Final answer:
Hand expansion valves, also known as needle valves, are used in engineering for precise flow control in systems such as refrigeration or hydraulic systems. They are manually operated and adjust flow by changing the opening size as the needle moves relative to the seat.
Step-by-step explanation:
Hand expansion valves, also known as needle valves, are mechanical devices used in engineering to regulate the flow of fluid within systems, such as refrigeration or hydraulic systems. They function by having a sharp needle-like plunger fitting into a conical seat, which when turned, allows for precise control over the flow rate. Due to the fine control they offer, hand expansion valves are commonly used where precise flow regulation is necessary, for instance, to control the level of superheat in a refrigeration cycle.
They are typically manually operated and offer a very high level of control over the flow. When the valve is turned, the needle moves closer to or away from the seat, which changes the size of the opening and thus controls the flow of fluid. They are not designed for simple on/off control, but rather for modulating the flow and adjusting it to meet specific system requirements.