The pipes of a VESDA smoke detection system are installed at the air intake of the air conditioner inside the computer room. Is this a good practice from an early smoke detection point of view?

This practice is not optimal for the earliest possible smoke detection. Relying solely on sampling at the air conditioner intake introduces a significant time delay, as smoke must travel from its source across the room to the air handling unit. This travel time is known as transport time. Furthermore, complex airflow patterns and high air exchange rates in data centers can lead to smoke dilution and bypass airflow, where a portion of the smoke does not get immediately drawn into the intake. This reduces the concentration of smoke reaching the detector, further delaying or even preventing an alarm, compromising the goal of very early warning.

A. The principles of early smoke detection are based on airflow dynamics and detector placement, not the type of suppression system that will be activated later.
B. While this approach may reduce installation costs, it compromises the primary objective of life safety and asset protection through the earliest possible detection of a fire event.
C. The air exhaust of an air conditioner supplies cooled air to the room; it is the incorrect location to sample for smoke originating within the data hall.

1. Xtralis (Honeywell). (2017). Application Note: Data Centres High-Tech & Telecom (Doc. No. 2819305).

Section 4.3, "Return Air Grille (RAG) Detection," states: "Sampling at the RAG of the ACU can provide a simple and cost-effective method of smoke detection for a protected zone. However, there are a number of factors that must be considered when using this method of detection. These include smoke dilution and transport time." This directly supports the reasoning that reaction time is longer and airflow issues (dilution/bypass) are a concern.

2. CENELEC. (2016). EN 50600-2-5: Information technology - Data centre facilities and infrastructures - Part 2-5: Security Systems.

Section 6.2.2.2, "Aspirating smoke detection (ASD)," discusses the challenges of detection in data centers due to high airflow. It notes that high air movement can "cause smoke to be diluted and dispersed (stratification)," which necessitates careful placement of sampling points. Relying only on the return air intake fails to account for this dispersion and dilution, leading to delayed detection, which contradicts the principles outlined in the standard for effective ASD systems.

3. Fahy, R. F., & LeBlanc, P. R. (Eds.). (2017). Fire Protection Handbook (21st ed.). National Fire Protection Association (NFPA).

Volume 2, Section 17, Chapter 2, "Detection I: Heat, Smoke," explains that for air-sampling smoke detectors, "The location of sampling points is critical to performance" and must consider airflow patterns to minimize transport time from the fire source to the detector. Placing sampling points only at a distant AC intake inherently maximizes this transport time for much of the protected space.