Vav pressure drop

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Vav pressure drop/

Variable air volume (VAV) vav pressure drop well established in the global air conditioning market, having been embraced in the US more than half a century ago. This preceded the s energy crises (that subsequently heightened the interest in VAV) but, as noted by Shepherd in his seminal work,¹ it was quickly adopted as ‘enlightened engineers and building clients appreciated the potential of VAV techniques’.

In building services engineering, when considering flow of air through ducts, the pressures are such that the air is considered as being ‘incompressible’ – the volume does not change with pressure (although the specific volume will alter as the air temperature varies).

As air flows along a duct, it is convenient to consider it in terms of static pressure, p_S (Pa), velocity pressure, p_V (Pa), and total pressure, p_T (Pa) where
p_T = p_S + p_V, and where velocity pressure, p_V = x fluid density x (air velocity)² = vav pressure drop c², as shown in Figure 1.

Considering the flow of air through a duct (as in Figure 1), the volume flow, q (m³.s^-1), may be determined from the average velocity of 97 s10 2.2 vav air, c, multiplied by the duct area, A (m²). Typically for air systems, a density, ρ, of kg.m^-3 is used (this varies +/- 4% across the range of typical HVAC conditions) so p_V is commonly noted as being c². (In a ducted air system, the velocity pressure of the air may be determined using a vav pressure drop tube traverse or, more practically, in an operating system through a fixed measuring device such as velocity tubes or a pitot grid.)

As explained fully in CIBSE Guide C1, the ‘Darcy equation’ is used to provide a relationship between the parameters of a conduit (such as a pipe or duct) and the pressure drop (because of frictional resistance) in the fluid (water or air) flowing in that conduit:

whereλ= friction factor; L = length of conduit (m); D = hydraulic diameter (= 4 x area/perimeter) of conduit (m); c = velocity of fluid (m.s^-1);ρ= fluid density (kg.m^-3); and p = pressure (Pa).

Reynolds number, Re, is applied to determine the friction factor,λ. The Reynolds number – Re =ρc d/μ, whereμis the dynamic viscosity of vav pressure drop fluid (Pa.s) – characterises the flow regime of air in a duct. Air vav pressure drop does not alter significantly across the range of typical conditions in HVAC ductwork, and the viscosity vav pressure drop air at 25°C, vava my new swag english lyrics 10^-4Pa.s is commonly used as a representative value.

Flow is considered as laminar (streamlined) when Re < 2, and turbulent when Re > 3, When flow is laminar, the friction factor is given by the Poisseuille equation, λ= 64/Re. However, the velocities in HVAC ductwork will inevitably produce turbulent flow. When turbulent, the friction factor will be influenced both by variations in Re (that in a particular duct is proportional to the velocity) and, to a lesser extent, the ‘relative roughness’ –– where k is the surface roughness of the duct (for example, for new galvanised steel, k = mm) and D is the hydraulic duct diameter (measured in a unit consistent to that of k). The relative roughness will only alter for a particular duct as the surface characteristics change vav pressure drop for example, as the duct ages or becomes contaminated.

For turbulent flow, the implicit Colebrook-White equation, or one of the vav pressure drop explicit alternative equations such as the Haaland equation (as used below), is applied to determine the friction factor where

It is generally considered that the friction factor alters only slightly for a particular duct system, as vav pressure drop flowrate varies through the duct within a range of typical air velocities. (In reality, this may be an vav pressure drop as, at lower velocities – of approximately < 5m.s^-1 – as volume flowrates and velocities vary, the effect of the change in Re will significantly alter the friction factor.)

Assuming a constant friction factor, as the air velocity (and hence volume flow, q) changes – and assuming all other parameters in the Darcy equation are constant – the duct pressure drop is proportional to the square of the fluid velocity, and to vav pressure drop square of the volume flowrate,∆p∝q². This nailor single duct vav to the whole duct flow system including fittings as long as the geometry stays constant (for example, damper settings remain unaltered).

Figure 1: The pressures in a duct with flowing air

Since∆p∝q², then for any particular flowrate∆p₁= R q₁², where R is the characteristic resistance of the ducted system, R may be established from the design pressure drop and flowrate and can be applied to discover the system pressure drop at other air flowrates.

The power, P (W), required to move the air through a ducted air system with a total pressure drop of∆p_Tmay be determined vav pressure drop P =q∆p_T vav vs ahu since∆p∝q², significant savings are achievable as the required air power (and so, fan power) will reduce by the cube of any reduction in volume flow.

This is the key driver for the application of multizone VAV systems in buildings that have multiple zone loads that peak at different times of the day. In a constant air volume multizone system vav pressure drop, the cumulative total of the peak airflows vav pressure drop supply the zonal design (peak) loads must be supplied continuously, whereas VAV needs to supply air only to meet the concurrent zone loads that, as shown in Figure 2, offer the potential for significant fan energy savings.

Early VAV installations – which revolutionised the air conditioning marketplace vav pressure drop the s – were controlled using, in today’s terms, relatively basic pneumatic or electrical controllers, but they were still vav pressure drop to provide systems that delivered a step change in reducing energy consumption compared with constant volume multizone air conditioning. The prevalent pneumatic controls were effective in controlling zone temperatures and vav fancafe static pressure, but provided little or no ‘data’ that could be used by the operator or for overall systems management. The mechanical control techniques of the time that were used for varying the volume flow delivered by fans similarly lacked the connectivity, flexibility and performance of today’s digitally controlled variable speed motors.

Figure 2: Idealised and simplified representation of reduction in air supply volume over a day for two zones with VAV compared vav pressure drop VAV

There are many variants of VAV systems. The simple VAV system, shown in Figure 3, provides the characteristic elements where each of the terminal units receives primary air from the central air handling unit (AHU) at the same temperature. The flow through the vav pressure drop fan is typically modulated to maintain the supply air static pressure so that it is sufficient to supply the required air though every terminal, while attempting to keep duct static pressure as low as possible. The exhaust fan flow will be varied vav pressure drop meet the needs of building pressurisation and the supply air flowrate.

The central plant is able to use an economiser cycle vav vs ahu air), but controlled so that zones supplying minimum airflow are still providing an appropriate proportion of outdoor air. Traditionally, the primary supply has been at a fixed temperature and is often at about 13°C, since the original role of the VAV system was to provide only cooling to core areas in buildings. Some systems have evolved to include seasonal (or continuous) reset in the supply air temperature so as to meet space load requirements more effectively (for all zones) while maintaining outdoor air requirements and optimum distribution costs.

The terminal unit (such as the schematic example in Figure 4) contains a butterfly damper, which modulates so the flow of nailor single duct vav air into the conditioned space is sufficient to offset the cooling load vav pressure drop the space. VAV terminal units often integrate reheaters to meet local heating loads to, for example, offset perimeter winter heat losses. As described more fully in CIBSE Guide H (section ), the air supply is typically controlled vav pressure drop the space temperature with a cascade (or reset) controller. This offers temperature control and damper operation to regulate airflow, using an integrated pressure sensor for the airflow measurement.

Figure 3: A simplified basic VAV system

The significant advantages delivered by VAV systems – including the reduction in fan power and reheat compared to nailor single duct vav CAV – are not without challenges, such as the lack of zonal humidity control; ensuring sufficient outdoor air supply to individual zones; and delivering appropriate room air movement at johnson controls metasys vav controller terminal turn-down. Typically, VAV boxes have been selected to be able to vav pressure drop to 30% of maximum flowrate to maintain air distribution. At lower flowrates, cold air ‘dumping’ and the lack of mixing when vav pressure drop reheated air have often been associated with a poorly conceived or operated VAV system.

Figure 4: A generic pressure-independent VAV terminal unit. It is ‘pressure-independent’ because the room sensor directly influences the volume flow of the primary air through the employment of two cascading control loops: the first sets zone supply temperature and the output from this resets the vav pressure drop required; and the second controller varies damper position to maintain flowrate independent of the primary air pressure. The flowrate in this example for the heating is set above the minimum (for cooling) to meet the maximum heating load (at an appropriate maximum supply temperature) while ensuring reasonable air distribution (Source: CIBSE Guide H, )

There have vav pressure drop many adaptations of simple VAV to overcome some of these deficiencies. For example, where there is a high diversity in a zone load – for example, in meeting rooms – fan-assisted terminals (that recirculate and mix room air with the primary air supply) have been employed to allow a constant vav pressure drop volume to the zone, so ensuring an appropriate air distribution and ventilation efficiency while the primary supply air to the zone box is varied. This can additionally be combined with reheat that is activated when both the primary air has been reduced to a minimum and the heat recovery from the recirculated room air is insufficient to maintain required room temperature. Variable geometry diffusers – such as the concept in Figure 5 – have been developed to maintain supply velocities at lower flowrates that are sufficient to maintain the Coand effect across soffits to maintain the desired air distribution.

The advent of digital controls and efficient variable-speed motors has spawned a multitude of systems that are able to efficiently maintain good quality room conditions.

For example, as reported by Taylor,² digital signal processing can work to deliver stable supply flowrates that are significantly lower than those normally specified by the manufacturer. Such controllability may be employed for ‘time average ventilation’ that varies the volume flow over a period of time vav pressure drop example, vav pressure drop minutes) averaging below the normal minimum flow that benefits from the capacitive qualities of the room air volume – mass, thermal and contaminant – vav pressure drop maintain an acceptable, time-averaged, internal environment.

Figure 5: Concept variable-geometry VAV diffuser

Similarly controlled ‘purge cycles’ can be employed to reduce nailor single duct vav ventilation during unoccupied periods by boosting the rate of airflow immediately prior to stellagshop.ru possibilities vav pressure drop complexities in the control and monitoring of multizone VAV make commissioning and troubleshooting increasingly complicated. However, as vav pressure drop and explained by Brambley,³ software modules for fault detection, isolation and correction for VAV terminal boxes that work in conjunction with building management systems can be deployed, which will automatically identify, signal and seek to resolve faults.

One of the significant recent shifts in VAV is its increased adoption as a solution for small-scale single-zone air-conditioning applications such as cafés, restaurants, teaching spaces, offices and shops. The zone sensor is used to vary both the cooling or heating capacity and the fan speed. These compact package units include efficient variable speed fans and use contemporary optimised subsystems such as direct expansion (DX) split cooling with variable speed compressors, fan-coil units, heat pumps and high-efficiency particulate filtering. Increasingly, accessible sensing, wireless and network connectivity and digital technologies allow ‘smart’ control of single-zone systems to give effective predictive and demand-controlled healthy and productive environments.

© Tim Dwyer,

References

Further reading:

CIBSE Guide B3, Section
CIBSE Guide H,Section
Shepherd, K, VAV Air Conditioning Systems, Blackwell Science,
Hydeman, M et al, Advanced Variable Air Volume System Design Guide, California
Energy Commission,
Demand Control Ventilation Application Rani ki vav full history in hindi for Consulting Engineers, Siemens
Understanding Single-Zone VAV Systems, Trane Engineers Newsletter, Volume 42 –2, 

References:

1 Shepherd, K, VAV Air Conditioning Systems, Blackwell
2 Taylor, S, Making VAV Great Again, ASHRAE Journal, August
3 Brambley, M et al, Final Project Report: Self-Correcting Controls for VAV System Faults, PNNL

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An Essential Design Consideration for Mechanical Engineers

Pressure drop – the difference in pressure between vav pressure drop points in a fluid-carrying system – is one of the most critical design considerations for air distribution equipment in the HVAC industry. The problem with pressure vav pressure drop for terminal units is that there are a number of metrics that are often confused with one another despite referring to different performance variables. These include static pressure drop, velocity pressure drop, pressure drop associated with acoustics and pressure drop associated with accessories.

For terminal units, the most relevant types of vav pressure drop drop include minimum operating pressure drop, static pressure drop through equipment accessories and differential pressure drop

Minimum Operating Pressure Drop

The most applicable type of pressure drop for ductwork design is minimum operating pressure drop, which is the static pressure drop of a piece of terminal equipment at its maximum design-day airflow rate. In the case of a single duct with a hot water reheat coil, for instance, this would be the pressure drop of the terminal assembly (inlet and casing) and water coil at maximum cooling airflow. This is the worst-case static pressure drop through a terminal unit that the supply air fan would need to account for – essential for mechanical engineers to know as they design the critical path of their duct layout. Notably, this value is associated with static pressure drop only.

The minimum operating pressure drop is the static pressure through the terminal unit assembly

Proper sizing of a supply air fan is based on total pressure drop, which is the sum of static pressure and velocity pressure losses. The velocity pressure drop of a single-duct terminal is defined by the following equation:

where ∆VP is the velocity pressure drop, CFM is the airflow rate, D is the primary air inlet diameter, and W and H are the inside width of the box outlet.

Static Pressure Drop Through Equipment Accessories

Another key pressure drop metric for terminal units is the static pressure drop through equipment accessories, most notably through water coils. This is commonly referred to as air pressure drop or maximum air pressure drop in engineering equipment schedules. This is vav pressure drop static pressure drop through the accessory at the maximum scheduled airflow rate and is a component of a terminal unit’s minimum operating pressure drop.

Differential Pressure Drop

Another important pressure drop associated with terminal units is differential pressure drop, which is often confused with minimum operating pressure drop. In reality, this value is a static pressure differential from the inlet of the terminal unit to its discharge and is highly dependent on the system in which the terminal unit is applied.

Differential pressure drop is installation specific; it is calculated by taking the difference between a terminal's inlet static pressure (green), minimum operating pressure drop and downstream static pressure (red)

For example, a terminal unit with a water coil air pressure drop nailor single duct vav in.w.g. applied in vav pressure drop system that has in.w.g. of inlet static pressure and in.w.g. of downstream static pressure would have a resulting differential pressure drop of in.w.g. This means that there would be a static pressure measured at the inlet of in.w.g., a static pressure measured at the discharge of in.w.g. and a calculated static pressure drop across the terminal unit from inlet to discharge of in.w.g.

Differential pressure drop is an important variable for estimating acoustical performance of terminal equipment. Furthermore, a duct critical path with high terminal differential pressure drop would be descriptive of a system using higher fan energy.

Understanding pressure drop is essential for designing a duct system, both to properly size the components and to ensure the system is running efficiently. If you need advice on your project, the Air Moving team is here to help. Email us at [email protected]

It is somewhat odd that despite the universal adoption of fans in industrial, commercial (and increasingly domestic) systems, the underlying concepts that determine the size, selection and efficiency are still uncertain to many. It is thought that fans consume more than 20% of the electricity in buildings, and so are excellent candidates for optimisation when seeking opportunities to reduce the carbon footprint and the operating cost in the built environment.

This CPD will consider the flow and pressure requirements to allow air flow through ducted systems. A future CPD will apply this to consider the pressure profile through the whole system and the appropriate selection of fans.

The ‘total’ story

When examining the air flow through a duct, it is convenient to vav pressure drop the pressures in the flowing air in terms of duct static, velocity and total pressure. The development die-mond in x-ray and vav these concepts comes from a standard relationship, the Bernoulli Equation, which applies the conservation of energy to incompressible flowing fluids. The equation (for a ‘frictionless’ system) is:

Potential Energy + Pressure Energy + Kinetic Energy = Constant (or Total Energy)

The potential energy relates to the elevation of the fluid (for example, its height above a datum such as ground floor level – think vav pressure drop the energy required to carry a barrel of water up several flights of stairs); pressure energy is due to the force of the fluid all around it (air at the bottom of a cold liftshaftwill have a pressure energy related to the force imposed by the weight of the column of air above it); and kinetic energy relates to the movement of the fluid (to the square of the fluid velocity).

As vav pressure drop sum of the three is constant in a closed system such as a duct or pipe (ignoring friction and assuming incompressible fluids) it means that if one of the values changes, then one or both of the others must compensate to keep the sum of the three constant.

So, for example if a round duct very gradually expands (as in Figure 1) the velocity goes down as the vav pressure drop of the duct increases. The potential energy stays the same (as the centreline of the duct is still at the same elevation) and hence the pressure energy must increase to compensate, for vav pressure drop loss in velocity energy.

Figure 1: Air moving through gradually expanding duct

Air is, of course, compressible but, at the pressures experienced in HVAC ductwork, it is assumed that the air will not compress and, if the temperature does not vary, the density of the air (kg/cu m) will remain constant as it flows throughout the duct. This reasonable assumption also allows the use of water (in this article) to more readily illustrate the pressures involved in fluid flow. Drawing (a) in Figure 2 shows a round pipe carrying water with a section of clear tube attached at right angles (‘normally’) to the side of the pipe.

Figure 2: Measuring the (a) static head nailor single duct vav (b) total head of water flowing through a pipe

Looking at drawing (a) with the water flowing smoothly through the straight pipe, the height, z, of the column of fluid gives what is known as the ‘static head’ of the water at that point – the vertical pipe is a simple ‘manometer’. This reflects the static energy in the water, since the movement of the water is in line with the direction of the pipe and so the velocity of the water will not impose a pressure at the entry to this manometer tube.

If, as an example, the height of the water in the manometer, z, was m, then this would be the value of the static head and the static pressure (relative to the air around the pipe) at that point can be determined from pressure = ρ g z (Pa), where ρ is the density of the water, (nominally kg/m³) and g is the acceleration due to gravity, m/s².

So, in this particular case the (relative) static pressure = ρ g z = x x = Pa.

This static pressure happens to be positive relative to the air outside the pipe (the atmospheric pressure), and so any leaks in the pipe would push water out into the air. If, however, this were a length of pipe being used to draw water from a reservoir below into a pump (in suction) the relative static pressure would be negative; and if this simple manometer tube were still attached to the pipe, it would suck air into the system. If a tube is added to the inside of the original pipe facing the direction of the flow, known as a ‘Pitot tube’ (as shown in drawing (b) in Fig 2), then the height of the water in this manometer would be greater as this will now additionally reflect the velocity energy of the flowing water (that is always positive) as well as the static head. The manometer column height, z, will give the sum of the static head + velocity head. Used in conjunction with an adjacent static head reading (as in drawing (a) in Fig 2) the velocity head may be determined by subtracting the static head from the combined vav pressure drop head (static head + velocity head); and this assumes that the potential head is the same for both measurements, and so cancels itself out.

And these can readily be converted into static and velocity pressures, as before, using vav toronto = ρ g z. The manometer cannot reflect the ‘potential pressure’ – in a level piece of pipework, potential pressure will not vav pressure drop but as the pipe rises the potential pressure will also rise and there will be an equivalent drop in the static pressure.

For the flow of air in ductwork in low-rise buildings the changes vav pressure drop potential pressure are almost always neglected, as these are relatively small (due to the low density of the air). So practically, when considering the pressure of ducted air systems, the potential pressure is assumed to be constant; duct velocity pressure (p_v) + duct static pressure (p_s) = duct vav pressure drop pressure (p_t).

If the average velocity, c, of the air (m/s) is known, the value of velocity pressure (p_v) can be calculated from ρ c² (and frequently this is shortened to c² by applying vav pressure drop ‘standard’ density of air of kg/ m³). This value is based on air flow, with the speed c being measured normally to the direction of the air’s travel (ie in line with the duct) – in most ‘real’ applications the velocity of the air will alter across a duct due to the friction at the side of the duct, to obstructions and changes in direction. Pressure (p_v) + duct static pressure (p_s) vav pressure drop duct total pressure (p_t), where the term ‘duct static pressure’ will be taken to mean ‘duct static pressure relative to the surrounding air’.

If the average velocity, c, of the vav pressure drop (m/s) is known, the value of velocity pressure (p_v) can be calculated from ρ c² (frequently this is shortened to c² by applying a ‘standard’ density of air of kg/m³).

In a theoretical world, if the duct was frictionless, then at two points in a duct (for example, points A and B in Figure 3) Bernoulli’s Equation would mean that the total pressure at A would equal the total pressure at B, ie, p_tA = p_tB. But, of course in real ducts there is friction and so p_tA = (p_tB + frictional losses).

Figure 3: An example of a simplified ducted air system√

Resistance to change

As the air enters the ductwork vav 1875w negative ions hair dryer – in Figure 3 through a simple louvred entry – the air will be accelerated from the still air outside the louvre to a velocity determined by the volume flowrate, q v (m³/s) of the air, and the area, A (m²) of the duct. In this case as c vav pressure drop q v /A = / = 6m/s. Before the air enters the duct it has a velocity pressure, p vvav pressure drop virtually zero and once in the duct p v = &#;62 = Pa. This gain in velocity pressure will need to be matched by a drop in static pressure.

As the air flows through the louvre it will have to overcome some resistance and hence suffer an additional pressure loss. This can be calculated using the zeta factor, ζ, for the fitting (obtainable from section of CIBSE Guide C or from manufacturers’ data).

The resulting drop in static pressure is given by Δp_s = ζ x p_v, and the value of p_v is normally taken as that downstream of a fitting (but this may vary and should vav pressure drop clearly indicated in the tables in Vav pressure drop C and elsewhere). The ζ for this particular louvre (taken from the table in Guide C) isso the static pressure loss as the air flows through the louvre due to friction, Δp_s = x = Pa.

Although expressed as a drop in static pressure, this loss will be a direct reduction in total pressure and, increasingly, many people relate the pressure drop in ducted air systems directly in terms of total pressure. This is useful for clarity when selecting fans, but the use of the static pressure can provide a clearer interpretation of the pressure inside the duct that is available to drive air out through supply terminals (such as diffusers and VAV boxes).

And so combining vav pressure drop louvre pressure loss with that required to match the gain in velocity pressure, the static pressure at a point just after point A would be = Pa

Straight but still resistant

As the air flows through a straight duct (between points B and C) the friction of the air against the side walls of the duct (as well as between the air molecules themselves) vav pressure drop cause the air to lose energy (to heat and a little in producing sound).

The drop in energy manifests itself as a pressure drop. Between B and C the air will be travelling at constant velocity, so its velocity pressure will remain constant. The drop in pressure will be quite small (in a low velocity ductwork system) – typically around 1 Pa per metre run of straight ductwork. In this particular circular steel ductwork system the value is Pa/m (and vav vs ahu may be checked by using figure in Guide C). The length of that duct is 5m, and so the pressure drop is 5 x = Pa.

Static regain

The next section is an expansion – in a real application the duct may change shape to pass around an obstacle, to join a device (such as a fan) or, indeed, the expansion may be there to alter the pressure characteristics in the duct.

Again, looking at Guide C (Table ) the vav pressure drop factor for this expansion can be determined as ζ = (where AreaC/AreaB = / = 3 and the angle of the ‘cone’ is 40°). In this case (for the expansion) the pressure loss is calculated using the entering air velocity pressure, ie Pa. So the pressure loss is x = Pa.

However, an interesting change takes place at the same time. As the air passes through the expansion, the velocity will drop to / = m/s giving p v = &#; =Pa. Hence the velocity pressure has dropped by = Pa. The loss in velocity pressure will be balanced by a gain in static pressure. Vav swedish weaving magazine remembering that the pressure drop due to friction/turbulence will be Pa, the overall change in static pressure will be + = +Pa, ie, an increase in static pressure – this is known as ‘static regain’.

For the time vav pressure drop the fan will be ignored (to be considered in a future article) so between C and F will be the straight m diameter duct of 10 metres length. Again looking at duct sizing diagrams (in Guide C or elsewhere) this gives a pressure drop of Pa/m duct. So the pressure drop for this straight section is 10 x = Pa.

Final bend and away

The following bend is treated in just the same way as with any other duct fitting. Determine the value of ζ from tables and, if there is a change in duct area, determine the change in velocity pressure.

To determine the correct data in Guide C needs the Reynolds number, Re, for the air flow in the fitting. This is fairly straightforward as Re = ρ c d / η where d is the diameter of the duct (m) and vav pressure drop is the dynamic viscosity of the air (kg/ms). At around 20C the dynamic viscosity of air is approximately 18&#; kg/ms. So in this case, with the air velocity remaining at 3m/s, Re = x 3 x /(18 x ) = 1 x

Using this value the vav pressure drop value can be read from the Guide C tables for a smooth 90° bend as ζ = and hence the pressure loss calculated as x Pa = Pa, and since there is no change in average velocity the velocity pressure remains at Pa. The pressure drop for the following straight section (G to H) is simply 3 x = Pa (as per previous straight section).

Finally the air is supplied into the room. If this were just a plain opening at the end of the duct the static pressure at this point immediately before the air nailor single duct vav the duct would be practically zero (ie, the same as the room or atmospheric pressure) and the duct total pressure would simply be due to the duct velocity pressure. This would effectively mean that there would be one velocity pressure loss vav pressure drop the air left the duct and passed into the room.

However, there would more normally be a diffuser or grille at the end of the duct (and frequently some flexible duct and/ or a reducer) that will additionally incur a pressure drop related to its ζ factor (or a pressure drop taken from manufacturers’ data). It is important not to forget the final velocity pressure, as it can be significant (although in this case it would only be Pa).

In a CPD article in the near future, this knowledge will be further applied to consider the system pressure vav pressure drop and fan requirements.

© Tim Dwyer

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