Pneumatic

[integrator]

Good-day,

We have two pneumatic valves that operate by two seperate stats. We are looking into just using one stat, to control both valves.

Does anyone know of a pneumatic device that can use a 0-10VDC \ 4-20MA input and provide two pneumatic outputs.

Thanks

[simux]

Johnson Controls has a transducer that accomplish this task. I think the part number is EP-8000-2 for voltage and EP-8000-4 for current.

[DiodeCutter_2K]

I suspect all controls companies make a DDC to pneumatic transducer.

In any event so does Kele and they work well.

Keep your air dry and clean by installing a small descicant filter in the panel.

[Control Man]

Why the need for 2 outputs , single output and TEE the branchline to the actuators.

[integrator]

Sorry about this, but I didn't explain the what we need.

We only have one output 0-10VDC or 4-20MA from the controller. One valve is the main heating valve and is controlled by the DDC. The other valve is the reheat, and is controlled by a pneumatic stat and not by the DDC. The issue we have is one is trying to cool and the other is heating up.

What I was looking for, is a device that at 0-5VDC could operate the first valve and 5-10 could control the reheat.

thanks

[Heavyevans]

Not sure about the device, but can you change the spring rate on one of the valves?

[jdd67]

I have not done pneumatic work in a long time, but I do remember using multi-purpose relays for similar situations. See attachment.

[printer2]

Bellofram I/P or E/P. Many people rebrand the device as their own. ControlAir Inc model is the 500X. Adjustable zero and span so you can configure it how you want, Also for split range aplications.

'(ControlAir part number 500-AC) can be recalibrated to provide a 3-9 psig or 9-
15 psig output.'

[viceman]

kele, mamac etc offer 0-10, 4-20 to pneumatic transducers.
These are sweet http://www.mamacsys.com/EP-311-313features.htm

you can use a signal convertor from kelehttp://www.kele.com/olcat/ot8/10372.html

with zero and span to alter the signal to the transducer for the pneumatic valve, (i think you have to tell them what your input vs. output is and they will configure it for you.)


or do what heavy sugested (which would probably be cheaper and more reliable) and change spring rate for the pneumatic valve . either way you will have a abortion.

trying to do it on the cheap usually costs more money. one call back to reconfigure something will cost you more money in labor than to do it right.

if you only have one AO available you can drive two electric valves off one AO singal . 4-20 ma wire it in series 0-10 wire it in parallel.

[jogas]

Sorry about this, but I didn't explain the what we need.

We only have one output 0-10VDC or 4-20MA from the controller. One valve is the main heating valve and is controlled by the DDC. The other valve is the reheat, and is controlled by a pneumatic stat and not by the DDC. The issue we have is one is trying to cool and the other is heating up.

What I was looking for, is a device that at 0-5VDC could operate the first valve and 5-10 could control the reheat.

thanks

I think you are describing a classic Constant Volume Reheat System.
Be careful...as not providing 55* supply air ( i.e. "Main [AHU] Heating Valves" ) to the re-heat coil inlet will prevent the required de-humidification required for occupant comfort...and you will be sorry when it's raining.
My 2 cents.

[teamkudzu]

Install a pilot positioner on one valve to set the stroke offset if both work at the spring range currently.If they are both "odd" spring ranges but a positioner on both.
Sorry about this, but I didn't explain the what we need.

We only have one output 0-10VDC or 4-20MA from the controller. One valve is the main heating valve and is controlled by the DDC. The other valve is the reheat, and is controlled by a pneumatic stat and not by the DDC. The issue we have is one is trying to cool and the other is heating up.

What I was looking for, is a device that at 0-5VDC could operate the first valve and 5-10 could control the reheat.

thanks

[blabath]

The simple solution, as suggested, is to have sequential spring ranges on the actuators for the valves. For example a 3-8psi spring range on the heating valve and a 8-13 psi spring range on the reheat. Simple matter at this point to set up your pneumatic transducer to deliver 3 psi at 0% call for heat and 13 psi at 100% call for heat, assuming reverse acting valves. Reverse that for direct acting valves.
This should work for what you describe. A little tweaking may be neccesary for your exact situation. For example, what type of air handler has a "main" heating valve and a reheat. You might mean pre-heat and reheat, or you might mean hot deck and reheat. Not sure what type of sytem you are describing.

[blabath]

O.K. The more I read this thread the more I MIGHT understand this. If you have DDC controlling a valve in an air handler and pneumatics controlling a reheat valve you most likely have a VAV system.
Is this so?
If so, are there several pneumatic stats controlling several VAV boxes with reheat, or just one?

[simsd]

Sorry about this, but I didn't explain the what we need.

We only have one output 0-10VDC or 4-20MA from the controller. One valve is the main heating valve and is controlled by the DDC. The other valve is the reheat, and is controlled by a pneumatic stat and not by the DDC. The issue we have is one is trying to cool and the other is heating up.

What I was looking for, is a device that at 0-5VDC could operate the first valve and 5-10 could control the reheat.

thanks

I don't understand what you are trying to say here. You say they are heating valves, but then you say one is trying to cool and one heating up - what does that mean? Can you explain clearly what you have and then what you want to do. None of this really makes any sense, yet my suspicion is that you actually have something very simple to solve.

[hueyman]

We used Honeywell pneumatics for years for controlling actuators. There are all kinds of inexpensive pneumatic relays, reversers, lockouts, selectors etc... that we used to stage sequences from one input or multiple inputs. Can you put up a diagram of what you have?

[integrator]

Here's the pic

[simsd]

Just as I thought - something really simple.

Okay, I see you are in Canada, so usually the valves are Normally Open on an air loss. The one in the unit should be N.O. in order to prevent a cracked coil but we need confirmation.

It looks as though the reheat to the room is Normally Closed but I can't tell becasue the writing is so small.

What is the spring range of each - this is very important. If you have the make and model number we can also go from there.

I am under the assumption you want to use an electronic room sensor and sequence the valves through your DDC system accordingly, using the heating coil in AHU as the first "stage" and if it cannot keep up then the reheat valve will start to heat up the room. Correct?

From what I can see, if the AHU valve is N.O. and the Reheat valve is N.C., then you will need a reversing relay to make this work AND the spring ranges of the valves.

[integrator]

hello,

Use we would like to sequence the valves through the DDC system. The first valve has a range of 9-13 PSI. The second valve - I'm assuming has a range of 0-15PSI as it's controlled by a room pneumatic stat.

What I would like to do - is from our DDC 0-5 VDC would produce 9-13PSI for the main valve. Then 5-10VDC would produce 0-15PSI for the second valve.

I was hopeing to keep this simple with one device, that could take in one input 0-10VDC and produce two pnematic outputs.

[WhoIsThat?]

The first valve has a range of 9-13 PSI.
Use a 0-5vdc input valve

has a range of 0-15PSI
Use a 0-5vdc input valve

from our DDC 0-5 VDC would produce 9-13PSI

0v from the DDC produces (5)9/13 = 3.46v for 9psi and
5v from the DDC produces 5v and 13PSI

Then 5-10VDC would produce 0-15PSI for the second valve.
5v from the DDC produces 0v and 0 PSI and
10v from the DDC produces 5v and 15PSI

one device, that could take in one input 0-10VDC and produce two pnematic outputs.

I can draw a circuit that does this, but finding someone who does this on an other-than-custom basis might be difficult.

This circuit would need a comparator, opamps and a level shifter. Parts cost a ten or so, labor a weekend.

Or, it would use a (commercially available?) programmable microprocessor card with 0v to 5v outputs and an A/D input, then you do the programming.

[hueyman]

If i understand correctly, both valve actuators are pneumatic. One controlled by transducer and one controlled by pnematic stat. Cheapest way would probably be to feed 2 seperate pneumatic controllers with the existing transducer output. adjust each controller output for the span and proportioning you need for each actuator.

[shifferbrains]

Looking at your drawing brings some questions to mind. The first one is...... Is the system really in need of a change? Or is perhaps something just a little out of whack?

Your drawing reflects a two pipe heating and cooling system??? Does the system shift from heating to cooling automatically?

The note at the Air Handler valve says “Valve Controlled by DDC to maintain Room Temp Setpoint” The note at the Reheat Valve says “Reheat Controlled by pneumatic Stat in Room”. Do we actually have two different controls doing the same job? Or should the note at the Air Handler say... Valve Controlled by DDC to maintain Supply Air Temperature SetPoint?

If the latter is true then perhaps do you have the supply air temperature setpoint set to high causing the room to overheat? Or could your room thermostat be a bit out of calibration?

These are just a few questions that came to mind....

[viceman]

http://www.mamacsys.com/EP-311-313features.htm