air compressor question..pls help

Try this web site for help with Comressed Air Systems:

http://www.compressedairchallenge.org/

Very good resource! I like the CAC sourcebook PDF file.

1. 6bar cut-out to 5.5 bar cut-in sounds reasonable. How does the shop operate at this setting, and what is the average amount of time between cut out and cut in, or how often is a cut in cycle? Usually a rotary screw loads and unloads slowly so this should not be a motor issue.

2. Oil-less air may NOT be contaminent free, it depends on what additional filtration and treatment is connected to the air system. It also depends on what the end-user considers to be a "contaminent". Cool, clean, dry CO2 would be OK for blowing dust off lenses, but would be a big problem for breathing air applications. Contaminents could either be particulate or chemical, depending on application. Know what the end-user needs or find out the specification for air requirements.

>> It sounds like the throttling valve on the 75HP compressor has settled on a steady flow point to maintain a constant load on the motor. What is the compressors load? Either KW or HP? Is that also constant? It also sounds like the 100HP loading valve is hunting and cannot find the steady position to maintain steady flow. Or perhaps it is connected to a varying demand and varies to meet the demand. Look at the downstream equipment and see if there is a production cycle that matches the compressor loading cycle.

3. See if you can get a flow rate chart from the manufacturer of your equipment.

4. I am not sure a pitot is the best way to evaluate pressure drop through the system. You could use a calibrated pressure gauge to messure pressure at several points (or just mount pressure gauges at several points) and see where the pressure drops occur. I would prefer to sketch out the system, and estimate the pressure drops in several segments. I do not go through a formal calculation, but use one of a couple of charts to quickly guess P-drops (I'm only looking for 10% "SWAG" accuracy). This will usually show where there might be a problem area (as long as the whole delivery system is not the problem area).

As I have gotten older, I have decided to borrow a trick from the electrical system designers to justify oversized piping. Most compressed air systems are designed with the idea if the compressor discharge is 2", then the main header is 2". The US National Electric Code required voltage drop to farthest point does not exceed 5%. A piping system can and should be designed with the same constraint. For a 6 bar system, the pressure at the far end of the plant should not be lower than 5.7 bar.

1. Half bar is ok if it is meeting your needs you should have an idea about what pressure you want shop at eg if running a lot of air tools that require 90 psi to operate efficently that should be your approximate mid point for your load unload settings(if regulators are not being used on hose ends). Their will also be an ideal setting for the compressor to supply air expressed as KwMin divided by cubic metres for efficency this should be taken into account as well

2. what Ried said.

3.compressors should be operating at about 80% load time and 20% unloaded. If you dont have any air recievers on site you should look at getting one for the 75 hp compressor at least, if it is running on load continuosly you have just about reached its maximum capacity at that pressure setting.

If you are having trouble with pressure drop in the factory if possible have ring mains try to avoid a main feed line terminating as this will cause problems with pressure.

A good web site that has online calculations for power usage, calculating pressure drop in piping, sizing air recievers and calculating cost of air leaks is www.kaeser.com.au and click on online toolbox link

I agree with all but the air receiver comment. Unless you have a high pressure differential in the system, receivers do not add much to a system except taking up space.

Air receivers do not add energy or capacity to a system. If there is a high pressure differential, they can store energy too balance load on a marginally sized compressor when the flow rate fluctuates. Otherwise, I try to avoid filling an equipment room with them.

hi to all.thanks for your kind respond on my problem.

post by Ried,

"6bar cut-out to 5.5 bar cut-in sounds reasonable. How does the shop operate at this setting, and what is the average amount of time between cut out and cut in, or how often is a cut in cycle? Usually a rotary screw loads and unloads slowly so this should not be a motor issue."

referring to the question 'how does the shop operate?',to be honest,i'm not quite understand what are you trying to say here.did you mean how the compressed air being consumed in the plant?the compressed air being consumed at a constant rate for a period of time,usually around 3hours and then workers will go for a rest.at this time,the compressor still running.as for the cycling time question,i really cannot determine the cycling time since it varies so much between reading.example,1st reading loading is 7minute,unload is 12sec.2nd reading gives loading 19minute,unloading 10sec.does these reading ok?or the compressor got problem?

by Ried,

It sounds like the throttling valve on the 75HP compressor has settled on a steady flow point to maintain a constant load on the motor. What is the compressors load? Either KW or HP? Is that also constant? It also sounds like the 100HP loading valve is hunting and cannot find the steady position to maintain steady flow.

i dont understand about the loading valve part in the explaination.as for the compressor load,from the data gathered by the energy analyzer,it shows that the KW consumed by the 75HP compressor is constant while the KW consumed by the 100hp compressor is 'up-down-up-down',like wave form..

by Ried,

As I have gotten older, I have decided to borrow a trick from the electrical system designers to justify oversized piping. Most compressed air systems are designed with the idea if the compressor discharge is 2", then the main header is 2". The US National Electric Code required voltage drop to farthest point does not exceed 5%. A piping system can and should be designed with the same constraint. For a 6 bar system, the pressure at the far end of the plant should not be lower than 5.7 bar.

can you pls give me the website link to the web you are referring about this matter?i'm interested to learn more.

Different types of compressors unload themselves in different manners.

A small HP recip usually unloads by simply turning on and off. Larger reciprocating machines will have a pilot that can lock the cylinder exhaust valves closed and let the motor run continuously. The motor is unloaded because it is no longer pumping air, and the motor does not have the stress of inrush current severral times per hour.

The rotary screw compressors I have seen unload with either throttleing the exhaust or inlet valve and limit the compressor capacity. If flow rate is fairly constant, the throttle position will find a set point and hardly move. If flow rate varies a lot, the control will hunt up and down allowing motor current to run up and down.

Since a 75HP motor will draw about 56KW at full load, take the measured current * measured voltage * 1.732 * Power Factor * motor efficiency to determine KW consumed. You should have a number lower than the 56KW. If you don't know PF or motor Eff, use 0.90 for each (SWAG).

Regarding the last point, I cannot point to specific web site, but I am treating the air system as being similar to an electrical system. In an electrical system voltage drop is always there and it is a loss of energy through heating the wire. The NEC limits this loss by establishing the "5% rule". In an air system pressure loss is always there and is energy you spent to compress air being lost by a lower available air pressure. My rational is by sizing the pipe to limit pressure loss, I limit the lost energy.

Try to get a copy of the compressor operation and maintenance manual and review the unloading system operation. It does vary somewhat by manufacturer of equipment.

Go to the International Fluid Power Web, Technical white papers.

http://www.ifps.org/Education/WhitePapers/index.htm

It is all there if you read them.

thanks again to everybody for giving me more answer.there is something that confuse me now.what is the different between nominal compressor working pressure,discharge pressure and system pressure?how to differentiate them?

regarding the rule of thumb which states that 1HP of compressor will produce 4cfm of compressed air at 100psi,how to know my system work at what pressure in psi?

anybody can show me how to do a load profile for my compressed air system?what should be taken into considerations?what should be done to get a good full picture of the system?

thanks

Water normally runs downhill, air normally moves from high to low pressure. When air moves in a pipe or through a piesc of equipment, it loses pressure.

I don't know if there are formal definitions, but see if this makes sense. Nominal working pressure is the range that a compressor is set to worrk around. Discharge pressure is the pressure at the compressor outlet. System pressure is the pressure out near the point of use.

The load profile is the load on the system over a time period, and is dependant on what equipment operates at which time and how it uses air during a cycle. First try to determine what plant equipment operates over a 24 hour period. Is all equipment running 24/7, or are different pieces turned on and off based on production schedules. Is the production process continuous or batch-type. In your particular case, due to lack of instrumentation you will have to intuitively determine what the peaks and flows of the production process and when, how, and how much air is consumed.

I would take a copy of a plant layout, make on it all the compressor locations, then trace and note the pipe size of the main comp. air lines. After this is mapped out, map the location of each air use point, with an estimate of how much air is used. Sum the estimates to get an approximate idea of volume of air in the mains. This is a somewhat tedious process, and can be confusing if there are hundreds of pipes running around. Expect some neck and eye strain as you try to sort it out. Invest in a good mag light to track the piping. I will scan and email a copy of a couple of 1965 charts that I have found to be extremely useful for determining adequate pipe size.

I take it the installation is a large factory, standard pressure losses of a correctly sized compressor and pipelines should come in at around .8 to 1 bar. If it is a large factory and you dont have a ring main and receiver you will have trouble getting equal pressure throughout the factory. You should size and fit a receiver they do save money and help to stabalise the pressure to the factory, even the small handyman compressors have receivers fitted. The idea of a receiver is to have the air stored ready for use, having you compressor running constantly and no reciever only adds to the electricity/fuel bill, maintenance costs and pressure fluctuations.With the 75hp compressor working all time do a leak check first to determine losses and fix them before you do anything else. The following procedure will give you a bench mark to work with.

Measure the load/unload times of the compressor in normal operation for 1 hour. Then stop using all tools etc turn off any machines that constantly use air during operation eg paint stirrers and repeat the test of measuring the load/unload times of the compressor. the difference between the two readings is the losses due to leaks in fittings pipework etc. Vl= Vc * t / T

Vl = volume of leak cubic metres

Vc = Volume flow of compressor cubic meters

t = sum of time units compressor ran underload

T = Total time for measuring procedure

Volume figures can be a guestimate as long as total time measured remains constant during all tests you can measure progress.

Repair your leaks then start working on pressure drop through piping etc

regarding to comment by Ried on load analysis,the production cycle in the plant can be regarded as a continuous process.the compressed air is being used extensively during the work hours and then it came to an abrupt stop in usage because all the workers go to rest.there are many areas that is using compressed air in the plant and i think i'm not able to cover all the area.this is because even the plant management doest know in specific what is the air consumption from each area that used compressed air.i'm planning to cover the area that used the most amount of compressed air.is this ok?or,should i analysis the whole plant air consumption?

as for the comment by Grochy,the plant management doest allow me to make the leakage test.they said if they were to allow me run 1 compressor while there is no production going,it will be a waste of energy.i hope you people know what king of company i'm dealing with now.they keep demanding me to analyze their compressed air system and yet,they know nothing about it!

i have one question here..what is the best way to determine the cycling time (load and unload) of a compressor?if there is no pressure gauge,how to know when the compressor is loading and when it unloads?the current method i'm using is by touching the compressor.there is a slight different in vibration on the compressor outer body and i take the time taken for the vibration to change.

this is my 1st time dealing with compressed air system and to be honest,i'm happy with my progress in understanding it.but i would be even more happy if the plant management can help me to help them analyze their compressed air system.everybody ignoring me here..keep demanding something impossible..i only have a flowmeter for lab usage (borrow from my faculty) and they keep asking me to measure the compressed air usage.its impossible right since the discharge pressure of 6bar is too great for the small flowmeter (T_T) (T_T) (T_T)

I am assuming the compressor is electric, You can data log the ampereage the compressor motor is pulling the higher reading will be when the compressor goes on load and the lower reading will be when the computer is off load their will be a fairly big variance in readings. Something will be switching the compressor from load to unload if it is a mechanical valve set up a switch with a cats whisker actuator and connect the switch to an hour meter. one reading a day will give you the load unload times. If its an electrical switch use a control relay for the same result. You could then use your rule of thumb to calculate usage

The biggest savings you can get with compressed air installation is to repair air leaks and ensure you are not oversupplying air to the factory. EG

3mm hole in air line

Air loss:0,5 cubic metre a minute (6 bar)

0.5 * 60 Min/H = 30 m³/h

30 m³/h x 8,000 h/year = 240,000 m³/year

240,000 m³/ year * $00.02/m² = $4800.00 per year from 1 leak (the 2 cents is what we worked out for our factories usage)

if you have the compressor cranked up to 7 bar to supply the leaks and a usable amount of air to the factory you are throwing money away. After repairing the leaks you will find savings in less load time on the compressor, you will be able to lower the pressure settings and keep the same flow, our factory is currently saving around $3500.00 a month on what we were paying.we did have the compressors set to 7.4bar with .5 bar pressure differential, we are now running at 6.9 bar with .5 bar differential. All we did was have a site audit and repair all leaks throughout the factory.

Following may be help ful

1. cfm delivery varies on the air temp at the intake of compressor. Lower the temperature , better is efficiency. It is better if intake is taken from outside the room,as rooms are warmer.

2 Rough estimation of consumption can be by calculatring in one or two hour , period of run and period of unload.Period of run , take as compressor output and no run as no output.

3.Screw compressors consumes higher energy during no load wrt to Reciprocrating units. If the compressor can be switched off and started automatically it is better. A VFD can be installed for start and stop.

4 To know pressure drop make pressure gauge points at several places on the line. Use the same pressure gauge too note peak pressure at each place. This is to be done on same day and similar ambient conditions.

5.Look at the cut off setting again and see if the range can be increased, so that you have less period of unload.

First comment to post 12 by Dynames07, you do need to limit what you can do within the time alloted to you. If your intuition suggests limiting the area, try to pick an area with the most waste; it you have the time, go for the whole plant approach. I am not surprised plant management has no clue about air consumption, in fact I would guess they are not sure how much the plant spends in energy or where the biigest energy usage is. It is a tough frustration, but I have seens it several times in my experience.

For Mr. C. Ranabir's item 1, cooler inlet air does improve mass flow through an air compressor, but the power demand of the compressor also increases with colder air. I once was on a project where the compressor overloaded the motor when air dropped below 40°F. Outside air may be a small help, but I do not think this will have a big impact.

For Item 2, concept will be close enough due to lack of other data. If this is all you can get use it with confidence since others have not measured and will know less than you. A rough approximation based on measurements is better than a shot in the dark guess.

For Item 3, it depends. In general, a well run recip can be more efficient than a well run rotary screw. BUT . . . most compressed air systems are not well run. A rotary screw is usually setup with a modulating inlet or discharge valve to balance the compressor output to plant demand. Check the compressor manuals to understand the compressors part load operation and determine it the systems are working properly.

For VFDs on rotary screw machines, in theory yes they are more efficient than constant speed operation. HOWEVER, you must very carefully evaluate IF they can be installed without damaging the air compressor. Answer the following questions before trying to install a VFD:

How is air cooled? Will fan speed be affected by VFD and reduce air cooling below acceptable levels?

How much oil circulation needed (for standard machines) and will reduced oil flow affect compressor warranty or longevity?

Are the motors on the air compressor inverter-duty rated? Probably not, since most compressors are bought on the basis of cheapest cost and invertor-duty motors will be more expensive.

Confirm with the compressor manufacturer's, but I doubt the existing compressors could be adapted to VFD operation.

4. Pressure gauges will probably be a bigger help than flow meters. Ideally, using the same gauge and moving it from location to location gives the best accuracy, but pragmatically this will be more work than it is worth, and you probably will be lucky to maintain 10% accuracy. Majority of new gauges are within 2-3%, live with it.

For each separate air system, have gauges on compressor discharge, air drier discharge, and 3-5 plant locations. Make a separate record sheet for each system. if possible take 2-3 readings per day of all pressures. Determine where and when there are high pressure drops in the system. A high pressure drop is a bottle neck that should be investigated and remediated.

thanks again for all the kind response in this thread.

referring to comment by Ried,i do concentrate my effort to evaluate the plant compressed air system but i concentrate it on the section that consumes the most amount of compressed air.is it ok?

2ndly,the company uses nozzle to create air jet for the blowing purpose.the system pressure do have effect on the nozzle orifice diameterand the amount of air being blown right?can anybody guide me through this?

If you think that will be the best chance to develop a cost savings, go for it. There is intuitive logic that supports following that approach.

For second question, which year of engineering studies are you in? I would suggest reviewing your fluid mechanics text books for examples of nozzle flow and the parameters that affect velocity of the jet.

Try this page for Air Flow Through Orificse:

http://www.trident.on.ca/orifice-air-flow.htm

thanks to BudT for giving such a nice table for me to refer to.but,i'm not familiar with it.what is the meaning by 1/64"? yesterday i go and measure the opening of my company nozzle and it is 5mm x 1mm.so,this is equivalent to?

the max working pressure for the compressor is 8bar,normal working pressure is 7bar while the cut in and cut off pressure is 5.5bar and 6.0bar respectively.so,which is the pressure i should take to get the pressure flow through an orifice from the table?

i found a formula which is Q=VA,where Q is the flowrate of the fluid,A is area and V is velocity of the fluid.does this formula sufficient for me to evaluate the plant compressed air consumption?

i also found some website that provide a nozzle calculator.and it mentions about 'throat' diameter.if the orifice is in the middle of a flow,the throat is the area which the fluid passes through right?but in the case of a nozzle,can i say that the throat size is same to the nozzle opening?

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