Screw compressor suction volume and discharge volume
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This post was last edited by skyhhl on 2016-5-26 at 10:04. The current project has a steam screw compressor; the model designed by the manufacturer requires a steam supply of 2–3 t/h to the compressor, yet the output volume of the screw compressor is only 1.3–2.2 t/h. Approximately 30% of the steam has to be discharged externally to prevent overpressure at the inlet. Does the screw compressor have such a characteristic? Is the mass of the gas drawn in by the screw compressor different from the mass of the gas discharged?
Isn’t mass conserved? It’s just that the volumetric flow rate decreases, right?
This post was last edited by skyhhl on 2016-5-27 08:29. There’s no doubt about this (my last statement was incorrect); what the manufacturer means is that only 70 wt% of the gas that enters the compressor exits it, while the remaining 30 wt% accumulates at the inlet. In other words, the screw compressor can handle only 70 wt% of the incoming gas volume, with the rest accumulating at the inlet. Do you agree with the manufacturer’s view?
It seems like a strange problem – why would a compressor be designed in this way? And what about steam compressors, water vapor compressors? Haven’t heard of it? Or some other medium. Even if the inlet pressure fluctuates, there’s no need to choose this approach; controlling the inlet flow rate should work, right? It seems the author could avoid including their own doubts when presenting the problem, and instead clarify the background of the problem as well as the relevant design details.
It is a water vapor compressor; the compressor controls the outlet pressure via frequency conversion, raising the pressure at its outlet to around 150 kPa before it is fed into the system’s low-pressure steam network
Law of conservation of mass is a law that must be obeyed. I suspect the issue you’re referring to is that screw compressors often have delivery efficiency issues: assuming a theoretical intake volume of 100 Nm3/min, only 70 Nm3/min can actually be delivered, as there are gaps resulting from the manufacturing precision between the screws, and steam leaks from the outlet side toward the inlet side along these gaps (with pressure acting as the driving force, causing flow from high pressure to low pressure). But it’s not necessary to worry about these; the manufacturer doesn’t necessarily have to tell you anything. In any case, they need to meet your actual compression requirements.
Thank you for your reply. Yes, there is a difference between the theoretical displacement and the actual displacement during the delivery process of screw compressors. However, manufacturers should design the compressors based on the actual displacement in order to meet the needs of users.
Water vapor gets compressed and turns into water, right?
Screw compressors are rarely used in domestic MVR systems; it’s not that they are unsuitable, but rather the precision of domestic manufacturing processes is not sufficient. Screw compressors require a higher level of precision, which leads to the situation you mentioned – it’s a matter of work efficiency.
It’s probably to ensure an adequate amount of steam at the inlet, to have a margin
Is the poster using a screw type or a Roots type? But whether it’s screw compressors or Roots compressors, I think there are some issues with the designs provided by the manufacturers. If they claim to ensure that pressure doesn’t exceed safe levels, then either the internal design of the compressor isn’t suitable for higher pressures, or the motor power assigned to it is insufficient. Additionally, it’s crucial to determine whether their design can meet your actual usage requirements. The compressor has leaks, but the manufacturer should consider this issue carefully on its own. Are there any specific process parameters or something? We can discuss that.