Regarding the issue of whether negative pressure will be generated during system operation
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A section of the process design for one project is as follows: The raw gas (biogas) coming from the gas holder enters at a pressure of 500 Pa; it passes through a desulfurization tower for dry desulfurization, and then goes through a Roots blower to have its pressure increased to 29.4 kPa. After the Roots blower, there are buffer tanks and other compression equipment. May I ask whether negative pressure will be generated in the pipeline ahead of the Roots blower while it is running? I believe that the feed gas comes from a 2000 m^3 gas tank; when the Roots blower operates, it starts to draw air from the gas tank into the system. Since the feed gas is continuously drawn from the gas tank into the system, no negative pressure is created in the pipelines ahead of the Roots blower or in the desulfurization tower. Is this explanation correct?
I can’t do the calculations, but in theory, the inlet pipeline of a Roots blower should be under negative pressure. The negative pressure generated by the Roots blower needs to be determined by taking into account the air supply pressure, flow rate, and pipe diameter of the air tank.
Why is there negative pressure in the inlet pipeline of the Roots blower? If the Roots blower is moved in front of the desulfurization tower, will there still be negative pressure at the inlet of the Roots blower? Thank you for your reply!
Negative pressure will also be generated. If the fan inlet is open, it might not be noticeable; but if it’s a pipe, the effect is obvious. Air is drawn in within a confined space, just as in the case of a centrifugal pump’s inlet. A centrifugal pump has a certain suction lift, and where there is suction lift, there is negative pressure.
It’s impossible to determine this intuitively, because there is resistance in the desulfurization tower and the pipes ahead of it; therefore, there will definitely be losses in pressure as it passes through those components. The Roots blower creates a vacuum at the inlet, and it’s not certain that the air supplied from ahead can compensate for that loss. Calculate or measure the resistance of the desulfurization tower and the pipes before and after it at the design flow rate; if this value is greater than 500 Pa, then there will be negative pressure in front of the fan
I think it depends on the amount of air that the fan can draw, the volume of air coming from the front, and whether the pipe diameter is sufficient.
Does this negative pressure at the inlet affect the normal operation of the Roots blower?
For rotary vane blowers, this means a higher pressure and greater power
The inlet pressure of the raw gas (biogas) to the gas holder is 500 Pa and remains essentially constant; whether it is a wet or dry gas holder, the pressure within the holder stays stable. Passing through a dry desulfurization tower, in extreme or faulty conditions such as the gate falling off, blockages occurring due to prolonged operation of the tower, or a vacuum forming at the inlet of the Roots blower. If the allowable negative pressure value of the desulfurization tower is not very high and is below the capacity of the Roots pump, the tower may be sucked flat. Under normal operating conditions, everything functions smoothly; the operation curve of the Roots blower should be similar to that of the water ring vacuum pump – as the vacuum level decreases, the volume of gas drawn in also decreases. As the vacuum level decreases, the pressure difference between the inlet and outlet of the tower increases; one trend is an increase in the ventilation volume ； However, as the flow rate of gas through the desulfurizer increases, the pressure drop also increases; the second approach is to reduce the flow rate. As long as the desulfurization bed is not overturned, the sum of these two trends, that is, dynamic balance, results in a moderate increase in the total flow rate. Achieve a balance!
For rotary vane blowers, when the inlet pressure decreases, the pressure increase at the outlet also decreases slightly!
If the pipeline is unobstructed, there should be no major problems