Selecting of compressor sizes (Energy saving)

If your looking for savings look into variable speed drive on rotary screw compressor.

While energy savings is one issue this must not be confused with minimizing cost of energy consumption. The total energy cost is a function of the energy charge plus the demand charge. From a demand viewpoint the larger number of compressors will allow you to manage the demand better than fewer compressors. The price you may pay for this is smaller efficiencies on smaller individual compressors thereby increasing the energy consumption. The variable speed compressor would appear to try to take advantage of both demand and consumption aspects.

If your 8 compressors are side-by-side then replaced by 2 or 3 modern efficient ones might work. If your compressors are spread all over the place you might create some serious pipework distribution problems.

Either way, compressors are most efficient when run at design load - and most inefficient when run off-load. So you shut them down. Then you start them when required. This has it's own problems because motors don't like frequent starts. They overheat and burn out.

As already suggested, the idea of a variable speed screw compressor makes sense - but the success lies in the control panel, so that if you have 2 or 3, they share the load in the most efficient way.

Also you need to ensure there is enough stand-by capacity (downtime for maintenance) if you need air continuously. With 8 compressors you are in a good position to ascertain the load pattern.

You might find will pay to install a modern computerised control device to link your existing 8 compressors to ensure the load is shared in the most efficient way.

As mentioned previously, it is your demand and usage that is important. How much air in cfm (or whatever your local units are) does your plant really consume? Does it vary between shifts or on the weekend? And by this, it doesn't count if you shut down all the compressors because no one is there. Make sure that if you replace little compressors with big compressors that the big ones aren't sitting there just idling or cycling on and off.

As far as variable speed compressors go, they are good if your demand changes a whole bunch throughout the day, but they aren't worth the money if you have consistent loads. From what I have seen, under full load conditions, the load/unload compressors will be more efficient than variable speed compressors. If you do choose to go with a variable speed drive, make sure it is the smaller of the compressors so that it is just used to meet the peak demands.

Your overall strategy does work on paper though. Based off numbers from Ingersoll-Rand's website (https://www.ingersollrandproducts.com/en-us.html) a 50 hp 100 psig compressor will put out 231 cfm and a 100 hp 100 psig compressor will put out 495 cfm (I picked these sizes of compressor because it makes the math easy). So replacing two 50hp's with one 100hp will net you about 7% in terms of cfm for the same hp consumption. Just be aware that the actual number WILL vary as far as performance in your facility. The published numbers are hard to duplicate outside of a lab or test cell. So all that being said you may not really see the improvement if the manufacturer did a better job with efficiency on the 50 hp package.

Lastly, consider the piping size leaving the compressors and the compressor room. As in the above example, the 50 hp package (per IR) has a 1.5" NPT flange, the 100 hp package has a 2.0" NPT flange, and a 300 hp package has a 4" NPT flange. Based on the higher outputs from the higher hp compressors, you may have to increase pipe sizes from the compressors to your air receiver.

Good luck!

It really is going to depend on the air requirement at the plant. A common strategy is to use multiple compressors in a system. One of these should be used as a trim compressor. As previously stated, most compressors are most efficeint at their design capacity. However, if the trim is a VSD driven compressor, this is likely the best scenario.

I'd recommend getting an air study done to determine what your facility's load profile really is. This will tell you what you really need. Check the DOE best practices for a list of reputable people. They may do it for free depending on the current Program they have running. You may also check with your local utility. Here in California we have many Programs to assist with energy evaluations.

Hello....Does anyone know if there are any charts or tables that will tell me what size compressor a household refrigerator of a certain size would need?