What is the primary function of check valves or non-return valves?

The primary function of check valves, also known as non-return valves, is to allow for the passage of fluid in one direction only. This is essential in many systems, including refrigeration and plumbing, as it prevents the backflow of fluid, which can cause damage to pumps and disrupt the operation of the system. By permitting flow in a single direction, check valves help to maintain the desired flow path and pressure levels, ensuring system efficiency and reliability.

Other options may seem relevant, but they do not capture the primary function of check valves. For instance, controlling flow rate pertains more to valves that can be adjusted to regulate the amount of fluid passing through, rather than merely preventing backflow. Preventing backflow in high-pressure conditions is certainly a secondary benefit of a check valve, but the core function is to restrict fluid movement to one direction. Similarly, increasing fluid velocity is not a role of check valves, as they function primarily to manage flow direction, rather than affecting the speed or velocity of the fluid.