logo

Galvanized Duct

Galvanized prefabricated rectangular ducts

Pre-made galvanized square ducts or air transfer ducts have been used for air condition and air transfer in air conditioning systems for a long time, and due to their strength, shape, beauty and variety of sizes, they are still considered the most high-quality and engineered air transfer system.

 

Industrial square duct (STI Super Seven)

Now, due to the increase in the speed of implementation and completion of construction and industrial projects, it is necessary to use the latest technologies in the world regarding the supply and implementation of various construction products. For this purpose, in 2017, Saman Tahvieh Iranian Company (STI) imported and set up a fully automatic line for the production of galvanized square ducts (STI Super Seven) or  mechanical air ducts in the country, using the experience and technical knowledge of the personnel. For the first time in the country, the production capacity of this product has reached the level of 1500 m2 ducts built per day. It should be noted that only 50 series of this line are available in the world.

 

Unique features of industrial galvanized duct

Due to the use of the most modern technology in the world, this production line has the ability to make ducts with TDF flanges , and this reduces air leakage, removes the iron flange, and increases the execution speed according to the desired quality. Duct construction standards. In order to completely air-seal the connection space of the flanges, air-sealing strips made of elastomer or fireproof thread are used.

Among the other advantages of this machine, we can mention the cutting of all shapes by CNC Plasma , which has the highest standards in the world and produces the desired geometric shape by using duct parts approved by Ashrae   . 
All the seams used in this type of ducts are of Pitts burgh type, which significantly reduces air leakage (almost zero percent) in all parts due to the use of galvanized sheet coil as the primary raw sheet and also benefiting from the highest The modern technology of this machine does not have any dimensional limitations in the construction of the duct and it is able to make and cut work from the thickness of the galvanized sheet from 0.5 to 1.25 mm.

 

In order to strengthen the sheet, this device creates Beads along the duct, which increases the mechanical resistance of the duct compared to the traditional system.

In the end, it is recalled that all the standards used are related to the production method of the Super Seven production line based on SMACNA, which provides the highest standard of duct construction and installation in the world.

 

 

The advantages of producing a square air condition duct with the Super Seven machine

 

  • Daily production of 1500 square meters of industrial galvanized air ducts, aluminum and rectangular steel along with accessories
  • High strength due to the presence of Beads
  • Removal of L profile for use in flanges
  • Zero air leakage as a result of removing the L profile and using the TDF flange
  • Lightening of the duct as a result of removing the L profile and using the TDF flange
  • The exact size and dimensions of the duct according to the map (maximum tolerance 5 mm)
  • The exact verticality of the duct due to the use of standard metal corners
  • Quick and easy installation due to the use of TDF flanges using bolts and clips as needed
  • The possibility of automatic drilling in the sides of the duct, in order to install reinforcing bars by the device
  • Cost reduction in case of insulation operation (elastomeric or glass wool) in the factory of Saman Tahvieh Iranian  Company

 

How to make and install galvanized air condition ducting

After signing the contract and receiving the galvanized air condition ducting plans in the form of an AutoCAD file, the installation and implementation experts of Saman Tahvieh Iranian  Company will be sent to the project to check the overall condition of the project, the duct crossing routes, risers and check the crossing route of other mechanical and electrical facilities. 

At the project site, all the required details are collected with the help of the project agents, and the Shop Drawing maps are prepared in the form of Auto Cad file and, if necessary, Revit, and after approval in this company, it will be approved by the employer.

After that, the approved drawings will be modularized and drawn into manufacturable parts for the Super Seven line as well as the CNC Plus machine by special software.

The purpose of modularization of straight ducts into straight galvanized duct pieces with a length of 1160 mm and preparation of a wide design of shapes including elbows, tees, shorts, S with the help of special CNC machine software.

It is worth mentioning that in the map of the ducts that have a lot of complexity and shapes, one or two parts that are known as critical parts by the company's experts will not be built and installed, and after the rest of the parts are installed, the dimensions of those critical parts will be accurately will be achieved After announcing the dimensions of the critical parts, these parts will be made in the form of prefabricated galvanized air transmission ducts in the factory and will be sent to the project site for installation.

The end part of the seam of each piece of galvanized industrial ducts is sealed and sealed with silicone. In order to seal air between two pieces of prefabricated galvanized ducts, you can use fireproof thread or STI Flex elastomeric insulation sealant type.

In addition, according to the standard, the thickness of the metal corners used in galvanized ducts should be one millimeter, and all four sides of the galvanized duct should have metal corners. The connection of galvanized ducts for air condition ducting is done with bolts and nuts and, if necessary, with special clips. In general, it should be said that this type of prefabricated industrial duct does not have any restrictions on the dimensions or installation of dampers, valves, duct installation to the device, etc.


Air condition duct design

The function of the industrial galvanized duct for air condition ducting is to transfer cold or hot air from the air production device to the space that needs to be ventilated, or to discharge unpleasant air from inside a space to the outside. To perform this operation properly, the system must be designed in such a way that limitations including available space, friction loss, speed, amount of air, noise, increase or loss of heat and air leakage are observed.


Division of duct design

The inlet and return air transfer duct systems are classified according to the air speed and pressure in the ducts:


  1. 1. Speed ​​- The air transfer system can be divided based on the initial speed in the flow duct as follows:
    in the air conditioning of normal buildings
    a) low speed - maximum up to 2500 fpm, usually between 1200 fpm and 2200 fpm
    b) high speed - More than 2500 fpm
    in the air conditioning of industrial buildings
    a) Low speed - up to 2500 fpm, usually between 2200 fpm and 2500 fpm
    b) High speed - 2500 fpm to 5000 fpm


Usually, for the return duct, both for normal and industrial buildings, a low speed is considered, which ranges as follows:
a) In air conditioning of normal buildings - low speed up to 2000 fpm, usually between 1500 fpm up to 1800 fpm 
b) in the air conditioning of industrial buildings - low speed up to 2500 fpm, usually between 1800 fpm and 2200 fpm 


  1. 2. Pressure - the total pressure of the system is the sum of the pressure drop along the duct, in the air conditioner(plenium box) in the entrance valve of the duct to the desired space and the required static pressure at the end of the duct.


The air transmission system is divided into the following three parts based on the overall pressure:
a) low pressure - maximum 3 3/4 inches of water
b) medium pressure – 3 3/4to 6 3/4inches of water
c) high pressure -6 3/4 to 12 1/4 inches of water
Duct design details


  1. 1. Transformations: We use transformations to gradually transform the cross section of the duct from large to small and vice versa. Conversion is also used when hitting construction obstacles such as beams, etc.


For high speeds, the ratio a/b=1/7
. For low speeds, the ratio a/b=1/4             


The following figure shows the types of transformations for the square duct system.
 



2. Elbows: It is used to change the direction or branch off the duct. The types of elbows are:
a) Round elbows: these elbows are made in flat, three-piece and five-piece types.
In this case, the ratio R/D=1.5 is the best ratio.            


    

 


  1. b) Square (rectangular) elbows:
    Square elbows are divided into two categories: long and short radius elbows. The ratio of the central radius of the elbow to the width of the duct is R/D=1.25, which is the best ratio.

  2. 3. Branching from the duct (Take off): When we want to divide the main duct of air distribution into several sub-ducts, a connection called Take off allows to divert a small part of the flow from the main duct to any other sub-duct. do.
    To calculate the take off number of a duct, the following relationship can be used: Takeoff = (branch cfm / main branch cfm) x the large side of the duct

 

 Types of branching (tee, four way, and shorts): In order to branch off air transmission ducts, tees, four ways, and shorts are used, which are shown in the following figures.


Three-way:





Four-way:


Three-way T:






 

The thickness of the air duct sheet

Determining the appropriate thickness for the design of air condition ducting plays a significant role in a proper design. If the thickness of the sheets is thin, they vibrate at high speeds and cause an inappropriate sound. Also, in order to increase the strength of the ducts, in the industrial method, in comparison with the traditional methods that used a bender to cross the line on the sheet, linear faces that have fixed and specific distances and have higher strength are used. will be The distance between the sides as well as their depth is the same and accurate in all levels of the duct.

In the following table, the minimum suitable thickness for industrial and aluminum galvanized sheet according to the maximum dimension of the duct section and the length of the duct, based on topic 14th of the National Building Regulations, is given:

 

Minimum thickness of galvanized sheet (mm)

Minimum thickness of aluminum sheet (mm)

Maximum dimension of duct section (inches)

The maximum dimension of the duct section (cm)

Maximum duct length (inches)

Maximum duct length (cm)

0.5

0.6

Up to 12 inches

up to 30 cm

100

250

0.6

0.7

30-13

75-33

100

250

0.75

0.85

54-31

137-78

80

200

1

1.25

84-55

213-140

80

200

1.25

1.4

High sizes

High sizes

60

150



Friction Coefficient:
The amount of friction on the friction chart is specified in inches of water per 100 feet of equivalent duct length. To determine the loss in each part of the duct, the total equivalent length in that part is multiplied by the friction coefficient, which includes all the elbows and connections in that part. It should be noted that there are tables that show the equivalent length based on the width and height of the duct.

 


Factors affecting the air pressure drop in the air condition duct

  1. A) Air flow speed
  2. b) duct size
  3. c) Roughness
  4. d) Length of ducts


How to calculate the equivalent length in elbows and joints:
The equivalent length of the duct is equal to the circular duct that passes the same CFM as the rectangular duct and has a drop per unit length. In other words, if we have a rectangular duct with dimensions a×b, we can instead choose a circular duct with a diameter of de, which creates the same drop in the air path. Therefore, we have:

 

   




In order to speed up the calculations, instead of the above formula, having the dimensions of the duct, the equivalent diameter can be obtained from the following table:


For example, according to the table above, the equivalent length of a duct with dimensions of 14 x 12 inches is 14.2



Pressure drop table along the galvanized duct:


Industrial galvanized duct design methods

The design of an air ducting system for building air conditioning is done by one of the following three methods:

  •  Slow down method
  •  Uniform friction coefficient method
  • The method of increasing surfaces to compensate for friction (reincreasing static pressure)


1- Method of reducing speed

In this method, the design of the  air condition ducting system is based on choosing a suitable speed at the beginning of the main duct and gradually reducing it at the same time as the duct progresses. For this purpose, we carry out the following steps in order:


A-1) Selection of the initial speed from the suggested speed table:
This selection is in such a way that we assume the speed to be high in the main galvanized ducts and the speed to be reduced as we get closer to the secondary ducts.
Recommended speed of the duct

 

Recommended speed of industrial galvanized duct

Recommended Velocities, fpm

Industrial

Buildings

Schools,

 Theaters,

Public Buildings

Residences

Designation

1200-1800

800-1000

800

1000-1300

600-900

600-700

700-900

600

500

Main ducts

Branch ducts

Branch risers

Maximum Velocities, fpm

1300-2200

1000-1800

100-1600

1100-1600

800-1300

800-1200

800-1200

700-1000

650-800

Main ducts

Branch ducts

Branch risers

Components

500

350

600

600

500

300

500

500

500

250

450

450

Outdoor air intakes

Filters

Heating coils

Cooling coils

500

1600-2400

500

1300-2000

500

1000-1600

Air Washers

Fan Outlets


Near the valve air velocity between 200 fpm to 800 fpm
for secondary ducts (aisle) air velocity between 1200 fpm to 1400 fpm
for main ducts air velocity between 1700 fpm to 1800 fpm


B- 1) Determining the cross section of the duct


We calculate the cross section of the duct from the following formula:
V: Air velocity in duct A=CFM/V


C-1) Determining the dimensions of the duct and the equivalent diameter (de)
D-1) Determining the coefficient of friction
E-1) By approaching the valve, reduce the assumed speed and repeat paragraphs b, c and d to calculate the dimensions of the duct and the value of the friction coefficient show
F-1) Determining the static pressure of the fan by calculating the pressure drop in the longest equivalent path.
The static pressure required for the fan to bring air into the room is calculated by using the longest duct path and taking into account the drop of the farthest diffuser, elbows and connections in the farthest path.


2- Same friction coefficient method

In this method, the friction loss per foot of duct length is considered the same for the entire system.

In order to design the duct using the same friction method, we perform the following steps

As with the speed reduction method, we calculate steps (1-a, b, c, and d) respectively


  1. a) Choosing V (air velocity in the duct)
    b) Determining the cross section of the main duct A=CFM/V
    c) Determining the dimensions of the duct and the equivalent diameter
    d) Determining the friction loss


3-The method of increasing levels to compensate for friction

The uniform friction coefficient method does not satisfy the design conditions of uniform static pressure in all branches and air intake diffusers to the rooms. To get the right amount of air at the beginning of each branch, it is necessary to use a branch valve to adjust the flow. It may also be necessary to install a control device (curtain, volume valve or adjustable volume controller on the air inlet grill to the desired space) to adjust the flow in each of the end valves and distribute the appropriate air. 

With the explanations given, we design the third method in such a way that determining the size of the industrial galvanized duct in a route in such a way that the increase in static pressure (increase due to the decrease in speed) in each branch or end valve is exactly equal to the friction loss inside Air condition ducting. In this method, the most important disadvantage is the increase in the cost of the design due to the increase in the surface of the duct, and for its merits, we can also mention the reduction of the fan power.


With these explanations, we proceed as follows to design the duct system using the method of increasing levels.


We repeat steps 1-a, b, c, d, e exactly like the previous methods.


E-3) We obtain the coefficient of friction in 100 feet of duct length and multiply it by (L1+L2).


L1: is the equivalent length of connections to the first branch 0.15+f/100*(L1+L2)=fan pressure drop
L2: is the length of the ducting to the first branch


Return duct design

The return duct system is generally calculated with the same friction loss method and in the same order as the outgoing duct. In many systems, the amount of return air is equal to the amount of air going to the room minus the amount of air required for room air condition (the amount of air that is needed to provide oxygen to the room). This amount of extra air creates a positive pressure in the room, which prevents the natural penetration of outside air into the room. Because the static pressure in the return duct is negative, special care must be taken to avoid creating a large pressure difference in the return valves near the fan. A large static pressure difference, when passing through the return valves, leads to excess air flow and noise. (due to the high speed of air passing through the grill). For this reason, the return duct system should be designed with the minimum length and minimum possible connections.
Again, in order to reduce the pressure drop, the return duct system must be designed so that the speed of the air passing through it is lower, in this sense, generally in the design of the return duct system, the coefficient of frictional pressure drop is about 75% to 80% of the coefficient of frictional pressure drop in the system design. The duct will be gone.


 

leave comment