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Determination of critical air velocities to prevent smoke backflow at a

stair door opening on the fire floor

(2)

Determination of critical air

velocities to prevent smoke

backflow at a stair door opening

on the fire floor

Tamura, G.T.

NRCC34065

A version of this document is published inSHRAE Transactions, 97, (2), ASHRAE Annual Meeting (Indianapolis,

IN, USA, June-22-91), pp. 627-633, 91 (Presented at the 1991 ASHRAE Annual Meeting, Indianapolis, IN, USA,

June 22-26, 1991):

The material in this document is covered by the provisions of the Copyright Act, by Canadian laws, policies, regulations and international agreements. Such provisions serve to identify the information source and, in specific instances, to prohibit reproduction of materials without written permission. For more information visit http://laws.justice.gc.ca/en/showtdm/cs/C-42

Les renseignements dans ce document sont protégés par la Loi sur le droit d’auteur, par les lois, les politiques et les règlements du Canada et des accords internationaux. Ces dispositions permettent d’identifier la source de l’information et, dans certains cas, d’interdire la copie de documents sans permission écrite. Pour obtenir de plus amples renseignements : http://lois.justice.gc.ca/fr/showtdm/cs/C-42

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DETERMINATION OF CRITICAL AIR

VELOCITIES TO PREVENT SMOKE

BACKFLOW AT A STAIR DOOR OPENING

ON THE FIRE FLOOR

G.T. Tamura, P.E.

Fellow ASHRAE

ABSTRACT

Tests to determine air velocities needed to prevent smoke backjlow were conducted in a 10-story experimental fire tower. They were measured at the stair door opening on the fire floor for burn temperatures ranging from 150°F (65°C) to 1 ,200°F (650°C) at increments of 150'F (83'C). The critical velocities ranged from 124 ftlmin (0.63 mls) at 150'F (65'C) to 460ft/min (2.34 m!s) at 1,200°F (650'F). Some of the other parameters inves-tigated resulted in decreases in the critical air velocity. These included venting, either by exterior wall vents or an exhaust fan, and opening stair doors in addition to the one on the fire floor. Reducing the angle of the door opening from 90' to 30° decreased by 50% the effective critical air velocities compared to an area of the door opening at 90'. There was no noticeable effect on critical air velocities when the method of pressurization of the stairshaft was changed from multiple injection to bottom injection.

INTRODUCTION

During a fire, stairshafts can become contaminated with smoke which can endanger the lives of occupants attempting to leave the building. Smoke contamination of

the stairshafts can be prevented by mechanically

pres-surizing the stairshaft with outdoor air. Various stair pressurization systems for protecting stairwells from smoke infiltration have evolved over the past several years (Tamura 1989). These systems are designed to maintain sufficient pressurization inside the stairshaft so that its pressures are higher than those in the fire floor caused by fire and other forces when all stair doors are closed. When stair doors are open, these systems are also de-signed to provide sufficient air velocities to prevent smoke backflow at the stair door opening on the fire floor. The objective of this project was to the determine the critical average air velocities required to prevent smoke backflow at the stair door opening for various fire conditions.

The critical air velocities required to prevent smoke backflow in a corridor were developed by Thomas (1970) in terms of energy release rate into the corridor. Shaw and Whyte (1974) dealt with the air velocity required to prevent contaminated air from moving through an open doorway in tbe presence of small temperature differences.

Heskestad and Spaulding (1989) dealt with airflow required at wall and ceiling apertures to prevent escape of fire smoke. All these tests were conducted in the labora-tory.

In this study, tests were conducted in an experimental fire tower simulating a typical multi-story, open floor plan building to investigate various parameters affecting critical air velocities. The parameters investigated were fire temperature, venting of the fire floor with exterior wall vents and an exhaust fan, angle of stair door opening, number of open stair doors, and method of air injection for stair pressurization.

TEST PROCEDURE

The plan view of the 10-story experimental fire tower

is shown in Figure 1. The tower contains vertical shafts

such as those found in a typical multi-story building,

including elevator, stair, smoke exhaust, service, supply, and return air shafts. The leakage areas of the experimen-tal fire tower were set for a building with average air-tightnesses and a floor area of 9, 700 ft2 (900 m'), or

seven times that of the experimental fire tower.

14.6 m

eセー・イゥュ。ョエ。エ@ tower

1 Building supply 2 Building return I exhaust

3 Smoke shaft

4 Elevator t stair lobby supply

セ@ セZセZセ@ セセrセセウエ@

7 Service shaft 6.5 m Elevator

[\]

Burners

Figure 1 Experimental fire tower

Grorge T. Tamurs is Senior Research Officer of the Institute for Research in Construction, National Research Council of Canada, Ottawa, Ontario.

(5)

The walls of the stairshaft are constructed of 8-in. (200-mm) poured concrete. Each stair door is 3 ft by 7 ft (0.914 m by 2.13m). The leakage area of each stair door is 0.25 ft2 (0.023 m2

) and that of the shaft wall for each

floor is 0.03

If

(0.004 m') represented by an orifice

located in the shaft wall on the corridor side 5 ft (1.52 m) above floor level. The supply air shaft is adjacent to the stairshaft (see Figure 1) with a supply air opening on each floor to permit injection of supply air on all floors or only at the top or the bottom of the stairshaft. The supply air shaft is connected by ductwork to a centrifugal fan with a capacity of 38,000 cfm at 2.6 in. of water (18 m3/s at

650 Pa) and with a variable-speed drive.

For a supply airflow rate of more than 2,000 cfm (940 Lis), the flow rate is measured at a measuring station located downstream of the fan inside a metal duct. The airflow-measuring station consisted of multi-point, self-averaging total pressure tubes and their associated

static pressure taps and an air straightener of honeycomb

panel located immediately upstream of the averaging tubes. Airflow rates up to 2,000 cfm (940 Lis) were measured with a metal orifice plate inserted in the duct upstream of the airflow-measuring station. All tests were conducted with wind speeds of less than 12 mph (20 kmb).

The tests to determine the critical air velocity to prevent smoke backflow at the stair.door opening were conducted on the second floor, which was designated as the fire floor. Static pressure taps to measure pressure differences across the wall of the stairshaft on the corridor side were installed at 1.3 ft, 7 ft, and 10 ft (0.396 m, 2.183 m, and 3. 048 m) above floor level. Thermocouples to measure temperatures inside and outside the stairshaft were also installed at these levels.

Measurements were made at fire temperatures of 150'F (65'C), 300'F (147'C), 450'F (232'C), 600'F (315'C), 750'F (400'C), 900'F (484'C), 1,050'F (565'C), and 1,200'F (650'C). The last three tempera-tures were obtained with strip propane gas burners (2.5 MW) fixed in the burn area of the second floor. Lower fire temperatures were obtained with four ganged small gas burners (total of 1.0 MW) set on top of the strip gas

burners, as shown in Figure 2. The test fire temperature

was measured with a thermocouple directly above the burners and 1 ft (0.305 m) below the ceiling and was controlled at the test temperatures by adjusting the propane gas flow rate. The locations of other ceiling thermocouples and the three thermocouple trees in the burn area are shown in Figure 3.

Initially, tests were conducted with the above fire

temperatures to determine the pressure differences across

the stairshaft wall caused by fire. This was followed by tests with the stairshaft pressurized and with the stair door on the fire floor fully open. The reference test condition was outside wall vents closed, no mechanical venting of the fire floor, stair door open only on the second floor at 90', and multiple air injection of the supply air for stair-shaft pressurization. The air velocities at the stair door opening required to prevent smoke backflow were deter-mined for the following test conditions:

I. Area of outside wall vents opening on tbe second

floor-0, 10 ft2 (0.929 m'), 20 ff(l.858 m2), and 30 ft2 (2.787 m');

628

Figure 2 1\vo sets of test gas burners

2. Mechanical venting of the fire floor-none, venting with exhaust rate of 9,800 cfm (4.62 rri'/s) to create a pressure difference across the closed stair door of 0.10 in. of water (25 Pa);

3. Number of open stair doors-one (second floor),two (first and second floors), and three (flfSt, second, and third floors);

4. Angle of opening of the stair door on the second floor-90', 60', and 30'; and

5. Method of air injection for stair

pressurization-multiple injection (floors 1,

3, 5, 7,

and 10) and

bottom injection (floor 1).

(Note: reference case highlighted in bold type.)

Both fire and nonfire tests were conducted for each of the above test conditions. During the fire tests, the supply air rate to the stairshaft was adjusted until no smoke backflow was observed as determined by running a smoke pencil along the top of the door opening. For each test, the supply air rate to the stairshaft correspon-ding to the point of no backflow was noted. During the

,,

i !1112

;

Stair

I

DD !!i10 m Ceiling thermocouples 0 Thermocouple trees Burners DA

Ji

liifS 1114 116 08 118 1117 1119 DC

(6)

following nonfire test with the stairshaft pressurized with the same supply air rate noted during the fire test, a 21-point hot wire anemometer traverse was used to determine the average air velocity at the stair door opening.

During the tests, pressure differences across the outside walls and vertical shafts and air temperatures for all floors were recorded with the data acquisition system.

RESULTS AND DISCUSSIONS

Pressure Differences across the Walls of the Test Stairshaft Caused by Fire

Fire tests were conducted with the supply air fan for the stairshaft shut down (no pressurization) and with all doors of the stairshaft closed and no venting of the fire floor. The pressure differences across the walls of the stairshaft (stairshaft pressure/bum area pressure) caused by the fire are shown in Figure 4 at 10 ft (3.048 m), 7.0 ft (2.13 m), and 1.3 ft (0.40 m) above floor level. From these pressure measurements, the neutral pressure level was found to he at 5.50 ft (1.68 m) above floor level. This is the level above which combustion gases of the fire floor flowed into the stairshaft and helow which air from the stairshaft flowed into the fire floor. The location of the neutral pressure level not only depends on the size and distribution of the leakage openings in the stairshaft walls but also on all other leakage openings in the fire floor enclosure.

Using 5.50 ft (1.68 m) above floor level for the neutral pressure level, pressure differences across the stairshaft walls were calculated using the following buoyancy equation: where P,- P1 = g h P, - P1 = ghp Hセ@ - T1)/T;

pressure difference across the shaft wall gravitational constant

distance from the neutral pressure level gas density

temperature Q

T

s

= shaft

i

=

outside the fire compartment

J

fire compartment.

(1)

Equation 1 assumes uniform space air temperatures inside the stairshaft and in the fire floor. The temperatures in the fire floor, however, varied greatly both horizontally and vertically, as indicated by the thermocouple readings given in Table 1. As an approximation, the control temperatures near the ceiling above the gas burners were used in Equation 1 to calculate the pressure differences across the stairshaft walls at the three levels. As expected, the calculated values, which are also shown in Figure 4, overestimated the pressure differences across the walls of the stairshaft. Based on Figure 4, the effective uniform fire temperatures in terms of Equation 1 are roughly about 75% of the control fire temperatures.

629 -0.1 ·0.08 :;; -0.06 1ii

0

"'

u -0.04 Mセ@ <li ·0.02 u c i" 0 w

"

u i" 0.02 セ@ m m i" 0.04 Q_ 0.06 0.08 Fire temperature, oc 0 100 200 300 400 500 600 700 -25 Measured values,

height above floor Calculated -Eq. 1 -20

10 It (3.05 m) calculated 7 It (2.13 m) 10 h (3.05 ml ·15

1.3 It (0.40ml -10 calculated B

7ft (2.13 m) ·5

0 calculated

1.3 It (0.40 m} 5

\

10 15 Reference pressure - Burn area

Neutral pressure level - 5.5 ft

(1.68 m) above floor level

L-.;.__l _ __j_ _ _ j _ _ . . J _ _ . . . l - _ . 1 _ - - l 20 0 200 400 600 800 1 000 1200 1400 Fire temperature, "F ro a. <li u c w :;;

""

'6 w 5 m m i" Q_

Figure 4 Pressure differences across stairshaft wall caused by fire

Venting the Fire Floor with Exterior Wall Vents

The critical air velocities plotted against the fire temperatures for exterior wall vent areas of 0, 10, 20, and 30 ft2 (0, 0.93, 1.86, and 2.79 m') are shown in Figure 5. This graph shows that the required air velocities to

prevent smoke backflow increased with fire temperature and decreased with size of the exterior wall vents. At 150'F (65'C) with the exterior wall vents closed, the

Fire temperature, oc

0 100 200 300 400 500 600 700

BOO 4

Wall vent area 700 • closed 3.5 600

1 0 It 2 (0.93 m 2) 3 E .E-

20 It 2 (1.86 m 21 ;i- 500 2.5 ·u

30 fl 2 (2.79 m 21 > _Q ID 400 o calculated 2 > ·@ 'iii 300 1.5 .g

8

200 0 0 0 200 400 600 800 1 000 1200 1400 Fire temperature, "F !!' E ;i-·u 0

"'

> ·iti 'iii .g

;s

Figure 5 Effect of outside wall vent area on critical air velocity

II

il

I

I

(7)

TABLE 1

Temperature Distribution on the Fire Floor for Control Fire Temperature of 1,200°F (650°C)

(See Figure 2 for locations of thermocouples} Test Condition

Outside wall vents closed;

Multiple air injection to prevent smoke backflow; Stair door on 2nd floor

Location 1 2 3 4 5 8 Temperature Of (DC) 1,170 1632) 900 1483) 1,074 1579) 993 1534) 1,065 1574) Location Temperature Of (DC) 7 921 1494) 8 9 972 1522) 10 853 1456) 11 430 1221) 12 945 1507)

Thermocouple Trees in the Burn Area Height above floor

ft 10 13.05) 8 12.44) 7 12.13) 6 11.83) 4 11.22) 3.5 11.07) 2 10.61) 1.3 (0.40) 1,022 (550) 993 (534) 889 (476) 511 (266)

critical air velocity was 124 ft/min (0.63 rn!s), and at the same temperature with the exterior wall vent open at 30 ft2

(2.79 m'), it was 91 ft/min (0.46 rn!s); at 1,200'F (650'C), the critical air velocities were 460 ft/min (2.34 rn!s) and 307 ft/min (1.56 m'). The critical air velocities

with an exterior wall vent opening of 30 ft2 (2. 79 m')were

about 70% of those with the exterior wall vents closed. Further increase in vent area is unlikely to result in any significant reduction in the critical air velocities.

918 (492) 880 (471) 748 (398) 505 (263) 0 ·0.6

セ@

-0.4 セ@ 0 .c u -0.2 .s

'"

u c セ@ ru

"'

0 '6 セ@

"

"

セ@ セ@ 818 (437) 855 (474) 701 (372) 855 (457) 558 (292) 430 1221) 323 (162) 368 (187) Fire temperature, oc 100 200 300 400 500 600

Wall vent area 1111 vents closed 0 10 It 2 (0.93 m2)

D a Non-fire tests

a. 0.2 Reference pressure セ「オイョ@ area

700 -150 -125 -100 -75 -50 ·25 0 25 50 75 0.4 100 0 200 400 600 800 1 000 1200 1400 Fire temperature, oF

"'

a.

'"

u c セ@ ru

"'

'6 セ@

"

"

"

セ@ a.

With the outside wall vents closed, smoke backflow

could not be prevented if the enclosure of the fire floor

were airtight. The pressures of the fire floor would then

increase to equalize with those of the stairshaft, resulting

in an exchange of air and smoke at the stair door opening caused by the buoyancy force. The extent of buildup of pressures in the fire floor would depend on the leakage area of the fire floor enclosure, and the buildup would be indicated by the pressure differences across outside walls and walls of the vertical shafts such as an elevator shaft. The pressure differences across the elevator shaft wall measured during the nonfire tests are shown in Figure 6. The buildup of pressures was greatest with the outside wall vents closed but was reduced significantly with a

vent area of 10ft' (0.93 m2

) and was negligible for vent

sizes of 20 and 30

ft'

(1.86 and 2.79 m2

). Airflow

through the stair door to prevent smoke backflow can cause buildup of pressures in the fire floor, which can

Figure 6 Pressure differences across elevator shaft walls with stairshqft pressurized to prevent smoke bacliflow

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result in an increased rate of smoke flow into elevator and

service shafts. On the other hand, this airflow can assist

firefighters entering the fire floor by increased visibility and reduced air temperature around the stair door.

Using the measured values of pressure differences across the stairshaft wall at the 7 ft (2.13 m) level of Figure 4, the corresponding air (smoke) velocities at the top of the stair door were calculated as follows:

yo

P, =

Pz

or, for air density at standard temperature and pressure,

(2)

where

P, velocity pressure, in. of water (Pa)

p air density, lb/ft' (kg/m3

)

V air velocity, ft/min (m/s)

C = 4,000 (1.29), ft/min/in. of water'h (m/s/Pa'h).

The air velocities thus calculated from Equation 2 are also plotted in Figure 5. This curve lies just above the curve for the outside wall vent opening of 30 ft' (2. 79 m'). With no pressure buildup on the fire floor, critical

air velocities can be estimated using Equations 1 and 2.

It should be noted that the critical air velocities

correspond to no smoke backflow at the top of the door

opening. However, smoke can flow into the stairshaft

through any crack openings in the stairshaft walls above the door opening.

Mechanical Venting of the Fire Floor

With the exterior wall vents closed, the fire floor was

mechanically exhausted at a rate of 5.5 ach to produce a

pressure difference across the stair door of 0.10 in. of water (25 Pa). Then, under fire conditions, the stairshaft was pressurized with the stair door open on the second

floor to conduct the smoke backflow tests. Critical air

velocities thus obtained, shown in Figure 7, were less than those with the exterior wall vents closed and were about the same as those with the exterior wall vents open

at 30 ft2 (2.79 m') as shown in Figure 5. The

correspon-ding required air supply rates to the stairshaft are shown

in Figure 8. The supply air rates were considerably less

with the fire floor mechanically exhausted than

in

the case

with the exterior wall vents open at 30 ft' (2.79 m'). Also, at a fire temperature of 150'F (65'C), mechanical venting alone induced sufficient airflow at the stair door

opening to prevent smoke backflow.

Angle of Stair Door Opening

The r?SUits of the tests with the stair door on the se<:ond floor open at angles of 30', 60', and 90' are shown in Figure 9. Because a velocity traverse could not be conducted with the stair door partially open, the airflow through the stair door opening was determined by subtracting the air leakage flow through the entire stair·· shaft walls from the measured supply air to the stairshaft.

631

Fire temperature,

oc

0 100 200 300 400 500 600 700 BOO I' f I I I I I 4

700 o mechanical venting 3.5

600

1-

Outside wall vents closed - 3

E Multiple air injection

セ@

2-セ@ 500

セ@ 2.5 セ@

'(j From figure 5 vents closed セ@ '(j

0 0 ill 400

GBBカOセᄋ@

セ@ 2 ill > > ᄋセ@ ᄋセ@ 03 300 セ@

--

- 1.5 03 u / / .

セ|MMMMᄋM

.'! ᄋセ@ Nセ@

<S

. セ@ 0 200 / / / ' ...

;;

.

/ ) ' /

...

100 _ 1 ··/ / : / From of 30 ft ヲゥセオイ・@ (2. 79 5 vent m 'I area 0.5

'

0 セMMセMMセセMMセセMMセMMMォMMMlMQセ@ 0

0 200 400 600 BOO toOO 1200 1400

Fire temperature, oF

Figure 7 Effect of mechanical venting on critical air velocity Fire temperature,

oc

0 100 200 300 400 500 600 700 BODO o no mechanical venting 7000 3.5 o mechanical venting 3 6000 0

"'

E 2.5 セ@ u 5000 ,;

iii

'iii 4000

>-a.

"- 3000 セ@ 0 Ul 2000 toDD 0

0 0

Outside wall vent

30 1t2 (2.79 m') 2 1.5 0.5 0 lMセMMMlMMセMMlMセMMMlMMセP@ 0 Figure 8 200 400 600 BOO 1 000 t 200 t 400 Fire temperature, oF

Comparison of required aiiflow rates to prevent smoke backjlow with and without

mechanical venting

The air leakage flow was calculated knowing the leakage areas of the stairshaft walls and the measured pressure differences across their walls on all floors. The airflow rates at the stair door opening thus obtained were divided

by the area of the stair door at a fully open position to

obtain comparative critical air velocities in terms of airflow rates. Figure 9 indicates that with the stair door open at 60', air velocities were about 85% of those with the stair door fully open and about 50% of those with the stair door open at 30'. Therefore, keeping the stair door

E

'"

iii

'iii

>-a.

"-セ@ Ul I I

(9)

Fire temperature, "C

0 1 00 200 300 400 500 600 700

800 rr-..,--...,--...,----,----,----,----,-"=1 4

700 - Door angle Outside wall vents closed 3.5

• 30 ° 600

60 ° 3 E

"'

.2- E セ@ 500 2.5 セ@ ·u セ@ 0 0 セ@ 0

ill 400 From figure 5 セ@

2 ill

> セ@ >

·o;; (door open 90') . . / ·o;; "iii u 300

セOO@

1.5

"'

セ@ /

0

セカセ@

0 200 100 0.5 0 0 0 200 400 600 800 1000 1200 1'100 Fire temperature, 'F

Figure 9 Effect of open door angle on critical air velocity

open to a practical minimum during a fire can minimize the possibility of smoke contamination of the stairshaft. Number of Open Stair Doors

The effects on the critical air velocities of opening several stair doors are shown in Figure 10. The top curve represents the critical air velocities with the outside wall vents closed and with only the stair door on the second floor (fire) open. The critical air velocities with the stair doors open on floors 1 and 2 and with the stair doors open on floors 1, 2, and 3 were about the same and about 75% of that with only the stair door on floor 2 open. This

was surprising,

as

critical air velocities greater than those

with only the stair door on floor 2 open were expected; these results could not be explained.

Method of Air Injection

Figure 11 compares the critical air velocities obtained with the stairshaft pressurized with multiple air injection on floors 1, 3, 5, 7, and 10 and bottom injection only on floor 1. The critical air velocities with the exterior wall vents closed indicated practically no differences between

the two methods of air injection.

SUMMARY

The critical air velocities for a given building can be calculated using equations for buoyancy and velocity pressure (Equations 1 and 2 ) for cases with no pressure · buildup in the fire floor caused by airflow through the

stair door into the fire floor.

The initial tests in the experimental fire tower were conducted to determine critical air velocities with fire

632 Fire temperature, "C 0 100 200 300 400 500 600 700 800 4 Number of open 700 stair doors 3.5 0 1st, 2nd floors 600

1St., 2nd, 3rd floors 3 E !{! .2- E >. 500 2.5 iC ·E 0 セ@ 0 From figure 5 ᄋセ@ 0 ill

{door open on 2nd floor) セ@

?!

> 400 . / 2 ·o;;

セOO@

·,; <ii ill .g 300 - 1.5 u ᄋセ@

8

/ 0

8

/

..

200 -/ / 100

! Outside wall vents closed 0.5

0 0

0 200 400 600 800 1000 1200 1400

Fire temperature, oF

Figure 10 Effect of number of open stair doors on critical air velocity

temperatures ranging from 150'F (65') to 1,200'F (650'C). The stairshaft was pressurized with multiple injections of supply air and the stair door was open at 90' on the fire floor with no venting on the fire floor (refer-ence test condition). For the range of test fire tempera-tures, the critical air velocities increased from 124ft/min (0.63 m/s) to 460 ft/min (2.34 m/s). The critical air velocities for other test conditions were compared with these values. Fire temperature, oc 0 1 00 200 300 400 500 600 700 BOO 4 Stairshaft pressurization 700

0 Multiple injection (floors 1 ,3,5,7, 1 0) 3.5

600

Bottom injection (floor 1) 3

E m .2-

E

iC 500 2.5 iC ·c; u 0 0 ill ill 400 2 > > c ·@ 'ii 0 <ii 300 セ@ 1.5 <ii u

·"

Zセ@

3

0 200 - 1 100 0.5

Outside wall vents closed

0 0

0 200 400 600 800 1000 1200 1400

Fire temperature, "F

Figure 11 Effect of method of air injection on critical air velocity

(10)

The critical air velocities

decreased to a minimum

セ|オ・@

of 70% of those with the exterior wall vents closed when the size of the

exterior wall vents was increased to 30 ft2 (2. 79 m');

decreased with mechanical venting at 5.5 ach to the above minimum value;

decreased with the angle of stair door opening at 60' and 30' to 85% and 50% of those with a door angle of 90', respectively;

decreased to 85% of those with only the stair door open on the second floor when the stair door on the first floor was also opened and were about the same when, in addition, the stair door on the third floor was opened; and

were about the same for multiple injection and bottom injection of supply air for stairshaft pres-surization.

The values of critical air velocities for various cases given in this paper can serve as a guide for estimating the supply air rates required for open door conditions in designing stair pressurization systems. The critical air velocities to use for that purpose are likely those obtained with the reference test condition at the design fire temper-ature.

DISCUSSION

J.R. Graves, President, GRACO Mechanical, Houston, TX: Do you feel there is a significant difference for critical velocity with four doors open versus three doors open? G. T. Tamura: The series of tests with open doors in-dicated that the critical air velocities with two and three doors open were about the same and less than with the stair

door open only on the fire floor. It is likely that critical air

velocities with four doors open would be about the same as those with two and three stair doors open. Critical air velocities are affected more by the fire temperature and the amount of venting of the fire floor either by use of exterior wall vents or an exhaust fan.

Richard E. Masters, P.E., Partner, Jaros, Baum &

Bolles, New York, NY: This was an excellent presentation with good data.

Tamura: The author is grateful to Mr. Masters for his kind comments.

633

ACKNOWLEDGMENT

The author is grateful for the support of the members of T.C. 5.6 in carrying out this research project. The author acknowledges the contribution of R.A. MacDonald in carrying out tests in the experimental fire tower and in processing the test readings; thanks are also due to D. W. Carpenter and V.A. Partington, who assisted in the tests.

REFERENCES

Heskestad, G., and Spaulding, R.D. 1989. "Inflow of air required at wall and ceiling apertures to prevent escape of fire smoke," pp. 533-540, FMRC J.I. OQ4E4.RU 070 (A). Norwood, MA: Factory Mutual Research Corporation.

Shaw, B.H. 1974. "Air movement through doorways-The influence of temperature and its control by forced airflow." Building Services Engineers, Vol. 42, December, pp. 210-218.

Thomas, P.H. 1970. "Movement of smoke in horizontal corridors against an air flow." Institutions of Fire En-gineers, Vol. 30, No. 77, pp. 45-53.

Kenneth Elovitz, Energy Economics, Inc., Foxboro, MA: How were the wall vents operated and controlled? Would the required/optimum vent area change with the size of the floor and the size of the fire?

Tamura: The specified sizes of exterior wall vents were installed before running the fire tests. The size of wall vents required to prevent pressure buildup in the fire floor would be about three times the effective leakage area of the stairshaft, including the open stair door on the fire floor, considering flow of air from the floor spaces into the stairshaft and out through the open stair door. The required vent size would then be this effective leakage area less the leakage area of the fire floor enclosure, neglecting the open stair door. This would mean that the required vent size for minimum critical air velocity for a given fire temperature would depend on the floor area and the height of the stairshaft.

Figure

Figure  1  Experimental fire  tower
Figure  2  1\vo sets  of test gas  burners
Figure  4  Pressure  differences  across  stairshaft  wall  caused  by fire
Figure  7  Effect  of mechanical  venting  on  critical  air  velocity  Fire  temperature,  oc  0  100  200  300  400  500  600  700  BODO  o  no  mechanical venting  7000  3.5  o  mechanical  venting  3  6000  0  &#34;' E 2.5 セ@u 5000  ,;  iii  'iii  4000
+2

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