How Thermal Glass Bulbs Work: Why They’re the "Heart" of Fire Sprinklers
Nov 20, 2025
As the core component of automatic sprinkler systems, thermal glass bulbs may be small, but they directly determine whether a sprinkler activates accurately in the early stages of a fire. They’re like the "heart" of a sprinkler—detecting danger and triggering action, serving as the critical "sensing organ" for fire safety. Today, we’ll break down how thermal glass bulbs work, their key role, and the design logic behind them.
1. First, What Is a Thermal Glass Bulb?
A thermal glass bulb is a "temperature-sensitive triggering device," typically composed of a thin-walled glass bulb, thermal expansion liquid, and sealed caps at both ends. Installed at the sprinkler’s water outlet, it serves two key functions: blocking water flow and sensing temperature.
Material: Special thin-walled glass with high pressure resistance but sensitivity to thermal shock.
Internal liquid: Usually kerosene, ether, or a mixed solution, chosen for its high thermal expansion coefficient.
Size: Common diameter 3-5mm, length 10-15mm, compatible with most sprinkler models.
2. How It Works: 3-Step Reaction from "Standby" to "Activation"
The core logic of a thermal glass bulb relies on "liquid expansion breaking the bulb." The entire process is automatic, no electricity required, and happens in 3 simple stages:
Standby State: At room temperature, the intact thermal glass bulb presses against the sprinkler’s seal flap, tightly blocking pressurized water in the pipe. The sprinkler remains "closed and ready" with no water leakage.
Temperature Sensing: When a fire breaks out, the ambient temperature rises rapidly. Heat transfers to the thermal liquid inside the bulb, causing it to expand quickly and exert increasing pressure on the bulb’s inner wall.
Crack & Activation: Once the temperature reaches the bulb’s preset activation threshold (e.g., 68℃ for common red bulbs), the expanded liquid’s pressure exceeds the glass’s strength, and the bulb cracks instantly. With the bulb broken, the seal flap is pushed open by water pressure, and water sprays evenly through the sprinkler’s deflector to extinguish the fire.
3. Key Design: Color-Coded Temperatures for Different Scenarios
A major advantage of thermal glass bulbs is "precision temperature control," with colors indicating specific activation temperatures. Choosing the right color ensures the sprinkler activates at the correct moment for your environment:
Color
Activation Temp
Application Scenarios
Red
68℃
Offices, residences, malls (most common for daily use)
Yellow
79℃
Kitchens, boiler rooms (spaces with naturally higher ambient temperatures)
Green
93℃
Industrial workshops, high-temperature warehouses
Blue
141℃
Metallurgical workshops, furnace areas (extreme heat environments)
Purple
182℃
Chemical reactors, high-temperature equipment zones
4. Why It’s the "Heart" of a Sprinkler?
Core Sensing Function: Without a thermal glass bulb, sprinklers can’t detect fires automatically. It acts like a "heart’s sensor," capturing temperature abnormalities—the first critical step to triggering fire suppression.
Reliable Automatic Response: No electricity, sensors, or manual operation needed. It works purely through physics, responding in seconds with an extremely low failure rate, standing by 24/7.
Safety Redundancy: Once broken, the bulb can’t reset—ensuring continuous water flow until the fire is controlled. Only the affected sprinkler activates, avoiding unnecessary water waste in unaffected areas.
Cost-Effective Practicality: Simple structure and low manufacturing costs, yet delivering "precision activation + reliable fire suppression"—it’s the most widely used triggering component in global automatic sprinkler systems.
5. Selection & Maintenance: Extend the "Heart’s" Lifespan
To keep your sprinkler systems working reliably, follow these tips for selecting and maintaining thermal glass bulbs:
Match to Scenarios: Choose red (68℃) for general commercial or residential spaces; opt for yellow (79℃+) for high-temperature environments to prevent false activation or failure to trigger.
Prevent Physical Damage: Avoid hitting or squeezing the bulb during installation or maintenance—this can cause premature cracking or seal failure.
Regular Inspections: Check bulbs quarterly. Replace them if you notice cracks, dirt buildup, or faded colors (recommended maximum replacement cycle: 5 years).
Proper Storage: Store unused bulbs in a cool, dry place—avoid high temperatures, humidity, or severe vibration to maintain their performance.
Conclusion: Small Bulb, Big Safety Impact
Though tiny, thermal glass bulbs are the "core hub" of automatic sprinkler systems. Using simple yet effective physics, they create a closed loop of "temperature sensing → automatic activation → precise fire suppression," acting as invisible guardians of building fire safety. Choosing high-quality thermal glass bulbs means equipping your sprinklers with a "healthy heart"—ensuring rapid response and effective fire control when disaster strikes.
If you need help selecting thermal glass bulbs for specific scenarios or want to learn about full sprinkler configurations, contact us! As a fire equipment manufacturer with 30+ years of experience, we offer a complete range of high-quality sprinklers and core components to safeguard your fire safety.
