What is Static Pressure in HVAC? Definition, Meaning, Examples, Calculations


Static pressure can occur in gases or liquids. The static pressure of a pipeline is measured using gauges attached to or placed near the pipeline. Pipelines are sometimes tested for integrity using hydraulic pressure. Water is used as the alternative to the normal pipeline fluid during this test. After the pressurization, the device is put back into service if it has passed the test.

What is Static Pressure in HVAC? Definition

Static Pressure Basics

Perpendicular pressure is force applied perpendicular to an object’s surface. In mathematics, it is defined as P = F/A, where P represents pressure, F represents force, and A represents area. Scalar quantities do not have directional vector characteristics and only have magnitude.

what static pressure hvac definition examples calculation
what static pressure hvac definition examples calculation

In practical terms, we can think of it as a force that acts equally on all surfaces it touches and that results from the combined energy of the gases and liquids involved. In general, there are two types of pressure – absolute and gauge – which are differentiated by the reference pressure against which they are compared.

Define Static Pressure in HVAC

When a fluid is not moving, static pressure refers to the pressure exerted by it. A static pressure is typically measured by multiplying a force by an area, or by measuring length units when using a pressure head. The combination of static and dynamic pressure can be used to determine the total pressure in a system, such as inside a pipe or a tank.

Workflow of Static Pressure

When air moves through an object, static pressure is the amount of pressure measured in inches of water. This concept is not relevant to home and business owners. Calculations and planning will be handled by the company installing the HVAC system. You should be knowledgeable about the subject if you are discussing your heating and cooling system.

A static pressure equals a gauge pressure when measured in relation to atmospheric pressure. There is however the possibility to measure the static pressure with a vacuum as reference, in order to determine the absolute pressure.

Whenever the fluid is at rest with respect to the measurement, the static pressure is measured. A piezometer can be attached to the wall of the pipe adjacent to the fluid flow to measure it.

Different Terms Related to Static Pressure

Define Absolute Pressure

There are different scales for measuring pressure, just as there are for temperature, and both have absolute scales. Absolute pressure reaches zero in an ideal vacuum. This prevents molecules from exerting pressure on the space. As a result, negative absolute pressure cannot be achieved.

Considering relative measurements of pressure makes things much more complex. In terms of terminology, there is a lot of confusion. Various software applications interpret pressure measurements in different ways, and it is common for them to disagree. 

Define Dynamic Pressure

The static pressure in the previous example did not include velocity effects. We will see an increase in pressure if we don’t ignore these effects. Typically, this is called dynamic pressure. Generally, dynamic pressure depends on the fluid velocity and density.

Define Total Pressure

A static pressure is combined with a dynamic pressure to determine total pressure, also called stagnation pressure.

Pitot tubes are stagnation points because their velocity is zero. The dynamic pressure can also be directly measured using a Pitot-static tube. There is a static pressure opening at the end of the Pitot tube.

Typically, the total pressure will be very close to the static pressure in an everyday situation. Generally, in order to prevent friction and heat loss due to fluid kinetic energy, most systems are designed to ensure low fluid velocities. There may not be any difference between static and total pressure in these situations.

Gain of Total Pressure

This is the difference between the total pressure at the inlet and the total pressure at the outlet. This pressure produces the airstream as a result. Pressure gain is directly related to the connection method.

Key Difference between Static, Dynamic & Total Pressure

Taking into account fluid mechanics knowledge, Pressure can be defined as the vertical force acting on the unit area of a body of fluid. There are three types of pressure in the flow of air along the inner wall of a duct: static pressure, dynamic pressure, and full pressure. There are different units for mmHg, kg/m2, and pA, respectively.

Static pressure

As a result of the irregular movement of air molecules, static pressure is generated on the duct wall. A static pressure calculated with absolute vacuum as the calculation zero points is called absolute static pressure. As the zero point of relative static pressure is atmospheric pressure, it is called relative static pressure.

When air conditioning is used, the static pressure refers to the relative static pressure. It is positive when the static pressure exceeds atmospheric pressure, and negative when it falls below atmospheric pressure.

Dynamic pressure

When the air flow in a duct has a certain dynamic pressure and is positive, the dynamic pressure is defined as the flow-generated pressure. The dynamic pressure is defined as follows: 0.5 * air density * wind speed 2.

Full pressure

There are static and dynamic pressures in 1m3 of gas, and the full pressure is the algebraic sum of those two. A positive or negative atmospheric pressure can be used as the starting point of the calculation.

Differences among static pressure, dynamic pressure, and total pressure

Divergent nature

  • Total Pressure is measured directly across the direction of the wind flow, directly opposite the wind flow direction.
  • Static pressure is calculated when an object is at rest or in uniform linear motion, its surface pressure is constant.
  • Dynamic pressure is calculated by Surface along the direction of the fluid movement when an object is moving in it. In a situation where the fluid velocity is 0, the fluid is completely blocked. As a result of this transformation, its kinetic energy is transformed into pressure energy.

Variable Features

  • In Total pressure there is a high wind speed at the fan’s outlet, high dynamic pressure, and low static pressure when installed near air conditioning or fresh air units. In anechoic static pressure boxes, engineers often install equipment to reduce dynamic pressure and raise static pressure. The same muffling effect occurs despite the changes in dynamic pressure and static pressure.
  • In static pressure there is no sound, vibration, or impact when there is static pressure.
  • Dynamic pressure can only be calculated by analyzing the directional flow of air. Dynamic pressure exists only in the plane perpendicular to wind flow pressure directions or obliquely to them. The plane bearing the maximum dynamic pressure in the vertical direction is equal to zero in the parallel direction. The dynamic pressure varies in the same flow section due to the wind speed variations at each point. The dynamics of pressure, or dynamic pressure, are always greater than zero, irrespective of the absolute or relative pressure.

Different utilizations

  • Total pressure is used in fan products and in air conditioning
  • Static pressure is used and applier in fluid dynamics
  • Dynamic pressure is used and applied in aerospace items, in the making of mimes and other products.

Define Pitot tube

A Pitot tube can measure velocity with Bernoulli’s equation in one of the most immediate ways. Originally named after the French scientist Pitot, the Pitot tube is among the most practical and useful instruments ever devised. The tube is bent at right angles to a line.

The amount of velocity can be accurately determined by pointing the Pitot tube upstream into the air flow and measuring the difference between the pressure sensed by the tube and the surrounding pressure. There is probably no more accurate method for measuring flow velocity on a routine basis than this method, and accuracy better than 1% is easily achievable. By using Bernoulli’s equation along the streamline that begins far upstream of the Pitot tube and rests in the mouth, we can determine that the Pitot tube measures stagnation pressure in the flow.

HVAC System & Static Pressure

A HVAC system is simply made up of heating, ventilation, and air conditioning. Commercial and residential buildings can use this system to heat and cool. HVAC systems provide comfort to the environment in everything from single-family homes to submarines. The use of fresh outside air in new construction is becoming increasingly popular because of the high quality of the air inside.

A process of replacing or exchanging air occurs when air is introduced into a space via the V in HVAC. Air quality is improved by removing moisture, smoke, odors, heat, dust, airborne bacteria, carbon dioxide, and other gases, while maintaining proper temperature and oxygen levels.

Workflow of HVAC System

In order to maintain acceptable indoor air quality and thermal comfort, the three main functions of an HVAC system should be interconnected. The heater or air conditioner in your home isn’t working, you will know promptly.

If you operate your HVAC system, then you should be familiar with the air return, filter, exhaust outlets, ducts, electrical elements, outdoor unit, compressor, coils, and blower.

Static pressure in HVAC systems is a measure of flow resistance. The static pressure must be reduced in order to improve the overall performance of the system. System static pressure is influenced by the materials’ surface conditions, the length of the system, and the equipment used in each case.

Importance of Static Pressure in HVAC system

For HVAC systems to provide heating and cooling, air handling units must overcome static pressure in the ducts. A fan’s performance and power consumption are largely determined by static pressure and airflow. Therefore, static pressure calculations are a crucial step in HVAC design.

The design of air ducts affects temperature control and energy efficiency in HVAC systems. A duct system is usually connected to packaged rooftop units and fan coil units. It is necessary to calculate static pressure accurately in order to specify these components correctly.

Bernoulli Theorem & Static Pressure

Bernoulli’s equation, when adapted to flow, can be seen as an expression of the conservation of energy principle.

P + 1/2 ρ v2 + ρgh

The equation is important and often used in fluid mechanics. An incompressible flow is governed by the relation between pressure and velocity. Bernoulli’s effect lowers fluid pressure (static pressure – p) in regions where speed increases. To account for head gains and losses, the Bernoulli equation can be modified. Most fluid flow problems can be solved using the resulting equation, known as the extended Bernoulli’s equation. 

Restriction of Bernoulli Theorem

The following points summarize the limitations of Bernoulli’s equation:

  • System with steady flow,
  • The fluid is incompressible because of its constant density,
  • In this fluid, there is no work done,
  • It is impossible for heat to be transferred to or from the fluid,
  • Internal energy does not change,
  • Two points on a single streamline are related by the equation; two points along two different streamlines are not related by the equation

P1 + ½ (ρv1)2+ ρgh1= P2 + (ρv2)2 + ρgh2

Fluid dynamics’ most famous equation is this one. This equation describes the qualitative behavior of flowing fluid, usually referred to as Bernoulli’s effect. A higher flow velocity lowers fluid pressure in regions where it occurs. It might seem counterintuitive to lower pressure in a constriction of a flow path, but it makes sense if you consider that pressure corresponds to energy density.

A constriction that moves at high speed will cause kinetic energy to increase at the expense of pressure energy. This is the dimension of kinetic energy per unit volume. 

Example of Using Static Pressure

Testing a ball in Jet

It is very stable to small perturbations in any direction, when it is placed in a vertical air jet. Whenever you push down on the ball, it returns to its equilibrium position; once you push it sideways, it quickly returns to its original position. There is a force on the ball causing its weight to be balanced by pressure differences in the vertical direction: the pressure on the rear half of the ball is generally lower than on the front half due to losses that happen in the wake.

A lot of flow energy is dissipated in the wake of large eddies. As a result of the jet’s maximum velocity at its center and a decrease towards its edges in velocity, you can understand the balance of forces in the horizontal direction. During lateral movement, the ball’s outer side moves into a region of lower velocity and higher pressure, while its inner side moves closer to the center where velocity and pressure are higher. This causes the ball to move back into the center where it is higher pressure.

Fast Moving Objects pulls nearby objects

Since most of the air around a fast-moving train moves in the direction of that train, a low-pressure zone develops close to the train because of the high velocity of airflow around the train; therefore, things are forced toward the train. Train platforms should be kept clear of people and their belongings so that accidents can be avoided.

Blew off Roofs due to Heavy Winds

The walls of the house stop the air velocity during heavy winds, but on the roof, the air velocity is quite high, which creates a lower pressure zone, while inside the house, the air velocity is much less, so the pressure is much higher as compared to the roof. High-pressure air inside the room tries to move toward a low-pressure area, and as a result, the roof sometimes blows away or is lifted.

Workflow of Chimney

The wind velocity outside the house is generally higher than inside the house because above the nozzle of the chimney, the pressure plunges, and the air rushes from the high pressure area into a low pressure area, making the smoke flow out of the chimney. Therefore, we can understand why chimneys perform better when the wind is strong.

Take of a Airplane

On the runway, the wings of an airplane are designed in such a way that air flowing over the top of the wing needs to overcome more distance than air flowing underneath it. Due to these differences, the air velocity on the upper side of the wing is greater than that on the lower side, and the pressure is lower, making the aircraft take off as the air moves from a higher pressure to a lower pressure state.

Mathematical Example Using Bernoulli Equation

Imagine a 100m-high tank filled with water – open to the atmosphere – that is hit by a bullet that pierces one side, allowing water to drain out. There is a hole 4m above the ground. How quickly will the water flow out of the tank if the hole is very small relative to the size of the tank?

Answer:  To determine the static Pressure, you have to use Bernoulli Equation

P1 + ½ (ρv1)2+ ρgh1= P2 + ½ (ρv2)2 + ρgh2

The top of the container will be considered point 1 in the situation described in the question stem, and the hole where water drains will be considered point 2.

A few things must be realized before simplifying things can begin. The atmosphere is exposed at both points. Therefore, the P terms on both sides of the above equation are equal to 1 atm and may cancel out. In addition, because the hole on the side of the tank is so small compared to the rest of the tank, the velocity of the water at point 1 is almost 0. As a result, we can drop the ½ (ρv1)2 term on the left side of the equation. 

ρgh1= ½ (ρgv2)2+ ρgh2

By eliminating ρ on both sides, we get

gh1= ½ (v2)2 + gh2

½(v2) = gh1-gh2

½ (v2)2 = g (h1-h2) = gΔh

(V2)2 = 2gΔh

(V2) = √ 2gΔh = √ 2(9.8m/s2) (100m – 4m)

= 43.37 m/s.

An ideal house should be able to withstand winds that reach hurricane force. v=44 m/s is the maximum wind speed. Suppose the roof has a surface area of 900 m2. How much force must the roof support be able to withstand if air density is 1.029 kg/m3?

Answer: A solution is obtained by using Bernoulli’s equation and the definition of pressure. To begin, obtain two Bernoulli points, A just inside the roof where the air is still and B just outside where the air is moving. The following terms will be eliminated:

PA + ½ (ρva)2+ ρgh1= PB + ½ (ρvb)2 + ρgh2

Due to the air is inside, ½ (ρva)2 = 0

ρgh can be cancel out to the same height, so we can rearrange:

PA-PB = ½(pvb)2

Since we have to think about the pressure in & out

ΔP = ½(pvb)2

ΔP = ½ * 1.029 kg/m3 * (44 m/s) 2

ΔP = 996 Pa

Force can be determined by area multiply with pressure

F= ΔP * A

F= 996 Pa * 900 * 1000000 N.

On a windy day, a kite boarder generates force with a kite. The kite has an area of 8 square meters. The density of air is 1.29 kg/m3. 18 m/s is the speed of the wind. How much force can the kite boarder expect the kite to generate if, on the inner surface, the air is stationary?

Answer:  Calculate the pressure difference on both sides of the kite using Bernoulli’s equation. A refers to the inner surface where the air is still, and B refers to the outer surface where the wind is at full speed.

PA + ½ (ρva)2+ ρgh1= PB + ½ (ρvb)2 + ρgh2

Considering both points have the same height, the ρgh is eliminated. In the case of point A, ½ (ρva)2 is 0 because the air is still at point A. Solve for PA – PB, since we have only to worry about the difference in pressure on the two sides of the kite.


PA – PB = ½ (ρvb)2 = ½ * 1.29 kg/m3 * ( 18 m/s ) 2

PA- PB= 208.98 Pa

Net Force can be calculated area multiplied by pressure difference

F= ΔP * A

F= 208.98 Pa * 8 m2

F= 1671.84N.

Docks are enclosed by a metal plate completely submerged and attached to an underwater wall. There is an exposed side of the metal plate that is exposed to both ocean, and tide-induced flow of water. The metal plate’s dimensions are 4.6 m in height and 38 m in width.  The maximum tidal flow of 9 m/s along a current determines how much force the water will exert on a metal plate.

Answer: A force causes the moving water on one side of the metal plate to exert more pressure than the still water on the other side.  Let’s write the Bernoulli equation first:

PA + ½ (ρva)2+ ρgh1= PB + ½ (ρvb)2 + ρgh2

To simplify the problem as much as possible, we chose two “Bernoulli points”. Point A should be on the still side of the water, while point B should be on the moving side. 

ρgh can be subtracted from both sides if they are at the same height. The velocity term on the right-hand side of the equation is zero since the water is still at point A. Rearrange the equation to find the pressure difference from side A to side B:

PA- PB=½ (ρvb)2

PA-PB= ½ * 1000 kg/m3 * (9 m/s)2

PA-PB= 40500 Pa

ΔP= 40500 Pa

This statement tells us that the pressure at point B is 40500 Pa less than at point A. Calculate the force by applying the definition of pressure:

F= ΔP * A

F= 40500 Pa * 4.6 m * 38 m

F= 7079400 N.

Imperfect Static Pressure

Causes

An imperfect static pressure might cause the motor to work harder and create a louder blower. You could also wind up with hot and cold spots in your home if you don’t have enough airflow.

You will have to run your system for a longer period of time if there is not enough heat or cool blowing out of the vents. Your home won’t be adequately heated or cooled by it.

Air quality issues will also affect you. Air conditioners and heat pumps remove humidity from the air by cooling it. In humid summer days, certain house areas could be sticky if airflow cannot reach them.

A humidifier or dehumidifier, which is air conditioning products, may not fully benefit these areas. There will always be some rooms that are too humid or too dry,

There should be no static pressure on the return side of the ducts, because it will make the blower work harder and wear out more quickly. The heat exchanger will prematurely crack if it cannot dissipate the heat rapidly enough when the system is heating.

Solution

One of two things can cause improper static pressure. On the supply side, there is either an insufficient amount of ductwork or insufficient return coming from the furnace.

Adding an addition to your home can open up a portion of it. Because the airflow to that part of the house will be too low, the airflow to the new room may be weak. Additional duct runs will reduce the amount of air going to other rooms as well.

The restriction of a filter usually results in insufficient return air. For example, you are running a marathon while wearing a mask is like running a marathon without enough return air. You can breathe through a mask. It’s just a lot more restrictive and a lot harder. That’s what’s happening to the furnace. The blower motor has to work a lot harder to move that air.

The flow of air is greatly restricted by a dirty filter. Filters that are supposed to be changed every 90 days should instead be changed every 30 days.

Ductwork that is not sized correctly is another problem. Insufficient air can be supplied to the furnace if return airdrops are too small. Because past HVAC systems were undersized, a lot of estimates include the replacement of return airdrops. Low static pressure can also be caused by an oversized return.

Problem & Solutions on Static Pressure

Difference between High & Low static pressure

Improperly sized equipment usually causes too high or too low pressure. If your furnace is too powerful, then it can cause high static pressure within your ductwork. Insufficient air flow from an undersized furnace makes you uncomfortable. There won’t be much force in the air coming out of your vents. Your home has a two-ton system, and your ductwork is designed to handle two tons.

You need two and a half to three tons of HVAC, according to a load calculation performed by an HVAC expert. It isn’t designed to handle the larger sizes, so the ductwork can’t handle them. Therefore, unless something is done, the pressure will be too high.  As a result, your furnace works harder, the blower motor works harder, and other parts may malfunction sooner than anticipated.

Why Static Pressure required in HVAC system?

Consider measuring static pressure in your HVAC system like measuring your blood pressure at the doctor’s office. The doctor can tell a lot about your health from your blood pressure reading. Static pressure readings are the same. A technician will be able to assess the overall health of our HVAC system by doing this.
It is easy for facility managers to compare and understand the static pressure in their HVAC systems by using these examples. As with a person’s body, if static pressure is too high in the HVAC system, the system will not function properly.

What is key function of Static Pressure in Bernoulli theorem?

Dynamic pressure represents kinetic energy, manometric pressure, 1Gh, represents gravitational potential energy, and static pressure, P, is due to molecular motion and thus represents thermal energy. Practical advantages can be gained from a number of variations of the basic equation.

How do you stop Static Pressure Problem?

Choosing an HVAC company that conducts a comprehensive assessment of your entire HVAC system is the first step to preventing static pressure problems. This includes all furnaces, air conditioners and heat pumps, and ductwork.

The proper step in regulating static pressure is determining what it should be. HVAC contractors can determine if your ductwork is adequate by doing a proper load calculation. An airflow calculator called a ductulator can also calculate duct flow. You can manually check the air duct to see anything that is bothering the static pressure of that system.

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