Smoke Detectors In Ducts

Written by Igor Neplohov, expert at System Sensor

Translated from Russian by Alexandra Igna (2019)

Almost all office and industrial premises are equipped with forced-air and exhaust ventilation systems or more complex automatic heating systems, veins air conditioning and filtration with humidification options. Exhaust ventilation air intakes are usually located on the ceiling of a room, i.e. in the area where the smoke tends to accumulate in an early stage of a fire. In some instances, the cooling system creates air flows that generally exclude the possibility of smoke accumulation underneath.

This triggers the need for smoke control in the ducts. A smoke detector mounted on the air duct often provides earlier fire detection than one installed anywhere else inside the room, since most of the smoke enters the hood without accumulating in the upper part of the room. In addition, fire in the duct is possible due to ventilation equipment failure, which is especially dangerous both at industrial facilities and in large shopping and entertainment centers, cinemas, theaters, sports facilities, etc. Duct smoke detectors provide a specific type of protection that cannot be duplicated by any other type of system.

Typical Scenarios

Duct smoke detection is useful to prevent threats such as personal hazards and property damage in the scenarios that will be listed below.

1. The HVAC fan motor is heated and the resulting smoke is detected by the smoke detector installed in the main air supply duct. At the signal of this detector, the supply voltage of the fan motor can be stopped immediately, before a significant amount of smoke reaches habitable areas.

The fire starts on the second floor of the building. Heating, ventilation and air conditioning are found from the first to the fourth floor, therefore, the smoke lines extend to all four floors. To detect smoke in this case, smoke detectors must be installed in the air ducts of the recirculation system on each floor, in front of the main recirculation chamber. When the amount of smoke in the pipe eventually increases to a concentration that is enough to activate the smoke detector on the second floor, a signal will be generated at the reception control device of a building's fire alarm system.

Of course, smoke pipe detectors do not eliminate the need to protect each room with regular firefighter tools. Duct smoke detectors can only detect a fire in which the smoke circulates in the piping system, although ventilation may not operate non-stop, but only during working hours. A ventilation system can be disconnected for repair or maintenance, as well as temporarily by turning off the supply voltage. In addition, smoke detectors cannot control large volumes of air that come from the extended area, therefore their effectiveness in this case may be poorer. In addition, dust-clogged air filters can restrict the flow of air, and this will affect the way smoke detectors work in the ducts.

Application Methods

These are determined by the national design standards of the countries. Smoke detectors used in air distribution systems must be located as follows:

downstream of the air filters and in front of any supply point branches in the air supply system with a capacity of over 944 liters per second;
on each floor, at the connection point of the common line of the recirculation air heating systems or fresh air recirculation points system with a capacity of over 7080 liters per second and serving more than one floor.

Change in Smoke Concentration

It is important to consider changing the smoke concentration when diluted with clean smoke air from other parts of the duct recirculation system. Simple math shows that the optical specificity of smoke density expressed in %/M decreases in proportion to the volume of smoke and clean air. For example, if the system is recycled, there are four branches and each receives an equal amount of air (cubic meters per minute). After connecting the specific option, the smoke density will be reduced fourfold.

In this case, a uniform distribution of smoke particles is ensured, which is an ideal situation. In reality, the concentration of smoke can vary from zero to very high in the transverse profile of the pipe. E.g, in the air recirculation pipe, delamination can be expected at the bottom right after each ventilation grille of the recirculation system. It is believed that an almost uniform distribution of smoke concentrations is possible at distances equal to 10 dimensions of the width of the air-water, downstream, in a channel with turbulent air movement. Some standards allow this limit to be reduced to 6 times the width of the ditch., but special conditions are required for 10 sizes across the entire width of the pipe.

Typical System Heating, Ventilation and Condition Rations

The conditioning section is usually a heating element such as a cooling element, a humidifier or any combination of the previous elements. These items can be installed in series, in parallel or they can be combined as part of a serial or parallel installation.

Smoke Detection In The Ventilation System

Heating, ventilation and air conditioning systems supply air in almost all sections of the building. Smoke detectors installed in the ducts should also cover the entire building and they should be located in the most suitable places for installation and maintenance in the main air downstream of the filter section so that they can automatically turn off the supply fans. Powerful ventilation systems require additional detectors for the recirculation system on each floor, exactly where they enter the general section of the recirculation or provide complete control of the system.

System Design

Smoke detectors in the recirculation system must be located at each air inlet of the recirculation system within the smoke chamber, where smoke exits from each smoke chamber, or in the air ducts, to the entry point at the recirculation system that communicates with several compartments.

As with conventional smoke detectors, high level of humidity or condensation inside the duct can cause false alarms. Smoke detectors placed inside the ducts must be located at a distance that is equal to ten dimensions across the width of the duct, away from the humidifiers, when looking downstream the airflow.

Source:

tinko.ru