Air conditioning

Problems

For reasons of comfort in the offices, the houses or even in certain technical fields and industrial like the medical laboratories or of manufacture of electronics components, the operating room suites, the computer rooms, it is important to maintain a level of treatment of the acceptable air and even sometimes pointed (operating room suite and laboratory of electronics components). For that one uses specific systems adapted to each field according to what one wishes beforehand to treat or control like:

renewal of the ambient air polluted (local to treat) by forced extraction of this air out of the room, either by forced introduction of new air (surrounding air) into the room, or to ensure partially the renewal of the polluted ambient air (addition of a box of mixture),
the temperature of the air according to the seasons (TO HEAT or To cool),
the control of the moisture of the treated air (humidification or dehumidification),
treatment of dust (filtration of the puffed up or taken again air),
the permanent maintenance of the conditions interiors (regulation).

One will use for that various systems the power stations of treatment of air (C.T.A) or the autonomous systems.

Example of a power station of treatment of air

Systems of air-conditioning

A system of air-conditioning must not only counter the thermal and hydrous loads of a room:

thermal loads: which are the contributions or the external heat losses and interns which are expressed in kilowatts,
hydrous loads: which them is generally internal intrusions of humidity expressed when with them in kilowatts or kilograms per hour (mass throughput).

But it must also ensure the quality of the air by the renewal of new air hygenic (maintenance of the content of CO₂ and the odors on an acceptable level defined by the standards in force), and of course the filtration of the puffed up air.

Enumeration of the various systems of air-conditioning

There exists in the field of the climatic genius several types of systems which one can classify in three categories:

systems any air:

- the power stations unizones (see example above),

- the power stations multizones,

- the autonomous systems (which will be the subject of a specific study a little later).

Renewal of air

There exist several technological solutions concerning the renewal of the air within a room:

1°) the new air is mixed with the external conditions, with part of the return air of the room by the means of a box of mixture (see diagram above),
2°) the new air is prepared in the specific conditions of the room (temperature, hygroscopy) by another power station, called generally central of treatment of new air.

Study of the systems any air

In this type of system in order to prevent that the surrounding air does not come to pollute that of the room, one slightly increases the interior pressure compared to the atmospheric pressure. The advantage of the box of mixture is to realize important savings of energies (respect of the environment).

One thus has in this case a mass throughput of puffed up air > to the mass throughput of return air

This type of process is generally used in the offices, the cinemas .....

Systems used in total recycling

In this type of process the renewal of new air will be obtained either by a mechanical ventilation system controlled or air mixes it will be carried out directly in the room,

is the new air will be prepared in a power station known as: " Power station of air neuf". This air is directly puffed up in the interior conditions of the room. A circuit of particular new air ensures the renewal of new air, and one will have in this case of figure a rejected air flow equal to the new air flow brought.

System functioning in any new air

In this type of process there is no recycling of the air of the room. According to the type of room it will be is in overpressure (in order to avoid any air pollution interior, operating room suite, laboratory of medicinal products…), with the atmospheric pressure is.

The disadvantage of this type of installation they is that it is generating very high thermal powers, therefore not very economic. Any time, in order to reduce the energy costs of these installations, one can install a recuperator of heat (with plates for examples) on these power stations.

System functioning with recuperator of energy

Study of the basic conditions

Before installing a system of air-conditioning it is important to define the conditions of interior and external temperature and hygroscopy.

Definition of the external conditions

These values depend on the season and the geographical location where will be located the buildings to air-condition. The already classified weather data will make it possible to fix the dry temperatures and the hygroscopies. These data will enable us to calculate the maximum loadings to fight in our buildings.

Definition of the interior conditions

The interior temperatures and hygroscopies depend on the type of room:

for the buildings like the individual dwellings, the offices, the department stores ..... (air-conditioning known as of comfort) , the temperature and the hygroscopy will depend on the seasons.

for the buildings of the industrial types, the temperature and the hygroscopy will depend on the use which one makes of the buildings. They can remain constant all the year (data-processing room for example).

Study of the loads of a room

At the time of the study of a project of air-conditioning, it is important in order to be able to correctly dimension the power station of treatment of air of which we have needs, to study as a preliminary the loads which our power station will have to fight. It will be necessary to take account of the loads known as significant and the loads known as latent .

a) Study of the significant loads:

With regard to the significant loads coming from external the , we will have a significant contribution of heat (by sunning in summer for example) and these loads will be positive on the other hand in winter as we will have losses they will be negative.

For the significant loads coming from the interior of the room they will come primarily:

- occupants,

- machines with interior of the room or buildings,

- lighting.

- ..................

b) Study of the latent loads:

The contributions of latent heat (release of moisture in the form of steam) come primarily:

- buildings (like the swimming pools for example),

- the material inside the buildings,

- and of course of the occupants (any human being releases from the odors and moisture).

mathematical Relation giving the hydrous loads named ':

øL = Mr. Lv'

M = represents the mass water to release per hour

Lv = représnte latent heat of vaporization of water.

c) Study of the total loads:

One call total loads the algebraic sum of the significant and latent loads named ' which can be positive or negative and which is given by the following mathematical relation:

øT = øS + øL'

Study of the energy assessment of a room

If the temperature and the hygroscopy of the room are constant the energy assessment of this one can be explained in the following way:

1°) the power brought to the room (puffed up air and internal contributions) is equal to the power lost by this one (return air or lost),

2°) the moisture brought to the room by the puffed up air and the intrusions of humidities interior is equal to the moisture lost in the form of condensation or of extraction of air.

Enthalpic study of the assessment

For that one will suppose that the mass throughput of puffed up dry air is equal to the mass throughput of return air thus:

to øair puffed up = to øair begun again

The power brought to the room is the sum of the power brought by the air in the room, i.e. with øT (see in the preceding chapter).

Conclusion: øT= qmas. has-harp

Knowing that one can determine the conditions of blowing.

Study of the conditions of blowing

If one wants to determine the conditions of puff blowing in the room it is necessary us:

- mass throughput of dry air to blowing (qmas,

- the rate of mixing τ ,

- the variation in temperature Δθ between blowing and the room,

- and of course to be able to trace the point of blowing, we need these coordinates, on a psychrometric diagram two values are enough to position the point blowing (enthalpy and water content for example).

The conditions of the point of blowing (more precisely the condition of comfort) will make it possible to dimension the elements of the installation:

- the mass throughput will make it possible to calculate the powers of the batteries and the water flow trapped by this one (wet cold Batterie), the water flow to be injected (humidifier vapor),

- the enthalpy, the temperature dries and the absolute humidity will make it possible to place the point on the diagram.

Positioning of the point of blowing

The positioning of the point of blowing compared to that of the room depends on the significant and latent loads (contributions or losses).

The conditions to maintain in the room are: θL , rL

The conditions of the point of blowing are: θs , rs

The significant loads can be: =0; <0 or >0

The latent loads can be: =0; <0 or >0

According to the values of the loads, one can consider 9 significant positions of the point of blowing compared to that of the room:

According to the heat balance (contributions or losses), one can thus envisage by advance the position of the point of blowing compared to that of the room.

Example of the point 5 .

Definition of the variation to blowing and the rate of mixing

The variation in temperature to blowing represents the algebraic difference between the temperature of blowing and the temperature of the room:

Δθ = θs - θL or Δθ = θL - θs . This variation is always positive whatever the position of the point of blowing compared to that of the room. It depends on the type of mouths used.

One can take the following values at first approximation:

Blowing be: Δθ = of 5 with 15K

Blowing winter: Δθ = of 5 with 20K

The rate of mixing represents the number of volume of air treated renewed in the room during one hour:

τ =qv/V or τ : rate of mixing in h^-1, V : volume of the room in m ³, and qv : volume throughput of blowing in m ³ /h.

It depends on the type of mouths of blowing installed. It does not exceed 15 in air-conditioning of comfort and can go up to 30 in industrial air-conditioning.

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