En razón de su función, los conductos reciben distintos nombres con el mismo apelativo que el aire que transportan, como se puede ver en la figura.

En ella, un climatizador provisto de enfriamiento gratuito,[4]​ impulsa el aire tratado en el local a climatizar. El aire retorna de nuevo al climatizador aspirado por un segundo ventilador. Una parte de ese aire se expulsa al exterior y la otra se recircula para mezclarla con aire del exterior. La mezcla se trata y comienza de nuevo el ciclo.

La ventilación es independiente. En ella un ventilador renueva el aire del local. El aire viciado es extraído hacia el exterior.

Todo ello no implica diferencia alguna en las características o método de cálculo de cada conducto, que dependen exclusivamente de las condiciones del sistema.

Los sistemas de distribución por conductos se clasifican atendiendo a estos dos parámetros:.

By pressure

They are classified based on the type of fan that is attached to the system. Therefore just like the fans:.

Por regla general, el sistema de distribución de conductos se diseña de forma que tenga el trazado más simple posible y simétrico en cuanto lo permitan las obstrucciones que implica la estructura del edificio.

En todos los métodos de cálculo,[5]​ se supone previamente calculado el caudal&action=edit&redlink=1 "Caudal (fuido) (aún no redactado)") total necesario para cubrir la demanda térmica sensible y latente y repartido proporcionalmente a la potencia de cada tramo.

linear head loss

The head loss is calculated by Darcy's formula.

In which is the hydraulic diameter[6]​ and is the friction factor.

Now, both the pressure loss and the friction factor depend on the properties of the fluid, which change with the length of the duct traveled, so taking into account the variations in the properties of the air with temperature, specific humidity, altitude and pressure, we arrive at:

In which is the head loss in section AB, it is a coefficient that depends on the material and is indicated in the following table, it is the length of the section and it is the speed.

pressure loss in accessories

The head loss obtained is known as linear head loss or head loss per meter of straight conduit. But when singularities occur in the duct, such as curves, reductions, branches, etc., an additional pressure loss is produced known as pressure loss in accessories.

Every accessory involves a change in speed or trajectory and therefore a variation in kinetic energy, which translates into a loss of pressure.

In which is the dynamic loss coefficient, different for each type of accessory[7]​ and is the speed at its entrance.

In many cases, what is known as Equivalent length of the accessory is usually used, which is nothing more than estimating the length of rectilinear conduit that produces the same head loss as the accessory and in the linear head loss formula, considering as the length of each section, its actual length plus the equivalent length of its accessories.

calculation methods

Low speed systems can be calculated by three main methods:.

It is a very simple method, but only applicable to very basic installations without major distribution requirements. It consists of selecting an output speed, taken from the attached table, at the fan discharge and reducing it in each section along the duct.

The diameter of the circular duct necessary for each section is calculated by:.

If the duct is going to be rectangular, the measurements are taken from the circular section of equivalent diameter.[8]​.

The head loss of each section is then calculated from the corresponding nomogram or using the formula:.

In which is the section flow, it is the perimeter of the rectangular duct and its section.

Finally, the pressure losses of the most unfavorable path are added.[9]​ The total corresponds to the static pressure required in the fan.

This is a better method than the previous one in that it is applicable to most of the most frequent installations and gives good results, especially if the distribution is symmetrical. If it is not, the system may be difficult to balance.[10]​ The method consists of calculating the ducts so that they have the same pressure loss per unit length throughout the entire system.

For the calculation, we begin by establishing the head loss, using the nomogram or Darcy's formula, corresponding to the total flow required and the recommended speed, taken from the previous table, or, a head loss can be set, known from experience as good.[11]​

Crossing the flow of each section with the fixed head loss and rounding to the speed closest to the recommended one, the diameter of the necessary conduit is obtained (see figure). It can later be converted to a rectangular duct.

It can also be calculated using the formula:.

Finally, the total head loss is calculated by multiplying the set head loss by the length of the most unfavorable path.

This method is based on Bernoulli's Principle, according to which, in a conduit through which a fluid circulates, the sum of the dynamic pressure due to its speed, the static pressure due to its pressure and the pressure due to its height is a constant value. Considering the horizontal duct, that is, without variation in height along its route, the static and dynamic pressure remain, in such a way that if the speed and therefore the dynamic pressure decrease in a section, the static pressure will increase by the same value.

This is the basis of the method, which consists of sizing the duct so that the increase in static pressure due to the decrease in speed in one section compensates for the friction losses in the next. In this way the static pressure remains constant and is the same at each outlet.

You begin by selecting an initial speed from the table of recommended speeds, but taking care that it is as high as possible, to prevent the final speed from being too low or the duct being too large. With this speed, the section of the first section is calculated and its head loss is determined, the same as in the previous method.

The following sections must comply with the foundation of the method. Therefore, for any section:

The first member is the sum of the linear head loss in the section plus the sum of the head losses in the accessories of said section and plus the head loss in the bypass or reduction at the entrance to the section whose entrance speed is . The second member is the dynamic pressure loss or static pressure gain when going from speed to speed.

This is an expression in which the only unknown variable is , but it will require an iterative calculation, which is why two graphs are usually used; In the first, the relationship for the section is obtained and in the second, by crossing this value with the inlet velocity, the exit velocity or the recovered pressure can be found, which allows us to calculate the section of the section.

Currently there are software programs with which a simple, fast calculation is achieved and, if the data is entered correctly, very satisfactory results.[12]​.

Como se ha visto, en una distribución de conductos se pueden encontrar algunos accesorios que forman parte de la misma y condicionan su comportamiento. Los hay tanto para conductos circulares, como ovales o rectangulares.

Elbows

The elbows change the direction of the current by a certain angle. The elbow can be; straight if the two directions intersect in a straight line or curved if they do so through a continuous curve. The curve can also be discontinuous and made up of three or more segments. Obviously, the smoother the change of direction, the less pressure loss and the less noise.

Derivations

They are used to divide or bifurcate the air stream. The branch current exits laterally to the main conduit and can do so; forming a right angle, which is not highly recommended or forming an angle between 15° and 45° with the main current and in the same direction, although later you have to change its direction with an elbow. It may be that the derived flow implies a reduction in section at the outlet of the main conduit or not.

When the main current is branched into two symmetrical equals, forming a Y, the branch is known as pants.

Gates and flow regulators

They are accessories that are used to completely or partially close the passage through the duct. The gates are usually formed by an aluminum frame, the interior of which is closed by flat or aerodynamic fins, each of which pivots jointly with a longitudinal axis that passes through the center of the fin and rests on both sides of the frame. The axes are unified on the outside of the frame with a single control to move them manually or by coupling a servomotor.

Regulators or flow registers are small gates that are attached to the outlet mouths to control their flow. They can be; shutter, as in gates, butterfly, when a single flap covers the entire passage section, or sliding, when a sliding screen is the one that closes the passage section.

Reductions

In a duct layout, the section is normally reduced after each outlet, except if the decrease in flow is very small. The piece that joins two sections of duct with different sections is called reduction and must be executed so that the transition from one section to the other is as smooth as possible. Slopes between 15% and 20% are recommended.

When the duct changes shape, for example from round to rectangular, the fitting is known as transformation.

When a large duct has to be branched to several small ones, it can be done through a large duct or box to which both the large inlet duct and the small outlet ducts are attached. This box or conduit is known as plenum and guarantees stabilization of pressure and a homogeneous distribution of flows.

Obstacles

They are not typical accessories of the ducts, although they are very common to find in the facilities. These are elements of the building or other installations, such as pipes, cables, etc. whose passage through the duct is not recommended, but in many cases it is irremediable. When this occurs, the obstacles must be covered with an aerodynamically shaped cover, which minimizes pressure losses and prevents noise.

Diaphragms

They are elements that are not currently used much, however they are very useful in balancing a duct system. They consist of sheet metal or even fiberglass plates, with the same shape as the section of the section in which they are going to be inserted. Perforations are made in the plate. The number of perforations and their diameter, as well as the thickness of the plate, determine the pressure loss that will occur in the air flow.

Mouths

The mouths are the exit and entry terminals of a duct distribution. There are only inlet openings other than those returning to the generator, or the outside air intakes, both generally made up of grilles.

The outlets can basically be; ceiling and wall. Both have many variants. Their selection is critical, since they constitute the fundamental element to guarantee a homogeneous distribution of the treated air in the premises to be air-conditioned and temperature and speed conditions that ensure thermal well-being[13]​ in occupied areas.

Its calculation and selection belong to an independent technique, which is known as air diffusion.[14]​.

En razón de su función, los conductos reciben distintos nombres con el mismo apelativo que el aire que transportan, como se puede ver en la figura.

En ella, un climatizador provisto de enfriamiento gratuito,[4]​ impulsa el aire tratado en el local a climatizar. El aire retorna de nuevo al climatizador aspirado por un segundo ventilador. Una parte de ese aire se expulsa al exterior y la otra se recircula para mezclarla con aire del exterior. La mezcla se trata y comienza de nuevo el ciclo.

La ventilación es independiente. En ella un ventilador renueva el aire del local. El aire viciado es extraído hacia el exterior.

Todo ello no implica diferencia alguna en las características o método de cálculo de cada conducto, que dependen exclusivamente de las condiciones del sistema.

Los sistemas de distribución por conductos se clasifican atendiendo a estos dos parámetros:.

By pressure

They are classified based on the type of fan that is attached to the system. Therefore just like the fans:.

Por regla general, el sistema de distribución de conductos se diseña de forma que tenga el trazado más simple posible y simétrico en cuanto lo permitan las obstrucciones que implica la estructura del edificio.

En todos los métodos de cálculo,[5]​ se supone previamente calculado el caudal&action=edit&redlink=1 "Caudal (fuido) (aún no redactado)") total necesario para cubrir la demanda térmica sensible y latente y repartido proporcionalmente a la potencia de cada tramo.

linear head loss

The head loss is calculated by Darcy's formula.

In which is the hydraulic diameter[6]​ and is the friction factor.

Now, both the pressure loss and the friction factor depend on the properties of the fluid, which change with the length of the duct traveled, so taking into account the variations in the properties of the air with temperature, specific humidity, altitude and pressure, we arrive at:

In which is the head loss in section AB, it is a coefficient that depends on the material and is indicated in the following table, it is the length of the section and it is the speed.

pressure loss in accessories

The head loss obtained is known as linear head loss or head loss per meter of straight conduit. But when singularities occur in the duct, such as curves, reductions, branches, etc., an additional pressure loss is produced known as pressure loss in accessories.

Every accessory involves a change in speed or trajectory and therefore a variation in kinetic energy, which translates into a loss of pressure.

In which is the dynamic loss coefficient, different for each type of accessory[7]​ and is the speed at its entrance.

In many cases, what is known as Equivalent length of the accessory is usually used, which is nothing more than estimating the length of rectilinear conduit that produces the same head loss as the accessory and in the linear head loss formula, considering as the length of each section, its actual length plus the equivalent length of its accessories.

calculation methods

Low speed systems can be calculated by three main methods:.

It is a very simple method, but only applicable to very basic installations without major distribution requirements. It consists of selecting an output speed, taken from the attached table, at the fan discharge and reducing it in each section along the duct.

The diameter of the circular duct necessary for each section is calculated by:.

If the duct is going to be rectangular, the measurements are taken from the circular section of equivalent diameter.[8]​.

The head loss of each section is then calculated from the corresponding nomogram or using the formula:.

In which is the section flow, it is the perimeter of the rectangular duct and its section.

Finally, the pressure losses of the most unfavorable path are added.[9]​ The total corresponds to the static pressure required in the fan.

This is a better method than the previous one in that it is applicable to most of the most frequent installations and gives good results, especially if the distribution is symmetrical. If it is not, the system may be difficult to balance.[10]​ The method consists of calculating the ducts so that they have the same pressure loss per unit length throughout the entire system.

For the calculation, we begin by establishing the head loss, using the nomogram or Darcy's formula, corresponding to the total flow required and the recommended speed, taken from the previous table, or, a head loss can be set, known from experience as good.[11]​

Crossing the flow of each section with the fixed head loss and rounding to the speed closest to the recommended one, the diameter of the necessary conduit is obtained (see figure). It can later be converted to a rectangular duct.

It can also be calculated using the formula:.

Finally, the total head loss is calculated by multiplying the set head loss by the length of the most unfavorable path.

This method is based on Bernoulli's Principle, according to which, in a conduit through which a fluid circulates, the sum of the dynamic pressure due to its speed, the static pressure due to its pressure and the pressure due to its height is a constant value. Considering the horizontal duct, that is, without variation in height along its route, the static and dynamic pressure remain, in such a way that if the speed and therefore the dynamic pressure decrease in a section, the static pressure will increase by the same value.

This is the basis of the method, which consists of sizing the duct so that the increase in static pressure due to the decrease in speed in one section compensates for the friction losses in the next. In this way the static pressure remains constant and is the same at each outlet.

You begin by selecting an initial speed from the table of recommended speeds, but taking care that it is as high as possible, to prevent the final speed from being too low or the duct being too large. With this speed, the section of the first section is calculated and its head loss is determined, the same as in the previous method.

The following sections must comply with the foundation of the method. Therefore, for any section:

The first member is the sum of the linear head loss in the section plus the sum of the head losses in the accessories of said section and plus the head loss in the bypass or reduction at the entrance to the section whose entrance speed is . The second member is the dynamic pressure loss or static pressure gain when going from speed to speed.

This is an expression in which the only unknown variable is , but it will require an iterative calculation, which is why two graphs are usually used; In the first, the relationship for the section is obtained and in the second, by crossing this value with the inlet velocity, the exit velocity or the recovered pressure can be found, which allows us to calculate the section of the section.

Currently there are software programs with which a simple, fast calculation is achieved and, if the data is entered correctly, very satisfactory results.[12]​.

Como se ha visto, en una distribución de conductos se pueden encontrar algunos accesorios que forman parte de la misma y condicionan su comportamiento. Los hay tanto para conductos circulares, como ovales o rectangulares.

Elbows

The elbows change the direction of the current by a certain angle. The elbow can be; straight if the two directions intersect in a straight line or curved if they do so through a continuous curve. The curve can also be discontinuous and made up of three or more segments. Obviously, the smoother the change of direction, the less pressure loss and the less noise.

Derivations

They are used to divide or bifurcate the air stream. The branch current exits laterally to the main conduit and can do so; forming a right angle, which is not highly recommended or forming an angle between 15° and 45° with the main current and in the same direction, although later you have to change its direction with an elbow. It may be that the derived flow implies a reduction in section at the outlet of the main conduit or not.

When the main current is branched into two symmetrical equals, forming a Y, the branch is known as pants.

Gates and flow regulators

They are accessories that are used to completely or partially close the passage through the duct. The gates are usually formed by an aluminum frame, the interior of which is closed by flat or aerodynamic fins, each of which pivots jointly with a longitudinal axis that passes through the center of the fin and rests on both sides of the frame. The axes are unified on the outside of the frame with a single control to move them manually or by coupling a servomotor.

Regulators or flow registers are small gates that are attached to the outlet mouths to control their flow. They can be; shutter, as in gates, butterfly, when a single flap covers the entire passage section, or sliding, when a sliding screen is the one that closes the passage section.

Reductions

In a duct layout, the section is normally reduced after each outlet, except if the decrease in flow is very small. The piece that joins two sections of duct with different sections is called reduction and must be executed so that the transition from one section to the other is as smooth as possible. Slopes between 15% and 20% are recommended.

When the duct changes shape, for example from round to rectangular, the fitting is known as transformation.

When a large duct has to be branched to several small ones, it can be done through a large duct or box to which both the large inlet duct and the small outlet ducts are attached. This box or conduit is known as plenum and guarantees stabilization of pressure and a homogeneous distribution of flows.

Obstacles

They are not typical accessories of the ducts, although they are very common to find in the facilities. These are elements of the building or other installations, such as pipes, cables, etc. whose passage through the duct is not recommended, but in many cases it is irremediable. When this occurs, the obstacles must be covered with an aerodynamically shaped cover, which minimizes pressure losses and prevents noise.

Diaphragms

They are elements that are not currently used much, however they are very useful in balancing a duct system. They consist of sheet metal or even fiberglass plates, with the same shape as the section of the section in which they are going to be inserted. Perforations are made in the plate. The number of perforations and their diameter, as well as the thickness of the plate, determine the pressure loss that will occur in the air flow.

Mouths

The mouths are the exit and entry terminals of a duct distribution. There are only inlet openings other than those returning to the generator, or the outside air intakes, both generally made up of grilles.

The outlets can basically be; ceiling and wall. Both have many variants. Their selection is critical, since they constitute the fundamental element to guarantee a homogeneous distribution of the treated air in the premises to be air-conditioned and temperature and speed conditions that ensure thermal well-being[13]​ in occupied areas.

Its calculation and selection belong to an independent technique, which is known as air diffusion.[14]​.