Temperature Measurement


Fire protection systems

December 2003 Temperature Measurement

Fenwal Control's Detect-A-Fire units play a central role in many fire protection systems. These devices have been something of a standard of the industry for over 45 years. Hundreds of thousands of these units are now in use controlling the release of fire suppression agents such as FM200, CO2, water, wet or dry chemicals and foam. In some systems the device is used as an alarm device, to sense overheating or fire. This alarm then alerts personnel. It is used as a release device in other systems, to sense fire and activates fire attack systems.

Some of the success of Detect-A-Fire units may be attributed to their rate compensation. This provides a unique advantage over both fixed temperature and rate-of-rise types of detectors, because the Detect-A-Fire unit accurately senses the surrounding air temperature regardless of the fire growth rate. The system is activated at the precise predetermined danger point.

Fixed temperature detectors must be completely heated to alarm temperature, and therefore a disastrous lag in time may occur with a fast rate fire. The rate-of-rise devices are triggered when there is an increase in ambient temperature and are subject to false alarms caused by harmless, transient thermal gradients such as the rush of warm air from process ovens.

The secret of this unit's sensitivity is in the design. The outer shell is made of a rapidly expanding alloy, which closely follows changes in surrounding air temperature. Designed to resist thermal energy absorption and sealed inside the shell, the struts, which are made of a lower expanding alloy, follow temperature changes more slowly.

A slow rate fire will heat the shell and struts together. At the 'set point' the unit will trigger, actuating the alarm or releasing the fire attack system, such as fire extinguishers. A transient rush of warm air causing a casing temperature increase rate of up to 72°C/min may expand the shell, but not enough to trigger the unit. By ignoring transient warm air excursions, the Detect-A-Fire unit virtually eliminates false alarms prevalent with rate-of-rise devices.

The Detect-A-Fire units are available in a vertical or horizontal format, and Temperature Controls is the sole agent in SA for Fenwal Controls.

For more information contact Andy Brown, Temperature Controls, 011 791 6000, [email protected]



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