Static Pressure

static pressure is the pressure insensity tries to brust the duct wall i.e. pressure on the wall of duct while velocity pressure is pressure due to flud velocity velosity presure=v²/2g

v= velosity of flude in m/sec

g= gravitation constant

The basic ratio describing current of an ideal liquid is equation Bernulli.

ρv²/2+P+ρgh = const.

ρv²/2 represents the dynamic pressure caused by movement of a liquid-kinetic energy;

Р- the static pressure which has been not connected with movement of a liquid (it can be measured, for example, a manometer);

ρgh - hydrostatic pressure, i.e. pressure of a column of a liquid upon the bottom basis.

P+ρgh-potential energy

So then how do you find static pressure in a duct? How does it differ from atmospheric pressure? It is usually 1" of water column or somewhere around there. What is atmospheric pressure in terms of water column?

I think you're confusing absolute pressure measurements with gauge pressure measurements...

In a HVAC duct the static pressure will be the pressure in the duct in relation to the atmospheric pressure...

So your pressure of 1" of water is a pressure reading of atmospheric pressure plus 1" water gauge...

With small pressures such as this its best to use gauge pressure readings, as you will always be measuring the static pressure relative to ambient pressure.

John.

Call an air balancing contractor and he would be able to give you the static pressure after taking some readings on your system.

PatyO:

Atmospheric pressure (14.7 psia) is equal to 33.905 feet of water column (WC), or 406.9" WC. In other words, 1" WC = 0.03613 psig differential pressure from atmospheric when read on a manometer or differential pressure gauge.

Rather than an actual water filled manometer, I preferred to use a pressure gauge (Dwyer Instruments) for ease of reading and use. (However it is much more expensive.)

Those numbers are accurate

Todd

I am designing a hood for silicon vapour absorbing duct system for forging machine.

What pressure (static pressure) almong the duct should be preffered?

And is there any relation between Static Pressure,Flow rate, flow velocity and face velocity?

for eg, 4" , 6" rectangular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

OR

6" circular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

And what the fan capicity should be choosen??

Coz I m fresh graduate and as soon as i start working .., I have this problem.

Thank You

Henry Caspers

hennrywebworm@gmail.com

You can also find static pressure drop of a duct given the duct size, volume of air flow (cfm) and velocity. You can also use a chart or an engineering design tool called a ductculator. For a given size of duct, lets say for example 24"x12" in a HVAC application. Industry standard of supply air flow from a standard rooftop unit is based on .08" of static pressure per 100 ft. of duct. At 24"x12" the flow rate is 1800cfm @ .08" of static pressure per 100 ft. with a velocity of 1050 fpm. If the 24"x12" duct is 250'-0" long the static pressure drop would be 250ft/100ft = 2.5ft (.08" SP"/100ft) = .2" of static pressure drop. This is straight run of 24"x12" duct that is flowing 1800cfm that is 250'-0" long. For every 90 degree or 45 degree turn, SMACNA lists the given pressure drop per fitting. This gets added on to the static pressure drop of straight runs of duct. An HVAC engineer will measure the duct length and add up all the fittings to determine the supply air fan size. Manufacturer's catalog, fan curve data will be examined to determine fan wheel size and horsepower given the known flow rate and static pressure drop. It's not a complicated process, but a mistake could cost alot of money given the cost of HVAC equipment, contractor labor etc.

It is basically the amount of air pressure drop caused by all the connected straight runs of duct and all associated duct fittings like 90 degree bends, filters, water coils etc. Anything connected to the fan causes a pressure drop is seen as static pressure drop as your fan is trying to push air through it. The static should be caculated by the engineer or designer of the system. If you had a 3000cfm system and 1.5" of total static pressure from ductwork and fittings etc., your fan selection would be a fan capable of 3000cfm at 2.0" of static to overcome the 1.5" of total static pressure of the connected duct system at a specific horsepower. An undersized fan would cause less than 3000cfm to be delivered and there would be a larger amperage draw on the fan. This could cause the fan to overheat and shut down. For heating or air conditioning, the problems of an undersized fan would also cause insufficient heating or cooling in spaces around the building making it impossible to maintain the wall thermostat setpoint.

I am designing a hood for silicon vapour absorbing duct system for forging machine.

What pressure (static pressure) almong the duct should be preffered?

And is there any relation between Static Pressure,Flow rate, flow velocity and face velocity?

for eg, 4" , 6" rectangular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

OR

6" circular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

Thank You

Henry Caspers

hennrywebworm@gmail.com

These guys are making your answer much more confusing than it should be.

What you are refering to is a pitot tube reading. You take the pitot tube and point it into the "wind" in the duct. A hole in the tip will give you velocity pressure (other tems may be used to describe this term but it is simply put as velocity pressure). This is the pressure produced by the velocity of the air.

If you look at the sides of the pitot tube, there are holes there as well. These holes are situated parallel to the "wind" in the duct and measure the pressure that is "static" or not caused by the velocity. That's why it is called static pressure.

Anyway, you don't even need to know this because you just have to hook the pitot tube up to a manometer (inclined manometers are more accurate) and you can take a direct reading off the manometer giving you the velocity of the air. You just need to make sure when you buy the manometer that it is for this purpose and it actually has velocity readings on it.

Your best bet is to ignore all this stuff, and get some sort of digital anemometer (mechanical or hot wire or whatever type you want). Stick that into the duct and take your reading for velocity that way. If you must measure the pressure for some other reason, fine...just do it that way.

Hehe, I didn't even want to mention to her about traversing the duct thinking that was too complicated for her. KISS baby!! Like she is going to understand that.

Clarification on pitot tube holes. The tip is actually total pressure, the holes in the side are static pressure. Manometer calculates the difference which is then velocity pressure. The Shortridge manometer is nice b/c it will correct for temperature and give you velocity readings.

PattyO

Most of the responses are right either in whole or part right, but I am not sure about the first response and I find them a little more technical and a little confusing. I think you might be looking for just a simple explanation.

Air is dynamic, it can be expanded or compressed. It is from this understanding of this idea the concept of static pressure comes into play.

Static pressure nothing more than simple back-pressure or resistance from the duct. What is happening are air molecules are expanding and contracting when forces are applied to them to move in the same direction. The problem is air is not a fluid movement like liquid through a pipe.

There are three factors that influence static pressure:

Velocity

: the faster you try to force air through a duct the more resistance you are going to experience. Normally with dust collection, the air velocity is normally 3,000 to 5,000 fpm

Elbows:

the number of bends in the ductwork will have a larger impact on increasing static pressure than velocity.

Surface Texture:

The texture of the inside walls will also have an even larger impact on increasing static pressure than elbows. Flexible hose is a great example of how much more static pressure is generated compared to a the same length to a straight duct..

I like to think of ducting and ventilation like. Just think of the cars as air molecules.

When the traffic comes to any turn, it might be normal to slow down before or during the turn. This will cause the spacing between the cars to become closer together (positive pressure) before the turn and farther apart after the turn (negative pressure).

Likewise if there is a rough section in the road traffic cannot move as quickly over it as a smoother section. The same forces of expansion and contraction come into play.

The amount of static pressure your system has means the more power you need in you blower to overcome those loses. You are correct it is measured in inches of water column, but it really does have much to do about the outside air pressure, because the system, in a sense, is sealed from it.

There are a number of sources available to help you calculate static pressure. The Industrial Hygiene Association has a book about industrial ventaliation systems. You can also log on to Dr. Steve Guffey's website www.industrialventilation.net. He is a professor of industrial hygiene at a university in Virginia.

If you look at a Grainger catalog, you might notice when the static pressure increases, the fans capacity decreases.

I hope I have answered your questions.

Good Luck,

Put simply, static pressure is the pressure on the duct walls as the air flows through it. It is often measured by sticking a static pressure tube into the duct with its opening parrallel to one of the sides of the duct. A differential manometer or anything that can measure pressure is connected to the end outside of the duct to measure the pressure

Some people put static pressure taps on the outside of the duct. It is like drilling a small hole in the duct wall and sealing one end of a tube over the hole, then you can measure the static pressure (pressure on duct wall) at the other end of the tube with any device that measures pressure.

When the static pressure is known, a fan and fan motor can be selected (sized) that is able overcome the static pressure and push the air though the the ducts with enough force that it comes out of all the vents in the house with enough speed (or force) to adequately distribute and mix it thoughout the whole house. If the fan is not big enough (have enough power) to over come the static pressure thoughout the duct, air may barely move through the ducts farthest from the funace and the air in those parts of the house will be stagnant and at a different temperature that the rooms with vents closest to the furnace. It this case, other that replacing the furnace fan with a bigger one, the only solution is to place portable fans around the house to distribute the supply air to the further regions.

I am designing a hood for silicon vapour absorbing duct system for forging machine.

What pressure (static pressure) almong the duct should be preffered?

And is there any relation between Static Pressure,Flow rate, flow velocity and face velocity?

for eg, 4" , 6" rectangular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

OR

6" circular duct with near hood velocity 2000 FPM ... wht will be the static pressure?

And what the fan capicity should be choosen??

Coz I m fresh graduate and as soon as i start working .., I have this problem.

Thank You

Henry Caspers

hennrywebworm@gmail.com