Pump Laws (Affinity Laws for Pumps) Explained

As HVAC professionals, understanding the fundamentals of pump laws, also known as affinity laws, is crucial for optimizing system performance and efficiency. These laws help predict how changes in pump operation affect flow, head, and power consumption, enabling better system design and troubleshooting.

What Are Affinity Laws?

The affinity laws describe the relationship between the speed of a pump and its operational characteristics when either the speed or impeller diameter changes. Let's break down these laws for both scenarios: constant impeller diameter and variable speed, and variable impeller diameter with a constant speed.

1. Constant Impeller Diameter with Variable Speed

• Flow: The flow rate of a pump varies directly with the speed change. For example, if the speed is doubled, the flow rate will also double.
• Head: The head generated by the pump varies as the square of the speed change. This means if the speed is increased by 20%, the head will increase by 44% (20% squared = 1.44).
• Power: The power required by the pump changes as the cube of the speed change. Therefore, a small increase in speed can lead to a significant rise in power consumption, emphasizing the need for careful speed management.

2. Variable Impeller Diameter with Constant Speed

• Flow: When changing the impeller diameter while maintaining a constant speed, the flow rate varies directly with the diameter change. A larger diameter leads to increased flow.
• Head: The head produced by the pump varies as the square of the diameter change. Thus, a slight increase in diameter can result in a considerable increase in head.
• Power: Similar to the speed change, power varies as the cube of the diameter change, highlighting the importance of selecting appropriate impeller sizes for efficiency.

Impact of Fluid Properties

In addition to pump speed and impeller size, it's essential to recognize that power also varies directly with the specific gravity or density changes of the fluid being pumped. As the density of the fluid increases, so does the power requirement for the pump, which is a key consideration in systems dealing with various fluids.

Conclusion

Understanding pump laws facilitates informed decisions regarding pump selection, system design, and operational adjustments, ultimately leading to enhanced efficiency and reduced costs. For HVAC professionals, mastering these principles ensures optimal performance in fluid handling systems.

By applying these affinity laws, you can effectively manage your HVAC systems to ensure energy efficiency and reliability.