﻿Calculation of air ducts for speed and flow rate + methods for measuring indoor air flow

# Calculation of air ducts for speed and flow rate + methods for measuring air flow in rooms

Checked by a specialist: Alexey Dedyulin
Author: Julia Oschepkova
Last update: January 2020

Balanced air exchange is the basis of well-being and working ability of people. Is not it? But in order to create comfortable conditions in residential and industrial premises, it is important to correctly calculate the air ducts for speed and flow rate and to ensure an effective mode of movement of air flows.

Next, we will tell you what you need to calculate the air ducts, what methods and devices are used to measure air flow rate.

## What is a duct?

Air duct - the main element of the systemair distribution. It is a collection of metal or plastic pipes placed to ensure air balance. The principle of operation of the duct is to supply and extract air using special fans.

Basic characteristics of the duct:

• shape (round or rectangular);
• cross-sectional area;
• stiffness (flexible,semi-flexible and tough).

The performance of the ventilation system and its functionality as a whole depend on these characteristics.

Choose a duct with a rectangular section, if it is important for you to make it invisible. Such an air duct is easy to hide on the upper surface of the cabinets.

The correct selection of the parameters of the duct, taking into account all the features of the room will ensure its long and efficient operation.

## Calculation algorithm

When designing, adjusting, or modifying an existing ventilation system, duct calculations must be performed. This is necessary in order to correctly determine its parameters, taking into account the optimal characteristics of performance and noise in current conditions.

When performing calculations, the results of measuring the flow rate and air velocity in the air channel are of great importance.

Air consumption - the volume of air mass entering the ventilation system per unit of time.As a rule, this indicator is measured in m³ / h.

Movement speed- a value that shows how quickly air moves in the ventilation system. This indicator is measured in m / s.

If these two indicators are known, you can calculate the area of ​​round and rectangular sections, as well as the pressure necessary to overcome local resistance or friction.

Drawing up a diagram, you need to choose the angle of view from the facade of the building, which is located in the lower part of the layout. Ducts are shown in solid thick lines

Most often, the following calculation algorithm is used:

1. Drawing up an axonometric diagram in which all the elements are listed.
2. Based on this scheme, the length of each channel is calculated.
3. Air consumption is measured.
4. The flow rate and pressure at each section of the system are determined.
5. The friction loss calculation is performed.
6. Using the desired coefficient, the pressure loss is calculated while overcoming the local resistance.

When performing calculations on each network sectionair distribution different results are obtained. All data must be balanced by means of diaphragms with the branch of greatest resistance.

### Calculation of cross-sectional area and diameter

The correct calculation of the area of ​​round and rectangular sections is very important. An improper cross-sectional size will not allow to provide the necessary air balance.

A duct that is too large will take up a lot of space and reduce the effective area of ​​the room. If you select too small a channel size, drafts will appear as the flow pressure increases.

In order to calculate the required cross-sectional area(S), you need to know the values ​​of flow rate and air velocity.

For calculations, the following formula is used:

S = L / 3600 * V,

wherein L - air consumption (m³ / h), andV - its speed (m / s);

Using the following formula, you can calculate the diameter of the duct(D):

D = 1000 * √ (4 * S / π)where

S  sectional area (m²);

π – 3,14.

If you plan to install rectangular rather than round ducts, the required length / width of the air duct is determined instead of the diameter.

All obtained values ​​are compared with GOST standards and products that are closest in diameter or cross-sectional area are selected

When choosing such a duct, an approximate section is taken into account. The principle used a * b ≈ Swherea - lengthb - width, andS - cross-sectional area.

According to regulationsratio width and length should not be higher than 1: 3. You should also use the table of typical sizes provided by the manufacturer.

The most common sizes of rectangular channels are: minimum dimensions - 0.1 mx 0.15 m, maximum - 2 mx 2 m.The advantage of round ducts is that they have less resistance and, accordingly, create less noise during operation.

### Calculation of pressure loss on resistance

As air moves along the line, resistance is created. To overcome it, the supply air fan creates a pressure that is measured in Pascals (Pa).

Pressure loss can be reduced by increasing the duct cross section. In this case, approximately the same flow rate in the network can be ensured.

In order to choose the right ngrate installation with a fan of the required capacity, it is necessary to calculate the pressure loss to overcome the local resistance.

This formula applies:

P = R * L + Ei * V2 * Y / 2where

R - specific pressure loss on friction in a certain section of the duct;

L - length of the plot (m);

Ei - total coefficient of local loss;

V - air speed (m / s);

Y - air density (kg / m3).

ValuesR determined by standards. Also, this indicator can be calculated.

If the duct cross section is round, friction pressure loss (R) are calculated as follows:

R = (X* D / B) * (V*V*Y)/2gwhere

X - coefficient.friction resistance;

L - length (m);

D - diameter (m);

V - air velocity (m / s), and Y - its density (kg / m³);

g - 9.8 m / s².

If the cross section is not round, but rectangular, an alternative diameter equal to D = 2AB / (A + B)where A and B are sides.

## Calculation Software

All calculations can be performed manually, but it is more convenient and faster to use specialized programs.

Using such programs, you can not only accurately perform the necessary calculations, but also prepare drawings.

If necessary, you can use special software to perform the calculations. This will eliminate possible errors that can play a fatal role during operation. Primary values ​​are entered into the program and after a few seconds you can get accurate calculation results

VentCalc - A functional application for calculating ducts. For calculations, the values ​​of air flow and speed, as well as temperature are used.

Magicad - Performs all types of calculations for engineering networks, images are presented in 2D and 3D formats.

Gidrv - A program for calculating all parameters of ducts. It is possible to select any combination of parameters to achieve the best performance.

Ducter 2.5 - A utility that accurately calculates the diameters of the cross sections of the ducts. Ideal for selecting their types.

The drawings that are compiled in these programs allow you to more accurately see the layout of all components of the system and ensure their most efficient operation.

## Speed ​​and air flow measurement

When taking measurements, it is important to choose the right instruments and methods, as well as follow the procedures for taking measurements.

### Instruments used for measurements

Most often, the following types of instrumentation are used:

• ultrasonic ZD anemometer - performs measurements based on changes in the frequency of sound between given points;
• Pitot tube - fixes the difference between static and total pressure;
• hot-wire anemometer - determines the flow rate based on the rate at which the sensor temperature decreases.
• winged Anemometer - performs measurements based on changes in the speed of rotation of the impeller.
• bolometer - determines the air flow due to the concentration of the flow at the measuring point, the cross section is set in advance.

Many devices on this list are quite expensive and rare. They can be rented and measured independently, but it is better to call an experienced adjustment engineer who knows all the nuances of performing measurement work.

Pitot tube is used in conjunction with sensors. It is an easy to use device. The tube is led open end towards the air flow, and its other end is connected to the pressure gauge

Speed ​​measurement is necessary not only to perform calculations, but also to control the hygienic parameters of indoor air. Over a period of time inevitably occurs ventilation duct contamination and ducts.

In such cases, the connections may be depressurized, equipment performance is reduced. In addition, measurements are necessary for scheduled maintenance, cleaning and repair of the ventilation system.

When performing measurements, a number of rules must be observed. Firstly, the air speed is regulated by building codes and standards. It is necessary to focus on these values.

Small deviations from these parameters are allowed in the presence of special technical circumstances. For example, when installing equipment, performing repairs, etc.

Secondly, when taking measurements, it is also necessary to take into account the norms of related factors - noise and vibration levels, which are indicated in regulatory documents.

Exceeding these standards indicates the shortcomings of the ventilation system.Air speed should not have any effect on these indicators.

### Methods for measuring air flow

On stagecommissioning it is necessary to take measurements of the air volume flow in the ventilation and air conditioning system. This will provide the opportunity for high-quality configuration of the system and its smooth operation.

Such measurements are carried out directly in the duct or on the inlet grille. There are several simple techniques.

#### Ceiling Diffuser Measurements

Most often for measurements using this techniquebolometer. Must close diffuserand the topconfuser attach to the ceiling. It is necessary to measure as the total amount of air exhaust from the roomsupply air.

The bolometer is highly accurate due to the fact that the built-in flow rectifier reduces the likelihood of error. Despite the fact that the device looks bulky, it is quite light - its weight is not more than 3 kg

In some sources, it is recommended to use a probe for measurements, inserting it between lamellas diffuser to obtain an average result.

This approach is ineffective for two reasons:

1. The turbulence of the flows is very high, so the real flow rate cannot be seen.
2. It is not possible to set the probe directly in line with the flow. The measurement results will be distorted in any case.

Thus, you should not waste your time on excessive manipulations with the probe. There are much simpler and more accurate measurement methods.

There is another way to take measurements using this technique. It provides for a straight section and a uniform flow. Measurements are made through pre-made holes.

This method is highly accurate, but there are not always conditions for its implementation. Not everywhere there are straight sections, sometimes it is impossible to prepare two holes for measurements. And yet, to implement this method, several people are needed: one must take measurements, the second - to keep the stepladder, and so on.

Given all of the above, if you need to get a quick and accurate result, without making unnecessary efforts - usebolometer.

#### Measurements on the ventilation grill

To perform control and measurement operations using this technique, hot-wire anemometer with an impeller from 60 to 100 mm in diameter. The impeller should be comparable with the dimensions of the grill.

The hot-wire anemometer is a multifunctional device that can be used not only for measuring air velocity, but also for measuring other parameters. Such a device will be very useful in the home. When purchasing a hot-wire anemometer, it is better to dwell on a device that has a function for analyzing and documenting data

This method provides for high accuracy of the results, and the number of measurements performed is minimal. In order to provide access to inaccessible places, you can use a special extension cord or telescopic probe.

#### Duct measurements

To perform measurements, a specially made working hole is used in the wall of the duct channel.

It is important to observe the following conditions:

• the cross-sectional size of this hole must exactly correspond to the diameter of the probe;
• the place for measurements needs to be selected carefully. The hole is drilled only in a straight section, the length of which should be at least 5 duct diameters. The hole must be positioned so that the distance to it is 3 diameters, and after it - 2 pipe diameters.

In case of measurements inside the duct, it is necessary to use a device with an impeller with a diameter of 16 to 25 mm. If the duct is high, a telescopic probe or extension cord will come to the rescue.

## Rules for using measuring devices

When measuring the air flow rate and its flow rate in the ventilation and air conditioning system, the correct selection of devices and the following rules for their operation are required.

This will allow you to get accurate results of the calculation of the duct, as well as to make an objective picture ventilation systems.

In order to fix the average flow rates, you need to perform several measurements. Their number depends on the diameter of the pipe or on the size of the sides, if the channel is rectangular

Observe the temperature regime indicated on the device certificate. Also monitor the position of the probe sensor. It should always be oriented exactly towards the air stream.

If this rule is not followed, the measurement results will be distorted. The greater the deviation of the sensor from its ideal position, the higher the error.

## Conclusions and useful video on the topic

Arrangement of ventilation at home:

You can find out how to measure volumetric air on ventilation grilles from the following video:

Thus, it is very important to observe the rules for performing measurements, because the slightest error can affect the results of calculations.

Correct calculations of the duct will allow you to choose its optimal configuration and the necessary components, which means that uninterrupted and productive functioning of ventilation will be ensured.

If you have questions or can supplement the material with valuable information, please leave your comments, share your experience. The communication unit is located under the article.

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