What is the difference between backflow and surge in axial flow main fans?

This post was last edited by wyt1234 on 2011-10-19 at 14:23. Backflow and surge both occur as a result of rotating stall. Surge is characterized by the operating point oscillating to varying degrees between two discontinuous characteristic curves following a sudden stall. Backflow occurs only when the volume of the piping network is large enough, or when the pressure in the network rises rapidly and remains high, causing the system to shift from positive to negative operating mode and resulting in a stable reverse flow; there are no oscillations as in the case of surging.

Reply to 2# wyt1234: What are the hazards of backflow and surge?

Reply to 3# 328104062: Check page 380 of \"Technical Q&A on Catalytic Cracking Units\"

So, isn’t this a better way to control backflow and surge in axial flow main fans?

I would like to ask everyone: is there an inevitable relationship between backflow and surge? Does surge necessarily lead to backflow, or does backflow not necessarily result in surge? For the main fan, which occurs first, surge or backflow?

During surge, the vibration frequency of the main fan matches its natural frequency, resulting in resonance. It causes mechanical vibration, which can lead to damage to the blades; in severe cases, it may even destroy the fan. During reverse flow, a large amount of primary air cannot be expelled; the work done by the primary fan is converted into the internal energy of the primary air, resulting in high temperatures in a short period of time, which damages the impeller……

Mild surge is still surge; severe surge will cause backflow

Surge usually occurs first; prolonged surge can lead to backflow in the unit. Generally, surge is caused by low inlet flow, high outlet pressure, and clogging of the inlet filter

What you mentioned focuses on practical aspects of operation, but theoretically, can it be said that backflow occurs when the system pressure – usually an abrupt increase in the pressure within the regeneration system – becomes higher than the discharge pressure at the outlet of the main fan? As a result, the gas flows in reverse direction. If the backflow prevention valves and check valves of the main fan do not close in time, gas will flow back into the operating main fan. The main hazard of backflow is that the regeneration system contains large amounts of catalyst; when backflow occurs, these solid catalysts can strike the impeller, causing damage to it； Surge is different; since the piping system has a certain volume, the gas pressure within it cannot drop rapidly. As a result, the pressure in the piping system can end up being higher than the outlet pressure of the main fan, causing the gas in the piping system to flow back toward the main fan. This forces the gas inside the main fan to be pushed out through its inlet, until supply is restored to the piping system, after which the flow rate through the main fan increases again. However, when the pressure in the pipeline network rises rapidly and the flow rate decreases, reverse flow of air occurs within the system. This back-and-forth between forward flow and reverse flow leads to periodic, low-frequency, large-amplitude axial air flow oscillations throughout the system, a phenomenon known as surge.