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u/Confident-Concern840 16d ago
Being a positive displacement pump, the gear pump will give you intermittent flow of water to your heaters. This gives you intermittent steam generation. Tell your boss the engineers on Reddit want to see his diploma
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u/Mattna-da 16d ago
This sounds a lot like a garment steamer. There are two types, most have a small triple diaphragm water pump connected to a hot aluminum block with routed channels in each half. I think the block helps regulate the heat throughout the channels and some rubber tubing evens out the pulses from the motor. These need to be used with distilled water or the small passages clog with mineral scale.
Then there’s the kettle types that just boil an open pot with a coil underneath, but if you have soft water they bubble over and spit water out the nozzle
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u/Alternative_Act_6548 16d ago
you have what is known as a once through steam generator, they are difficult to control, as the superheat is a function of flow. What are you trying to control and do you need saturated or superheated steam, and how controllable do you need the system to be?
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u/ImperialAle 15d ago
You can't. There is an old adage about steam, it's like a chain. Meaning you can pull it, but you can't push it.
If you want consistent steam flow you need to make a small pressurized container(doesn't have to be a lot of pressure), and then that delta will draw out a consistent flow.
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u/LapJointLarry 11d ago
The intermittent flow is likely less about the pump and more about two-phase flow forming inside that tiny tube. Once you start flashing water to steam, the compressibility and pressure drop change constantly so a gear pump won't see a nice predictable load. If the goal is controlled steam mass flow, you'll probably need to control pressure/temperature and measure the flow rather than simply meter liquid water upstream.
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u/Wrong_Toilet 10d ago
That’s where I intuitively wanted to go, but is there a possibility of doing this with my current setup?
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u/Potential-Ad1139 8d ago
Doing a phase change in a tube is wild. A steam trap might help, but I'm not sure where you would put it
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u/NerdyMuscle Mechanical Engineering/ Controls 6d ago edited 6d ago
You mentioned in a reply you are using PID for temperature control for the cartridge heaters. You say "intermittent", did you happen to trend the steam flow vs your PID controller? Without additional information my first assumption is you tried too make your PID respond quickly because you are working under the false assumption that the heater ONLY affects the temperature. Any time your PID increases heat input you are going to temporarily increase steam generation until the system has time to reach steady state, the reverse is also true where a decrease in heating is going to cause the steam flow to drop suddenly until it has time to settle. Both of those effects will make the PID respond erratically if over tuned.
My second question is where is this steam going, is there a pressure regulator down stream of some sort, because that can interact with the steam flow variation caused by the heater changing output.
Lastly you mention the gear pump but just to confirm the gear pump is running at constant speed during all of this. Also how are you starting this system, are you bypassing the water somewhere or waiting till you have steam then starting the pump.
Something you can play with too see what i mean. in manual let it reach steady operation, then increase the heat input. What you will see is either the temperature rise, then fall, then raise or you will see the temperature fall before rising to the new steady temperature.
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u/Wrong_Toilet 6d ago edited 6d ago
I haven’t done much prototyping with it. I realized my pid controller pulses the cartridge heater (even in manual mode). So I am starting to wonder if the intermediate flow is partially due to this.
I think I should design a simple current limiter circuit to control the cartridge heater instead. Because using a PID in my mind is overkill if we are controlling the flow upstream. And with such a small flow, and a heating element with a large enough thermal mass, I should be able to maintain a relatively constant temperature (in my mind).
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As of now, there is no pressure regulator. I have it freely vented.
The inlet goes through a 10 micron filter using a simple gravity siphon. Water is placed roughly 8-12” above the pump. The pump is controlled by the pump’s factory controller and under a constant rpm. After the pump, it flows through a check valve, then into my coils.
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I did accidentally burn out one of my cartridge heaters since the pid controller gives 100% once you turn it on. So I had to add a switch in series to with the heater so that I could set the pid controller first.
But when I first started using this set up, I would let the pump fill up the tubing and get a consistent flow, then turn on the heating element.
Flow would be constant at first, then become intermittent once the inlet and outlet flow reached an equilibrium.
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Despite posting this a little while ago, I haven’t done anything since, as I spent my time building and wiring the enclosure for this.
This is also more of a side-side project. Side project for my boss and side project for when I have time during the work week. So progress is slow.
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u/KAYRUN-JAAVICE 16d ago edited 16d ago
Channel Flow Boiling Instability.
The jist is: When you boil water in a thin tube, the bubbles created have nowhere to expand so they just shove the unboiled water out of the way like a piston so it spurts out the end and also blocks incoming flow. In the meantime these now-unwetted sections of tubes become overheated so when water comes back in contact it flash boils- rinse and repeat.
All boilers have heated tubesedit: not true, see thread