Problems with starting the centrifugal main fan

For the main fan, the inlet valve is slightly opened before startup to primarily prevent vacuum formation in the unit. Keeping the outlet valve fully open prevents surging in the unit, thus avoiding overcurrent during startup.

An excessively large opening of the inlet will result in a high starting current, causing a trip

The fan controls the flow rate through the inlet valve

If the main fan inlet valve is set to fully open at startup, it will cause an overcurrent shutdown.

The inlet is kept slightly open to prevent a vacuum and avoid overloading, while the outlet is kept fully open to prevent backpressure

The difference between these two medium forms lies in the huge disparity in their compressibility coefficients. The compressibility coefficient of the liquid pump medium can be approximated as zero; therefore, when the outlet pressure reaches its design maximum, the force is zero, and the inlet pressure remains constant. Under these conditions, the current required to start the pump is at its minimum. The medium behavior of centrifugal air pumps is exactly the opposite to that of liquid pumps, with a very high compression coefficient. If the flow control is set to zero, the inlet or outlet valve must be closed. If the inlet is closed, an excessive vacuum level is created; when a certain level of centrifugal force is reached, gas must flow back in from the outlet to compensate, resulting in periodic fluctuations – this is known as \"surge\". If the inlet is fully open while the outlet is fully closed, pressure will surge sharply. When the pressure at the outlet exceeds the centrifugal force generated, the remaining pressurized gas at the outlet will expand towards the inlet. After this expansion, the centrifugal force becomes greater than the outlet pressure, leading to compression, further expansion, and reverse flow – thus creating terrifying periodic fluctuations; this is what is known as \"surge\". To minimize the starting current as much as possible (since, at the beginning of operation, the device must overcome mechanical and pressure-related resistances, which requires a large amount of energy in a short period of time), the most effective way to reduce this current is to lower the flow rate. Therefore, a compromise is adopted: the minimum opening degree at the inlet, which is sufficient to prevent \"surge,\" is considered the optimal value. It should be noted that a liquid pump operates as an energy conversion device; therefore, after it starts working, the flow rate needs to be increased promptly. Otherwise, this energy is converted into heat, which can cause the liquid to turn into gas. A phase change like this isn’t very desirable: lol (imitated language: lol, for reference only)

We have the inlet slightly open while the outlet is fully open. After starting, it was observed that the motor speed was very high; by increasing the inlet opening further, the current gradually decreased, otherwise there would be constant overloading.

Although I only understood it partially, I grasped the main ideas. Thank you

What should be done if another fan is started while one centrifugal fan is already in operation? (The outlets of the two centrifugal fans are connected to each other, with no isolation doors.) How can operation be carried out so as to prevent the newly started fan from rotating in reverse?

What should be done if another fan is started while one centrifugal fan is already in operation? (The outlets of the two centrifugal fans are connected to each other, with no isolation doors.) How can operation be carried out so as to prevent the newly started fan from rotating in reverse?