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What Exactly Is Positive or Negative Pressure?? 1

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edtpro

Industrial
Nov 15, 2010
24
We provide maintenance and facility management for healthcare facilities throughout the southeast. I am NOT an engineer, so I apologize if my question doesn't rise to the technical level it should. Please forgive.

My question involves checking ventilation for ORs, Procedure Rooms, Soiled Rooms and Clean Rooms. I know these are required to maintain either positive or negative pressure. My company has us check that in a couple of ways. First, there is always the tissue test. If the room is pressurized enough either way it will flutter in or out. Mostly, though, we use a meter off of an Alnor EBT 721 with a tube that slides under the door and get a quantifiable reading.

So my question is, does a reading of .0001" to say even .0005" wc really mean that the room is "positive?" At that level the tissue doesn't even move. Wouldn't that be considered a technically neutral room? Also, when reading the codes in states like NC and FL, I see that they call for Positive or Negative, but I don't see where they quantify that (ie - what constitutes "positive")

Is it a certain CFM of supply over Exhaust/Return? Is it a minimum .001" wc? Would something as low as .0001" wc qualify? Same questions for negative rooms too. Just need some not-over-my-head-technical type guidance.
 
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Positive pressure will occur when the pressure in the pipe/duct/room is greater than the pressure outside of it - so flow would go out of a room. Negative pressure means the pressure in the pipe/duct/room is less so flow would go into the room. These are all gauge pressures, which can be positive or negative since they are merely measuring a relative pressure compared against some constant pressure, typically atmospheric which is about 14.7 psi.

There could be a slight negative or positive pressure, but not enough to cause the tissue to move, because the tissue is less sensitive than a gauge that could read .0001" w.c. It is like measuring the volume of water with a 1/4 teaspoon or a 55 gallon barrel. In the barrel, it would look like a wet spot, but would fill the 1/4 teaspoon.

Hope that helps.
 
Positive pressure is only a means to an end; the absolute value of the pressure differential is irrelevant, so long as air flows from the higher pressure to a lower pressure. Since pressure is easier to measure than air flow, people tend to be lazy and only measure pressure. To me, the tissue test is a more meaningful exercise.

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529
 
If you have only an exhaust fan in the room, then you have created a negative pressure and the way you can see that is use a smoke tube around door or window cracks and you should see the smoke entering the room. If you have only a forced draft fan into the room then in all likely hood you have created positive pressure in the room and the smoke from a smoke tube will be leaving the room thru cracks. This is as simple of an explanation as I can put it without all the other details from the above replies.
 
To quantify positive or negative, I have seen 10% excess supply/exhaust air flowrate used for positive/negative.

But if this exceeds approximately 100 cfm/door, there may be issues with doors standing open or slamming shut.

 
Thank you all for your input, but I'm wondering if I just didn't word the original question correctly. I'm going to try again. Please forgive my first attempt.

AIA/FGI Guidelines (since 2001?) call for at least .01" wc pressure in ORs when measured against a non-like area such as a corridor. Therefore, I'm assuming a tissue test is out since you would have to meet a quantifiable measurement, not just a qualitative one. Of course, some states adopt all or part of the AIA/FGI Guidelines. I believe ASHRAE notates .01" wc as well...but, I'm not confident of this. In industry literature for the medical field, .01" wc is recognized as a target of best practice for infection control.

The .01" wc target for an O.R. seems easy to measure at the door. We just stick a meter under the door crack. If it doesn't meet the target, we know there could be various reasons (high returns, loose room, corridor pressure, etc).

But, when a code calls for just positive or negative pressure how do you determine that? In respect to, say, a positive room I've heard that if you have 50 cfm more supply than exhaust/return per door that meets the intent of the code. But if I have that in the room, yet my meter (yes, we calibrate annually) shows the room measures as low as .0001" or even .0004" wc, is the room arguably "positive?" Does a reading that low at the door satisfy the intent of the code when it calls for a "positive" room?

It would be hard for me to accept this because I've had such a situation and then put a flutter strip against the door and watched it just sit there...even get sucked/pushed into the room when a distant door is opened/closed or someone walked by. To me, that seems like at best a neutral room.

So maybe the question is more like, "What specific measurements are necessary to satisfy a code's intent of a positive or negative room?" For instance, I've looked in the NC Code for ventilation requirements of the physical plant of Ambulatory Surgery Centers and can see where it calls for either, but doesn't explain what either actually looks like.

So, I'm looking for something like, "A room is positive when supply exceeds exhaust/return by 100 cfm and has a differential pressure of at least .001" wc to the corridor." Or, "A room is positive when supply exceeds exhaust/return by 50 cfm/door and a flutter strip blows outward."

Thanks for any clarification and I apologize if you already explained this and I just didn't understand it.
 
You seem to have answered your own question. Air flows from high to low pressure. Positive pressure is specified to insure that air flows in a defined direction. 0.01" is barely enough to ensure this. 0.001" wc can be created by the draft of someone walking by, so this cannot insure airflow will not be reversed by drafts. Think about how small a pressure a 0.001" layer of water is. This is a real physical quantity that is less than the thickness of sweat on your skin. It is not just a number on the digital display of your meter.
 
0 pressure is vacuum. so by definition you only have positive pressure unless you de-pressurize perfect vacuum.

what you mean is gage pressure, which refers to the pressure difference to a reference pressure. In most cases the gage is open to atmosphere. so you reference pressure is 1 atm (or 14.7 psi).
so if your gage pressure is - 1 psi, you have 13.7 psi actual pressure. Negative to atmosphere.



 
Again guys, I'm not an engineer (which I know you're thinking is the problem, LOL). I actually thought a differential pressure of .01" wc from one reference room to another was significant. It is the AIA/ASHRAE standard for both ORs and Airborne Infectious Isolation rooms.

Again though, if you were designing or testing a room, when would you call if positive? What has to be present? I',m assuming it starts at a mechanical design of supply exceeding exhaust. But by how much. If supply > exhaust by 1 cfm does that qualify? If exhasut > supply, but the tissue blows outward is that ok?
 
Again, positive pressure is a means to an end, which is some design value of outward airflow. Under the premise that no one has faffed with the construct of the room, then there will be a design positive pressure that is intended to achieve the desired airflow. When you measure at least that pressure, the construction is compliant to the design. This would generally apply to a room where outward airflow is mandatory, as in semiconductor clean rooms.

So, if you are designing such a room, you would start with the required outward air flow, and assume various configurations of gaps, openings, external environment, etc., and then design the air handler to accommodate that demand, which then results in a desired air pressure difference.

From a testing perspective, if the requirement is non-existent or nebulous, the fall-back would be to measure the pressure under some room configuration variations and see if measured value is at least 10x the rms noise/error of the instrument. This fallback is basically so that the measurement would be repeatably positive, given noise/error in the instrument.

TTFN
faq731-376
7ofakss

Need help writing a question or understanding a reply? forum1529
 
I think there is a mixing in your question between positive pressure as a physical term and pressurized room.
positive pressure as a physical term is any pressure value over an index point such as atmosphere pressure. (it is like when you say +1 or -1, both use 0 as an index point)
pressurized room is a room that the pressure inside the room is greater than the pressure of room surrounding, we could reach this by making the amount of air entering the room is greater than the air leaving the room, even though the difference could be about 0.00001, it meant to prevent air from room surrounding to come in, and if the tissue does not move that does not always mean it is not pressurized room. the tissue has a mass should be taken in the account too.
to check a room, I think you need to use special tools not a tissue.
 
Agreed: special tools are needed. A balancing report that said "the tissue blew outward" should be rejected by any reputable engineering firm.
 
i believe you should separate issue of how to design from the issue on how to measure pressure differential.

when designing, all begins with something like what wilbur say, you plan for excess supply or exhaust, it can be 10% or 5% depending on absolute figures, tolerances.

than the issue on how to control it is often related with what specs require - in pharmaceutical industry specs are always detailed in practice it least to pressure differential sensor tolerance class, procedure for results recording and archiving, procedures for regular maintenance, calibration and correct functioning validation.
 
0.01 inches WC is too low. You need to maintain 0.05 inches of WC to avoid contamination. ASHRAE studies show reduction in contamination migration above 0.05 inches WC differential pressure between adjacent areas. It all depends upon what the adjacent rooms are. Your pressure differential regime should answer the following questions.

1. Clean to unclean area
2. Areas of different cleanroom classification
3. Is it important to save the product (safe material)
4. Is it improtant to save operators (containment)
5. If the two adjacent areas are of same class, then which one has more dust.


 
As far as I know, to provide negative pressure in a room means, you have to maintain at least -0.02 to -0.03 wc with respect to the adjacent room. There is lot of thumb rule calculation. Why because this pressure deference is directly dependent to the leakage area of the room. If your room is air tight other than the door you can calculate it perfectly.

The formula for the calculation is Q= 2610 * A * sq. rt delta P.

Where Q is the volume in cfm,
A is the total leakage area of the room.
delta P is the difference in pressure.

Hope this will help you.
 
Which is essentially the formula for flow through an orifice:
Q = C_d * A * v
v = 4005 * sqrt(delta_P)

C_d = discharge coefficient.

With C_d = 0.65:
0.65 * 4005 = 2603.

Rounding up: ~ 2610, as in the previous post.

 
I think the Op is referring to the exact set point needed between two rooms, especially in OR suites or TB isolation rooms.
Well, welcome to the world of limited liability.

See, no code (and no ASHRAE does not say anything) indicate set point for negative or positive pressure, especially in hospitals

We get this question all the time when we design an TB isolation suite - "how much negative do you what?" the contract documents do not states the exact set point for the specified pressure differential.

So, we engineers will send you to the Authority Having Jurisdiction, because even the codes and guidelines (IMC, ASHRAE 170, etc) do not dare indicate an exact set point - Reason? Liability

Even the CDCP does not indicate the exact negative or positive set point of pressure differential sensors.

It is common "practice" to have a set point between -0.02 and +0.02 - This practice is as volunteered by instrument manufacturers such as TSI, so if you use TSI instruments, they will recommend you a set point. "Usually 0.01" - Then again, take this advice at your own risk.

 
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