What Type of Compressor Should be Used

Most of the questions asked here should be directed to the internet search engines or Search GlobalSpec FIRST!!!!!!!

You give no meaningful information (your question sounds like it came from a test) that would be helpful in recommending a compressor type.

Based on this, I'd go with the, "plant technical deptt" recommendation. At least they have an opinion.

Gas compressor - Wikipedia, the free encyclopedia

screw

thanks but can you please guide how much less energy it will consume when compared with reciprocating compressor

Unless you want a bank of compressors that will fill a large room you're going to have to use a design made for large tonnage. Probably 3 or 4 of them. Or as Mr Tornado says 1 Centrif. If that 1 goes down you have no cooling. If you have 4 screws and one goes down you still have 75% capacity. If your sole criteria is energy efficiency I suggest you you hire a consultant and do a long term study on initial cost and maintenance.

or just read this and go high tech

http://www1.eere.energy.gov/femp/pdfs/etcs_vscompressor.pdf

Any reason centrifugal is not in the mix?

R-11 is in short supply these days

no special reason but they want to select from the 3 options i have mentioned

I would actually consult the data on bhp (bkw) versus refrigerating capacity from a few manufacturers, especially those who offer several options.

One large compressor (of whichever type) may be more efficient than several smaller one, but if a single compressors fails, you will still have much of your capacity.

The efficiency of a compressor will depend on how many stages of compression you want to go with.

My experience is with air compressing. Your application should be similiar. A recip compressor is probably the best since they will likely be two stage. (If they are only single stage their efficiency will be low, too.) Screw compressors, centrifugals and scroll compressors are pretty much all the same--they are the equivalent of a single stage and their efficiency is not as good as a two stage recip.

You can get better efficiency if you go to a dual stage screw or similar and also you need to investigate the part load efficiency of all of the compressors. Recips are pretty good since they are either pumping or unloaded, and don't offer part load output.

Screws, for example, offer operation at part load but the efficiency can be horrible. Some screw compressors, when operating at low output can pull like 60% of full load hp when putting out 20% output. If part output is necessary, consider variable speed screw compressors. I don't know if similiar options are available in centrifugals or scrolls.

I have always preferred screw compressors over recips because they are far more reliable. However, with operating costs becoming more important, the reliability issue is starting to not compare so well.

Other responses to this thread suggest you consider multiple compressors for redundancy considerations in the event of a compressor failing. That is a good idea. It also allows you to modulate your load by shutting down compressors if the load is low.

You should consider engaging the services of a consultant who can help you with the detailed pros and cons of the various types of compressors available for your application. You also need to get a handle on the competency level of the plant's maintenance department and just what is available in your area for (a) replacement parts and (b) compentent service personnel. Maybe the plant is very comfortable in maintaining and repairing these big compressors. How important is the continuous operation of the chiller? If a compressor dies and a replacement is half a world away will there be some panic? There are a lot of questions to be properly answered before you can get close to recommending a compressor.

Jon.

"The efficiency of a compressor will depend on how many stages of compression you want to go with.........Screw compressors, centrifugals and scroll compressors are pretty much all the same--they are the equivalent of a single stage and their efficiency is not as good as a two stage recip."

I do not agree with the above statement. The efficiency of a compressor will slightly improve for multi stage compression, only if there is inter-cooling between the stages. Centrifugal compressors are also available in multi stage and their efficiencies are much better than recips.

Well, in air compressing, we usually consider a decent single stage screw efficiency at about 4 CFM/hp. A 2 stage recip can deliver 5 CFM/hp. That's a decent efficiency boost.

Yes, you are correct--cooling between stages is important! If there is no inter-stage cooling you might as well have a single stage machine.

I don't have any experience with centrifugals--I understand they need a lot of skill to maintain, as there are parts that run at very high speeds. I also understand they do not like modulation-- they are happiest running at 100% output. I suppose that if they are slightly more efficient that other methods of compressing it is becase the turbine shape can be optimized for minimum flow losses. But the thermodynamics for all the types of single stage compressing is the same-adiabatic compression.

Jon.

This thread is about refrigerant compressors for chillers. The compression ratio in this case is low to moderate, and not really suitable for multistage scroll/recip/screw compressors. I'm not sure about centrifugals, though.

reply to #16

"But the thermodynamics for all the types of single stage compressing is the same-adiabatic compression."

This is only true for adiabatic compression. If you actually want to obtain real adiabatic compression, you will find that it is actually very difficult, and in practice is not attainable. So the real question is, how close to adiabatic, or more to the point, how close to isothermal compression do you want to get? With this question, the differences between different compression methods become substantial. Cost raises its ugly head (as usual) and the choices become complex and dependent on many details (not provided). If you actually want an advanced design, you must share details of your goals.

"centrifugals........do not like modulation-- they are happiest running at 100% output."

Centrifugals can be modulated in three ways- suction throttle, variable opening inlet guide vanes and variable speed through VFD or Magnadrive. Out of these variable speed is the most efficient way of running at part load. But running centrifugals below 70% capacity is not an efficient option.

"CFM/hp" can be a measure of comparing efficiency for machines running at identical suction and discharge conditions, otherwise not. For refrigerant compressors kW/Ton of refrigeration is more meaningful for similar set-up.

For OP's question, I recommend chiller with multiple screw compressors (3 or 4 nos), so that part loads can be efficiently addressed. We have 250 TR units with 4 screw compressors from Carrier. In another locations we have 320 TR units with one screw compressor from York and 350 TR units with 3 screw compressors from Danambush.

Thanks for the informative reply.

I too prefer Screw compressors with built in VFD to save power. Reciprocating compressors old horse. I think normal technician can service reciprocating compressors and spares are less expensive and screw compressors just opposite.

Huh? Recip rebuilds are in the skill range of the average teenage hot-rodder.

Refrigerant screw compressor shaft seals are more critical than air screw compressor shaft seals (and hence more expensive). A dab of air leakage is not very harmful, unlike some refrigerants.

Thanks for the reply but the plant wants to stick to reciprocating and that too without VFDs. In this case do you agree that reciprocating compressor for chiller will consume more energy ( how much in %age) as compared to scroll/ screw. By the way the chiller they are buying has 4 compressor each of 60 Ton= 240 Ton

For chiller unit with 4 compressors, VFD is not so neccessary. Depending on load number of compressors to be operated with some automation to start/trip.

How much more energy recip compressor will consume, I can't say. As per link article by Fredsky at #8, difference between screw and recip is not significant. But recip compressors are maintenace oriented, down time, oil consumption and refrigerent consumption are tend to be higher.

Comparison of energy consumption for centrifugal and screw chllers I can give, which are presently in operation. It depends on size, and make too.

500 Ton chiller with single centrifugal compressor (York) : 0.65 kW/Ton

250 Ton chiller with single screw compressor (York) : 0.60 kW/Ton

310 Ton chiller with four screw compressors (Carrier) : 0.75 kW/Ton

The best way of comparing compressors is through SCFM. Since all the measurements are calculated back to a set of reference standard conditions, it levels the playing field among the multitude of different manufacturers.

No, specific power consumption of compressors cannot be compared with SCFM alone because power consumption depends on pressure ratio also for same fluid handled. For refrigeration systems kW/TR is the best way to compare overall performance. Obviously, it includes performance of condenser and evaporator too.

thanks for further updating me with your comments

Each Type of compressors has its merits and demerits. Selection depends on many factors such as initial cost , ease of maintainability, availability of spare parts, running cost and the type of application.

This depends on the running pattern of the compressor. If part load runs for even for 40-50% times then in over all reciprocating will consume less power then screw compressor. There is almost no running maintenance of screw compressors but overhaul and seals are costly afairs.

Running maintenance of reciprocating compressor will be there but it is not too costly.

In HvAC application, the most interesting parts is equations and rules of thumb, in most commons is cooling load calculation where below procedures will apply;

1. Location.
2. Orientation N, S, E, W, NE and etc.
3. External / internal shading.
4. Occupancy type and time of day (if applicable).
5. Size.
6. Colour.
7. Shape.
8. Constructions and material properties.

Then we will decided on what type of centralized chiller will be apply, either negative or positive chillers.

Most common for plant application, they will install negative chiller, as the heat recovery obtain very fast (i.e. cooling load is high) however; speaking on energy wise is also high.

PLEASE, I NEED SOME INFORMATION ABOUT YOUR COMPRESSOR

- DELIVERY PRESSURE

- CAPACITY

-USED, FOR AIR INSTRUMENTATION, ETC

- AIR OIL FREE

Total capacity is 240 ton ( 4 compressor each of 40 ton ) and the chilled water is used for cooling purpose.

4x40=160

Extremely sorry it is 4*60 as stated earlier.Thanks for the correction.