Greetings from the soon to be frozen "Eastern Wilderness" @1140 feet above mean sea level.

The rule of thumb for any type of machinery is that the dumber it is the easier it is to take care of, repair, and trouble shoot problems.

You can buy a lot of buffer tankage set up in series or parallel storage for your intended use.

You will just have to be sure to drain the tanks every day of air to remove all the accumulated moisture otherwise you will lose compressed air capacity.

We managed very well with an old Joy radial piston air compressors with a Square D pressure switches and large buffer tanks that shut off when not needed for the work load.

Perhaps you should look at the radial piston air compressors used by Electro Motive and General Electric in their locomotives,

as they operate for millions of cycles and are proven units that have been in use for most of the last century.

Assuming you have three phase power the motor size needed will be dependent on the maximum rpm via v belts and v belt pulleys.

I am unsure as to who is the manufacturer but a phone call to either locomotive builder or googleing locomotive air compressors will solve that mystery:^)

lzaharis ■

Good day Mr dror,

The application of an oil free screw compressor with or without a variable speed drive (VSD) is just calculating and comparing the performance of both options.

This calculation will show the range of speed control, the starting currents, the starting time, the necessity of a no-load starting device, resulting exhaust temperatures at various speeds and loads, the size of a buffer tank, etc.

Also the application is important, f.i. in pneumatic conveying it is in most cases sufficient to have a constant air delivery.

If you inform us of the relevant data, you have so far, we might be able to help you further.

best regards

teus ■

dear Dror,

I doubt, whether a screw compressor can have a speed range from 0% to 100% without problems.

The volumetric efficiency at very low speeds will go to 0%, increasing the outlet temperature, and the (forced) lubrication will have failed before that.

The cooling of the electric motor is also a concern.

If you can supply the required maximum volume and the required constant pressure, I can make the approximate screw compressor curves you need.

At the same time I calculate the performance curves for a lobe blower (Roots type)

I do not have experience with Hybrid Permanent Magnet motor. It seems an expensive piece of equipment to me. Manufacturers should be able to explain the advantages.

let me know

teus ■

Dror,

your purpose is much better served if you bottle your own

breathing air or purchase it from a gas distributor in large

volumes as the economies for it out weigh the cost of an oil free

screw compressor, buffer tanks, and many regulators versus one

large storage tank and regulator for the entire system and feed it

through a manifold to your crew-I am assuming you are spray

painting.

This simplifies the whole process as much less money is spent on

physical plant and breathing air purity-the case in point is

breathing air bottled for fire fighters and scuba divers.

Many fire trucks built for rescue work have on board

compressors to bottle their breathing airand have been used

for many years.

You will save a considerable amount of funds for your purpose as

it is purpose specific and designed specifically for the task at

hand and you will avoid the potential health problems from oil

mists in breathing air streams.

The local fire truck distributor and or the local bottled gas

suppier/distributor BOC or Linde in your area will be able to help

you with this.

With manifold distribution of bottled gasses flow is guaranteed

at low pressure -breathing air pressure at 1 atmospere Hg. and

purity is gauranteed plus moisture is eliminated in the process. ■

Dror,

If you are setting up multiple work stations you are going to be using parker or Aeroquip compressed air male/female connectors with full face apparatus this will be even easier as the load for breathing air will be very low since you are working at sea level you cannot push more than 14.28 psi through the entire system and you will be required to use stainless steel anyway to maintain breathing air quality. ■

Dror,

The option of having a bigger receiver in you plant to take care of any sort of fluctuation or to cater the plant air demand when compressor goes to a unload in practically not viable since for 100CFM demand you would require a 20M^3 (Approx) Air receiver so that the pressure drop inside the air receiver is not more than 0.5 kg/cm^2G

Further the compressor that you are evaluating comes with HPM motor as a standard supply from the manufacturer. You are absolutely right that in VFD compressor at part loads the compressor efficiency dips i.e because the motor efficiency goes down drastically when it is made to run at RPM lower than the RAted RPM of the motor. In case of HPM Motor this efficiency drop due to reduced RPM is minimum in fact the difference between minimum and the maximum efficiency is hary 5 to 6 % hence at part loads these kind of motors are more efficient than the normal induction motor.

Further in HPM Motor any size there is not restriction of start and stops that the motor can take per hour hence they are designed to take part load as well as frequent on/off situation with out even allowing the machine to unload.

One point you should remember that Rotary screw compressor are best used when they are at full load as far as possible Unloading condition should be avoided that is the reason why VFD is required bassically to avoid unloading in the compressor.

Hope this is in line with your requirements

Best Regards ■

Hello Dror,

before we get our Unterhosen all bunched up over efficiency it is necessary to review the pressure in conjunction with the flow you need. I doubt that a buffer tank will help you, if you are talking about a oil free single stage screw compressor with a maximum of 3.5BarG pressure.

Instrument air usually means a pressure of 8-10BarG. That would be a twin oil free rotary screw compressor. If your system already exists, I would contact your two compressor manufacturers and ask for an "air audit". Sucg audits reveal the snap shot of how much air (even leakage rates) you really need and you usually get a recommendation as to how to proceed with and without a buffer tank.

My personal inclination is to always go with a VFD as they provide with the maximum of flexibility - they is nothing worse than wasting HP by compressing to a higher pressure than you need and for accruing unneccary idle run time.

Permanent magnet motors are something relatively new, but the techniology is sound. They are harder to replace and repair. Standard customary squirrel cage motors can be acquired and fixed easier. ■