Air Compressor Size

Something to consider as an aside is whether a 150CFM machine would be a less wise choice than 2 x 75CFM. If a machine requires downtime, with 2 x 75CFM machines one of the conveyors will keep running; with 1 x 150CFM both conveyors will be off.

I have to disagree with 2x75 that is an unnecessary amount of power consumption.

I would opt for 200cfm unit. You can always install valves in such a way as to allow

yourself to isolate one or the other for maintenance.The reason for a larger unit is

to reduce the cycle times of the compressor this saves wear and tear.

My 2 cents worth would be to have three 75 cfm units. Two would run as needed for production, with the third as a standby for breakdowns or planned miantenance such as filter or oil changes. It is a waste of energy to by over-capacity equipment thinking you might need it someday. Constant load and continuous operation are usually easier on equipment than "on-off" or cycling.

when you say your company is thinking of installing 2# 75 CFM compressors for 2# air conveyers. this I assume to have arrived by an expert within your company. keeping this in mind, you still haven't indicated, whether this CFM is an exact volume for your usage or has some extra capacity for future plan as well. if not, then in that case some additional capacity is recommended. here I agree with one of the reply to this question of yours.

two separate compressors, with their separate drives and control systems etc. would be preferred, provided the capital cost for the equipment is sufficient and the floor space is also enough for these two units, (including receivers, aftercoolers, moisture separaters and electrical control gear etc).

as an alternative, the items in bracket can be common to both compressors, with a change over arrangement, so one can switch over air delivery from it to the other one, as and when required to do so. this is assuming one air conveyor is operational at a time.

in case of both the units are operational at the same time, in that case, still the common items concept hold good but the control of air supply to conveyors will become somewhat tricky, because it is not necessary that after sometime the compressors will be delivering, each conveyors their exact requirements, because of compressors wear and tear, leading to increase of internal leakges etc, supplying lesser air than required. this is why one can aim for a sightly larger capacity compressor than an estimated one.

so in brief, three things are emerging from your question posed - space available to house two compressors with their associated accessories, whether two compressors are running simutaneously or not, and thirdly the cost of equipements afforable or not, plus the service and maintenence angle involved with it.

If all of your devices collectively require 150 cfm, then that is your system requirement. You will want 2 compressors, 150 cfm each, one primary, one back up. You can cycle each compressor to run one day each or one week each.

At some point, if you plan to expand in the near future, then you will want to go up to the next largest size, based on your projected needs.

Ingersoll Rand and Sullair manufacturer great "screw type" compressors. Screw compressors develop a large volume of air in relative short amount of time. But both generate a significant amount of condensate if they run "unloaded" as opposed to "loaded", so you will also want to base your size on this detail as well.

If you prefer a "reciprocating" compressor, then you will not have to consider "unloaded" vs "loaded" in terms of condensation generation.

Typically, you would want any compressor to be "loaded" 80% of the time and "unloaded" 20% of the time to maintain an optimum operating temperature.

Most compressor manufacturers offer VFD's for the drive motors to improve energy efficiency, so be sure to look into that avenue as well.

Good luck with this project!

Jeff

Everyone here has made good points for each of their positions and their experience in this area shows. One of the assumptions we all have made is that both air conveyors will be operated simultaneously. Having worked for an air compressor manufacturer for years, I too know some of the pitfalls associated with installing air compressors. If the 75 cfm is the absolute maximum usage each air conveyor will ever use, then 150 cfm is all you need. If, however, 75 cfm is an average value, you need to have enough capacity to cover the peak usage. In that case, Mr Osborne's approach is better.

If these conveyors are critical to the company's daily operation (meaning if the conveyors are shut down, the company is shut down) then you must have a backup compressor somewhere. There are several ways to do that.

One way is to install a 200 cfm compressor and a spare 200 cfm compressor. That is the most expensive way. The compressor manufacturer and compressor distributor will love you for that one.

Another is to have a 100 cfm base machine and to have a 75 cfm machine as a "trimmer".

The most energy efficient way is to have a single variable speed (vfd) 200 cfm machine and a maintenance agreement with the local distributor with an emergency backup agreement where the distributor supplies a compressor to keep your facility operating until your equipment is returned to operation.

Talk to reputable distributors. They will be able to assess your operation and give you a recommendation.

First let it be known that I have nos specific knowledge about air compressors.

A couple of principles that I would consider however are:

1. Never buy anything that just matches your percieved reqirements, what if the air conveyers actually require more that 75 CFM, or what happens as the compressor wears and it becomes less efficient and unable to deliver the rated output unless of course the manufacturer used very conservative ratings for their products.
2. If the air conveyors are essential to your operation you can not afford the down time to have the supplier bring in and connect a spare. You would alread need the plumbing, electrical capacity, and space to accomadate the spare while repairing and testing the failed unit anyway. This would really dictate a hot spare capable of carrying the full load unless you can afford the downtime to repair the failed unit or to have the vendor bring in and install the interim machine.

The cost of a hot spare may appear to be high when considered by itself but when compared to the cost of total plant down time it is probably quite cheap. In the olden days when I was working in the mainframe computer world, the cost of an uninteruptible power supply seemed high but when compared to just the hourly cost of 1000+ terminal operators sitting on their hands the picture changed considerably.

I´m afraid that we are all missing the point. The stated requirement is for a conveying application whereas you have all referred to compressed air. Compressed air is not used for bulk solids conveying. This takes a blower not an air comprssor. A blower generates high air flow and volume at low pressure. There can be multi-branch installations served by a common blower. This depends on the product mix and rates according to each user needs. If Guest refers to installing 2 air conveyors this means moving two different products. The question is, Do you need to move them simultaneously? If you don´t then you can probably get away with installing only one 75CFM and save yourself some change.

In the above post I stated that compressed air is not used for pneumatic conveying. For the record this is not correct. Compressed air is used in dense-phase conveying whereas a blower is used in dilute-phase conveying. Care to give more details of your application, Guest?

You confuse your readers by referring to air conveyors then compressed air they require totally different types of equipment. What do you really need?

We are looking at using two of these EXAIR units.

http://www.exair.com/index.php/products/air-operated-conveyors.html

They create a vacuum by injecting air from the compressor through a nozzle. We are looking at these because we have limited space and these look like they will do the best job. Both conveyors will have to operate at the same time.

Thank you for providing a URL for the EXAIR site, I found it very informative. Does your plant have a centralized compressed air supply to serve a variety of functions? It would make sense from purely a reliability perspective to have several compressors connected in parallel. The number would be at least N+1 where N is the number required to actually supply the needs of the plant. The redundant compressor(s) would allow failed machines, or machines requiring routine maintenance to be isolated from the rest of them without compromising the air supply to your plant. Part of the justification for such an approach would be the fact that production would either cease or be severely cut back if the compressed air supply was subject to reduction by the loss of a single unit. The over capacity resulting from such a configuration would also mean that individual units would not be working at the limits of their ratings.

Just a thought from someone who is a little paranoid about failure possibilities.

You can choose either a Rotary Screw Compressor or a Recipcrocating Compressor. One of the many differences is that a Rotary Screw can operate at a 100% duty cyle , while a reciprocating compressor can only operate at 70% to 75% duty cycle. The key to the system is the air receiver size, which serves two functions. The first is that the air reciever is used as a storage reservoir for the compressed air to prevent a continunous run cyle. Second it cools the air which allows the water vapor in the air to condensate into liquid water, which can be drained from the receiver prior to being introduced into the system.

The answer to your question depends as stated by others. If you wish to have one operating conveyor in the event of a problem with the second conveyor, I would recommend (2) seperate compressors. Do you have the space for two compressors and a air receiver? If not then you may be limited to a single compressor.

What is your air pressure requirement? This will determine the compressor size.

I am asuming that the conveyors operate continuously all day.

If you could provide alittle more detail, I can better advise you.

Let me know if you want additional advise.

Best of luck.

In short, A compressor with 150 CFM at the correct air pressure air would work just fine, provided the air receiver is correctly sized for the application.

Given additional data I could compute the correct size air receiver.

Hello,

You could rent a 150 cfm compressor and calculate the actual demand from the duty cycle before you buy. If you found you could get by on 125 cfm you would save considerable $$$ on utility bills. On the other hand if you found you needed 175 cfm you could go after the Equiptment Mfg. for a discount. Either way you'll come out lookin good. IMHO there are too many variables to answer your questions, ex: I had a customer with 2 identical plants 1 in Los Angeles (sea level) and 1 in Reno Nv. (4500 ft) The Plant in Reno couldnt operate at full capacity because the plant air pressure slowly dropped until the process failed. The compressor in Los Angeles Ran At a 80-90 % duty cycle with no air related problems. Compressor cfm ratings are not unlike mpg ratings on cars. Elevation, ambient air temp, and humidity are all being optimized for the test.
Any Compressor salesman should be able to apply rental charges against your purchase.

The least expensive option would be a single compressor of the rating you need for both conveyors. If the process is critical 3 units each sized to operate 1 conveyor would afford you a decient level of redundancy. I would avoid compressors with built in dryers and set the units up as three seperate stand alone systems plumbed into a common header, ie: compressor, tank, dryer, header. Also, avoid dirty or hot environments for the compressor and any refer dryer to live in. Most refer dryers don't like ambient above 100-105 f same with air cooled compressors. Lastly, leave space around the compressors and dryers to acommidate service. Service intervals are normally 2000 - 4000 hours.

Best of luck in you project,

Ronnie

Ronnie

As you have stated that your using it for your air conveyor. Seem you din't state your pressure requirement(assume 110psi)

I would suggest to take 2units of 22kw air compressor (around 125 cfm each) 1 running and , 1 standby

Then opt with 1 unit 1000lit receiver.

Trust me. It work wonder

Thanks