Using a hybrid design approach to stairwell pressurization
While the design process for a stairwell pressurization system should account for many building specific variables and is anything but simple, the system used to achieve the performance criteria doesn’t have to be complicated
- Identify some of the variables that can impact the design and performance of stairwell pressurization systems.
- Understand the pros and cons of different smoke pressurization system configurations.
- Become familiar with the benefits of designing systems for commissionability and reliability with reduced lifetime maintenance requirements.
While advances in building system technologies have provided opportunities for smarter stairwell pressurization systems, a simplified design approach can deliver a more reliable solution. A smoke pressurization system using tried-and-true design principles coupled with elements of new technology can create a building life safety system capable of easily being calibrated during commissioning and require minimal long-term maintenance, thereby providing dependable performance long after system acceptance.
Of the many design considerations and features addressed in high-rise and tower construction, the ability for occupants to safely egress during a fire event with minimal exposure to harmful combustion products is paramount. In these types of buildings, the time required for the occupants to travel from the higher floors to the exit discharge may be substantial and drive the need for additional safety precautions.
In the terms of building egress, these protected smokeproof enclosures are considered building exits. Once an occupant has entered the stairwell, even if on the 40th floor, the occupant is considered as having exited the building.
When selected by the designer, the code allows for a few alternatives to these requirements. Alternatives include: natural ventilation alternative in 2015 IBC Section 909.20.3, mechanical ventilation alternative in 2015 IBC Section 909.20.4 and stairway and ramp pressurization alternative in 2015 IBC section 909.20.5. This article will discuss design approaches for one of these alternatives, which mitigates the infiltration of smoke via positive pressurization of the stairwell enclosure.
NFPA codes and standards
In jurisdictions where NFPA 101: Life Safety Code is adopted, such as across select federal agencies, requirements for smokeproof enclosures are also provided. Per NFPA 101, smokeproof enclosures are required in airport traffic control towers and in high-rise buildings. In these scenarios, NFPA 92: Standard for Smoke Control Systems becomes applicable when the designer elects to provide enclosure pressurization as the means of maintaining the smokeproof enclosure.
Pressurization of the smokeproof enclosure maintains a tenable environment for occupants during evacuation by creating a pressure differential across the enclosure boundary. This restricts the migration of smoke from the fire event into the stairwell, allowing occupants additional time to safely travel through the protected enclosure.
Stairwell pressurization analytical process
In its simplest form, a stair pressurization system uses a supply fan to blow outside air into a stairwell, creating a positive pressure differential across the enclosure boundary. However, the design of these systems can depend greatly on the codes and standards used in design. Per IBC Section 909.20.5, the differential pressure limitation between the stairwell and the egressing level when all stairway doors are closed and when the building is under maximum stack effect and wind effect conditions shall not be less than 0.10 inch water gauge or greater than 0.35 inch water gauge.
When NFPA 92 is used, the stairwell pressurization system design criteria requires specific minimum pressure differential limits based on building design factors including ceiling heights, and whether automatic sprinklers are provided. Further, the maximum pressure differential limit is determined as required to maintain the smokeproof enclosure door opening forces to less than 30 pounds-force, the maximum permitted by NFPA 101. Additionally, NFPA 92 requires that this criterion be achieved with both zero and the design number of doors open during system operation.
There are challenges associated with achieving these requirements. The primary challenge is determining the correct volumetric flow rate of the supply fan, without knowing all the static and dynamic leakage conditions that may be present in the stairwell during an emergency. If the pressure differential across the enclosure boundary is too low, smoke may begin to infiltrate the stairwell. If the pressure differential across the enclosure boundary is too high, the egress doors into the stairwell may be pressurized closed, making it difficult for egressing occupants to open the doors.
During an engineering analysis of the stair pressurization system, the system designer should consider many building conditions including: shaft construction type, normal and reverse stack effect, wind and climate effects, interior and exterior walls, openings between floors, the anticipated number of stairwell doors open during egress, system commissioning and long-term system maintenance, among other factors.
As mentioned above, the designer should consider how the occupants will egress the building to identify the number of open doors assumed in the system design. The designer should not only ask how many doors will be open simultaneously during egress, but which doors. Will it be a door on the fire floor and one level above, or maybe just the door at the level of exit discharge? Do we design around one, two or even three doors open?
Additionally, the designer should consider if the doors open during evacuation should be considered partially or fully open. During evacuation of a moderately occupied floor, will the average free area of the door opening be only one-third that of a fully open door? Does the designer consider the occupants egressing through the door as an obstruction to the airflow?
While some of the doors may be considered open or partially open, the designer should also evaluate and manage the effects of the system when all the doors are closed. These door variables alone create an ever-changing dynamic effect on the airflow requirements to meet the performance criteria.
Another variable often forgotten is pressurized stairwells in a small footprint building, such as an air traffic control tower. In these cases, the designer should evaluate if the leakage from the floor plate is enough, or if exhaust off each floor is required to maintain the required differential pressurization over a several minutes of system operation. In some cases, small floor plates or tight construction, such as windowless floors, can create conditions where the floor itself becomes overpressurized during extensive system operation and the differential pressure across the stairwell enclosure can no longer be maintained.
Arguably one of the more important factors the designer should consider is how the system will be commissioned to demonstrate system performance and pass final acceptance. The designer should bear in mind the potential variables that may shift during construction, resulting in impacts to the final acceptance test. Additionally, any variables may affect the required maintenance over the life of the system.
Early system design approach
Traditionally, a supply fan would pressurize the stairwell enclosure and any excess pressure would be relieved through leakage paths in the shaft enclosure or a barometric relief damper. These barometric relief dampers are often found near the top of the shaft and are used to maintain the pressure differential across the enclosure below the specified limit. Earlier stairwell pressurization systems typically included a constant volume supply fan to pressurize the enclosure and a weighted barometric relief damper to relieve the excess pressure.
While a relatively simple and robust system, this approach sometimes ran into pitfalls during installation and commissioning. When these types of systems were designed, much of the information available on leakage rates through various wall, floor and door assemblies was not readily available. This resulted in the overdesign of supply fans to account for the degree of uncertainty in the design, often leading to potential headaches during commissioning.
With a constant volume fan, changes in airflow were limited during commissioning by the available fan and motor sheave selection. Adjusting fan and motor sheaves often would allow for the system performance to be close to what it needed to be, but not necessarily provide the exact airflow required to achieve the performance criteria. If other unforeseen conditions arose, such as a very leaky shaft enclosure, a larger fan may have been required, resulting in a larger power supply than planned, causing a potential domino effect of construction phase issues.
Modern system design approach
Over the past few decades, the introduction of empirical leakage rate data for engineering analysis and design, along with variable frequency drive fans, have allowed for more modern design approaches. Using VFD fans in a system design introduces the ability to modulate the airflow into the shaft based on changing conditions through differential pressure sensors located throughout the enclosure.
In some cases, VFD fans have eliminated the need for a barometric relief damper to prevent overpressurization within the shaft enclosure. This technology allows the system to increase and decrease airflow as the leakage rates in the shaft enclosure change over time. However, this design approach is not without its weaknesses.
When used in building heating, ventilation and air conditioning applications, pressure sensor-controlled systems increase the efficiency of the systems. When these pressure sensors fail or fall out of calibration in HVAC applications, it is quickly noticed and reported by uncomfortable building occupants and the problem is corrected within a reasonable timeframe.
However, in a stairwell pressurization system, where the system ideally never operates outside of system testing, the performance of the system and impact of a failed or out-of-calibration sensor is seldom identified. As a result, the fans may not perform during an emergency as originally designed, resulting in under- or overpressurized stairwell enclosures.
Another weakness of modulating airflow with a VFD fan is the impact the centrifugal fan blade inertia will have on the pressure within the stairwell enclosure. As an example, when a door is closed in the stairwell enclosure the system sends a signal to reduce the airflow of the supply fan. This change is not instantaneous, due to the inertial energy of the spinning fan blade, thereby causing excessive airflow and momentarily overpressurizing in the stairwell enclosure. This overpressurization may result in egressing occupants not being able to open the doors into stairwell, and in restricting egress until the pressure in the stairwell is reduced.
Hybrid system design approach
Considering the advances in empirical leakage rate data, industry research, system technology and the potential pitfalls highlighted in the early and modern approaches above, a hybrid approach of traditional methods and new technologies may present an optimal stairwell pressurization solution. The required supply and relief airflow for a dynamic system can be determined during design with a fair amount of precision when using modern modeling packages such as CONTAM by NIST.
With these airflows identified, specification of a VFD supply fan and a weighted barometric relief damper can accomplish the design criteria with a desirable level of precision. The difference with a VFD fan in this approach is the VFD fan would be tuned to a specific frequency during commissioning to provide a constant, specific flow rate into the shaft enclosure. The weights on the barometric relief damper would then also be configured during commissioning to relieve excess pressure in the shaft enclosure and maintain the pressure differential within the allowable limitations.
Table 1: This shows a summary of the pros and cons of each stairwell pressurization system design configuration. Courtesy: Burns & McDonnell[/caption]
While the design process for a stairwell pressurization system should account for many building specific variables and be anything but simple, the system used to achieve the performance criteria in the enclosure doesn’t have to be complicated. A stairwell pressurization system designer should bear in mind that in addition to designing to meet the performance criteria, the system should deliver the required functionality with a high level of flexibility, reliability and low long-term maintenance.