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|Issue 73: Effects of Radiant Heat Flux on Clean Agent Performance for Class-C Fires|
By Romil Patel & James A. Milke, P.E., Ph.D., FSFPE
Data server room and telecommunication facility fires can have significant financial impact.1
Interruption of service can erase permanent and temporary memory due to
the physical loss of memory storage devices, costing companies
millions. Average downtime costs for computing infrastructures are
estimated at $42,000 per hour.2
Current fire protection standards provide guidance for protection
hazards based on class, e.g., Class A, B, C, D and K. However, for some
classes of hazards, the guidance is not supported by performance data.
The lack of performance data may lead to over or under-design of fire
Class C Fires
In standards such as NFPA 20013 and ISO 14520,4 concentrations of extinguishing agents for Class C fires are based on concentrations for other classes of fires. The use of the Class A or B concentrations for the protection of Class C fuels is done because of the lack of test data to provide the technical basis for minimum extinguishing concentrations to suppress energy-augmented electrical fires. However, applying a multiple of Class A or B extinguishing concentrations for Class C fires is questionable as there is no data to support the relationship between either of these two classes of fire with Class C fires.
Energy augmentation refers to the effects of external heat flux on
the behavior of a fire. Blade server power densities has been a concern
in the fire protection industry because radiant heating can enhance
energized equipment fires, as demonstrated from studies conducted by
Linteris5 and Steckler et al.6
The total heat dissipated by energized electrical equipment is
related to the power consumption of the equipment. Hence, an increase
in power consumption, such as that associated with the trend of using
higher performing, smaller blade servers to efficiently maximize today’s
technology needs, will cause an increase in heat dissipation to nearby
Blade server equipment found in data processing facilities are made
of units (CPUs), chipsets, storage devices, memory, voltage regulators
and power suppliers. Since CPUs consume the majority of all power
supplied, ASHRAE sought out to determine the trends in power consumption
by CPUs in blade servers. Figure 1 shows the trends in CPU power
consumption by year 2020 for four processor types; high power and low
power high-performance computing (HPC) and two-socket and four-socket
Similarly, in 2010, Ponemon Institute conducted a survey of 453
individuals who have responsibility for data center operations in the
U.S.8 One of the questions asked for predictions of power
density (in kW) per rack in data centers in two years. Poll results from
the respondents showed an average value of 11.4 kW per rack power
Several assumptions were made to determine the radiant heat flux
levels emitted from the blade servers. First, it is assumed all racks
hold 42, 1U blade servers. The "U” term refers to 1.75 in (44.5 mm)
vertical height within a rack between blades. Since most blade server
fires originate in a single component, the radiant heat flux augmenting
that fire is from the surrounding two blade servers. Two
horizontally-stacked blade servers are shown in Figure 2 with a cable
running between them.
With power consumption per rack of blade servers expected to be 11.4
kW by year 2012, power rack densities of 10 kW and 20 kW are used in
calculating radiant heat flux emitted by each blade server. Radiant heat
flux emitted by blade servers is calculated from equation 1:
The radiant heat flux emitted by blade servers of various surface areas and power densities is presented in Table 1.
Table 1: Radiant Heat Flux Emitted by Blade Server in respect to Surface Area and Power Density
The Radiant Enhanced Extinguishing Device (REED) apparatus was
developed to determine the relationship between extinguishing
concentrations and radiant heat flux. The design of the REED apparatus
takes features from both the cone calorimeter and cup-burner tests. The
apparatus provides a radiant heat flux from coils at the top of the
device and has similar design characteristics to a cup-burner test,
allowing clean agent to be introduced to the fire at a laminar flow
Experiments were conducted on clean agents, IG-100, IG-55, IG-541,
HFC-125, HFC-227ea and FK-5-1-12. Testing results for all the agents are
presented in Figures 3 and 4.
Figure 3 shows results for clean agents tested from the 0-40 kW/m2
range. In the results, the agent concentration increased at a more
rapid rate with increasing heat fluxes. Beginning at 10 kW/m2, the curves start to plateau. This trend is similar to results seen from Steckler et al.6
Figure 4 shows results for clean agents, IG-100, IG-55 and IG-541. Results were conducted at the 0-5 kW/m2 heat flux levels. At these lower heat flux levels, the plateau effect seen from higher heat flux levels are not seen.
Due to the actual method clean agents are released into the fire hazard and the effects of flame being weakened by the REED testing method, it was important to develop an alternate testing protocol.
Dubbed the "Impulsive Agent Release" (IAR) method, the testing
procedures were modified. The proposed method reduces the 100 second
time delay between increases in agent concentration used in the REED
apparatus to two seconds. This method was conceived to eliminate the
process of the flame being weakened before it succumbs to
Two tests were conducted using the reduced time between agent
increases at each specified heat flux level. The resulting extinguishing
concentrations were averaged. The average extinguishing concentration
was then verified by running a test with the IAR technique. With the IAR
method, clean agent is immediately supplied to the REED apparatus at
the predefined flow rate based on extinguishing concentrations
determined from the two second delay test. The idea was to introduce the
clean agent to the flame at a known concentration.
A comparison of the IAR testing results versus the original method is shown in Figure 5. It is evident that extinguishing concentrations in the new IAR testing method exceed the original method by approximately 30 to 45 percent. Also, the curves for the inert gas clean agents have a more curved behavior which plateaus at 2 kW/m2. In the original testing method, the curves didn’t start to plateau until roughly 10 kW/m2. It was also noticed that extinguishing concentrations of clean agent HFC-227ea were greater than HFC-125 in the IAR testing method. In contrast, the original testing method resulted in HFC-125 having greater extinguishing concentrations than HFC-227ea.
Romil Patel is with Siemens Industry, James A. Milke is with the University of Maryland
The authors would like to acknowledge the generous support of 3M in supporting the work described in this article.
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