7 replies to this topic

[empower70]

I need your assistance, particularly people who have experience in N2 booster compressor(Reciprocating type) sizing.

The feed is Nitrogen from upstream Nitrogen generation plant at 5 kg/cm2(g) and discharge pressure is 35 kg/cm2(g).The upstream nitrogen plant(PSA type) will also be set up in phasewise manner.

During different phases, nitrogen requirement are:
1) 39 NM3/Hr
2) 77 NM3/Hr
3) 128 NM3/Hr
4) 269 NM3/Hr
5) 350 NM3/Hr

In first phase 130 NM3/Hr Nitrogen generation with adequate turndown will cater nitrogen requirement upto 128 NM3 /Hr and in second phase 220 NM3/Hr Nitrogen generation plant will be set up to meet additional nitrogen requirement. So that both plant together caters 350 NM3/Hr.

My question is how do we select compressor capacity, so that it works in all condition from min. 39 NM3/Hr to 350 NM3/Hr.
Points to be considered should be fixed cost, ease of operation, standby availibility, running cost, parellel operation.
Here conventional option can be to go for capacity control by bypass control valve or unloaders with 2-3 same capacity compressors.
Another option( I am not sure), is to select single compressor of 350 NM3/Hr. The capacity can be varied by changing motor speed and pulley, which will change capacity. I doubt whether single compressor can have such a large turndown. If not available, one can opt for two or max. three compressor with similar arrangement.Here fixed cost may be more, but one unit can work as standby.
U may suggest your valuable viewpoints and suggestions.

[Art Montemayor]

empower70:

Yours is a very conventional, and easy to apply reciprocating compressor application. I have done this one more than once in my past.

What I recommend is:

Employ one compressor at the initial outset with a capacity of 350 Nm3/h. (Here, I have to assume that you mean that the stated capacity is the capacity of the suction stream at NORMAL conditions of 0 oC and 1 atm. You have not stated your suction conditions specifically and concisely, so I have to assume them.) This compressor would be electric motor driven through a V-belt drive. It would probably be 2-stage with an inter- and after-cooler. It should have non-lubricated cylinders with Teflon rings and have fixed, unloader pockets on each stage. The unloader pockets should be designed to allow the compressor to turndown its capacity by opening the pockets. If you increment your future capacity, then you can carry out the unloader pocket settings manually.

You should have the capability of changing the speed of the compressor as well – by changing the motor pulley – or changing the motor speed (if you select a synchronous motor). A synchronous motor is very desirable because you can vary the speed at will and control the power factor as well – a great power saving feature. However, synchronous motors may not be available if the horsepower required is too small. I recommend a synchronous motor where possible. This would give you excellent capacity variance with great flexibility and power savings – using both the speed and the pockets.

Use PEEK type of construction in your compressor valves. If you are interested, I can recommend a supplier. These are the best and most dependable of the compressor valves.

You have not given your project’s Scope of Work, but I have to assume that you would prefer to make an investment in a compressor until you need the compressor. That is why I recommend you start with one compressor (over-sized and running underloaded). The ultimate and optimum solution is to have two compressors – both identical (for obvious maintenance reasons) with one serving as a standby unit ready to replace the other when valves have to be changed or when maintenance has to be done.

I hope this experience helps you.

[empower70]

Thanks Art.


First, let me try to answer your questions:The suction and discharge both are at ambient temperature(say 40 deg.C).Suction and discharge Pressures are already mentioned.
Our scope is to select and engineer compressor/s for this application.

To refine further:

1)Practically, whether models are available to vary capacity(cylinder volume only) from 350 NM3/Hr to 35 NM3/Hr, by merely changing location of cylinder unloader pockets? Or variation in speed is also required, to achieve this large turndown?
2)If capacity variation by fixed pocket unloader is not possible, then we have to go for variation in speed.U mentioned synchronous motor for varying speed. Here we have to weigh three options in terms of installed/running power, fixed cost, running cost:
Option a:As we select compressor for 350 NM3/Hr, motor will be corrspondingly be for same capacity induction motor and instead of changing motor speed, one can change only motor pulley. In this case, motor will be unnecessarily wasting power, when operated at lesser capacity.
Option b:Another option is motor rating and motor pulley sizing shall be done in three stages i.e. 1st stage ( 35 – 70 Nm³/hr ) , 2nd Stage ( 113 Nm³/hr ) & 3rd Stage ( 237 - 350 Nm³/hr ). i.e. during 2nd stage, change Motor & motor pulley and same way for 3rd stage.
Option c:Third option is what you suggested of selecting single synchronous motor for highest capacity(350 NM3/Hr)with motor speed variation(by varible speed drive?).Here again motor will be wasting power, when operated at lesser capacity.
3)Suppose turndown can be achieved by both option individually: either by fixed pocket unloader or by speed variation, which one is more effective in terms of installed power/abosrbed power.

Pls. guide.

[Art Montemayor]

I believe that you are not understanding the design and operating features of a reciprocating compressor and its motor drive.

Reciprocating compressors can be specified and purchased as per the manner that I described them in my original post. You can employ the clearance pockets as external or internal ones. You can also employ “Plug” valves – which include clearance volume. The main thing to understand here is that the clearance control, while being a marvelous way to control capacity, is a STEP control feature. The capacity is varied in increments, and those increments are limited depending on the design.

I recommend that BOTH clearance pockets and a synchronous motor be employed to vary the capacity because of the wide range that you desire. I would not employ an induction motor. In fact, you might have an application where the synchronous motor is directly connected to the crankshaft. This has been done frequently in the past. Experienced compressor manufacturers will verify what I say.

Do not assume that you will be free to use a V-belt drive as you please. There are extraneous factors involved in the application such as: total horsepower, configuration, and speed. Only the manufacturer can decide if a V-belt drive is the recommended one. V-belt drive is usually only applicable up to 100 horsepower.

You are absolutely wrong when you state that a synchronous motor will be wasting power when the compressor is running at reduced capacity. A clearance pocket doesn’t waste power. Additionally, as I have stated, the synchronous motor runs at a reduced speed in order to control the capacity of the compressor also. Unlike the clearance pockets, it varies the capacity DIFFERENTIALLY. Therefore, the synchronous motor saves you energy costs in TWO ways: by running at a reduced speed (controlling the capacity) and raising your power factor.

Please read my original post carefully. I tried to include all the major points I could in it.

[empower70]

Again Thanks Art for elaborating.

The approximate power required will be approximately 60 HP. So, V-Belt option is also viable.Secondly, regarding speed variation of synchronous motor, I hope it is by variable speed or frequency drive. Pls. clarify.

Further to your viewpoint,I would like to explore following possibilities; adhering basic philosophy suggested by you.

1) Option 1: Instead of running one full capacity(350 NM3/Hr) compressor and one 350 NM3/Hr compressor as standby, how about running two compressors of 175 NM3/Hr each and keeping one additional 175 NM3/Hr standby( i.e.2 working + 1 standby). What are the points to be kept in mind while running two recip.compressors parallel and what controls are required.

2)Option 2:Instead of recip compressor, can one opt for oil free screw compressor? The main advantage is longer life and less maintenance.

Pls. suggest your view points.

Thanks,

[Art Montemayor]

empower:

The ability to vary the speed of a synchronous motor is probably one of the oldest methods of controlling a reciprocating compressor’s rpms. The oldest method I know of is steam driven reciprocating compressors – but these are probably not available anymore. This a common and well-known feature of a synchronous motor. If you check with an electrical engineer, he/she will confirm all that I have stated.

I have already stated very plainly and directly what MY recommendations are regarding the installed capacity and what I would install if it were MY project. However, it is YOUR project and your decision is what you will do. There is nothing extraordinary about operating two or more reciprocating compressors in parallel. The main, important point to bear in mind is the safety interlocks and instrumentation – which are all dependent on the Scope of Work on your project.

If you prefer to install an oil-free screw compressor that is your decision. I do not recommend it for obvious reasons which I have previously detailed to others on these Forums in the past. These threads can be researched through our Search feature. In my experience, you are not correct in stating “The main advantage (of an oil-free screw compressor) is longer life and less maintenance”. I have re-installed and adapted reciprocating compressors that out-lived several engineers – and they will probably out-live me also! No other type of compressor has a longer longevity than a reciprocating compressor. I do not believe your application is one for an oil-free screw compressor.

[sachindhopade]

hope this helps u
http://www.processcalculator.com/

[Art Montemayor]

sachindhopade:

Please stop cluttering all our threads with your one, solitary suggestion to use one, solitary website to solve all problems. You have even inserted the same comment in a thread that is SIX years old!

It is obvious that you are either not an engineer or you have no opinion of your own regarding the solving of engineering problems. Therefore, if you cannot contribute to a thread in a direct, positive method, please don't simply keep adding the silly recommendation that almost always has no bearing on the subject or topic.

I will keep on deleting your silly posts if they do not apply to the topic, until you stop it and start to contribute with at least a reasonable, logical comment.