4 replies to this topic

[A mukherjee]

Hi everyone,
In our plant we are producing gaseous nitrogen at 1.2 barg pressure.Gaseous nitrogen is compressed by screw compressor to 7.65 barg pressure.
Now screw compressor is positive displacement kind of machine.Thus the volemetric flow rate delivered by it will remain almost constant.Hence if the suction pressure goes high the power consumption will be lesser,due to low pressure differential.
Thus my question is why vendor has provided trip of compressor at high high suction pressure.

Regards,
Arpan

[A mukherjee]

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[Art Montemayor]

Arpan:

You are forgetting about the simple gas law that can be applied to the suction nitrogen: the higher the pressure (assuming a constant suction temperature), the higher the gas density. Therefore, for a given volumetric flow rate (which a PD machine, such as your screw compressor, puts out) the amount of MASS rate through the compressor is increased.

If the mass rate increases, the work done is increased. If the work done is increased, the amount of horsepower required is increased – and the electrical amperage is increased. All of this has the capacity to overload the electric motor electrically and burn it out, unless you either stop it or reduce the mass flow rate to the machine. It is very difficult to reduce the mass flow rate to a PD machine UNLESS you have capacity controls – such as a sliding vane capacity control on your screw compressor. If you don’t (which many cheap – or “low capital cost” machines are) then you can only resort to shutting down the compressor rather than risk burning out the motor.

I hope this explains what you are confronting.

[Jiten_process]

Dear Art

Your answer is very clear cut. But i have a doubt, with increase in pressure ofcourse mass rate increases as u explained but on contrary compression ratio is also decreases (outlet pressure - inlet pressure) that means compressor have to increase pressure lessen than the earlier it was, isnt this also reduces the power consumption. I assume we have to do the simulation study and will have to check the overall effect. Am i right ?

Secondly all PD compressor has a volumetric efficiency which corresponds to volumetric capacity of compressor. I guess with increase in compresor suction pressure, internal leakages will descrease to some extent which will increase the volumetric efficiency and so the volumetric capacity. That means with increase in pressure, volumetric capacity of the same compressor will be increased to some extent. Is my understanding correct ?

[Art Montemayor]

If your compression ratio is decreased while your mass rate is increased, then another variable comes into play. If you are compressing pure nitrogen, and you can obtain a Mollier Diagram or thermodynamic tables for it, you can see if the enthalpy difference is increased under the new conditions. (you can use the free NIST thermo database for this) This will directly and quickly give you a relative idea of the new power requirements. In my opinion, accurate enthalpy differences (assuming adiabatic, isentropic compression) yield more accurate horsepower requirements than simulators using analytical equations.

Internal leakages don’t necessarily decrease due to increased suction pressure, in my opinion. This depends on other factors – such as the compression ratio. Within a reasonable design capacity range, you can usually assume the volumetric efficiency to remain essentially constant. This can be verified or confirmed with the machine’s manufacturer.