air compressor sizing 2

[babajaan]

Hi All,
this is my first question on the forum. and it might be a pretty basic question, but any help on the topic will be great. I'm trying to size an air compressor for the pneumatic actuators in a water plant. I'm having a bit of difficulty finding the right logic to doing it. I've come up with a total volume required for the actuators to open & close, but am lost as to what to do with this volume. Any ideas?
thanks,
Sarah

 

[TD2K]

Are these actuators on control valves that are going to be opening and closing more or less constantly to control the process or are they on/off valves that will open, sit for a while and then close?

If you have mostly control valves, typical approach is to assume each valve consumes so much air constantly, I've seen 1 scfm used. Add up all your valves and that gives you an idea of the size of the system you need along with a design margin.

If you have a bunch of valves that switch from one postion to another, you need to consider how many valves could 'switch' at one time and how fast the valves should switch to determine the air flow rate. If the valves aren't moving, the air consumption should be very low or zero.

In either case, your control system vendor should be able to help you out.

Remember also on an instrument air system you have an air receiver to provide some backup volume. If you have short peaks of use, the air receiver will supply additional air as the system pressure drops so you don't need to size your compressors for that short, peak load (you can quickly estimate how much air the tank holds based on its volume, its normal working pressure and minimum air pressure you want to have. Even a fairly large tank doesn't hold a lot of scf of air though). Once the air 'demand' is over, the compressors can repressurize the receiver for the next air demand cycle.

 

[MJCronin]

baba,

TD2K gives good advice....

If you have say,..... 50 valves, I would use a "service factor" of 0.20 (valve operation only 20% of the time)
So the continuous demand would be only 50(.2)= 10 SCFM.

However, my experience has been that the plant's compressed air system is the one system that changes most during the life of the plant.

Someone always wants to add an air-operated diaphragm pump or another location for a service air drop.

Contact a helpful vendor of compressors (like KAESAR or Sulair) and ask the tech rep for help....they have standard methodologies for system sizing. Make sure that you get a realistic list of probable future air-users from your boss.

(then add 25% to get your final SCFM estimate !!!)

MJC

 

[ugwuegbuchima]

Air compressor sizing depend on the vol. of air which the compressor can disperse(cfm)and the pressure at which the air is being dispersed(psi).Since you have gotten the required vol. for your job,I believe you don't have much problem as regards to compressor size.
1. If your vol. is within the range of 5cfm @ 100psi,use a 2-5 Hp air compressor.
2. If 5-20cfm, use 5Hp.
3. If 20-40cfm,use a10Hp air compressor.
If your vol. did'nt correspond with these one's, let me know.

 

[1969grad]

My personel philosphy is to size the air compressor to provide sufficient air as if every user is operating simultaneously. I do not normally count intermitent users such as air tools. Frankly, I do not want to get any phone calls about the plant shutting down because the air system pressure was too low.

Using this philosphy will normally result in the air compressor cycling on and off. If the air compressor runs continuously then it is too small.

If the service is critical then provide two air compressors with a total air output equal to the above. Then, link the two air compressors together with lead-lag control.

Using lead-lag control, the first air compressor starts on falling pressure. If the pressure continues to fall then the second air compressor starts. Lead-lag controls also switch which unit is the lead unit so that operating hours are equal on the two units.

Always include an air receiver. For small air compressors they can be furnished tank mounted.

I also provide air receivers near big users or a group of users located considerable distance from the air compressor to reduce the impact of header pressure losses at high flows.

Further to the previous thread, recip type air compressors are used in hp sizes less than about 30 hp. For larger size air compressors use a screw type.

 

[ugwuegbuchima]

Just want to give you the other volumes and air compressor size needed for the volumes.

(A) For (40-60) cfm vol. At 100psi,use (10-15)Hp air compressor.

(b) For (60 –81)cfm vol. At 100psi,use 18Hp air compressor size.

(c) For (81-137)cfm vol. At 100psi, use (30-31)Hp air compressor size.

(d) For (137-196)cfm vol.At 100psi, use 44Hp air compressor size.

(e) For (196-254)cfm vol. At 100psi, use 56.5Hp air compressor size.

(f) For (254-338)cfm vol.At 100psi, use 75Hp air compressor size.

(g) For (338-548)cfm vol.At 100psi ,use 122Hp air compressor size.