Impact of ambient temperature on sizing 1

[compguy22]

Hello,

In determining size of screw type air compressor, I am facing the following problem.

If I consider max. ambient temperature at site which is 35 deg C, then I need to select bigger frame size to match scfm peak flow demand. This is normally the sizing case. However the site temperature during the year is rarely at 35 deg C (says less than 5% of operating time), means a couple of days per year on average. The yearly average is much between -35 to 15 deg C.

The combination of this occurrence (35 deg C ambiant temperature) and flow demand peak is therefore relatively limited.

So my question, what methodology (statistical) should I follow to include or take into account the worst case scenario but also factor in its prevalence (or frequency) in the final sizing of the compressor. Or otherwise, should the worst case be strictly applied.

If the installed capacity does not match demand but is still relatively close, this means outlet pressure will drop below nominal pressure which may be acceptable for limited duration.

Thank you for any insight or any reference that could help me.

 

[georgeverghese]

From a suction flow perspective, there is only a 7% volumetric throughput shortfall between 15degC and 35degC. So given that plant air demand tallies are usually conservative, it would be okay to use 15degC.

However, if the shift from 15degC to 35degC means more stages (due to stage discharge temp limitations), then it means that on rare days when it is at 35degC, these compressors would trip out on high temp, shutting down the whole plant. If so, using 35degC seems unavoidable, unless you have (a) some contingency plans to draw the entire required plant demand for compressed air from some where else on such days or (b) you can get by operating the compressors at a lower discharge pressure to keep within discharge temp limit for the smaller compressor option on such days.

 

[compguy22]

Thanks for the input. High temperature at discharge is not an issue, machine is water cooled and it can be coped with.
Max. ambient temperature is rarely to occur at 35 deg and your indication seems to make sense.

As for me to substantiate selecting a new temperature to be used for sizing, is there some rule of thumb or any good practice / reference I can recall and which would set say a certain threshold of max. temperature occurrence % over time period; I can think of it as ambient temperature yearly distribution.

Thanks

Edit: as precision, I would need to substantiate the method because it will be subject to client review/approval.

 

[georgeverghese]

If the plant air tally includes intermittent utility air, then you could say that discretionary utility air use could be postponed till such time when it is cooler.
You've got to be sure that higher discharge temp can be handled by the watercoolers - coolers will need to be sized for the higher cooling duty.

 

[SB1976]

Hi,

You might consider installing this compressor inside an enclosure, which may be fitted with Air Conditioning Units. The compressor inlet shall be from locally inside. Once air ambient temperature (outside enclosure) reaches 35°C, you can switch the AC on with lowest possible setting (typically 18°C). Compressor sizing can be done corresponding to this lowest temperature local ambience. This way you can save on Compressor sizing, optimize driver rating and all related auxiliaries.

Trust this helps.