Impact to frequent load/unload cycles on rotary screw air compressors?

[YungPlantEng]

Have two 100 HP rotary screw air compressors that modulate via loading/unloading. Last compressor analysis indicated very frequent load/unload cycles (maybe once a minute per compressor). This is because we don’t regulate pressure after the compressors and various processes use a lot of it.

Am I underrating the potential issues this causes? We deal with significant oil carryover but I wasn’t sure if the switch between loading/unloading would really impact that.

 

[goutam_freelance]

Do you have one or more adequately sized air receivers? If so, what is the capacity in minutes of flow?

 

[YungPlantEng]

1.5 minutes of max flow. I don’t think they’re inadequately sized but since the pressure isn’t reduced from compressor pressure we’re very susceptible to high use periods.

 

[LittleInch]

So what is your dead band on the compressors?

Can you increase it?

 

[pierreick]

Hi,
Consider these resources.
Pierre

 

[goutam_freelance]

1.5 minutes of max flow

But I feel this is low. If possible try to increase to 3-5 mins.

 

[georgeverghese]

You may have to install backpressure regulators on some of the high transient usages if these users can manage with a reduced supply. Backpressure regulator setpoint at each of these high users should be a little lower than the LOAD setting for the compressors.

 

[SB1976]

Rotary air compressors operating in 1 operation + 1 standby configuration employs a load/unload valve being connected in parallel at the suction piping side. They help deal with frequent load/unload without adversely impacting compressor or motor from frequent start/stop.

Remember motors are designed for a fixed number of starts per hour.

 

[YungPlantEng]

I am not talking about turning the motor on and off. I am talking about the action of loading & unloading itself (switching of the unload valve at the suction) and whether the frequency of that can contribute to issues like oil carry-over, etc. Gut says yes because I've seen cases where it swaps from unload to load and then the temperature rapidly increases before stabilizing.

 

[georgeverghese]

Oil carryover in compressed air is unavoidable in oil flooded machines, but becomes excessive due to corrosion of carbon steel internal components of the onboard oil-air separator. Ask for a replacement separator with full SS316 if this is the case.

 

[pierreick]

Hi,
The reason why we are using oil free compressor in API manufacturing process.
example attached.
Pierre