Compressors operating downstream of air storage 1

[vladc]

Hi there,

My plant has all existing compressors operating upstream of the storage tank (air receiver). I am trying to prove that only two would be enough upstream and the rest downstream, operating at a lower pressure, in order to save energy. Downstream of the receiver there is also a pressure/flow controller, so that the pressure downstream is maintained constant (530 psi). In the receiver, the average pressure (in time) is about 580 psi. I need to come up with some accurate figures and a solid basis for proving the savings.

Other than that, I am not sure what the working principle for a pressure/flow controller is (for instance, whether at a given moment in time, volumetric flowrates up- and downstream of it are different - based on the different pressures up- and downstream of it).

Can anyone help me with this?

Thanks,

Vlad

 

[zdas04]

Let me make sure I understand your situation, are you:
- Using compression to keep a receiver charged
- Supplying your loads from the receiver through a pressure regulator.

Now, it looks like your modification is to supply your loads with several compressors discharging at 530 psig and only two compressors operating at 580 psig to keep the receiver charged.

Compressing atmospheric air to 580 psig (at sea level) takes 210 hp/MMCF.

Compressing it to 530 psig takes 202 hp/MMCF. Is a 4% reduction in power requirements worth the capital cost of modifying your system? Can your end devices tolerate increased oil contamination? Can your end devices tolerate increased pulsation?

The answer to all of these questions may very well be "yes" and it might be a great project, but you really need to explicitly answer all of the questions prior to proceeding.

David

 

[vladc]

Thank you for your post David. The answers to all questions is yes. Before the receiver and pressure/flow controller were put in place, all compressors were discharging into a header directly connected to the load, so oil contamination was not a problem.

As for the pressure regulator, it's actually a pressure/flow controller (you can see the exact type at

http://www.sullair.com/Files/Sullair/Global/US-en/industrial/LIT_FLOWLOGIC_20040200.pdf).

There will be no increased pulsation, as the compressors that I plan to have operating downstream of the receiver are running at least 95% of the time anyway, so I plan to have them running continuously).

To be honest, I am not fully aware of the working principle of the controller and I do not have a manual for it. I have just started work as an engineer. Can you (or anyone else) let me know where I can find detailed information about the working principle of such controllers and also about pressure regulators. I really want to understand the theory behind it. The question that bothered me for a few days now is: for such pressure/flow controllers and also for pressure regulators are the volumetric flowrates upstream and downstream of them equal?

I hope no one will get annoyed by these questions. No need to answer in detail if you don't have time, just point me to the right direction where I can find the information (internet would be the best source, but books are also helpful).

Thanks,

Vlad

 

[zdas04]

My undergrad Fluid Mechanics prof said it in a way that will stick with me forever. When asked that same question he said "if the [mass] flow rates were different, fluid would stack up on one side and you'd have a vacuum on the other side, see anyplace for fluids to infinitely stack up?"

Pressures are different. Velocities are different. Volume flow rates (at actual conditions) are different. Mass flow rate (and its surrogate, Volume Flow Rates at Standard Conditions) will be exactly the same on both sides of a regulator, flow controller, step-down station, pump, compressor, or meter station.

The link you provided is awfully non-technical. It probably contains the reason for the modification to your system that you are currently trying to reverse--for intermittent loads a receiver and flow regulator are far more efficient than directly feeding the load from a compressor. This allows a smaller compressor to recharge the system while demand is low (and possibly while electric costs are lower) while still having adequate air for demand. It works really well in their example of 5 seconds of demand every few hours. From the little you've said about your case it sounds like a hybrid system like you've proposed could be better.

I'm not familiar with that particular flow controller, but I've had good experiences with other Sullair equipment.

David

 

[dcasto]

Junk the regulator and replace it with a VFD on the air compressor(s).

 

[vladc]

Thanks David and dcasto (however, with regards to VFD, will it work in case of reciprocating pistons compressor - it's true, I did not mentioned that before).

Other than that, please help me with the following. I am doing some calculations, I'm pretty sure I'm wrong, but I don't know where:

1. We have one HP compressor (among others) which takes 300 HP at 650 psi to make 730 CFM.

2. We have two LP compressors:
- 200 HP at 125 PSI to make 890 CFM
- 100 HP at 125 PSI to make 450 CFM

I am thinking, why use the two LP compressors, when I can consume the same total power by using the HP compressor (300 HP) and regulate the pressure down to 125 PSI, making more CFM:

LP air flowrate using HP compressor:

Q = (650 PSI/125 PSI) * 730 CFM = 3795 CFM

I know for many of you this is a simple problem of thermodynamics, so please let me know where I am wrong.

Thanks,

Vlad

 

[zdas04]

Yes, a VFD can work very well with a recip, varying compressor RPM on recips is done all the time all over the world.

There are zillions of combinations of options to optimize compression. First I'm going to assume (in spite of your last calculation) that all of your "CFM's" are "SCFM". If your CFM's are at actual conditions as your "equation" implies then figuring out what you are talking about is far more work than I'm willing to do for free.

No one can answer your questions without knowing the LP and HP loads. If you need 730 SCFM of 650 psig air (I'm not sure what happened to the 580 psig service, but ok) then you don't have any extra air to step down to 125 psig to replace the LP compressors. In any case, even if your demand for HP air is zero, you don't have enough HP gas by half to supply your LP loads.

As a rule of thumb, any compression hp not expended has 100% efficiency. It takes 427 hp to take 1340 SCFM from atmospheric pressure to 650 psig at sea level. It takes 195 hp to compress the same volume to 125 psig. This means that if you need 1340 SCFM of 125 psig air then compressing it up to 650 psig and regulating it down is a waste of 232 hp or more than 50% of the input energy is given up in the pressure regulator. Never an optimum solution.

David