Which component of a pressurization system prevents the cabin altitude from exceeding airplane altitude?

The function of a negative pressure relief valve in a pressurization system is to prevent the cabin altitude from exceeding the airplane altitude by allowing cabin air to escape when there is a risk of creating negative pressure. This is important for maintaining safe pressurization levels within the cabin and to avoid structural damage or situations where the cabin cannot be properly pressurized.

This valve specifically opens during instances where the pressure inside the cabin drops too low compared to the outside atmospheric pressure, allowing the cabin to stay in balance with the outside environment. It ensures that the pressure differential does not become excessive, which could lead to undesirable flight conditions.

The other components listed, while important in specific functions within the pressurization and air management system, do not directly regulate the cabin altitude in relation to the airplane altitude. The outflow valve, for instance, regulates the amount of air leaving the cabin but does not specifically act to prevent cabin pressure from falling below a certain level relative to outside pressure. The delivery air duct check valve functions to prevent backflow of air, and the thermostatic expansion valve manages the flow of refrigerant, neither of which involve the crucial role of maintaining cabin altitude limits.