Air Compressor - Pros and Cons for Rotary Screw an

I'm in a position of making decision for purchasing two 7.5kW air compressors for our production plant.

We are expecting water & oil free air at 7-8 bar pressure.

I could easily find comments from compressor manufacturers in the internet, but you know their comments often putting a lot of positives on their product and negatives on others.

Can anyone share your independent experience and comparison between rotary screw and rotary vane compressors with me?

Thanks!

Albert

What were the problem you had when it was new? Was the working envorinment dirty so it required a good intake filtration?

Quote: We use the waste heat from the heat exchangers to heat 20K sq ft in upstate NY.

We were thinking of doing this for our plant as well, but for 85K sq ft with high ceiling. All compressor contractors I contacted saying it isn't a good idea for our situation. However, they suugested us to exchange the heat to hot water. What are your thoughts?

Thank you. =)

Most Sullairs I have worked with have run a bit on the hot side. They also have had some drive gear issues, since Sullair seems to think they need little or no heat treating. Generally, they are well built though.

Most installations can be properly served by an adequately sized refrigerant type air dryer. This will give you a dew point of a few degrees about freezing, which is fine for 99% of most customers. These dryers require relatively cool, clean air to shed the heat that they are removing from the compressed air system. They also have condenser coils like an AC system which must be kept clean. This is an easy job, as long as the dryer is installed in an area where it can be conveniently accessed. Putting it up high like a space heater invites neglect and premature failure of the unit. If you can get to the condenser coil with a blowgun to clean it out periodically, that is about all the attention these units require. The only other concern regarding these units, is that the air going into them must be at 100F or below. Otherwise, you will overload the cooling circuit and cause the refrigerant compressor to oveload and shut off. Many screw and vane compressors will have an air aftercooler built into them, but if not, a purpose built aftercooler put inline between the compressor and air tank, or compressor and dryer, will remove enough heat to prevent overloading the dryer. they also do a good job of knocking out a lot of water. Air tanks can often be effective heatsinks if demand is not excessive. You can buy high temperature dryers which have aftercoolers built into them, but they are more expensive.

If you need really dry air, the only solution is a twin tower dessicant dryer, which will give you dewpoints from -40C to -100C, which is pretty much food grade or pharmaceutical grade air (with proper filtration). These do use about 5% (heated) to 10% (non-heated) of your air production to drive off moisture from the dessicant beds, so if you are near, or at your compressor's air delivery limit, these may push you over the edge. They are a bit more mechanical than refrigerant dryers (valves, solenoids, timers controls etc) but are relatively trouble free. Hankison makes good dryers, and most of the branded dryers are just rebadged Hankison's (Kaeser, DeVair,DeVilbiss).

I know that all this sounds like a lot of work, and it is, which is why there are so many really bad compressor installations throughout industry. Compressed air system problems can dive even the most seasoned maintenance professionals crazy, considering the severe impact that compressor system downtime has on any production process. It is far easier to do it right from the beginning, than chase problems (sometimes for years) using half measures and guesswork in place of real solutions.

Thank you very much for your input (of course thanks for all others as well).

Here is an update for our purchase status and current situation. Our current air demand is at approx. 35 cfm average and peak 60 cfm.

We have filtered many quotations and end up two options: 1) CompAir HV07RS Hydrovane and
2) Ingersoll Rand UP5-7-8 Rotary Screw

I do understand basic pros and cons for both types. But things I need help is the power consumption when they are off load and the air quality.

Power consumption: Ingersoll Rand salesman said to me that 7kW RS will take 7kW when it is on load and 3.5kW when it is off load, but 7.5kW RV (50%) will take 7.5kW when it is on load and 5.25kW (70%) when it is off load. Is this about right?

Air Quality: We wish to have oil and water free air in the system. Which one will provide better quality?

Please share your porfessional input with me again. Or, any input from any one in this discussion room will be also appreciated.

Many thanks!

ALBERT

Thanks for your good input (of course thanks for all others as well).

Here is more info for our plant and my purchase status. Our current air demand is at approx. 35 cfm average and peak 60 cfm.

We have filtered many quotations and end up two options: 1) CompAir HV07RS Hydrovane and
2) Ingersoll Rand UP5-7-8 Rotary Screw

I understand most basic pros and cons for both types in this discussion here. But things I need help now is the power consumption when they are off load and the air quality.

Power consumption: Ingersoll Rand salesman said to me that 7kW RS will take 7kW when it is on load and 3.5kW when it is off load, but 7.5kW RV (50%) will take 7.5kW when it is on load and 5.25kW (70%) when it is off load. Is this about right?

Air Quality: We wish to have oil and water free air in the system. Which one will provide better quality?

I would like to hear your professional input. Or, any input from any one in this disscussion room will be also appreciated.

Thanks!

ALBERT

1) Plain bearings for the driveshaft. No expensive and difficult to replace taper or roller bearings to wear out. No preload or shimming issues, or end clearance problems when rebuilding. They often last for the life of the machine if they are not seized up from lack of maintenance.

2) Due to the fact that the compression element is not end loaded like a rotary screw, there are no high thrust loads to worry about. When a rotary screw is left to run past its bearing replacement interval, the rotors can travel the length of the case and seize up catastrophically.

3) Hydrovane rotor/stator units can run for hundreds of thousands of hours with simple routine maintenance, no bearing replacement required.

4) A Hydrovane is cheap to rebuild when the need arises. They are also pretty energy efficient, and can be had in variable speed configurations, but perhaps not in a low hp machine as you are considering purchasing.

6) They have a small footprint compared to most rotary screws, which can save you money when it comes time to build a compressor room.

7) Hydrovanes are relatively simple. Most competent mechanics could rebuild one with just a factory manual in a day or two.

8) Hydrovanes usually enjoy a significant cost advantage over rotary screws of the same Kw rating.

Having said that, some people would gouge their eyes out rather than buy another Hydrovane. Usually, they have worked with them in an environment where they did not recieve proper maintenance, or where they installed in a poorly designed compressor room where they were allowed to run too hot and embrittle their seals (they leak oil ferociously if neglected, they are British machines afterall..)

You won't go wrong buying a good quality rotary screw either. Unfortunately, in this HP range, you will find mostly belt driven machines, which raise their own maintenance issues. I am a Kaeser factory trained technician, and while they are superb machines, the belt drive can be a maintenance problem if they are operated in a dusty environment. Rotary screw air ends are usually spinning quite rapidly (unlike a Hydrovane, which is directly flange coupled to a 1740 rpm electric motor), and so they place quite a bit of stress on the belts. In a dusty environment, it is not uncommon for the drive pulleys to become severely worn in short order. Many small rotary screws are also used for a wide variety of HP ranges, and can sometimes be spinning at terrific speeds (8000-10000 rpm). this does not bode well for when someting goes amiss (ingesting a pop rivet from a cheap air cleaner cover for instance....). If you decide to go the rotary screw route, see if you can buy a direct drive unit. Quality brands are Quincy, Kaeser, Gardner Denver, CompAir (Makers of Hydrovane also), RotoComp, Atlas Copco, Sullair and, possibly, Ingersoll Rand. Avoid computer controls if possible, though the Kaeser Sigma controller is pretty trouble free. Rotary screw compressors are the workhorse of most industries and are not especially troublesome. They are more complex, more expensive, and require regular bearing replacement (about every 50,000 hours). Rotary vanes are cheaper, simpler, easier to rebuild, more compact,and quieter (arguably). They are also much less common than screws are.

• Don't Neglect A Rotary $crew Airend!