Vacuum feedback control / PID

[drewbot]

Hi all, new here. I've been pouring over installation manuals for sensors, controllers, drivers, and pumps that just don't seem to fit together. I would appreciate just a little direction if possible in order to figure out a feedback single setpoint control method.

My aim is to achieve a constant vacuum in an evaporation chamber over a distillation process. It's a small rotary evaporator machine with overpriced and underperforming out-of-the-box vacuum controller and vacuum.

The vacuum is driven by a 1 phase 300W scroll pump, so I expect that I would need to use a vacuum pressure sensor and gague, controller, and a drive (VFD).

My question is what is the smartest way to start picking out components? Do I search for a communication protocol that is standard to vacuum gauges and go from there or should I find a device which integrates a gauge and controller? I'm just trying to find a starting point.

 

[NetNathan]

Several stand alone vacuum gauges are on the market that support almost my communication protocols in addition to analog output across the range.

If the vacuum gauge is seeing other gases, my strong would be a "Capacitance Manometer" or CM.
A CM pressure reading is not affected by the type of gases in the residual atmosphere at one of the most accurate vacuum gauges.
MKS is one of the best, they call their gauges "Baratrons".
Inficon makes some good ones also.

It would help to know the vacuum range you expect your equipment to operate at.

 

[drewbot]

Several stand alone vacuum gauges are on the market that support almost my communication protocols in addition to analog output across the range.

If the vacuum gauge is seeing other gases, my strong would be a "Capacitance Manometer" or CM.
A CM pressure reading is not affected by the type of gases in the residual atmosphere at one of the most accurate vacuum gauges.
MKS is one of the best, they call their gauges "Baratrons".
Inficon makes some good ones also.

It would help to know the vacuum range you expect your equipment to operate at.

Thank you for your reply! I forgot about the pressure range (oops). It's around 10^-4 torr to atmospheric. I expect that if I could get a gauge that's chemically resistant I'd probably opt to use it.

I've been looking at MKS and I did a quick check of the Baratrons-
One scenario I'm envisioning using a digital Baratron conected to an "Allen Bradley" Powerflex a/c drive fitted with a Powerflex DeviceNet adapter. Does MKS have motor drive capabilities? Is that the obvious piece I'm missing?

Should I just go with analog operation? I see many more options and a lower pressure range overall. Probably more ebay controllers in analog as well. This is going to be a project that continues to expand and I'm trying to learn which devices and protocols I aught to start messing around with. Thanks again for the advice!

 

[NetNathan]

MKS makes flow control system that go with their Baratrons.
However you should be able to feed the analog signal from the CM to the PF (Powerflex) VFD and let the VFD control pressure with its internal controller. Pressure control is a common application for a VFD, so there is already a start-up configuration built in to most VFDs. Pressure control is close enough to the same as vacuum control (vacuum is still a "type" of pressure) however the PID control direction may be direct and not reverse.
You can hook DeviceNet (If you already using DeviceNet) to the PF also for comms. to a PLC if needed.

 

[iant]

how crucial is the vacuum - do you need it to be constant (PID control) or just above a preset value (Greater than.
the requirements dictate what you actually need.

 

[TConnolly]

It is very difficult to control vacuum. This is because the difference between 10^-4 and 10^-2 torr represents a tiny minuscule amount of gas but at the same time it is a hundred times the pressure. The best way I've found is to place a motor operated vacuum rated butterfly valve in your foreline and drive the valve to a predetermined position. You usually have to determine this position empirically. Then introduce a sweep gas and modulate the flow of the gas to control pressure. A distillation process compounds the problem because you have a product that is converting to gas, and if that conversion rate changes you will probably find that you need multiple sets of PID gains to use at different points in the process.

 

[NetNathan]

It is very difficult to control vacuum. This is because the difference between 10^-4 and 10^-2 torr represents a tiny minuscule amount of gas but at the same time it is a hundred times the pressure. The best way I've found is to place a motor operated vacuum rated butterfly valve in your foreline and drive the valve to a predetermined position. You usually have to determine this position empirically. Then introduce a sweep gas and modulate the flow of the gas to control pressure. A distillation process compounds the problem because you have a product that is converting to gas, and if that conversion rate changes you will probably find that you need multiple sets of PID gains to use at different points in the process.

I believe this method will work better than trying to control vacuum level by controlling scroll pump speed.
If your pump can handle the additional sweep gas.
It sounds like a pretty small system, a 300W scroll pump only has about a 100L per minute pumping speed (3.5 cfm).
Maybe you can use a valve with "control orifice" valve be used to control pressure rise caused by only the evaporated gases.

I have used these methods (the motor drive or "proportioned air" controlled butterfly valve and/or a "control orifice" valve) far more than a VFD on the vacuum pump, however I regularly use a VFD on vacuum "booster" pumps, when they are required.

However...using a VFD on scroll pumps is a growing control method. There are some manufactures that sell scroll pumps with built in VFDs now.

 

[TConnolly]

Nathan, by booster are you referring to a roots type pump or something else? Most of the systems I work with consist of a roughing pump(which might be a scroll type or a rotary piston (Stokes) type, a roots booster on the roughing pump inlet and a diffusion pump for high vac. Because the roots booster is so sensitive to thermal expansion when pumping at near atmospheric pressure I usually don't allow it to turn on until below 10kpa (~75 torr). Once its low enough to run safely I want to run it at full speed, so you statement has me curious. If it is a roots type booster, what is the advantage a drive gives you?

(sorry for the hijack)

 

[shooter]

I would go for a bleed valve, controlled by a simple electronic pressure sensor, not the very expensive precise ones as this is way overkill, the amount of vacuum is not very important in a destillation process.

 

[NetNathan]

Nathan, by booster are you referring to a roots type pump or something else? Most of the systems I work with consist of a roughing pump(which might be a scroll type or a rotary piston (Stokes) type, a roots booster on the roughing pump inlet and a diffusion pump for high vac. Because the roots booster is so sensitive to thermal expansion when pumping at near atmospheric pressure I usually don't allow it to turn on until below 10kpa (~75 torr). Once its low enough to run safely I want to run it at full speed, so you statement has me curious. If it is a roots type booster, what is the advantage a drive gives you?

(sorry for the hijack)

Yes, I am reffering to the roots style pump as a "booster" pump.
In out normal setup we use a Stokes 412 mechanical pump (400 CFM) and a Stokes 615 (Roots style - 2000 CFM) booster pump.
The 615 usually has a normal start point of 15-20 Torr utilizing a pressure switch. If you have one of these systems you hear the "hard" start of the motor instantly wanting to spin up this pump. Lots of bearing and belt wear caused by this hard start.
I use a VFD running in "Current Limit Mode" (as I call it)...meaning when the motor starts, it goes to max set current and will not go over.
I start the pump at about 730 Torr and it slowly speeds up as the pressure lowers, caused by reducing motor load.
This eliminates the hard load on the bearings and the belts, reducing "wear and tear"....Maintenance loves this benefit...
It also reduces the pump down time. On our normal vacuum system, the time to 20 Torr is reduced by 15-20 minutes.....Production loves this benefit...
Win, Win..

 

[iant]

Why wouldn't you use a Soft starter instead of a VFD - cheaper

 

[NetNathan]

Why wouldn't you use a Soft starter instead of a VFD - cheaper

Once a soft start "Starts", it does not go back into current limit if the motor current increases, the initial current limit time is close to 30 minutes or more, soft starts do not do well at limiting current for that long, plus it has a few more limitations in this application.
I also actually intend to "over speed" the motor below 20 Torr as the load reduces, further increasing pumping speed. I am experimenting with this now. The motor is a 1750 rpm and I plan on testing running it at faster speeds (higher frequencies) to see what gains there are from 20 Torr to 5x10-2 Torr which is where I "cross Over" to high vacuum pumping and the mechanical/booster pumps become fore pumps for a Diffusion Pump..

 

[iant]

30 mins is a very long start time
- thanks for the info

 

[Highland Controls]

One thing that I don't think anyone has mentioned is that you can't use a VFD on a single-phase motor.

 

[NetNathan]

One thing that I don't think anyone has mentioned is that you can't use a VFD on a single-phase motor.

OOps....minor detail.
Oh yea...that too...

Back to the control valve system....