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.
