Estimating the Response of Thermal Detectors

Abstract

The paper considers the prediction of time to operation of sprinklers, as well as the response of heat detectors to changing fire conditions. The discussion concentrates on devices having a heat-sensitive element which operates at a preset temperature, though rate-of-rise devices are also mentioned.If a thermal device is heated slowly and is not subject to heat losses, (so that it is close to being in thermal equilibrium with its surroundings), it will operate at a gas temperature equal to its set temperature. This is alternatively known as the actuation temperature or nominal operating temperature, Tnom. If the device is heated rapidly, its temperature will lag behind that of the surroundings. The faster the rate of change of ambient temperature, the greater the time lag will be. This arises because the device has a thermal capacity and cannot respond instantly to changes in the surroundings. The quantity which is used to characterise the time lag is known as the thermal time constant r. For an isolated body temperature T,, subject only to convective heat transfer from a hot gas flow at

Variability of sprinkler response time index and conduction factor using the plunge test

“…The radiation effect in the early stages of a growing fire is generally considered to be small because the configuration factor in the radiation calculation is small and also smoky gases can block out some of the radiation received by the sprinklers. 11 There will be some radiation heat transfer between the ceiling and the sprinkler but Beever 12 notes that this radiative component is usually small compared with the convective component although Sako and Hasemi 13 suggest that radiation can be significant when a sprinkler is relatively close to a fire or for rapidly growing fires.…”

“…14 These temperature and velocity magnitudes are generally reproducible in a wind tunnel, thus sprinklers can be tested under realistic simulated environments. 12 Tests such as the plunge test 2,5 and the ramp test 11 were developed to determine the sensitivities of sprinklers by simulating fire conditions in a wind tunnel.…”

Review of residential sprinkler systems:

“…17,18 Important contributing research was also performed at the British Fire Research Station and the National Institute of Standards and Technology (NIST). [19][20][21][22] The progress in this area climaxed late in 1990, when an agreement was reached within the working group on sprinkler and water spray equipment of the International Standards Organization (ISO) for a standardized approach to sprinkler sensitivity requirements and testing. The agreement, included in ISO 6182/1, "Requirements and Methods of Test for Sprinklers," uses a combination of sprinkler test procedures developed by laboratories in the United States and Europe and establishes the three ranges of sprinkler sensitivity characteristics shown in Figure 5.…”