Describe the different types of fire alarm initiating devices, including their operating principles and applications.

Fire alarm initiating devices are components of a fire alarm system that detect the presence of a fire and initiate an alarm signal. There are several types of initiating devices, each with its own operating principle and application. Smoke detectors detect the presence of smoke, which is an early indicator of fire. There are two main types of smoke detectors: ionization smoke detectors and photoelectric smoke detectors. Ionization smoke detectors use a small amount of radioactive material to ionize the air in a sensing chamber. Smoke particles entering the chamber disrupt the ionization process, triggering an alarm. Photoelectric smoke detectors use a light source and a photosensor. Smoke particles entering the chamber scatter the light, causing it to be detected by the photosensor, triggering an alarm. Heat detectors detect the presence of heat, which is another indicator of fire. There are two main types of heat detectors: fixed-temperature heat detectors and rate-of-rise heat detectors. Fixed-temperature heat detectors activate when the temperature reaches a predetermined level. Rate-of-rise heat detectors activate when the temperature increases rapidly over a short period of time. Manual pull stations allow occupants to manually initiate a fire alarm signal. These devices are typically located near exits and are activated by pulling a lever or breaking a glass. Flame detectors detect the presence of flames, which are a direct indicator of fire. Flame detectors use sensors that are sensitive to the infrared or ultraviolet radiation emitted by flames. Duct smoke detectors are installed in HVAC ducts to detect smoke circulating through the building's ventilation system. These detectors can shut down the HVAC system to prevent the spread of smoke. Each type of initiating device is selected based on the specific hazards and requirements of the building. NFPA 72 provides guidelines for the selection and placement of initiating devices.