temperature, with the exterior wall vents at the 10 ft2 _{(0.9 m}2

) fire floor open to

simulate broken windows. Corresponding critical velocities were 350 fpm (1.8 m/s) and 438 fpm (2.2 m/s), respectively.

Conclusion

The available literature contains little information on the performance of stair pressurization systems with overpressure relief features to deal with door operation during a fire. The num-ber of doors open at a given time and the significance in terms of smoke contamination depend mainly on the type of evacuation (whether uncontrolled total or controlled selective), building. use and occupant density.

Some codes specify the open door condition in which a stair pressurization system must maintain the required pressure. Others specify the air velocity required at the door opening. Most codes specify the required amount of minimum and maximum pressu-rization. VELOCITY PRESSURE, Pa

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EXTERIOR WALL VENTS OPEN

0 0

-0.04 -0.02 0.00 . 0.02 0.04 VELOCITY PRESSURE, INCHES OF WATER

Figure 2. Centerline velocity pressure profile at open stair door

(90') on fire floor.':: .·:::;; ·:•;' • ' ' ,,·,; > ' ,,';_') • ＮＧＮＺＺｏＺＧ［Ｇ｣ｾＭＳＬ＠ ｾＧ＼＠ •/\>,.;o,; '/•' ·.·.,·:. -;'\ ... -. '•' 15 2.5

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-0.04 -0.02 0.00 0.02 0.04 .·. 0.06 VELOCITY PRESSURE, INCHES OF WATER

Figure 3. Centerline velocity pressure on fire floor wlth stair-shaft pressurized to prevent smoke backf!oW.

----="'===--=--=-=-= = J

Stair pressurization systems

The variation in the code requirements can be partially attributed to the lack oftest data on the performance of stair pres-surization systems with operation of stair doors. Such informa-tion can be obtained from fire tower tests by determining the critical air velocities required to prevent smoke back flow for var-ious fire conditions.

Also from fire tower tests, various types of overpressure relief systems can be evaluated by observing smoke backflow with progressive opening of a number of stair doors. Stair pressuriza-tion systems installed in buildings can be evaluated by measuring air velocities at stair door openings and comparing them with crit-ical air velocities determined in the fire tower.

In this way, the capabilities and limitations of stair pressu-rization systems can be assessed for a number of open stair door configurations and fire conditions. ASH RAE Research Project 559RP (which is underway) is intended to provide such informa-tion to develop effective design and to assist in formulating rational requirements for stair pressurization systems. The results of this project will be published in a future issue of ASH RAE

Journal.

Acknowledgment

The author is grateful for the support of ASH RAE Techni-cal Committee 5.6 in conducting this research project. The author

acknowledges the contribution ofR.A. MacDonald in conduct-ing tests in the experimental fire tower and in processconduct-ing the test readings. Thanks are also due to members of the National Fire Laboratory, who assisted in the tests. II

References

1. Standards Association of Australia. 1979. ''Australian Standard 1668, Fire precautions in buildings with air"hand!ing systems." SAA Mechanical Ventilation and Airconditioning

Code. Pt. !. North Sydney, New South Wales, Australia.

2. BOCA. 1984. The BOCA Basic/National Building Code. Section 818. Homewood, Illinois: Building Officials and Code Administrators International Inc.

3. Bryan, J.L. 1983. ''A. review of the examination and analysis of the dynamics of human behavior in the fire at the MOM Grand Hotel, Clark County, Nevada, as determined from a selected questionnaire population." Fire Safely Journal. Vol. 5, pp. 233-240. 4. British Standards Institution. 1978. "Smoke control in protected escape routes using

pressurization." Code of Practice for Fire Precaution in the Design of Buildings. Pt. 4.

5. City of New York. 1979. "Standards for the pressurization ofstairshafts." Local lAw

No. 84.

6. Heskestad, G.H., Spaulding, R.D. 1989. "In!1ow of air required at wall and ceiling aper-tures to prevent escapeoffiresmoke." FMRC 1.1. OQE4.RU 070 (A). Norwood, Massa-chusetts: Factory Mutual Research Corporation.

7. Hobson, P.J., Stewart, L.J. 1973, Pressurization of Escape Routes. Bracknell, Berkshire, England: Heating and Ventilating Research Association.

8. Klote, J.H., Fothergill, 1:-N. Jr. 1983. Design of Smoke Control Systems/or Buildings.

Atlanta, Georgia: ASHRAE.

9. NFPA. 1986. ''Concept of egress design." Fire Protection Handbook, 16th ed. Section 7, Chapter 3, pp. 7-20-7-40. Quincy, Massachusetts: National Fire Protection Association. 10. Shaw, B. H., Whyte, W. 1974. ﾷｾｲ＠ movement through doorways-the influence of tem-perature and its control by forced airflow." Building Services Engineers. Vol. 42, Decem-ber, pp. 210-218.

II. Southern Building Code Congress International. 1985. "Set:tion 506." Standard

Build-ing Code. Birmingham, Alabama.

12. NRCC. 1985. "Measures for fire safety in high buildings." Supplement to the National

B11ifding Code of Canada. Chapter 3, pp. 87-111. Ottawa, Ontario: National Research

Council of Canada.

13. Tamura, G.T. 1989. "Stair pressurization systems for smoke control: Design consider-ations." ASHRAE Transactions. Vol. 95, Pt. 2, pp. 184-192.

14. Thomas, P.H. !970. "Movement of smoke in horizontal corridors against an air flow."

Institutions of Fire Engineers. Vol. 30, No. 77, pp. 45-53.

!5. Thornberry, R.P. !982. "Designing stair pressurization systems." Technical Report

82-4. Boston, Massachusetts: Society of Fire Protection Engineers.

16. International Conference of Building Officials. 1988. Uniform Building Code. Whit-tier, California.

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ASHRAE Journal July 1991 _