The Risk of Underprotection—Understanding Application Density

What is “application density or concentration” and “design application density or concentration”?

You often hear these terms used when discussing fire suppression agents and systems. They are used broadly to address most agents and their various modes of application. For example, if you want to know how much aqueous film-forming foam (AFFF) you need to extinguish a fully involved 100-foot diameter storage tank using an “over the top” application with foam monitors, there is an application density formula for this exact scenario.

In short, application density or concentration is the amount of agent per unit of volume that will reliably extinguish fire. The term concentration is typically used in association with gaseous systems. In this discussion, we will be referring to application density as it applies to condensed aerosol fire suppression agents.

Understanding Fire Suppression Application Density and Design Concentration

The NFPA defines “extinguishing application density” as: “minimum mass of a specific aerosol-forming compound per cubic meter of enclosure volume required to extinguish fire involving a particular fuel under defined experimental conditions excluding any safety factor.[1]” It is important to note that this is the “minimum mass,” and the conditions are experimental. Therefore a “safety factor” is mentioned in the definition.

To account for unforeseen variables, additional agent over the minimum is required. For most special hazard systems, including condensed aerosol agents, the safety factor is 30% (X 1.3). When the 30% safety factor is added, this amount is known as the “design application density.”[2] This is the amount of agent that should be installed to protect a particular risk inclusive of a safety factor.

Who determines design application density?

Design application density determinations almost always start with research and development by the agent or system manufacturer. Through many live-fire trials, the manufacturer determines what they believe to be an effective application density.

Then testing follows at an independent testing organization such as Underwriters Laboratories® (UL). UL conducts a battery of tests, many of which include purposely obstructed live fire tests, to determine the viability of the extinguishing agent. If the agent is successful, it may receive UL listing for design application densities for various classes of fire under various circumstances.

Other organizations—such as Factory Mutual (FM) and VdS in Germany—also perform application density testing. The NFPA and other standards organizations often cite or adopt these findings as part of their standards. In the case of condensed aerosol agents, the prevailing standard is NFPA 2010: Standard for Fixed Aerosol Extinguishing Systems®.

Why is design application density important?

Design application density is the amount of condensed aerosol required to put a fire out and prevent reignition of the fire over a specified period of time. Should you use less agent, the risk of an unsuccessful extinguishment increases, the fire could cause additional damage, and the investment in fire protection will be money wasted.

So, it is important that you understand the design application density for the space and hazard you wish to protect. Your first stop should be the NFPA 2010 Standard. If you are considering a condensed aerosol system, it pays dividends to be familiar with this standard. It will help you enormously when working with authorized contractors and Authorities Having Jurisdiction (AHJ’s) involved with local permits and final installation approval.

A second source of information is the contractor themselves. All reputable installers should be able to provide extensive details about how they calculate design application density, along with third-party test results that substantiate their numbers. Any hesitation on the part of the contractor to produce this type of documentation should be a red flag. Some AHJ’s (for example FDNY) may have professional fire protection engineers on staff to verify contractor design calculations and can refuse final system approval if they deem the suppression system under designed. Professional contractors will be familiar with local fire, building and electrical code requirements as some states require installations involve state licensed fire protection contractors.

It is most unfortunate that some unscrupulous installers will attempt to sell an under-designed system to make their bid price more attractive. They may recommend smaller and/or fewer generators. They may also inform you that the 30% safety factor is not required for their agent or this specific application. These are high risk strategies that nullify the value of the fire protection system.

If the project specifies a large, protected space, the bid price for an underperforming system can be far less than one properly sized. The salesperson may provide assurances that you and their system will be fine. What they are banking on is that you will never have a fire and find out the truth. This way of doing business is highly unethical and bordering on criminal when considering the risks involved.

Therefore, you need to perform your own due diligence. It starts with arming yourself with the most information possible. If your contractor is a manufacturer’s authorized dealer/distributor (and they should be!), ask your dealer/distributor to provide a technical contact at the manufacturer. Ask the factory representative for additional information and contact references for other users of similar systems. Most fire suppression systems manufacturers offer training and certification programs for their authorized dealers/distributors and will issue factory certificates for successful completion of training classes.

The bottom line is that you should trust, but verify, what contractors tell you. If the information you are receiving does not look or feel right, it probably is not.

How is the design application density determined for my space?

In order to determine the design application density, several tasks need to be accomplished first. You will need to determine:

  1. How the area is used, and if floor, ceiling, and/or walls are fire-rated per code
  2. The hazard’s fire classification (e.g., Class A, Class B, Class C, etc.).
  3. The appropriate design density based on fire classification
  4. The leakage rate of the hazard enclosure.
  5. The geometric dimensions of the protected area: volume, total area, height. (Note: protected equipment volume is not deducted from the total volume.)
  6. If any large obstructions exist in the hazard.
  7. If additional agent will be required to compensate for leakage or obstructions.
  8. Provision for automatic closure of access doors, windows and room air exchange forced ventilation systems

Note: These tasks are equally applicable for both large spaces (e.g., rooms) or small enclosures (e.g., cabinets)

A reputable contractor will assist you in compiling this information. They should also be able to provide you with software output in which the data is entered and arrives at a final value for application design density.


For purposes of explaining the methodology behind these calculations, we will briefly describe the equations and formulas. The following example is specific to Stat-X® condensed aerosol fire suppression systems:

Required Mass of Aerosol Required for a Specific Volume

This is calculated using the following formula: M = K1 • K2 • K3 • V • q

M = the total mass of aerosol required to protect the hazard
K1 = a ratio based on the non-uniformity of aerosol distribution according to the height of the protected enclosure
K2 = the ratio based on the calculated leakage rate and leakage distribution for the protected volume
K3 = the ratio based on specific parameters for cable tunnels
V = the total volume of the protected area in cubic meters
q = the design application density of aerosol required to extinguish the hazard class

K-factors are quantity adjustment multipliers to increase agent quantity for tall, protected spaces, spaces with fixed openings, and special cable tunnel hazards.

Per UL testing (UL 2775), Stat-X has the following design application densities (these numbers include the 30% safety factor discussed earlier):
Class A (combustible material) hazard: 97 g/m3
Class B (flammable material) hazard: 55 g/m3
Class C (electrical) hazard: Use 97 g/m3 if the other hazards are Class A prevalent
Use 55 g/m3 if the other hazards are Class B prevalent

Calculation of Leakage Rate

To determine the leakage rate, use the following formula:

LR = leakage rate %
Σ ∶ Aopen = sum of the area of unclosed openings (windows, doors, etc.)
ATotal = total surface area of the bounding structure, including floor and ceiling

Note: The following equations are based on calculations in metric units

Determining Value K1

Formula value K1 is determined according to the height of the protected enclosure as follows:
When the leakage rate is ≤ 1%:
K1 = 1 when the height of the enclosure is ≤ 3.5 meters
K1 = up to 1.16 when the height of the enclosure is 3.51 – 5.0 meters
K1 = up to 1.26 when the height of the enclosure is 5.1 – 8.0 meters

When the leakage rate is 1% – 2%:

K1 = 1.00 when the height of the enclosure is ≤ 3.0 meters
K1 = up to 1.16 when the height of the enclosure is 3.1 – 4.5 meters
K1 = up to 1.26 when the height of the enclosure is 4.51 – 6.0 meters
When the leakage rate is >2%, factory assistance is required.

Determining Value K2

Formula Value K2 is determined by the relationship between the leakage parameter (LP) and the distribution of leakage in the protected enclosure (LH).
To determine the leakage parameter, use the following formula:

LP is a value, which characterizes the leakage of the protected enclosure as a ratio of the sum of the area of unclosed openings to the volume of the enclosure

To determine leakage distribution, use the following formula:

LH is a value expressed as a ratio of the area of permanent unclosed openings in the upper half of the protected enclosure (A upper) to the sum of the area of permanent unclosed openings

Determining Value K3

K3 = 1.5 for cable structures
K3 = 1.7 for cable structures where the longitudinal axis of the cable structure is situated at an angle > 45 degrees to the horizon (vertical, inclined cable collectors, tunnels, passages, and cable wells)
K3 = 1.0 for all other structures.

Calculate the Number of Aerosol Generators Required

To determine the number of aerosol generators required, use the following formula:

N = M/m
(when using generators of one size only)

N = the number of generators required
(If the value of N is fractional, it is rounded up to a whole number.)
M = the total mass of aerosol required
m = the mass charge of the individual aerosol generator
Stat-X units cannot provide fractional mass quantity. The selected final mass quantity will usually exceed the required mass of aerosol.
Again, it is important to note that reputable installers will provide an automated, software printout or screen that is used to determine these values. The formulas for hand calculations were provided here to highlight the mathematical methodology involved to provide precise determination of design application density[3].

What other considerations affect the design application density?

The above calculations are proven and accurate. However, at Stat-X, our experience in supplying over 700,000 generators has demonstrated that there are other factors that must be considered to assure superior, effective coverage and avoid problems during and following discharge.

Area Coverage Review

Each Stat-X generator has been tested and listed with a unique “footprint” for area coverage. Once the number of generators required to provide the necessary mass of aerosol has been determined, the area coverage of each unit selected must be evaluated to ensure the system falls within listed parameters. If not, additional units shall be provided to make certain that the final system configuration conforms to the Stat-X listing.

In potentially explosive atmospheres, Stat-X electrical “EX” generators are UL-listed for installation in these atypical applications. Prior to generator selection, verify the hazard classification of the enclosure. The activation of a non-listed generator in an explosive environment can lead to a detonation or deflagration.

Stat-X electrical “EX” generators are certified for use in UL Class I, Division 2 spaces[4]. This certification verifies that installation of a Stat-X electrical EX generator in the classified hazardous space will not precipitate ignition or detonation of a flammable atmosphere.

Excess Pressure

In general, few enclosures are completely tight and therefore excess pressure is not an issue at normal design application density. However, in sealed enclosures (LP=0), an evaluation of the structures ability to withstand temporary positive pressure buildup should be made, and it is recommended that louvered pressure venting be installed if deemed necessary.
Venting should be sized to provide an effective open area during discharge that calculates to an LP=0.001. If venting is added, the design calculation must be recomputed, including the vent open area, to ensure proper design density is established for the protected space.

Significant Obstructions/Agent Distribution

In cases where there is a large ratio of fixed equipment to total volume exists, or where the protected equipment is located in such a way as to present a barrier to the free flow and distribution of aerosol throughout the hazard area, the use of a larger number of smaller aerosol generators is preferred.

Aerosol units installed in underfloor or false ceiling spaces may have restricted height and necessitate the units to be mounted and oriented in the horizontal position.

This will allow for strategic placement of the aerosol generators and improved distribution characteristics throughout the area.

Total Flooding in Normally Occupied and Unoccupied Areas with Personnel Present

In total flooding installations for normally occupied and/or unoccupied spaces where personnel may be present, cross-zoned detection (requiring two different detection points and/or technologies to reach an alarm state, prior to initiating a system activation) and a 30-second time delay shall be included to allow personnel egress time prior to system discharge. In these areas, during maintenance activities, a system isolation switch shall also be installed outside the hazard area to ensure that activation of the system is “manual only” when maintenance personnel are present.

Shutdown of Air Handling and Power Supply

Upon detection of a fire, the ventilation system for the protected area must be shut down prior to discharge, to ensure the required application density is delivered and that the fire is not intensified by continuing airflow. In addition, electrical power to protected equipment must be shut down to minimize the potential of re-ignition from a live electrical source that initiated the electrical fire.

Mounting Considerations

Stat-X aerosol generators are listed for both sidewall and ceiling locations and may be mounted on any secure surface, provided that the unit:

In general, the aerosol generators should be mounted in rooms at or near ceiling height and angled to discharge down toward the floor at an angle to insure three-dimensional distribution of aerosol.

The aerosol generators also must be mounted in such a way as to have a clear discharge path and must not discharge onto walls or equipment as this may result in the aerosol particles collecting and sticking together (agglomeration), thus decreasing its effectiveness.

To ensure maximum distribution of aerosol throughout the hazard area, the maximum height of generator placement above the floor or between tiers must be limited. In facilities with walls higher than the maximum heights, the generators must be installed in tiers on multiple levels to assure complete coverage. Further, generators should be spaced as evenly as possible around the hazard area and directionally positioned to promote a circular, three-dimensional flow pattern.

Aerosol generators must never be positioned to discharge directly at each other. This can cause agglomeration of the aerosol particulate, reducing the aerosol’s fire extinguishing effectiveness. For the same reason, aerosol generators in total flood applications should also be positioned to ensure that the aerosol stream does not impinge directly on the walls or sides of the equipment being protected.


Condensed aerosol is a superior fire extinguishing agent that is simple to install and maintain. The system relies on producing an effective amount of aerosol to extinguish the fire. This is known as “design application density.” Failure to install the correct application density may result in the fire not being extinguished or potential spread of the fire.

It is therefore important to understand this requirement and perform due diligence to ensure your space is protected by an adequate mass of aerosol. In most instances, a reputable installer will be able to assist you in these calculations. However, you should always question a vendor who tries to assure you that a lesser design application density will suffice (and will also be less expensive). The vendor is attempting to sell underprotection.

At Stat-X and with our trained distributor partner network, we are completely transparent about system requirements and provide data that allow you to independently cross check our calculations. We cannot speak for other vendors, but at Stat-X we are 100% confident we will never recommend or install an undersized system protecting your assets.

[1] Source: NFPA 2010
[2] Source: NFPA 2010
[3] Source:
[4] Stat-X is also ATEX and IECEx certified by UL.
[5]Required clearance is .25” to 1” from combustible materials.

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