Compressed air humidifier control equipment help?

Rsanderson · ‎07-21-2010

Hello all!

First off, I apologize if this is in the wrong forum board. I'm not too sure where it belongs.

I'm a recent college graduate Mechanical Engineer with very limited Labview experience - mostly controls class lab with pre set up systems and very simple vi programming.

At the company I now work for I've been tasked with building a compressed air Humidifier and Heater unit to test our product under harsh real world type conditions. I've been given a fairly good budget, though it will need to cover operating hardware as well as the control hardware (I should already have the Labview licenses I need). Since I realize I have just enough Labview experience to make me grossly overconfident in my abilities and to get me in a mess, I'm hoping I can come to these boards for a lot of advice over the next several weeks. Though I'm going to have to figure out the programming later, right now I'm trying to select hardware.

A brief description of the system: It’s going to be a portable pressure, temperature, humidity, and liquid water flow control system. Compressed air will enter the system and flow through piping where warm liquid water will be injected. The air and water will be heated in the piping and the water should evaporate. They’ll enter a coalescing filter chamber where any remaining liquid water should be absorbed. The coalescing filter will act like a “wet rag” and allow some of the absorbed water to evaporate and humidify the air. It will also prevent any liquid water from continuing through the system. (so the amount of liquid water that is injected can be accurately controlled)

After the coalescing filter, the humid air will reenter pipes, where liquid water will be injected. The humid air/liquid water should leave my system and enter the attached test set up at a set pressure, temperature, humidity, and liquid water flow rate.

I think I'm going to need to use Labview for five control operations.

1) Air Temperature

2) Air Pressure

3) Water Temperature

4) Water (Humidity) Injection Flow Rate

5) Water (Liquid) Injection Flow Rate

I’ve attached a power point file with the equipment and control diagrams I’m putting together; hopefully they’ll make sense. If you need any further detailed explanation of what my system is trying to do, just let me know. For each of the control operations I need to run, I’ve done some research into what I’ll need:

1) Air Temperature: I plan to use either flexible heating tape wrapped around copper piping and the aluminum coalescing filter chamber, or a combination of heating tape around the aluminum chamber and a tubular heater of some kind inside the copper piping. The aluminum chamber should act as a temperature stabilizer somewhat, but I’d really like to implement some kind of PID or TBH control instead of just an on/off for the heaters. The problem I’ve run into is that DC heaters really don’t exist for the application I need. At least, I haven’t been able to find any. So I need some way to control the power into AC heaters. Further research is indicating to me that since I can’t afford a lab grade controllable AC power supply, I should use a PWM circuit of some kind, but that’s as far as I’ve gotten. Can you help me figure out what PWM device I can use to control an AC heater, and what else I’ll need to make that work?

2) Air Pressure: This should be simpler. I only need to control the input Air Pressure when the humidifier is hooked up in a lab that already has a compatible pressure regulator. Otherwise we’ll control the pressure manually. So what input and output devices will I need to send a voltage or current signal (not sure which is uses yet) to the pressure regulator, and to measure input signals from a pressure gage?

3) Water Temperature: I want to preheat the water before it’s injected into the system to prevent a temperature drop in the air as much as possible, and to improve the warm up time of the humidifier. I’m likely going to use an inline immersion heater, and again it looks like it’ll need to be AC powered, likely with a PWM.

4) Water (Humidity) Injection Flow Rate: I’m injecting water before the humidifier that will be evaporated in the coalescing filter chamber. I’m still working on finding a pump that meets the system’s requirements, but it seems likely I will have to hook it up to a VFD, and that VFD will have to be able to receive an input from a computer. What input and output devices will I need to be able to take in a relative humidity measurement and a flow rate measurement, and output a pump speed control signal?

I’ve found a possibility that might simplify things:

http://www.laballiance.com/Pdf/active/pumps/Flash150DataSheet.pdf

This pump, modified slightly to decouple the individual piston pumps, will likely work very well for my system. If I use it, it would simplify my control set up, as I would not have to use RH% and flow rate measurements to generate a pump speed, but would only have to use RH% to generate a flow rate control. Whether I can use this pump will depend on its price, so advice based on either configuration would be great.

5) Water (Liquid) Injection Flow Rate: This would be the same setup as 4), but the control would be based solely on the injection flow rate.

I think that’s all I’ve got. If you can provide any help, suggestions, or advice at all, I would greatly appreciate it.

Thanks,

Ryan Anderson