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Pressure Puzzler
I have a situation where we need to control the pressure in 2 ducts. This would typically be as simple as measuring both and using the average (or highest) in software, but in this case it is a retrofit operation and there is only one input available. Our first thought is to use two separate 4-20 sensors and arrange them in a circuit that would give us an average on a single 4-20 signal. However, my question is could this be done with one sensor piped to both ducts. ...I just don't what reading that is going to give me.
So my puzzler.... In this layout, what would be the reading at the sensor??? Any help or insight would be appreciated.
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All of my instincts tell me that the answer is (B) ...but I don't really have any science to back it
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Pressure moves from high to low and since the reference is atmosphere, it will read 1.5" (B.)
The open tubing (between the ducts) allows the 1.5" to exert itself on the sensor input. So if the tubing were capped at the 1" duct, it would be a no brainer, right? But the tubing is open to the 1" duct so the 1.5" pressure exerts itself into the 1" duct also. At that point it is a 0.5 delta P to the 1" but to atmosphere it's still 1.5".
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Originally Posted by
icy78
Pressure moves from high to low and since the reference is atmosphere, it will read 1.5" (B.)
The open tubing (between the ducts) allows the 1.5" to exert itself on the sensor input. So if the tubing were capped at the 1" duct, it would be a no brainer, right? But the tubing is open to the 1" duct so the 1.5" pressure exerts itself into the 1" duct also. At that point it is a 0.5 delta P to the 1" but to atmosphere it's still 1.5".
Thank you! This is exactly what I was thinking.
The higher pressure wins out, and air will actually be moving into the 1"WC duct from the 1.5" duct through the tube.... and still just 1.5" at the sensor.
Thank you for responding 
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I'm not a controls guy, but it seems to me that you want a differential pressure transducer, like the kind that are used in VAV box controllers. They are designed to tell you exactly what you want to know. I went to kele.com and searched on "differential pressure" and got a ton of options.
It will be a lot more accurate than what you proposed in the schematic.
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Are you trying to keep both ducts at the same pressure?
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Can you now measure the pressure in both ducts? If so you probably have control of both fans. If you have control of both fans and can measure the pressure in both ducts simple select one fan and duct as lead and have the other fan follow the selected duct pressure. You don't need another static pressure input to do it that way. It's a matter of control configuration.
No man can be both ignorant and free.
Thomas Jefferson
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I really don't understand the OP's question.
Are these two ducts on the same system? Or two completely different systems?
Regardless, normally you would have two sensors each with it's respective controller.
Regarding your A-E question, that would depend on where the tee is. If the tee is closer to the low pressure duct, then it will be sensing closer to the low pressure.
I do a triple evac with nitro to remove non condensables.
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At first I was wondering the same thing BBeerme but if the ducts were on the same fan the pressure would self equalize. If the problem is the fans are on the same duct and need to track each other and are not pressure control isn't going to do that.
If the problem is fan tracking we need to know what control input is available before we can solve that problem.
I didn't get very far into how to control the fans in post #7 because we don't really know the problem for sure.
No man can be both ignorant and free.
Thomas Jefferson
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The only thing we know for sure is that we don't know. Could be the same system, but pressures different due to duct sizing or the sensors down stream of VAV boxes. Maybe one unit on a two story building?
Since this is a retrofit, might it be said that not all aspects were thought out beforehand?
Originally Posted by
WAYNE3298
At first I was wondering the same thing BBeerme but if the ducts were on the same fan the pressure would self equalize. If the problem is the fans are on the same duct and need to track each other and are not pressure control isn't going to do that.
If the problem is fan tracking we need to know what control input is available before we can solve that problem.
I didn't get very far into how to control the fans in post #7 because we don't really know the problem for sure.
I do a triple evac with nitro to remove non condensables.
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I don't know why they would be sensing the downstream pressure of the VAV boxes. I certainly wouldn't want to use that pressure for control.
Hopefully we will get an answer to what he is trying to accomplish. Until then we can't do much for him.
No man can be both ignorant and free.
Thomas Jefferson
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I just realized that I may have misunderstood what you are looking for when I said to use a differential pressure transducer. Now I read this as you need to know the pressure in both ducts, but only can use one transducer.
If your two air lines are of equal length, you will get the average of the two pressures. For most laboratory testing of HVAC equipment where the duct static pressure has to be measured you put a tap in each wall of the duct and run 4 lines of equal length to get the average of the pressure on each of the four walls. It does not matter how long the lines are (up to a point) but they must be equal. The same would apply here.
You need to join the lines with a T-fitting.
Here is a thought experiment: Let's say one duct is at 2 in. wg and the other is at 1 in. wg. Then you join the two with a small diameter straight duct and you use really good transitions that have negligible pressure loss. Air will flow from the higher pressure to the lower pressure duct, starting at 2 in. wg and ending at 1 in. wg static pressure. Velocity pressure stays the same along the length of the duct. The static pressure loss will be linear and will be the average of the two systems at the midpoint of the run regardless of the length of the run.
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One of my thoughts was something along these same lines, but each time I came up with a new thought, all it did was raise more questions.
Originally Posted by
coolcoil
I just realized that I may have misunderstood what you are looking for when I said to use a differential pressure transducer. Now I read this as you need to know the pressure in both ducts, but only can use one transducer.
If your two air lines are of equal length, you will get the average of the two pressures. For most laboratory testing of HVAC equipment where the duct static pressure has to be measured you put a tap in each wall of the duct and run 4 lines of equal length to get the average of the pressure on each of the four walls. It does not matter how long the lines are (up to a point) but they must be equal. The same would apply here.
Here is a thought experiment: Let's say one duct is at 2 in. wg and the other is at 1 in. wg. Then you join the two with a small diameter straight duct and you use really good transitions that have negligible pressure loss. Air will flow from the higher pressure to the lower pressure duct, starting at 2 in. wg and ending at 1 in. wg static pressure. Velocity pressure stays the same along the length of the duct. The static pressure loss will be linear and will be the average of the two systems at the midpoint of the run regardless of the length of the run.
I do a triple evac with nitro to remove non condensables.
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Anything is possible, heck, you probably know that better than me, especially when it comes to air flows and pressures, LOL!
I've seen a few whacked out things along these lines, but all in all, those are really few and far between.
Originally Posted by
WAYNE3298
I don't know why they would be sensing the downstream pressure of the VAV boxes. I certainly wouldn't want to use that pressure for control.
Hopefully we will get an answer to what he is trying to accomplish. Until then we can't do much for him.
I do a triple evac with nitro to remove non condensables.
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This may be one of those wacked out things BBeerme. If so it may be fun to help him resolve the problem.
One of the wackiest things I ran into was VAV boxes with reheat that required a minimum downstream duct pressure before the heat would energize. That was a balance nightmare because the box manufacturer wouldn't relent on that permissive and when balanced per NEBB requirements we had no heat. I had to balance one box with all the diffuser dampers throttled back. I was called in to troubleshoot and only got one VAV to heat and don't know what the balance guy did after that.
No man can be both ignorant and free.
Thomas Jefferson
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When you have a single gauge to measure the water pressure on both the inlet and outlet of a circulator. If you have both valves open at the same time. You read the average pressure, not the highest pressure.
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On a circulator (fan) with both valves open you read differential pressure.
No man can be both ignorant and free.
Thomas Jefferson
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Originally Posted by
WAYNE3298
On a circulator (fan) with both valves open you read differential pressure.
If thats true, then you only need 1 port on a manometer and a barb tee to read the pressure drop across an air filter.
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If you have a single gage and take one reading on the supply with the low side sensing atmospheric pressure that gives you the supply static pressure. If the low side is connected to the return (low) side the result is differential pressure because the reference pressure is lower than atmospheric.
I never measured DP because I had to put both pressures in the report and separate measurements are more reliable. I measured filter DP the same way.
No man can be both ignorant and free.
Thomas Jefferson
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Originally Posted by
WAYNE3298
If you have a single gage and take one reading on the supply with the low side sensing atmospheric pressure that gives you the supply static pressure. If the low side is connected to the return (low) side the result is differential pressure because the reference pressure is lower than atmospheric.
I never measured DP because I had to put both pressures in the report and separate measurements are more reliable.
That would be a dual port manometer.
Not a single port that always references atmospheric preesure.
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