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The essential component to prevent backflow in a steam system with an automatic feeder is a backflow preventer. This device is specifically designed to stop fluid from flowing backward through the system, ensuring that water does not flow back into the potable water supply, which could cause contamination and other issues. Backflow prevention is crucial in maintaining the safety and efficiency of the steam system, especially when automatic feeders are involved, as these systems require a direct feed of water to maintain proper levels and pressure.

While other components like pressure gauges, expansion tanks, and siphon filters serve important functions in steam systems, they do not specifically address the issue of backflow. A pressure gauge monitors the pressure within the system but cannot prevent reverse flow. An expansion tank is used for accommodating thermal expansion of water within the system, and a siphon filter may help in filtering but does not provide any mechanism to prevent backflow. Thus, the backflow preventer is the critical component for this function.