GENERAL INFORMATION ON AIR DISTRIBUTION SYSTEMS

The proper air diffusion to the environment (closed room) is one primary factor in order to achieve desirable comfort conditions. In fact, among the basic causes that bother people in the environment are:

  • The excessive air's velocity (draught)
  • The different (dissimilar) distribution of air flow in relevant to the existing thermal charges (positive or negative)
  • The excessive temperature's changes at vertical and horizontal level

All the above can be charged to an improper study or manufacture of the distribution system or air diffusion.

The draughts can be defined as located sense of heat or cold, to a part of body, that are due to the movement and the temperature of the air, at stable conditions of relative humidity and of medium temperature radiation.

In the winter, the draughts are more disturbing than they are in the summer. On the other side excessive low air's velocities, from 0,08 m/s and below, provoke the sense of stationary air that is also disturbing, so they should be avoided

In fact the ideal air's velocity for human comfort is about 0,13 m/s but in most of the cases this rate is difficult to be obtained

The temperature's difference between two areas is also very important for the comfort. Usually a maximum difference of 1,5 o C is tolerable. At vertical level, for people that are doing sedentary work, a difference of 2 o C between the floor (feet) and the pelvis causes displeasure to 10-20 % of the cases.

Also the direction with which the air flow comes towards people must be examined. Experience shows that the best result is achieved when the air's flow is directed towards the face. In case that the air flows towards the head at side, the sense that is caused is less pleasant. Also in case that the air flows towards the back of the neck, even with acceptable velocity, unpleasant conditions are caused. A slow flow of air downwards is generally acceptable, while when the air flows upwards is more disturbing.

At a study of distribution and diffusion system must be taken in mind all the above demands. The distribution as and the diffusion of air must be well studied in order to succeed in manufacturing the perfect installation from cooling and heating side.

After evaluating the cooling and the heating charges that exist in the area, the study includes the following stages:

A)- The air's supply is defined for every area during the heating and the heating. Between the two rates, we choose the biggest. The indicated air's velocities are estimated, considering the basic equipment of the foundation.

TABLE 1 - TYPICAL AIR'S VELOCITIES (m/s) IN THE INSTALLATION EQUIPMENT

EQUIPMENT

INSTALLATIONS

 

DOMESTIC

MERCHANTS

INDUSTRIAL

entrance's external duct

2,5 - 4

2,5 - 4,5

2,5 - 6

filter

1,3 - 1,5

1,5 - 1,8

1,8 - 2,5

cooling units

1,5 - 2

2 - 2,5

2,5 - 3,8

heating units

2,3 - 2,5

2,5 - 4

3,5 - 5

fan's entrance

3,5 - 4,5

4 - 5

5 - 7

fan's exit

5,8 - 8,5

6,5 - 11

8 - 14

basic air ducts

3,5 - 6

5 - 8

6 - 11

branching ducts

3,5 - 5

3 - 6,5

4,5 - 9

B)- The number and the type of air-grilles is defined, considering the following:

1)- AIR'S FLOW VELOCITY

At the case stand the indications that were made previously. Also the flow velocity influents the sound level. At table No 2 are mentioned the effluence's velocities that are actually acceptable for every area as and the acceptable sound level.

TABLE No2 - INDICATED EFFLUENCE 'S VELOCITIES FOR VARIOUS APPLICATIONS

APPLICATION

VELOCITY (m/s)

SOUND LEVEL

SOUND STANDARD

Radio and TV. studios, libraries

1,5 - 2,5

25 dB(A)

NR 20

houses, hotels' rooms, banks, theatres

2,5 - 3,8

30 - 35 dB(A)

NR 25 - 30

offices (without sound elaboration)

2,5 - 5

35 - 40 dB(A)

NR 30 - 35

industries

7,5 - 10

65 - 75 dB(A)

NR 60 - 70


2)- PRESSURE'S DROP IN THE AIR-GRILLES
Excessive pressure's drop decrease the static pressure that is available to the fan and can set in dangerous the right distribution of air and in addition causes the sound's level increase. It is recommended not to exceed 7,5 Pa.

3)- PLACE OF THE HEATING CHARGES IN THE AREA
The air's flow must be directed in a way that counterbalances and exterminates the located heating sources or cooling radiation (for example big surfaces of glass, domestic machines, office's machines)

4)-RANGE OR RADIUS VECTOR OF DIFFUSION (AIR THROW)
The range is the horizontal distance from the air-grille to the point of the draught that the air's velocity is decreased to 0.25 m/s. It is not necessary to be equal to the length of the area (except if heating sources are located at the opposite side). Usually we choose ranges equal to ? of the horizontal distance between the air-grille and the opposite wall.

5)-MAXIMUM VELOCITY OF AIR AND DIFFUSION OF FLOW

6)-AIR'S EXTRACTION SYSTEM
It is defined for every area if one or more extraction's and ventilation's grilles are set. For this purpose are considered :

  • Maximum acceptable velocity of air
  • Maximum acceptable pressure's fall for air's crossing
  • Sound that is produced

The air moves towards the extraction's grilles from all the directions in such a way that his velocity decreases sensibly with the distance. The air's velocity in the extraction's air-grilles should not exceed 1,5 m/s.

At table No3 are mentioned the velocity's rates with which the air-grilles are crossed for various applications. The air-grilles must be set near areas that permit a movement of air with the indicated velocities. They must never be set across supply's air-grilles.

TABLE No3 - MAXIMUM AIR'S VELOCITY IN THE VENTILATION AND EXTRACTION GRILLES

SETTING

MAXIMUM VELOCITY (m/sec)

extraction's grilles

industrial areas

merchant areas :

above occupied area

in occupied area

•  Far from sitting places

•  Near sitting places

in corridors

domestic areas

4 and above

4

3,5

1,5

3,5

2

overpressure air grilles

industrial areas

domestic areas

5 and above

1,5 - 2

door transit grilles

static pressure before the grille

mm H.Y. 0.6

1.25

2.25

6 and above

1,5

2

2,5

3

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