Centrifugal fans connected in series: the effect of changes in one fan on the other
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As shown in the figure above, two centrifugal fans are connected in series, and the entire system operates under negative pressure. Question: When fan 1 remains unchanged while the speed of fan 2 increases: (1) How do the pressures at the inlet and outlet of fan 1, and at the inlet and outlet of fan 2 change? (2) How do the pressure differences of Fan 1 and Fan 2 (i.e., outlet–inlet pressure) change? (3) How does the system air volume change? In my opinion: (1) The inlet negative pressure (absolute value, the same below) of fan 2 increases, and the outlet pressure (which should be slightly positive) rises as well ； The negative pressure at the outlet of fan 1 increases, and the negative pressure at the inlet also increases ； (2) The pressure difference between the inlet and outlet of the fan is an indicator of the fan’s performance capacity. Therefore, the pressure difference of fan 2 should increase, while the pressure difference of fan 1 is unclear ； (3) The system air volume will increase because the drawing air volume increases (regardless of which drawing air volume it is).
For example, before the speed of fan 2 increases, the inlet pressure of fan 1 is -5000 Pa, and the outlet pressure is -200 Pa； The inlet pressure of fan 2 is -7000 Pa, and the outlet pressure is 100 Pa. So, when the speed of fan 2 increases, the inlet pressure of fan 2 might rise to -8000 Pa, while the outlet pressure could be 120 Pa. The outlet pressure of fan 1 might increase to -1000 Pa. Then, how will the inlet pressure change? Will the voltage rise of fan 1 increase or decrease?
Can centrifugal fans be connected in series? The operating conditions of both turbines have been changed; could there be problems in terms of mechanics, manufacturing processes, or safety? First, the suction pressure at the main inlet is below the design value; the inlet pressure of the second fan exceeds the design value, and the overall outlet pressure is above the design value. There may be a risk of leakage when in use. It’s hard to say whether the loads on the motors of the two devices are acceptable. Personal opinion, for reference only. It is noted that there are similar multi-stage fans, with both single-axis and multi-axis versions available. Similarly.
By adjusting the negative pressure in the furnace in this way, there won’t be very significant changes. If it were possible to increase the speed of fan 1 more significantly, furnace exhaust fans use the opening degree of the inlet valves to regulate flow. Fans 1 and 2 are connected in series; by adjusting fan 2, fan 1 functions as if it were an inlet valve, which limits the flow. It’s better to adjust fan 1 directly
It is certainly possible to use them in series, but two issues must be addressed, issues that you have overlooked: 1. The pressure difference has a fixed value (approximately)； 2. Temperature – crucial ； Any work done will cause the temperature to rise... When the inlet is under negative pressure, gas density also needs to be taken into account... So, look at the pressure difference! Subtracting the pressure difference from atmospheric pressure gives the maximum negative pressure that this fan can achieve… You need to understand that a fan is essentially a device for converting energy, turning potential energy into kinetic energy; however, due to the properties of gases, the conversion of kinetic energy back into potential energy is quite limited……
-------------------------------------------------------------------- How can a conclusion be drawn without defining the air volume and pressure difference capacity of each exhaust fan? 1. Fan 1 is large; Fan 2 operates in a series configuration ； 2. When the induced draft fan 1 is small and the induced draft fan 2 is large, it exhibits a series characteristic again ； 3. When the 1st and 2nd phases of the exhaust fan are the same, it is another characteristic ； How to define it?
Well, what you say makes sense. But I work in the cement industry. Sometimes, when fan 1 can no longer increase its output, the company will boost fan 2 to assist fan 1.
They can be connected in series; this is a practical issue. In actual production, there are cases where two fans, or even three fans, are connected in series. It’s just that each fan has to overcome a different level of resistance. You have thought this through very carefully; I just want to discuss the trend in these parameters without taking factors such as safety into account :)
1. The pressure difference has a fixed value (approximate). If the flow rate remains constant, the product of the pressure difference and the flow rate is the work done by the fan. But in this case, initially, the inlet pressure of fan 1 is -5000 Pa and the outlet pressure is -200 Pa; therefore, the pressure difference is 4800 Pa ； When the speed of fan 2 increases, the inlet pressure of fan 1 is unclear; the outlet pressure is -1000 Pa. If the pressure difference remains constant, then the inlet pressure of fan 1 should increase to -5800 Pa, right? But with all other conditions unchanged, if the pressure at the fan inlet increases, isn’t that because its resistance loss has increased? So the traffic has increased? 2. Agree
I’m sorry; I didn’t expect it to be this complicated. My understanding of fans isn’t very deep – I just wanted to discuss the trends in some parameters. Suppose: Fan 1: rated pressure of 7500 Pa, rated flow rate of 930,000, rated speed of 995 rpm. Fan 2: rated pressure of 9000 Pa, rated flow rate of 900,000, rated speed of 995 rpm
1. After fan 2 increases its speed, its characteristics change. When connected in series with fan 1, by combining the characteristic curves of the two fans, it is possible to determine the inlet pressure and flow rate of fan 1 under the operating conditions of fan 2； 2. In a series configuration, the inlet pressure of fan 1 cannot be calculated in this way; first, it is necessary to take into account that the air volume handled by fan 2 is greater than that of fan 1. At this point, it is required to determine the characteristic changes of fan 2, with its inlet pressure corresponding to the pressure at the time when the air volume flowing through it is equal to that of fan 1 ； 3. In simple terms, the pressure difference of fan 1 will not increase, but the actual operating pressure difference will decrease – this is reflected in a reduction in shaft power! 4. With the rotation speed remaining constant, the flow rate of fan 1 will not increase; what changes is the pressure, and as the pressure changes, the volume will naturally change as well.