بسم الله الرحمن الرحيم
اخوتي الاعزاء أقدم لكم هذا الموضوع الشيق الذي اتمنى من الله أن ينال اعجابكم والذي ربما لما ينوه أليه حتى الأن في موقعنا الرائع ستجدون الملف بالمرفقات انشاء الله

Terminology
Air Distribution
The transportation of a specified air flow to or from the treated space by means of ductwork.
Air Diffusion
Distribution of the air in a space, called the treated space, by means of devices, called air terminal devices, in a manner so as to meet certain specified conditions, such as air change rate, pressure, cleanliness, temperature, humidity, air velocity and noise level.
Terminal Velocity
The point at which the discharged air from an outlet decreases to a given velocity usually 0.25 m/s or 50fpm.

Throw
The distance, measured in meters or in feet that the air stream travels from the outlet to the point of terminal velocity.

Drop
The vertical distance between the centre of an outlet and the bottom of the air stream at the end of horizontal throw.

Envelope
The geometrical surface of the points of an air jet corresponding to terminal velocity.

Spread
The maximum total width of the air pattern in the envelope.

Air Flow
The Air flow is the rate of quantity of Air passing through the Air outlet to the room so as to achieve the desired design conditions such as temperature, Noise level etc.

Static Pressure
Pressure inside the duct which is necessary to overcome friction resistance measured in Pa or IN-WG.

Total Pressure
Sum of at the static & velocity pressure.

Induction
Process by which the primary air sets into motion an air volume, called secondary air, in the room.

Coanda Effect
Also called ceiling or wall effect. Tendency of an air stream to follow a wall plane when the stream is in contact with the wall. This effect increases throw and reduces drop.

Noise LevelsDecibel is the unit used to measure sound. It is logarithmic ratio of two sound pressure levels (SPL Lp) (or) sound power levels (SWL Lw) where one is a reference level, The most commonly used criteria are Noise criteria curves (N.C. level), Noise rating curves (N.R. levels) and dB (A).

Air Terminal Devices
A device located in an opening provided at the boundaries of the treated space in order to improve a predetermined air movement within the space.

Supply
The air flow entering the treated space.

Exhaust
The airflow leaving the treated space by means of following methods. Extraction, Relief, Recirculation & Transfer.

Register
A grille with a damper.
Grille
An air terminal device with multiple passages for the air,
usually placed on side walls, bulkheads.

Linear GrilleA grille with fixed linear blades, usually used in large continuous lengths.

Adjustable GrilleA grille with aerofoil blades (Louvers) which can be adjusted to vary the air diffusion direction.

DiffuserSupply air terminal device square rectangular or circular usually placed in the ceiling and consists of deflecting members for diffusion.

Slot Air Terminal Device A device with one or multiple slots for continuous long rectangular opening with or without adjustable member to vary the air flow rate & direction.

NozzleAn air terminal device designed to generate a low energy loss and thus produce a maximum throw by minimum entrainment.

DamperA device used to control the volume of air passing through a terminal by varying crossectional area.

Louvre
A device used for intake air from atmosphere with bird screen and the blades at 45° inclination to eliminate rainwater.

Fire Damper
Device which is installed in an air distribution system between two fire separating compartments and is designed to prevent propagation of fire and smoke.

Sound Attenuator
Device which is inserted into the air distribution system and designed to reduce airborne noise which is propagated along the ducts.




Introduction
The design and selection of air distribution equipment in today's buildings presents one of the more unique challenges for mechanical designers. Unlike other mechanical equipment required for these environmental systems, the air distribution equipment selection must combine a proper choice of engineered products efficiently provide conditioned air to the space while adding architectural features which compliment the interior design.
With today's emphasis on occupant comfort, air quality and energy conservation, the proper selection of air outlets cannot be overlooked. It is the intent of this section to provide a concise approach to the proper selection of air distribution outlets. The general information is given below while the actual selection procedures of the air outlets are given in the respective technical guides.

Comfort Criteria
Comfort
The true measure of the performance of any environmental system is that it maintains comfort of the occupants of the space it serves. Provided the total amount of heated or cooled air required to thermally satisfy the requirements of the space is available, the comfort level within the space becomes totally dependant upon the space air distribution. In general, comfort when related to anatomy can be described as the condition that exists when the heat generated by the body is balanced by some of the metabolic heat transfers through convection, conduction and radiation with the air, wall surfaces and other heat transfer mechanisms in the space. The definition of "comfort" allows us to identify and control the factors which influence the metabolic heat transfer within the space. These factors are:

• Dry bulb temperature
• Relative humidity
• Mean radiant temperature of the surroundings
• Air movement
• Metabolism
• Clothing worn by occupants


Of these, metabolism and clothing are predetermined by the function of the occupants within the space. Dry-bulb temperature and relative humidity are controlled by the thermal effects of the air distribution system. The air movement is a function of the selection of the air diffusion devices employed and is the primary controllable factor by which comfort is affected in the space. Proper air diffusion prevents thermal stratification and stagnation in order to approach a homogenous mixture of room air. Mean radiant temperatures can also be affected by proper location of outlets to prevent substantial heat transfer to and from room surfaces.
Draft Temperature
Extensive studies have resulted in relationships between local temperatures, velocities and comfort reactions. On the basis of the temperature and velocity at a specific point, an effective draft temperature can be calculated for that location. The draft temperature is calculated by the equation:
Ø = (T - Tc) - 0.07 (V - 30)
where:
Ø = draft temperature
T = local temperature
Tc = control temperature
V = local velocity
Research indicates that a high percentage of people are comfortable where the effective draft temperature difference is between -3°F and +2°F and the air velocity is less than 70 fpm. This comfort zone is illustrated as the shaded area in Figure I








Space Air Diffusion
Proper selection of air diffusion devices requires a basic knowledge of the mechanics of room air distribution. (Figure 2) illustrates the interactions of the major factors influencing room air distribution. These factors are described below:
Primary Air
Primary Air is defined as the conditioned air discharged by the supply outlet plus entrained room air which lies within an envelope of arbitrary velocity, usually taken as 150 fpm. It is this air which provides the motive force for room air motion.

Total Air
Total Air is defined as the mixture of primary air and entrained room air which is under the influence of supply outlet conditions. This is commonly considered to be the air within an envelope of 50 fpm (or greater) velocity. Temperature difference between total air package and the room air creates buoyant effects which causes cold supply air to drop and warm air to rise.
Drop
The drop of the cool total air, as shown in figure 2, is the result of the vertical spread of the airstreams due to entrainment of room air and the buoyancy effect due to the density differences between the total air package and the surrounding primary air. The term “density” is very important as drop is primarily dependent upon the mass flow of the total air. Drop can be minimized by spreading the air uniformly over the ceiling surface, thus reducing the mass flow per unit surface area.
Spread
The "Spread" of an outlet is defined as the divergence of the airstreams in a horizontal or vertical plane and is a function of the outlet geometry.


Surface Effect / Coanda Effect
Drop can also be effectively reduced by use of the surrounding ceiling surface. When the supply air velocity is sufficiently high, a negative or low pressure area is created between the moving air mass and the ceiling at or near the supply air outlet. This low pressure area causes the moving air mass to cling to and flow close to the ceiling surface. This principle is known as the "Coanda Effect". Good air distribution design makes use of room surfaces to help keep the supply air outside the occupied zone.
Occupied Zone
The Occupied Zone is usually defined as the area within 6 ft. of the floor and not within 1 foot of the boundaries of the space (walls, etc.) . As this is the area of occupancy, it is desirable to avoid excessive draft velocities and temperature differences within the space.
Room Air
Finally, we come to the medium through which all metabolic heat transfer occurs and thus the most critical factor in controlling human comfort - the room air. The room air consists of all the other air within the space which is not included in the total air package. Proper air distribution attempts to condition this room air to maintain draft velocities and temperatures within those ranges shown in figure 1. This velocity of air within the occupied zone is known as "Room Velocity".
Room air movement is created by its gradual induction toward the primary and total airstreams. It is this constant mixing that provides the mechanisms for heat transfer between the supply and room air. When air movement does not occur (usually as a result of insufficient outlet velocities or poor outlet location), a stagnant layer of room air is formed. Above that layer (or below in the case of overhead heating), proper heat transfer does not exist and temperature stratification occurs. This is illustrated by the temperature gradient curve shown in figure 2. It is always desirable to keep the stagnation layer above the occupied zone in cooling and as near to the floor as possible when heating from above.
Convection Currents
The total air package can easily be influenced by several factors within the space. One of these factors which occur in exterior zones of buildings is the natural convection. currents which result from a hot outside wall during cooling (Figure 2) or a cold outside wall during heating. The upward movement of air in the vicinity of the hot surface tends to oppose the total air movement in overhead cooling. This can act to reduce the outlet throw values or even cause the colder total air to leave the ceiling and create drop into the space. The downward movement of cold air in the vicinity of a cold surface can create cold draft within the occupied space. In the case of overhead heating, the only effective way to minimize these drafts is to direct a high velocity jet of warm air over the wall surface to reduce the difference between the temperature of the surface and that of the room air. Maintaining surface temperatures as near that of the space as possible also minimizes radiation heat transfer potential between the surface and the occupants, resulting in improved comfort response.
Return
The return air inlet has very little effect on room air diffusion regardless of inlet type or location. However, return air inlets should be located at sufficient distance from the supply outlet so that short circuiting of supply air does not occur, It may also be desirable to locate the returns in the stagnant zone to remove unwanted warm or cool air. For cooling, a high sidewall or ceiling return will remove warm air from the space.



Selection Fundamentals
Several factors influence the selection of an air outlet. These factors can be divided into two groups: design factors and performance factors.
1. Design Factors:
Function
The most obvious question is that of primary function - supply or return. Diffuser designs, grille blade profiles and blade spacing are all performance derived, thereby making the outlet's function "non-interchangeable", in most cases.

Location
Our field of selection is further narrowed by knowing where the outlet is to be located within the space (i.e. sidewall, ceiling or floor). This more than anything else determines the generic type of outlet to be used. Two factors can influence outlet location, desired performance and/or architectural/physical constraints. Performance - location considerations could include spot cooling or perimeter heating. Physical/ architectural constraints could, for example, consist of an inaccessible ceiling forcing the use of sidewall or floor outlets.
Air Volume
The actual air volume required to handle the heating/cooling load will also have an effect on the model of outlet used. Generally, the larger the volume per outlet, the more difficult it is to handle comfortably. Some air outlets, due to design considerations and basic concept, are more effective at handling large air volumes.
Natural Convection
Natural convection air currents created by cold walls/windows or localised heat sources should also be considered when planning room air motions. For example, convective air-velocities of over 50 fpm can be expected on some cold window applications and will, if unchecked, dominate the room air motion. The air outlet may be required to blow warm air down the window or wall from the ceiling or up from the floor. Each situation would require different treatment and totally different outlet performance. The buoyancy effect of warm air makes it more difficult to blow down the cold window while it would assist the throw of a floor outlet.


2. Performance Factors:
Throw
Throw is by definition the distance from the outlet face to a point where the velocity of the airstream is reduced to a specified velocity, usually 150, 100 or 50 fpm (Figure 3) . Throw is primarily a function of mass flow and outlet velocity and therefore can be reduced by decreasing either of these values.
Drop
Whenever cool supply air is introduced into a warmer space, its natural tendency will be a downward movement. If the supply air projects into the occupied space, uncomfortable drafts will occur. Drop can be minimized by utilizing the surface effect of ceilings. Outlets located in or near the ceiling will exhibit less drop than outlets located on exposed ductwork. Reducing the supply air volume and increasing supply air temperature will also reduce drop.
Spread
Delivering the air in a spread pattern tends to reduce both the throw and drop of an air outlet. Dissipating the air stream over a wider area increases entrainment and reduces the mass flow per unit surface area (Figure 4) .
Sound / Pressure Drop
Pressure drop and sound are important to the overall system design and performance; however they do not directly affect the room air distribution.




Noise Control Fundamentals
Noise control in air conditioning systems is, in many cases, just as critical as the environmental conditioning itself. Sound is transmitted through the duct system and is also generated by some of the duct elements including diffusers, dampers, volume control and straightening devices, terminal boxes, fans and even the ducts themselves. Noise is defined as objectionable or unwanted sound. The absence of sound created by the air conditioning system can also be annoying. Properly designed, this sound can mask unpleasant noise of conversations, office machinery, etc. to create a more workable environment.

Sound Pressure
Basically sound is a pressure variation that the ear can detect. These pressure variations are caused by a vibrating object or surface, or by turbulent air or gas.
As sound measurement is primarily concerned with human comfort, its measurement reference points reflect the limits of human hearing. That is to say, the lower limit is 20 µPa (20 millionths of a Pascal), the pressure change at the threshold of human hearing. The human ear can withstand pressure changes more than a million times this lower threshold. This would result in a rather large and awkward measurement scale. To avoid this problem a logarithmic scale called the decibel (dB) scale was developed. This decibel measurement is simply a logarithm of the ratio of the measured sound pressure and an agreed reference pressure (in this case f the threshold of human hearing 20 µPa):
Where:
Lp = 20 LOG (P/Po)
Lp = Sound Pressure Level dB
P = Pressure of Sound Wave(Measured)
Po = Reference Pressure
This result in a more compressed and manageable measurement scale.
Sound Pressure vs Sound Power
To this point we have discussed sound pressure levels measured in dB based on a reference pressure (.0002 micro bar or 20 µPa). Sound power levels of an outlet reflect the total acoustical energy output of a source.
A sound source has a certain amount of sound energy or power level. The sound pressure measured in the room will be a function of the sound power generated by the source as well as distance from the source to the measurement point and the amount of sound energy absorbed by the walls and transferred through the walls, windows, etc.
Sound power levels cannot be measured directly. It is usually calculated using sound pressure levels measured in the laboratory with known sound absorption characteristics. Sound power levels are most com-monly referred to in decibels (dB), similar to pressure levels, only the reference is to a power (usually 10-12 watts) instead of a pressure. Most performance data will be presented in sound power levels as it establishes a common reference point and eliminates room absorption variables.
Frequency
The pressure variations that make up sound occur over several frequencies. The frequency is measured in cycles per second or hertz (HZ). Although the normal range of hearing for a human is between 20 HZ and 20,000 HZ, it is most sensitive to sounds between 2 KHZ and 5 KHZ and is less sensitive at higher and lower frequencies. Most naturally occurring sounds produce several pressure waves each at its own frequency and amplitude (dB). This is what is known as broad band noise, as opposed to sound made of a single frequency termed a pure tone. The equipment we deal with produces broad band noise.
To better analyze the characteristics of a sound it is necessary to divide the sound into several bands, each of which contains a limited range of frequencies. The sound pressure level is then measured in each frequency band or octave band. Figure 5 illustrates the frequency range and center frequency of octave bands 1 to 8.






Typically for air distribution systems only bands 2 to 7 are considered.
NC Curve
A method has been developed that allows us to generate a single number to represent the sound level of a device.This single number is called the NC or noise criteria and it makes allowances for the various dB levels in each of the octave bands and human sensitivity to each of these frequency bands.
Sound pressure levels are determined in each of the desired octave bands.These pressure levels are plotted on a standard NC curve from (see fig.6).The highest pressure level when measured against the NC curve,regardless of frequency,determines the NC of the outlet.
NC values give a weighted value of the octave band analysis as perceived by the human ear.A single number NC is often used by manufactures to rate the noise of their equipment,particularly grilles,registers and diffusers.

Addition of dB Levels
Sound power levels and sound pressure levels expressed in decibels(dB)are logarithmic functions and therefore cannot be added directly.Rather than determining the combined effect of two sound sources mathematically,simpler approach is provided in Figure 7.
Note that when two sounds sources are equal value,the resultant will be 3dB higher than either source.If the difference between two sources is greater than 10 dB,the contribution from the quietest source can be ignored.fig 7 can also be used when several sources are to be considered.The different sources are added in pairs as shown below.







Selection Procedure
Outlet Selection
:
The outlet type and size must be carefully selected so as to provide uniform air temperatures and satisfactory velocities within the occupied zone. In addition noise levels must be acceptable. If these criteria are met then a high level of comfort will be achieved for the occupants of the conditioned space.
In order to properly select and size an outlet, a number of performance factors must be taken into account.

Throw

Achieving the proper throw for a specific application is critical to proper outlet selection. Throw data is usually presented at terminal velocities of 50 fpm (0.25m/s), 100 fpm (0.50m/s) and 150 fpm (0.75m/s) Generally outlets should be selected so that the throw at 50 fpm (0.25m/s) terminal velocity equals the distance from the outlet to the boundary of the conditioned space. In most cases this criteria will produce acceptable results.
When an air stream strikes a surface it tends to spread and follow the surface until the velocity dissipates. The total horizontal and vertical distance travelled by the air stream is equal to the tabulated throw of the outlet (Figure 8). For high ceiling applications it may be desirable for the throw to exceed the space boundary (ceiling) and travel down the wall toward the occupied zone. However penetration of the occupied zone should usually be avoided.


In addition to physical boundaries created by walls or partitions, boundaries can be created by the collision of two air patterns (Figure 9). Where two patterns will meet, the outlets should be selected so that the throw is equal to one half the distance between the outlets. For high ceiling applications it may again be desirable for the throw to travel downward toward the occupied zone. Throw is again equal to the horizontal and vertical distance travelled by the air stream.
It should be noted that most catalog throw data is presented for isothermal conditions, i.e. supply air temperature equals room temperature. During cooling the denser supply air will shorten the horizontal throw to approximately 75% of tabulated values.
When selecting outlets for VAV application, both minimum and maximum air quantities must be considered for throw. Although many models of outlets provide excellent horizontal air pattern at extremely low flows, throws may be reduced below acceptable limits. Slot diffusers tend to maintain reasonable throw at low air volume and are therefore a good choice for this application.
Spread
Spreading the air pattern dissipates the air stream over a wider area and increases entrainment. This reduces the mass flow per unit surface area, which in turn reduces throw. Some outlets are designed to produce a spread pattern due to their geometry while others have adjustable vanes.
Air Volume
Throw is directly related to mass flow, therefore a reduction in air volume per outlet will reduce the throw. This can be achieved by utilizing more outlets with less air volume per outlet. For linear diffusers or grilles the same thing can be achieved by dividing the outlet into active and inactive sections (Figure 10). Each active section handles smaller quantity of air, thereby reducing the throw. In order to effectively separate the air pattern, the outlet should be divided by minimum inactive length as illustrated in Table 1.


Drop
Since a velocity of 50fpm (0.25m/s) or less is desirable in the occupied zone, drop should be limited to a terminal velocity of 50fpm (0.25m/s), 6 feet (1.8m) from the floor. At several method can be used for reducing drop for sidewall grilles.
1. Mounting the grille close to the ceiling creates a surface effect and reduces drop.
2. Deflecting the air pattern upward reduces drop.
3. Spreading the air pattern reduces drop.
4. Reducing the air volume reduces drop.
5. Reducing the throw reduces drop.
Drop for ceiling diffusers is rarely catalogued as it is normally of no concern. However for applications with low ceiling heights or when large volumes per outlet are required, the drop should be considered.
ADPI
By definition the ADPI is the statistical percentage of the points when measured uniformly within the space whose draft velocities and temperatures fall within the comfort criteria established in Figure 1. The higher the ADPI rating the higher the comfort level within the space. Generally an ADPI of 80 is considered acceptable.
Through extensive testing, relationships have been developed between ADPI and the ratio of throw over characteristic length (T / L). The throw is the isothermal throw at a selected terminal velocity taken from the catalog performance charts. The characteristic length is the distance from the outlet to the nearest boundary. Table 2 provides definition of characteristic length for various outlet types.


Table 3 illustrates the range of T/L values which will result in optimum comfort conditions for various outlet types at several room loads. By selecting a throw from the catalog data which produces the required T/L ratios, an acceptable ADPI rating can be achieved.
Noise Criteria
Most air outlets are catalogued with a single NC (Noise Criteria) sound pressure rating based on a 10 dB room absorption. This NC value assumes an average room and an approximate distance of 5 feet from a single sound source. These assumptions are reliable for most applications.
Table 4 illustrates the ASHRAE recommended space NC values for many commercial air conditioning applictions. Outlets should be selected so that the tabulated NC levels are within these design goals.
All outlet sound data is for a single source. Allowance must be made for multiple outlets when this occurs in a space since the overall noise level may be the resultant of more than one outlet. Table 5 illustrates the additive effect of multiple outlets of equal sound.