Plunge Test Paper

25
Fire Safety Journal 39 (2004) 191–215 Establishing heat detectors’ thermal sensitivity index through bench-scale tests Soonil Nam a, *, Leo P. Donovan b , Jieon Grace Kim c a FM Global Research, 1151 Boston-Providence Turnpike, Norwood, MA 02062, USA b FM Global Engineering Hazards, 1151 Boston-Providence Turnpike, Norwood, MA 02062, USA c Summer Intern Student, Department of Mechanical Engineering, Worcester Polytechnic Institute, 100 Institute Road, Worcester, MA, 02062, USA Received 10 June 2003; received in revised form 18 September 2003; accepted 3 November 2003 Abstract The current study has established a means of quantifying heat detectors’ thermal response sensitivities through the use of plunge-tunnel tests, so that a fire safety engineer can estimate the detector response times with an acceptable accuracy. Eleven samples of fixed temperature- type detectors were chosen and the response times of the detector under varying test conditions were measured. An analyses of the collected data showed that response time index (RTI) provided the most consistent thermal sensitivity index throughout the different test conditions, among other possible candidates of the indices. In order to check the practical applicability of the RTI values assigned to each detector type, full-scale fire tests were conducted with heptane pan fires under a 3-m high ceiling. The matches between the measured and the estimated response times were excellent for all the detectors, except a ‘‘rate- compensated’’ high-temperature rating detector. Full-scale test results show that the RTI values, which were obtained through the bench-scale tests established with this study, will provide a very practical means of predicting detector-response times with an acceptable precision to fire safety engineers. r 2003 Elsevier Ltd. All rights reserved. Keywords: Heat detectors; Thermal sensitivity index; Prediction of response time ARTICLE IN PRESS *Corresponding author. Tel.: +1-781-255-4964; fax: +1-781-255-4024. E-mail address: [email protected] (S. Nam). 0379-7112/$ - see front matter r 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.firesaf.2003.11.001

Transcript of Plunge Test Paper

Page 1: Plunge Test Paper

Fire Safety Journal 39 (2004) 191–215

Establishing heat detectors’ thermal sensitivityindex through bench-scale tests

Soonil Nama,*, Leo P. Donovanb, Jieon Grace Kimc

a FM Global Research, 1151 Boston-Providence Turnpike, Norwood, MA 02062, USAb FM Global Engineering Hazards, 1151 Boston-Providence Turnpike, Norwood, MA 02062, USA

c Summer Intern Student, Department of Mechanical Engineering, Worcester Polytechnic Institute,

100 Institute Road, Worcester, MA, 02062, USA

Received 10 June 2003; received in revised form 18 September 2003; accepted 3 November 2003

Abstract

The current study has established a means of quantifying heat detectors’ thermal response

sensitivities through the use of plunge-tunnel tests, so that a fire safety engineer can estimate

the detector response times with an acceptable accuracy. Eleven samples of fixed temperature-

type detectors were chosen and the response times of the detector under varying test

conditions were measured. An analyses of the collected data showed that response time index

(RTI) provided the most consistent thermal sensitivity index throughout the different test

conditions, among other possible candidates of the indices. In order to check the practical

applicability of the RTI values assigned to each detector type, full-scale fire tests were

conducted with heptane pan fires under a 3-m high ceiling. The matches between the measured

and the estimated response times were excellent for all the detectors, except a ‘‘rate-

compensated’’ high-temperature rating detector. Full-scale test results show that the RTI

values, which were obtained through the bench-scale tests established with this study, will

provide a very practical means of predicting detector-response times with an acceptable

precision to fire safety engineers.

r 2003 Elsevier Ltd. All rights reserved.

Keywords: Heat detectors; Thermal sensitivity index; Prediction of response time

ARTICLE IN PRESS

*Corresponding author. Tel.: +1-781-255-4964; fax: +1-781-255-4024.

E-mail address: [email protected] (S. Nam).

0379-7112/$ - see front matter r 2003 Elsevier Ltd. All rights reserved.

doi:10.1016/j.firesaf.2003.11.001

Page 2: Plunge Test Paper

1. Introduction

It is essential for a fire safety engineer to have a reliable means of predicting heatdetector activation times associated with a fire growth rate, in order to design properfire protection systems. Our current knowledge of heat detectors, however, is solimited that no one can perform this task with an acceptable precision.

The set point temperature and the maximum coverage spacing, which are specifiedby listing organizations, currently characterize the performance of heat detectors.The listed spacing values can be used as a tool, following the procedure given inNFPA 72, to estimate a fire size at detector activation. However, the uncertaintiesassociated with this practice are too great for the estimated fire size to be used in anyreliable manner. The confusion created by the leading listing organizations, such asUL or FM Approvals, by assigning the independent spacing values that may notagree for the identical type of detectors, does not alleviate the situation.

These issues, among other concerns, prompted the NFPA 72 Committee tointroduce paragraph 2-2.1.3 of NFPA 72 [1]: ‘‘Heat-sensing fire detectors shall bemarked with their operating temperature and thermal response coefficienty . Therequirement for the marking of the thermal response coefficient shall have aneffective date of July 1, 2002.’’ As of today, the issue regarding the thermal-responsecoefficient (TRC) still remains unresolved.

The following two sub-tasks were performed in this study in order to answer theconcerns mentioned above:

(1) The first task was establishing a bench-scale test that would assign a detectorresponse sensitivity index for each fixed temperature heat detector. The theorybehind the test, as well as a method for practical implementation of the theory,was developed by utilizing the plunge-tunnel [2], which has been used to assignresponse time index (RTI) of automatic sprinklers. The RTI principles were firstapplied to predicting response of heat detectors by Heskestad and Delichatsios[3] in the late 1970s. Following the same principles, Bissell [4] measured the RTIvalues of several heat detectors in the late 1980s. However, somehow these earlycontributions had not been fully materialized and the RTI values in heatdetectors have not been adopted yet as a part of industry standards.

(2) The second task was conducting full-scale fire tests in order to validate themethodology developed in the first task.

2. Theory

The plunge tunnel tests used in this study rely on the following three basicassumptions: (1) the temperature at the heat sensing element of a heat detector israised solely by forced convection; thus, any contribution from radiation heattransfer is negligible; (2) any heat losses from the heat sensing element to connectingelements are negligible, i.e., conduction heat loss is negligible; (3) any latent heatassociated with the melting of eutectic metal in a heat detector, if there is any, isnegligible.

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215192

Page 3: Plunge Test Paper

Using these assumptions, the energy balance equation on the heat sensing elementbecomes

mcdTe

dt¼ hAðTg � TeÞ; ð1Þ

where m is the mass of the heat sensing element, c is the specific heat of the element,Te is the element temperature, t is the time, h is the convective heat transfercoefficient, A is the element surface area, and Tg is the surrounding hot airtemperature.

Eq. (1) is transformed to

dTe

dt¼

Tg � Te

tð2Þ

by introducing the time constant t � mc=hA . As the Nusselt number for a bluntbody involved in a cross flow convection heat transfer can generally be expressed asproportional to Ren, where Re is Reynolds number and n is a power index dependingon the shape of the blunt body. The time constant can be expressed as

t ¼ t0u0

u

� �n k0

k

� �nn0

� �n

; ð3Þ

where u is the velocity, k is the thermal conductivity, and v is the kinematic viscosityof the surrounding air. The subscript 0 represents a reference state. Then Eq. (2) canbe expressed as

dTe

dt¼

unC0ðT ; nÞTRC

ðTg � TeÞ: ð4Þ

Here TRC � t0un0 and C0ðT ; nÞ � ðk0=kÞðn=n0Þ

n; which is generally a weak functionof temperature. When nE0:5; C0ðT ; nÞ is very close to 1. Thus TRC becomesidentical to RTI, which is a sprinkler property describing the thermal sensitivity.

Like the RTI, the TRC is an index showing the sensitivity of a heat detector. TheTRC can be a function of (1) the shape of a heat detector’s sensing element and (2)unlike the RTI, the choice of reference temperature. In order to reduce any potentialinfluence of n and C0ðT ; nÞ on the determination of the TRC of each detector,the reference temperature should be close to the median point of the detector’soperating temperature range. The TRC values of the heat detectors can be obtainedin a manner similar to the way in which the RTI values are obtained for thesprinklers [2].

3. Plunge-tunnel tests to obtain TRC

3.1. Test sample preparation

Various types of fixed temperature heat detectors were chosen for test samples.Figs. 1a and b show the photos of the 11 types of the test samples mounted on thetest plates. They are denoted as Types A–K. Among them, seven types, Types A–G,

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215 193

Page 4: Plunge Test Paper

were restorable, the rest were non-restorable detectors. The rating temperatures ofthe detectors are given in parentheses as follows: A (57�C); B (77�C); C (57�C); D(93�C); E (88�C); F (71�C); G (88�C); H (57�C); I (90�C); J (57�C); K (93�C).

In order to minimize the possible heat conduction effects to each detector samplethrough the plates, marinite was chosen as the test plate material. Each mounteddetector sample was plunged into the tunnel, which is shown in Fig. 2, until thesample activated.

ARTICLE IN PRESS

Fig. 1. (a) Detector samples Type A through Type F used in the tests. (b) Detector samples Type G–Type

K used in the tests.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215194

Page 5: Plunge Test Paper

3.2. Test conditions

In order for the TRC values to be acceptable for practical applications, thefollowing characteristics are desirable:

(1) The measured TRC values should be applicable to a wide range of conditionsthat can be imposed by fire. Thus, tests under various temperatures and

ARTICLE IN PRESS

Fig. 1 (continued).

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 195

Page 6: Plunge Test Paper

velocities should be conducted and the TRC values obtained through theseconditions should be reasonably consistent.

(2) Once condition (1) is met, one test condition should be picked as arepresentative of all the test conditions. The ideal qualifications for the newlypicked condition are: (1) the TRC value measured under the condition should beclose to the average of the TRC values measured across the various testconditions, and (2) the reproducibility of the tests under the condition should beexcellent.

(3) Different test samples from an identical model should provide reasonablyconsistent TRC values. This condition (3), however, is more relevant with aquality control issue at a manufacturer’s production site than with the validityof a test method.

A multiple series of plunge tests were carried out to see if the test results wouldsatisfy all the above requirements. In order to check item (1) mentioned above, eightdifferent plunge test conditions were used as shown in Table 1. In order to minimizethe ratio of the radiational heat loss by a test sample to tunnel walls to the convectiveheat gain by the test sample through the air flow in the tunnel, the air temperatureswere set high, 197�C and 291�C. Each test sample was plunged into the hot airflowing through the tunnel and the time required for a detector sample to activatewas measured. Every sample was tested 6 times each under conditions 1–6, five timeseach under conditions 7 and 8.

ARTICLE IN PRESS

Fig. 2. The plunge tunnel used in the project to measure the detector response times. A sample is

immersed in hot air while the air flows from left to right in the figure. A test plate with an extruded silver

handle is seen next to the instrument panel.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215196

Page 7: Plunge Test Paper

3.3. Computing TRC using the test data

A fourth-order Runge–Kutta numerical scheme was adopted to solve Eq. (4) inconjunction with the response time and the initial detector temperature. The shapesof the thermal sensing part of the tested detector samples indicate that the most likelyvalue of n in Eq. (4) would be 0.5. However, n was varied between 0.4 and 0.6 inorder to see the influence of n on the final TRC values at each detector.

Table 2 shows how the different n values affect the TRC values of each detectorsample in the tests. %x is the average of the 46 TRC values obtained from the 46 teststhroughout the eight test conditions. s is the standard deviation of the 46 TRCvalues across the eight test conditions and s= %x is the standard deviation normalizedby the average TRC value. The degree of the scattering of the TRC values among thedifferent test conditions was judged by the normalized standard deviations. Notethat only restorable detector samples could be participated in the comparison.

Table 2 shows the following:

(1) The degree of the scattering of TRC values is lowest with n=0.5 for detectors Aand B; with n=0.4 for detectors C and D; and with n=0.6 for detectors E, F,and G.

(2) The degree of the scattering of the TRC values is substantially higher fordetectors C and D with n=0.6, compared with the degree of the scattering of theTRC values with n=0.4 or n=0.5.

ARTICLE IN PRESS

Table 1

Plunge test tunnel conditions

Condition

number

Air temperature (�C)/

air velocity (m/s)

Condition

number

Air temperature (�C)/

air velocity (m/s)

1 197/0.51 5 291/0.49

2 197/0.91 6 291/0.91

3 197/1.55 7 291/1.57

4 197/2.56 8 291/2.54

Table 2

Effects of n to the average of the TRC values across the eight plunge test conditions

Detector ID n=0.4 n=0.5 n=0.6

%x s s= %x %x s s= %x %x s s= %x

A 118 12.8 0.11 124 9.2 0.08 130 11.0 0.09

B 110 7.9 0.07 114 4.6 0.04 119 9.2 0.08

C 34 6.5 0.19 36 9.0 0.25 39 11.6 0.30

D 25 2.0 0.08 26 3.3 0.13 27 5.1 0.19

E 18 2.1 0.12 18 1.5 0.08 19 1.5 0.08

F 23 2.5 0.11 24 1.4 0.06 25 1.2 0.05

G 25 2.9 0.12 26 2.2 0.08 26 1.9 0.07

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 197

Page 8: Plunge Test Paper

(3) The degree of the scattering of the TRC values is substantially higher fordetectors E, F, and G when n=0.4 is used, compared with the degree of thescattering of the TRC values with n=0.5 or n=0.6.

(4) Although the degree of the scattering of the TRC values is lower for detectors E,F, and G with n=0.6 than the degree of the scattering of the TRC values withn=0.5, the difference in the degrees of the scatterings at each detector sample isinsignificant.

Thus, it was decided that the computation of the TRC values for all the detectorsto be conducted with n=0.5. Once n=0.5 was chosen for all the detector samples,Eq. (4) could be further simplified by assuming that C0ðT ; nÞ ¼ 1:0 . Then TRCvalues are the same as RTI values. Thus, the term RTI was used hereafter as athermal sensitivity index of heat detectors.

Figs. 3–9 show the average RTI value with error bars at each test condition andthe average RTI value across all the test conditions of test sample A–G. The errorbars correspond to the highest and the lowest RTI values measured under each testcondition. The figures indicate that the average of the RTI values of each detectorsample across all the test conditions seemed to be an acceptable representation of thedetector sample’s RTI value. The normalized standard deviations of all the testsamples, except detector samples C and D, were not greater than 8%. The RTIvalues of detectors C and D under the higher temperature with high velocities (i.e.,conditions 7 and 8) were considerably higher than their corresponding average RTI

ARTICLE IN PRESS

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

120

140

RT

I (m

.s)1

/2

Test Condition Number

Average RTI at the given test condition

Average RTI over all the test conditions

Fig. 3. Average RTI values of detector sample A.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215198

Page 9: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 6 7 8 90

20

40

60

80

100

120

140

Test Condition Number

Average RTI at the given test condition

Average RTI ovel all the test conditions

RT

I (m

.s)1

/2

Fig. 4. Average RTI values of detector sample B.

0 1 2 3 4 5 6 7 8 90

10

20

30

40

50

60

Test Condition Number

Average RTI at the given Test Condition

Average RTI over all the Test Conditions

RT

I (m

.s)1

/2

Fig. 5. Average RTI values of detector sample C.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 199

Page 10: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 6 7 8 90

10

20

30

40

Test Condition Number

Average RTI at the given test condition

Average RTI over all the test conditions

RT

I (m

.s)1

/2

Fig. 6. Average RTI values of detector sample D.

0 1 2 3 4 5 6 7 8 90

10

20

30

Test Condition Number

Average RTI at the given test condition

Average RTI over all the test conditions

RT

I (m

.s)1

/2

Fig. 7. Average RTI values of detector sample E.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215200

Page 11: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 6 7 8 90

10

20

30

Test Condition Number

Average RTI at the given test condition

Average RTI over all the test conditions

RT

I (m

.s)1

/2

Fig. 8. Average RTI values of detector sample F.

0 1 2 3 4 5 6 7 8 90

10

20

30

40

Test Condition Number

Average RTI at the given test condition

Average RTI over all the test conditions

RT

I (m

.s)1

/2

Fig. 9. Average RTI values of detector sample G.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 201

Page 12: Plunge Test Paper

values, which resulted in noticeably higher normalized standard deviations thanthose of the other detector samples. Although the case of detectors C and D calls formore study to find out the cause of the large deviation of RTI values under condition7 and 8, the overall average RTI values of detectors C and D were also deemedacceptable. The detectors are likely to respond well before the surroundingenvironments ever reach the conditions resembling condition 7 or 8, should a firebreak out.

The next step is finding a test condition among the eight conditions that wouldserve as the standard test condition. The RTI values of detector samples will bemeasured only under this condition and the average RTI value will be assigned as theRTI value of the given detector type. The ideal qualities of the condition are: (1) theRTI value measured under the condition should be close to the average of the RTIvalues measured under all the eight test conditions, and (2) the measured RTI valuesobtained through multiple tests under the condition should exhibit a low degree offluctuations, i.e., a good reproducibility of tests.

The test data showed that the best test condition that meets the abovequalifications varies depending on the test samples. Although there is no outstandingtest condition that can be equally applicable to all the samples, either condition 3 or4 seemed to be an equally acceptable choice. The test condition 3 was chosen for thestandard test condition for future tests because the detector response time under thecondition would be longer than that under condition 4. That would reduce themagnitude of a potential error associated with the time required to plunge the testsamples into the tunnel.

3.4. RTI measurements of multiple samples of the same model

For any practical usefulness, the RTI values should be reasonably consistentamong detectors of the same model. In order to assess the consistency of the RTIvalues among the detectors of the same model, seven sets of restorable detectors(Types A–G) and four sets of non-restorable detectors (Types H–K) were testedunder test condition 3. For the restorable detectors, 10 series of tests were conducted.

Figs. 10–16 show the average RTI value of each sample and the average RTI valueobtained through all the five samples for each restorable detector type, Types A–G.The average RTI value of each sample is the average of 10 RTI values measuredthrough the 10 tests and the overall average of the five samples is thus the average of50 RTI values.

The figures show that the normalized standard deviations associated with thedifferent samples of the same model are reasonably low except the samples fromTypes D and E. As this is an issue more relevant to quality control of the productsthan to the concept associated with RTI, the high degree of deviation seemed towarrant more caution when we need to test these products. Overall, the normalizedstandard deviations associated with the different samples of the same model arewithin the boundary established by the normalized standard deviations associatedwith a sample of the same model under the different test conditions. Thus, assigning

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215202

Page 13: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 60

20

40

60

80

100

120

140

Sample ID Number

Average RTI of each sample

Average RTI of all five samples

RT

I (m

.s)1

/2

Fig. 10. Average RTI values of detector samples A1–A5.

0 1 2 3 4 5 60

20

40

60

80

100

120

140

Sample ID Number

Average RTI of each sample

Average RTI of all five samples

RT

I (m

.s)1

/2

Fig. 11. Average RTI values of detector samples B1–B5.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 203

Page 14: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 60

10

20

30

Sample ID Number

Average RTI of each sample

Average RTI of all five samples

RT

I (m

.s)1

/2

Fig. 12. Average RTI values of detector samples C1–C5.

0 1 2 3 4 5 60

10

20

30

Sample ID Number

Average RTI of each sample

Average RTI of all five samples

RT

I (m

.s)1

/2

Fig. 13. Average RTI values of detector samples D1–D5.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215204

Page 15: Plunge Test Paper

ARTICLE IN PRESS

0 1 2 3 4 5 60

10

20

30

Sample ID Number

Average RTI of each sample

Average RTI of all five samples

RT

I (m

.s)1

/2

Fig. 14. Average RTI values of detector samples E1–E5.

0 1 2 3 4 5 60

10

20

30

Average RTI of each sample

Average RTI of all five samples

Sample ID Number

RT

I (m

.s)1

/2

Fig. 15. Average RTI values of detector samples F1–F5.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 205

Page 16: Plunge Test Paper

a RTI value to a detector based on the measurements with a number of selectedsamples under one test condition was deemed acceptable.

Multiple samples of non-restorable detectors were also tested under condition 3.Twenty samples each from Types H–K were mounted to the plates and plunged intothe tunnel. Some data points obtained with the samples of Types J and K weredeemed unreliable—the timer either did not trip at all or did take unreasonably along time before it tripped. In order to prevent the rest of data from beingcontaminated, those data points were discarded. Eighteen data points for Type Jdetectors and 17 data points for Type K were used in the analysis. Figs. 17–20 showthe RTI value of each sample of the non-restorable detectors used in the testprogram and the average RTI value of all the test samples per type. The figures showthat detector Types H and K have quite higher degrees of the standard deviationscompared with those of Types I and J.

4. Full-scale fire tests to measure detector response times

4.1. Test description

In order to make a comparison between the detector-response times estimated byusing the RTI values obtained through the bench-scale tests and the real responsetimes, full-scale fire tests were conducted. The detectors used in the tests were the

ARTICLE IN PRESS

0 1 2 3 4 5 60

10

20

30

Sample ID Number

Average RTI of each sample

Average RTI of all the samplesRT

I (m

.s)1

/2

Fig. 16. Average RTI values of detector samples G1–G5.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215206

Page 17: Plunge Test Paper

ARTICLE IN PRESS

0 5 10 15 200

10

20

30

40

50

Sample ID Number

RTI of each sample Average RTI of all the samples

RT

I (m

.s)1

/2

Fig. 17. RTI values of detector samples H1–H20.

0 5 10 15 200

10

20

30

Sample ID Number

RTI of each sample Average RTI of all the samples

RT

I (m

.s)1

/2

Fig. 18. Average RTI values of detector samples I1–I20.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 207

Page 18: Plunge Test Paper

ARTICLE IN PRESS

0 5 10 15 200

20

40

60

80

100

120

140

Sample ID Number

RTI of each sample

Average RTI of all the samples

RT

I (m

.s)1

/2

Fig. 19. Average RTI values of detector samples J1–J18.

0 2 4 6 8 10 12 14 16 180

20

40

60

80

100

120

140

160

180

200

Sample ID Number

RTI of each sample Average RTI of all the samples

RT

I (m

.s)1

/2

Fig. 20. Average RTI values of detector samples K1–K17.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215208

Page 19: Plunge Test Paper

same types of the detectors used in the plunge tests described earlier. The responsetime of each detector was estimated by solving Eq. (4). The TRC value in theequation was substituted with a RTI value, C0ðT ; nÞ ¼ 1 , and n=0.5 were used in thecomputations.

Seven restorable detectors and a sprinkler were mounted on the ceiling. They werearranged in such a way that each detector could maintain an approximately 3.4-mradial distance from the center-axis of the plume that was generated by a 0.76-m-diameter-heptane-pan fire, as shown in Fig. 21. The dimensions of the test room were18.3 m by 12.2 m by 4.6 m high. The detectors and the instruments attached on theceiling were: velocity probe 1, detectors A–G, velocity probe 2, and a sprinkler,respectively, from left to right in the figure, approximately 0.15 m apart each other.The pan was placed on a 1.6-m high platform, so that the ceiling clearance from thepan was 3.0 m. Temperatures and velocities of the ceiling jet flows from the fireplume were measured by the two velocity probes, which were equipped with pressuretransducers and thermocouples. Fig. 22 shows a comparison of the measured and thecomputed response time of each detector and the sprinkler, which is represented asType S in the figure. The comparison of the detector response times showed thefollowing:

(1) In general, the matches between the computed and the measured response timesof the detectors were excellent.

(2) The discrepancies between the measured and the computed response times weremore pronounced with the high temperature rating detectors, i.e., detector typesD, E, and G, than the discrepancies with the low-temperature rating ones.

ARTICLE IN PRESS

Fig. 21. The pan-fire and the detectors used in the full-scale tests.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 209

Page 20: Plunge Test Paper

(3) Detectors D and E took substantially longer times to activate than the timesestimated by the computations. Detector G, however, activated sooner than thecomputation indicated; it responded even before the ceiling jet temperaturereached the fixed rating temperature of the detector.

Additional tests with the identical settings also confirmed the same trendmentioned above, i.e., excellent matches except the cases with detectors D, E, and G.

Detectors F and G were ‘‘rate compensated’’ fixed temperature detectors. Theyrespond at a pre-set fixed temperature when the surrounding temperature risesrelatively slowly. However, when the surrounding temperature rises fast, they tend toactivate sooner than the surrounding temperature reaches the pre-set fixedtemperature. The temperature measurement indicated that detector F might haveactivated at the pre-set temperature. But it was clear that detector G was activated bythe rate of temperature rise rather than by the fixed temperature, which made theestimated time using the RTI based on a fixed temperature rating irrelevant.

In order to see the behavior of the high-temperature rating detectors more closely,the fire source was moved closer to the detectors. In the following tests, the center ofthe pan was located 3.0 m away in the radial direction and 3.0 m away in the verticaldirection from the detectors. Figs. 23 and 24 show, respectively, the ceiling jettemperatures and the ceiling jet velocities measured at velocity probes 1 and 2.Fig. 25 shows a comparison of the measured and the computed response time of eachdetector.

ARTICLE IN PRESS

A B C D E F G S0

20

40

60

80

100

120

140

Res

pons

e T

ime

(s)

Detector Type

Measured Computed

Fig. 22. Comparison of the measured and the computed detector response times where the test fire was

3.4 m away.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215210

Page 21: Plunge Test Paper

ARTICLE IN PRESS

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 15020

30

40

50

60

70

80

90

100

110

120

130

Cei

ling

Jet T

empe

ratu

re (

°C)

Velocity Probe 1 Velocity Probe 2

Time (s)

Fig. 23. Ceiling jet temperatures measured at velocity probes 1 and 2 (R=3.0 m).

0 10 20 30 40 50 60 70 80 90 100 110 120 130 140 1500.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Cei

ling

Jet V

eloc

ity (

m/s

)

Time (s)

Velocity Probe 1 Velocity Probe 2

Fig. 24. Ceiling jet velocities measured at velocity probes 1 and 2 (R=3.0 m).

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215 211

Page 22: Plunge Test Paper

The comparison showed the following:

(1) In general, the matches between the computed and the measured response timesof the detectors were excellent.

(2) Compared with the previous tests where the radial distance of the fire source was3.4 m away from the detectors, the match between the measured and thecomputed response time of detector D was exceptionally good. The match indetector E was also much improved compared with that in the previous tests.detector G again activated much sooner than the estimate indicated.

(3) Detector G’s activation long before the surrounding temperature reached thefixed rating temperature invalidated the time estimate based on the RTI that wasassigned for a fixed temperature rating detector.

Additional tests under the identical settings confirmed the same trend mentionedabove.

5. Summary and conclusions

A method to quantify heat detectors’ thermal response sensitivities that wouldenable fire safety engineers to estimate the response times of heat detectors withreliable accuracy was established by this work. Multiple samples from 11 detector

ARTICLE IN PRESS

A B C D E F G S0

20

40

60

80

100

120

140R

espo

nse

Tim

e (s

)

Detector Type

Measured Computed

Fig. 25. Comparison of the measured and the computed detector response times where the test fire was

3.0 m away.

S. Nam et al. / Fire Safety Journal 39 (2004) 191–215212

Page 23: Plunge Test Paper

types, all fixed temperature rating ones, were used in the study. The project wascarried out with two subtasks: bench-scale tests measuring thermal responsesensitivities of the samples and full-scale tests validating the methodology developedin the first task.

A plunge-tunnel similar to what has been used to measure response time index(RTI) of automatic sprinklers was utilized in the bench-scale tests. As a true thermalresponse sensitivity index of a detector should be independent of surroundingconditions that can be caused by different sets of fires, eight different plunge testconditions were used to measure the detector response times. The eight conditionsconsisted of two set temperatures, 197�C and 291�C, with four velocities, rangingfrom 0.5 to 2.5 m/s, at each set temperature. An ideal sensitivity index of eachdetector obtained through these measurements should be an invariant. Multiple testsacross the eight test conditions were conducted with one sample per each restorabledetector type.

Among the several candidates of the proposed sensitivity index, which differedmainly based on functional relationships between the Nusselt number and theReynolds number associated with the heating of the sensing elements of thedetectors, RTI turned out to be the most consistent value across the eight testconditions for most of the tested detector samples. Thus, RTI was chosen as thethermal response sensitivity index of heat detectors. In the second stage of the bench-scale tests, the RTI values of five samples from each restorable detector type weremeasured under one test condition (T=197�C , u=1.5 m/s), and then the averagevalue was assigned as the RTI value of the given detector type. For non-restorabledetectors, RTI of 20 samples/detector type were measured under the same testcondition.

Although there were various degrees of scattering of the RTI values per detectorsample across the eight test conditions, the RTI values of each sample werereasonably consistent. The scattering of RTI values of multiple samples of the samemodel measured under one condition were also within an acceptable range.However, the data indicated that the quality control among the different typesof the detectors was by no means uniform, and some detector types may requiremuch closer attention than the others when they come for the product approvaltesting.

In order to assess the usefulness of the measured RTI values in real fieldapplications, full-scale fire tests were conducted. Seven detector samples that wererandomly chosen from the seven types of the restorable detectors used in the bench-scale tests were installed on a ceiling in such a way that they can maintain the sameradial distance from a test fire source. A 0.76-m-diameter heptane pan fire under a3.0-m high ceiling was used as the fire source.

In the first set of tests, the center of the pan was placed a 3.4-m-radial distanceaway from the detectors. The response time of each detector was estimated byutilizing the measured temperatures and velocities of the ceiling jet in conjunctionwith the RTI value of the detector assigned through the bench-scale tests. Thecomputed response time of each detector was compared with the measured responsetime. The comparison showed excellent matches between the measured and the

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215 213

Page 24: Plunge Test Paper

computed response times for the low-temperature rating detector samples, but lessthan the excellent matches for the high-temperature rating ones. When tests wererepeated with the fire source closer to the detectors by maintaining the radialdistance of 3.0 m, the matches between the measured and the computed responsetimes for all the detector samples were excellent except for one detector sample, arate-compensated high-temperature-rating detector. The detector activated muchsooner than the surrounding temperature reached the fixed rating temperature. Itwas clear that the detector was activated not because of the sensing element havingreached the fixed rating temperature but because of another activation mechanismassociated with a rate of temperature rise, which invalidated the prediction using theRTI values based on the fixed-temperature activation.

With the exception of that particular detector, the full-scale test results showedthat the RTI values obtained through the bench-scale tests will provide fire safetyengineers a truly reliable means of predicting detector response times with anacceptable precision.

Some additional observations were as follows:

(1) When the shape of the detectors were relatively simple, such as that of detectortypes A, B, F, and G, the measured RTI values showed substantially lowerdegree of scattering per test conditions as well as per different detectorsamples of the same type, compared with the degree of scattering of RTIvalues of detectors with more complicated geometry, such as detector types C, D,and E.

(2) The relatively high degree of scattering of the measured RTI values fromdetector C, D, and E, as mentioned in item 1, could be related to (i) thesensitivity associated with the detector orientation with respect to the oncomingplunge-tunnel air flow, and or (ii) the boundary layer effect associated with thedetector frame. Detector Types C, D, and E all have frames, and some detectors’heat sensing parts were very close to the center cover of the frames.

(3) The measured RTI value of detectors in conjunction with the detectors’temperature ratings can provide fire safety engineers a valuable means to choosethe most appropriate type of detector at a given occupancy.

(4) The simple bench-scale tests introduced in this work can be utilized bymanufacturers as a tool for improving detector performance. The bench-scaletests also can be used as a tool for quality control of the products.

(5) The simple bench-scale tests can be used as a screening device to weed out someless desirable detectors.

6. Future work

The heat detectors investigated in this project were all fixed temperature ratingdetectors. Although the line-type detectors can be grouped as a fixed temperaturerating detector, they were not included in this study due to their unconventionalshapes. They should be analyzed in the next phase of work because they are beingused in many locations.

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215214

Page 25: Plunge Test Paper

A way of assigning the RTI values to the detectors that are activated by a rate oftemperature rise rather than a fixed rating temperature also should be developed inthe next phase of the work. A detailed study should be conducted for the rate-compensated fixed temperature detectors, too. Depending on the rate of temperaturerise, a set of dual RTI values can be used for the estimation of the response times.

References

[1] NFPA 72, National Fire Alarm Code, 1999 ed., National Fire Protection Association, 1 Batterymarch

Park, Quincy, MA, USA, 1999.

[2] Heskestad G, Bill RG. Quantification of thermal responsiveness of automatic sprinklers including

conduction effects. Fire Safety J 1988;14:113–25.

[3] Heskestad G, Delichatsios MA, Environments of fire detectors—phase I: effects of fire size, ceiling

height and material, vol. II—analysis. NBS-GRC-77-95, National Bureau of Standards, Gaithersburg,

MD, USA, 1977.

[4] Bissell WG, An investigation into the use of the factory mutual plunge tunnel and the resulting RTI for

fixed temperature fire detectors. Master thesis, Worcester Polytechnic Institute, Worcester, MA, USA,

1988.

ARTICLE IN PRESSS. Nam et al. / Fire Safety Journal 39 (2004) 191–215 215