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How they work

Each closed-head sprinkler is held closed by either a heat-sensitive glass bulb (see below) or a two-part metal link held together with fusible alloy (soldering material). The glass bulb or link applies pressure to a pip cap which acts as a plug which prevents water from flowing until the ambient temperature around the sprinkler reaches the design activation temperature of the individual sprinkler. Because each sprinkler activates independently when the predetermined heat level is reached, the number of sprinklers that operate is limited to only those near the fire, thereby maximizing the available water pressure over the point of fire origin.

The bulb breaks as a result of the thermal expansion (change in volume) of the liquid inside the bulb. The time it takes before a bulb breaks is dependent on the temperature. Below the design temperature, it does not break, and above the design temperature, it takes less time for higher temperatures. The response time is expressed as a response time index (RTI), which typically has values between 35 and 250 m½s½, where a low value indicates a fast response. Under standard testing procedures (135 °C air at a velocity of 2.5 m/s), a 68 °C sprinkler bulb will break within 7 to 33 seconds, depending on the RTI. The RTI can also be specified in imperial units, where 1 ft½s½ is equivalent to 0.55 m½s½.

From Table NFPA13 2007 Edition indicates the maximum ceiling temperature, nominal operating temperature of the sprinkler, color of the bulb or link and the temperature classification.

Maximum Ceiling Temperature
Temperature Rating
Temperature ClassificationColor Code (with Fusible Link)
Glass Bulb Color
100°F / 38°C135-170°F / 57-77°COrdinaryUncolored or BlackOrange (135°F) or Red (155°F)
150°F / 66°C175-225°F / 79-107°CIntermediateWhiteYellow (175°F) or Green (200°F)
225°F / 107°C250-300°F / 121-149°CHighBlueBlue
300°F / 149°C325-375°F / 163-191°CExtra HighRedPurple
375°F / 191°C400-475°F / 204-246°CVery Extra HighGreenBlack
475°F / 246°C500-575°F / 260-302°CUltra HighOrangeBlack
625°F / 329°C650°F / 343°CUltra HighOrangeBlack

ESFR, or ESFR Sprinkler, denotes a special type of fire sprinkler. Early Supression Fast Response sprinkler heads were developed in the 1990s to take advantage of the latest fast-response fire sprinkler technology to provide fire suppression of specific high-challenge fire hazards. The sprinklers are specifically designed to fully suppress a fire that is within the design limits of the piping system. Prior to the introduction of these sprinklers, protection systems were designed to control fires until the arrival of the fire department.

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