air flow through a flexible hoe at 100psi 1

[SammyG]

I have a simple question, I need to pump 500 cfm of dry air into a drying station for painted parts. I am in Costa Rica, and do not have time to wait for a solution, due to a lack of painted parts at this time. I would like simply to run an air hose from our HUGE compressor system (multiple air dryers) into the heated drying tank without any restrictions. The tank will have very little back pressure, and this job does not need to be accurate, the parts simply need to dry as fast as possible. I have looked everywhere, but have not found any manufacturs data as to what the flow rate would be at 100 psi( about 25 degrees celcius ambient temperature)(it will change from morning to night etc) for certain hose sizes. Does anyone have a rough idea what size hose would work? 1/2, 3/4, 1 inch? Thanks,
Sam

 

[Montemayor]

Sam:

You're headed straight for a "choked flow" condition and you're going to need an industrial quantity of 1" hoses to try to handle 500 cfm (even if you mean Scfm @ 60 oF and 1 atm). It'll be a mess or a war zone - whichever suits your imagination.

You haven't stated the pressure of the air you're trying to hose up, but I assume you have instrument-type compressor/dryer installations doing the supply. What you need is an axial fan or blower - but this can't give you the pressure to supply an air dryer - which you haven't described or defined.

You're going to require a HUGE hose if you insist on going that route, unless you are dealing with very high pressure air in the hose and expand down as you enter your drying room. Is that what you want to do? You haven't explained in detail your basic scope and data, so you're forcing us to guess and re-guess. That's going to take up a lot of your precious (& our costly) time.

Give us all the details, please.

 

[SammyG]

The drying station only need to recieve fresh air and does not require pressure. Yes the blower is what one would want, but concidering we have nearly 100 percent humidity reght now, the air is not drying the parts fast enough. Our 3 compressors are as as big as 20 foot shipping containers, and have enough capasity to easily handle the output of our hose. I unfortunatelly did not mis-type, and do need 500cfm of airflow. I may try to leave the blower (we presently have one attached that gives 300 cfm of ambient 35 deg cel 95 percent humidity air) as it is, and only add an extra 200 cfm of dry air with the compressor hose to help accelerate the drying process.

Thank you very much for your input,

Sam

 

[Montemayor]

Sam:

Thanks for the rapid feedback. First, let me assure you that I believe I recognize the dilemma you are facing. I suspect you find yourself with a parts painting project located in the lowlands of Costa Rica (Puerto Limon or Puerto Quepos?) instead of the highlands (like San Jose). Why a paint project was located in the lowlands, notorious for high humidity and high temperatures is something I prefer not to get into for now. Suffice it to say that I believe your use of high-priced compressed air might be your best temporary solution –assuming the costs be dammed for now. I know Costa Rica well and I can appreciate what it means to be in a taxing situation with no backup or front office help.

Your compressed air, I assume, is obtained from air compressors and is after-cooled down to a condition where it finds itself in a tank at ambient temperature (35 oC) and 100 psig. If you have cooled the air down to 35 oC after compressing it to 100 psig, you have removed the bulk of the humidity it had originally before it entered the air compressor. Therefore, this air is relatively “dry” compared to your ambient air and could offer you a quick drying cycle for your painted parts. I believe that you understand perfectly that this is going to $cost$ and the bill will be high when you receive the electric costs for the air compressor(s) operation. Now let me explain what you are up against so that you can take the appropriate steps to operate as you suggest – but doing it in a safe and logical manner.

The maximum velocity of air in a pipe is limited by the velocity of propagation of a pressure wave which travels at the speed of sound in the air. Since pressure falls off and velocity increases as air proceeds downstream in a pipe or hose of uniform cross section, the maximum velocity occurs in the downstream end of the pipe or hose and the exit velocity will reach the velocity of sound. This effect occurs when the “critical pressure ratio” exceeds approximately a value of 2. In other words, when the upstream pressure is approximately twice that of the downstream pressure, you will get sonic (or “choked”) velocity. When this velocity is reached, you will get a maximum of air mass flow rate flowing – and no more than that. Therefore, it behooves you to make the hose as big as you possible can – and the valve that is connected to the air tank should match that same large size, at least. What I suggest you do is put a large globe valve at your air tank – approximately a 4 or 6” size and rated for the full 100 psig you will impose on it. Connect a pipe or hose to this block valve. Make this hose of a much larger size and as short as you can before it enters your painting tank. I would make the pipe or hose 10” or much bigger, if possible. The bigger, the better because it will build up less pressure and allow more air through. You should throttle the block globe valve slowly, attaining air expansion and sonic flow through it and into the pipe/hose and on into your paint tank. Make sure your block throttle valve is located as high up on your air pressure tank as you can put it. The reason for this is that there will be condensed water in the bottom of the air pressure tank and you don’t want to entrain it over. You want to initially throttle the air into the paint tank carefully to make sure the connecting pipe or hose can withstand the pressure build up within it due to friction pressure drop. That’s why the bigger the hose the more air flow you will have and the faster the drying. Feed this air on one side of your tank, allowing it to pass through all the painted parts in order to maximize its drying action before exiting out to the atmosphere. You may get a lot of noise and vibration from this type of bailing-wire type of operation, but that is why I would prefer you use short-length, large diameter pipe instead of hose.

My concern would be about safety in the strength of the connecting pipe and the manual throttling. I would weld steel pipe to the paint tank and use flanged connections of no less than 10” size. If you can use a large expander diffuser at the entrance to the paint tank, this will reduce the outlet velocity and allow for better controls. Don’t forget, you’re dealing with sonic velocity and that is why you should use as big an entrance pipe as you can justify. The estimated velocity of the air as it leaves the throttle valve will be 1,100 ft/sec! Without knowing what size valves & piping you have available, precise calculations can’t be done. However, you can throttle and try to get as much as you can get through. You should realize that you are limited by the sonic velocity going through the throttle valve.

This sonic limiting effect is what you run up against with compressible flow such as gases and air. Sorry, for the bad news but that’s what I was trying to communicate to you on my first post. That’s why a blower with a low discharge pressure can move more air – albeit very humid air in your case. I going to presume that you will try to dry the compressed, 100 psig air with refrigeration dryers or adsorption dryers in order to use as little compressed air as you can and dry as fast as possible.

Good luck and make sure you understand the safety concerns when you throttle through a sonic condition of expanded air.

 

[SammyG]

Thanks so very much!! The compressors we have here have air driers attached to them, we are a company of over 1,300 people running a rubber factory, this paint project is part of this process, later we need to bond the rubber to the painted metal parts. We do not need to run very many parts through this drying station, only parts beirng made for our australian martket. We are located however near Jan Jose, but right now we are entering the rainy season, and the humidity is rising (where the drying station is loocated, there are many heated waterbased chemical tanks which add to the humidity as well. I will leave the low pressure blower attached, and will run a seperate air pipe into the tank (If I can add 200 to 250 cfm into the tank, it should be more than sufficient, the tanks are heated to 105 deg centigrade as well. I am only haveing big problems with drying times, the paint needs to dry within 5 min, or it leaves noses an the parts where there are drips, and presently I need over 15 minutes to do the job. We designed a new system being installed presently in Texas using vacuum drying stations, this works very well, but is presently here not feasable.

Thank you again for everything, and if you visit C.R. again, let me know!!

Thanks,
Sam