6. RESULTADOS
6.4. Morfometría de los plastídios de Tagetes erecta L. mediante TDI
A single injection system is one that has pressuriza- tion air supplied to the stairwell at one location. The most common injection point is at the top, as illustrated in Figure With this system, there is the potential for smoke feedback into the pressurized
through the pressurization intake. Therefore, the capability of automatic shutdown in such an event should be considered.
tall stairwells, single injection systems can fail when a few doors near the air supply injection point are open. All of the pressurization air can be lost through these open doors, and the system will then fail to main- tain positive pressures across doors farther the injection point. To this, some smoke control designers limit the height of single injection
to eight stories; however, other designers feel this limit can be extended to twelve stories. Careful design is rec- ommended for single injection stainvells in excess of eight stories.
There is the potential for failure of a bottom injec- tion system when the exterior door is opened. Some of the supply air can short-circuit the system by
directly out the opened doorway. It is recommended that supply inlets be at least one floor above or below rior doors.
Duct Shaft
Duct
Figure by injec-
tion located at
level.
Figure 10.3 by multiple
Figures and are two examples of many possible multiple systems can be used to overcome the of single injection
Figures 10.2 and 10.3, the supply is shown in a sep- arate shaft. However, have built that have eliminated the expense of a separate duct shaft by locat- ing the supply duct in the stairwell itself. If the duct is located inside the stainvell, care must be taken that the duct does an obstruction to orderly building evacuation.
of Smoke Management
Many multiple injection systems have been built with supply air injection points on each floor. These resent the ultimate in preventing loss of pressurization air through a few open doors; however, that many tion points may not be necessary. There is some ence of opinion as to how far apart injection points can be safely located. Some designers feel that injection points should not be more than three floors apart, while others feel that a distance of eight stories is acceptable.
For designs with injection points more than three stories apart, the designer should determine by computer analy- sis that loss of pressurization air through a few open doors does not lead to loss of stairwell pressurization.
Compartmentation
An alternative to multiple injection is
of the stairwell into a number of sections, as illus- trated 10.4. The stairwell is divided into a number of sections or compartments, each compartment being from one to about eight floors high. The compart- ments are separated by walls with normally closed doors. Each compartment has at least one supply air injection point. The main advantage of
tion is that it allows satisfactory pressurization of stair- wells that are otherwise too tall for satisfactory
is inappropriate for densely
Roof Level
lated buildings, where total building evacuation stairwell is planned in the event of a fire.
can be an effective means of providing stairwell pressurization for very tall buildings, when a staged evacuation plan is used and when the system is designed to operate successfully when the maximum number of doors between compartments are open. This maximum number of doors open between compartments would need to be by an evacuation analysis. Com- partmentation does have a disadvantage from an archi- tectural standpoint in that it probably cannot be achieved without increased stairwell landing space at the location of the compartmentation doors.
Vestibules
A number of pressurized stairwells have been built with vestibules, which can be either pressurized or not pressurized. Vestibules provide an additional barrier around a stairwell and, to some extent, a vestibule can reduce the possibility of an open-door connection exist- ing between the and the building. An evacua- tion analysis can be to determine the extent to which both vestibule doors are likely to be opened simultaneously.
Analysis of a pressurized stairwell with an surized vestibule can be performed using the same methods employed for analyzing a system without a vestibule except that the effective leakage areas from the stainvell to the building would be used. These effective areas can be detemiined by methods presented in Chap- ter 5. No formal method of design analysis has been developed for pressurized stairwells with pressurized vestibules, and this topic is beyond the scope of this manual.
Supply Air Intakes
In the pressurization systems illustrated in Figures 10.1, 10.2, and 10.3, centrifugal fans supply pressuriza- tion air to the stainvell. A shield around the intake should be considered to reduce adverse effects of wind on the fan performance. This is especially important for propeller fans, which are more susceptible to wind effects than are other types of fan. Roof-mounted pro- peller fans should have wind shields as illustrated in Figure Because the horizontal component of wind is about ten times greater than the vertical component, wall-mounted propeller fans are
susceptible to wind effects. If wall-mounted propeller fans are to be used, design analysis should address wind effects to minimize the probability of these fans being overpowered by the wind.
Outdoor movement that might result in smoke feedback into supply air inlets depends on the location of tire. location of points of smoke leakage
Chapter 10 Pressurization
from the building, wind speed and direction, and on the temperature difference between the smoke and the out- side air. At present, no formal method of analysis has been developed for this complex outdoor airflow. How- ever, some general recommendations can be made. The supply air intake should be separated from exhausts, outlets from smoke shafts and roof and 'neat vents, or open vents from elevator shafts or other build- ing openings that might expel smoke during a fire.
These smoke outlets include the outlets from a zoned smoke control system. Ideally, this separation should be as great as is practically possible. Because hot smoke rises, consideration should be given to locating supply air intakes below such critical openings. A commonly used approach is to have all of the supply air intakes near the bottom of the building and smoke outlets above roof level. Another approach is to have the supply air intakes on one side of the building and the outlets on the other side and on the roof.
PRESSURE PROFILES
The pressure differences across a normally vary over the height of the Analysis of the pressure profiles of unpressurized shafts was presented in Chapter 5. The analysis of pressure differences in stairwells presented in this chapter is slightly more com- plicated in that pressurization is incorporated.
To facilitate analysis, the following discussion is limited to buildings that have the same leakage areas on each floor. Figure 10.6 shows pressure profiles for pres- surized stainvells located in three buildings
ent leakage characteristics, all of which have the same stairwell and outside temperatures. These profiles repre- sent winter conditions; that is, an outside temperature less than the inside temperature.
In a building without vertical leakage between floors or througli shafts other than the the pressure profile of a pressurized stain\-ell is a straight line. The slope of that straight line depends on the tem- perature difference between the stairwell and the outside and on the building leakage areas. This relation is dis- cussed later in this chapter.
Figure shows typical pressure profiles of pres- surized stainvells in a building with leakage between the floors in building without leakage tloors
are except at the top and the bottom of buildings. The extent of the deviation depends on the magnitude of leakage area between floors. The pres- sure depend on the leakage areas of the stair-
well, the and exterior as well as
tlie building, the and outside air. Analysis of a building is
and generally only the aid of a computer.
Figure Stairwell pressurization by propeller fan.
Top of Stairwell
Building
Between (Except at the ends.
this curve is the same as that for a
building without vertical leakage Building Vertical Leakage
P"
Building Vertical Between
Leakage Through
an Elevator
,
I
I , of StairwellPressure Difference
Figure 10.6 profile for
three buildings different leakage characteristics.
The pressure difference across a stairwell at one height can be much larger than at another height. There- fore, in addition to being concerned with the average pressure difference across a a designer should also be concerned with both the and the mum pressure differences.
STAIRWELL ANALYSIS
In this section, a method of analysis is presented for a pressurized stairwell in a building without vertical leakage floors. This is the same zero floor leak- age that was used for the analysis of stack The performance of pressurized
Principles of Smoke Management
stairwells in buildings without elevators may be closely approximated by the method of analysis developed in this section.
Neglecting the effects of leakage through floors and other shafts increases the spread between the minimum and maximum pressure differences. In this sense, the analysis is conservative. This analysis considers only one pressurized stairwell in a building; however, it can be extended to any number of stairwells by use of the concept of symmetry, as discussed in Chapter 6 . The ini- tial analysis does not include consideration of open stairwell doors, but they are addressed later in this chap- ter.
This analysis is for buildings where the leakage areas are the same for each floor of the building and where the only significant driving forces are the stair- well pressurization system and the temperature differ- ence between the indoors and outdoors.
Pressures
For many applications of pressurized stairwells, the vertical flows within the stair shaft are low. enough so that friction losses can be neglected. This is particularly true of the simple stairwell system, which has closed doors. Therefore, the absolute pressure in is considered hydrostatic and can be represented as
= - (10.1)
where
= absolute air pressure in stairwell at elevation in.
= absolute air pressure in stairwell at bot- tom, in. (Pa);
= air density within the stainvell,
y = elevation above bottom, (m);
= constant, (9.8).
For the case where the wind velocity is essentially zero, the outside air pressure, is also hydrostatic and can be expressed in the same manner.
where
absolute air pressure at elevation y, in. (Pa);
= absolute air pressure at stainvell bottom, in.
= air density outside the
The pressure difference from the to the outside can be expressed as = and substi- tuting Equations I) and ( l is
where
= pressure difference at elevation in.
= pressure difference at the bottom of the stair- well, in. (Pa).
The above analysis assumes no change in densities, and with elevation resulting in a slight
diction of pressure difference. The magnitude of this overprediction increases with elevation and, for a story building, the resulting error would be less than 4%. For purposes of this book, this overprediction is 'insignificant. By substituting the ideal gas law into Equation can be expressed as a function of temperature.
and where
b = temperature factor, in. (Palm);
= absolute temperature of outside air, (K);
= absolute temperature of stainvell air, (K);
K, = 7.64 (3460).
The effective flow area from the through building to the outside is expressed on a per floor basis as
= effective flow area behveen the stairwell and the outside, (m2);
= flow area behveen the stainvell and the building,
= flow area behveen the building and the outside, The areas in this equation are those of the entire floor. In such a case, the pressure difference, bcnveen the stairwell and the building can be expressed as
Chapter l 0
-
Stairwell PressurizationThe pressure differences and are related
as follows: = (10.14)
where
+ '
= volumetric flow rate of air to which can be rewritten as
cfm
Pressurization Air
For the case where a stairwell is positively pressur- ized throughout (i.e., the direction of air flow is from the stairwell to the outside the entire stairwell height), the flow from the stairwell to the outside can be written in differential form as
CA,, .
The term is the distributed effective flow area per unit height, which is uniform vertically. This distrib- uted flow area is expressed as
where
distributed effective flow area per unit
H = stairwell height, fi (m);
= number of floors.
Substituting Equations (10.4) and (10.1 into Equation gives
This can be integrated from = to = H give the total flow, from the stairwell to the building and to the outside:
flow area between the stainvell and the building per floor when stairwell doors are closed,
= number of floors;
= pressure difference between the stairwell and the building at the stairwell top when all the stairwell doors are closed, in. (Pa);
= pressure difference between the stairwell and the building at the stairwell bottom when all the stairwell doors are closed, in. (Pa);
Because there is no vertical flow in the building,
= This is the flow rate of supply air to the stairwell necessary to maintain the pressure differences,
at the stairwell bottom and at the top.
In a building with vertical air leakage, the exact evaluation of the system would require that the effect of three or more of air at different temperatures be included. Such an analysis is cumbersome and, for prac- tical purposes, a computer is needed. For this reason, the method of analysis presented in this section is based on a building without vertical leakage. In order to make this analysis conservative when applied to buildings with vertical leakage, the temperature is replaced by the building temperature. Thus, Equation (10.5) becomes
where ( l
= absolute temperature of outside air, "R (K);
= absolute temperature of air in the building, "R where is the difference between the stair-
well and the outside at the stainvell top = Because the is a linear function of as expressed in Equa-
tion Equation 0.13) can be in of For a building temperature of 70°F (2 and for pressure from the stairwell to the building. For C = 0.65, conditions, the temperature factor b can be
this becomes obtained from Figure 10.7.
Principles of Smoke Management
The average pressure difference can be defined as a pressure difference over the stairwell height that would result in the total flow as a nonuniforni
average pressure across the effective flow area, in.
density of air, 776
effective area can be either the area between
The subscripts SB and SO have been eliminated this equation because it is applicable to flow stairwell to either the building or the outside. When applying Equa- tions (10.16) and to flow from the stairwell to the
building, A, = = and When
applying these equations to flow from the stairwell to the
outside, A, = and Equa-
tion (l 0.17) can be approximated by
The maximum error in this relation is approxi- mately 6 % and occurs when = 0.