Blades of a fast centrifugal pump impeller are typically backward facing.

Explanation:

Centrifugal pumps are widely used in various industries to move fluids from one place to another. The impeller plays a crucial role in the pump's operation as it helps to increase the fluid's velocity and pressure as it passes through the pump.

In a centrifugal pump, the impeller consists of blades or vanes that are mounted on a central shaft. These blades are designed to rotate rapidly, creating a centrifugal force that propels the fluid outwards.

The blades of a centrifugal pump impeller can be categorized into different types based on their orientation and shape. The options given in the question are propeller type, radially spreading, forward facing, and backward facing.

- Propeller type: This type of impeller is typically used in low-speed applications where a large volume of fluid needs to be moved. The blades of a propeller-type impeller are shaped like a propeller or a fan, and they rotate in the same direction as the fluid flow.

- Radially spreading: This type of impeller is designed to spread the fluid radially as it flows through the pump. The blades of a radially spreading impeller are curved or twisted, which helps to increase the fluid's velocity and pressure as it flows through the pump.

- Forward facing: This type of impeller is designed to push the fluid in the forward direction. The blades of a forward-facing impeller are angled in such a way that they direct the fluid towards the outlet of the pump.

- Backward facing: This is the correct answer. The blades of a backward-facing impeller are angled in the opposite direction to the rotation of the impeller. This design allows the impeller to generate a high velocity and pressure by trapping the fluid between the blades and the pump casing. The backward-facing blades also help to minimize the possibility of cavitation, which is the formation of vapor bubbles in the fluid due to low pressure.

Overall, the backward-facing blades are the most commonly used design for fast centrifugal pump impellers as they provide efficient fluid movement and minimize the risk of cavitation.