Air conditioning processes
Contenido
En instalaciones completas de climatización, el acondicionamiento del aire se hace mediante Unidades de Tratamiento de Aire (UTA), que son aparatos modulares en los que en cada módulo se realiza un tratamiento y se agrupan en función de las condiciones finales de aire requeridas. El tratamiento de aire más completo, es la climatización, en la que se necesitan la mayor parte de los módulos existentes, para garantizar las condiciones del bienestar térmico") de las personas. Probablemente por esta razón, las UTAs se conocen normalmente como climatizadores. Los módulos de calor y frío, funcionan con baterías de agua caliente y fría respectivamente, que obtienen de generadores independientes; la producción de agua caliente suele confiarse a calderas "Caldera (calefacción)") y la de agua fría a máquinas frigoríficas llamadas enfriadoras.
Ventilation module
The module located at the head of any AHU is always a fan that moves a mass flow of air taken from the environment to be treated, passes it through all the modules installed in its suction and propels it, once treated, back into the environment.
In those installations in which there is a large return network or in those in which there is free cooling (free-cooling) of the air, two fans are installed; one in the delivery and another in the return, which usually have the same flow rate and with an available pressure corresponding to the pressure loss of the part of the distribution network they supply.
Filtered
The filtering function is fulfilled in the filtration module and in filtration stages installed at key points of the distribution. It consists of treating the air using appropriate filters in order to remove dust, impurities and suspended particles. The degree of filtering necessary will depend on the type of conditioning installation to be carried out.
To clean the air, filters are used that are normally of the mechanical type, composed of porous substances through which the air is forced to pass and in which it leaves the particles it carries in suspension.
In common comfort installations, polyurethane filters, glass wool, synthetic or metal wire microfibers woven with different steel or aluminum mesh soaked in oil are used.
The filter is the first element, and very commonly, also the last to be installed in the air circulation, because it not only protects the conditioned premises, but also the conditioning equipment itself.
Sensitive heating
In sensible heating, the air passes through the heating module, which consists of a coil through which water generally circulates from a boiler "Boiler (machine)"). During the passage, the air increases its temperature from to and its enthalpy without modifying the specific humidity, so that at the entrance:
And at the exit:
Subtracting member from member:.
is the thermal energy received for each kg of air, to go from to . The maximum humidity corresponding to will have increased, so its relative humidity will have decreased.
The power of the heat battery will be the product of the energy received per kg, multiplied by the mass flow.
In systems with direct expansion, the hot water coil is replaced by the condenser of a refrigeration machine or heat pump, so that it is the condensation of the refrigerant that provides the necessary heat, by direct exchange with the air.
Mix
In sensitive heating, the decrease in relative humidity combined with the continuous recirculation of a flow, without any type of renewal, means that after a short time, the air deteriorates, and especially in comfort conditioning (air conditioning), the user feels the dryness and thinning of the air.
Because of this, heating-only installations are not the most advisable. An improvement to the system is to incorporate ventilation, that is, an intake of a quantity of outside air that dilutes the concentration[3] of contaminants in the air.
If a quantity of outside air is added, at some point a mixture of two air flows with different conditions occurs. The new air from the building or ventilation air enters through an air intake grille, located outside, in an area called mixing module, where the new air is mixed with the return air from the premises, regulating their respective flows through manually or automatically operated shutters.
Carrying out a balance of matter and energy we obtain:
By conservation of mass:.
By conservation of steam quantity or water balance:.
Where is the humidity ratio (kg water vapor/kg) of the currents.
For energy conservation:.
Where is the enthalpy in (J/kg) of the currents.
From these three equations we obtain:
It is also used with a pretty good approximation:
Sensitive cooling
In sensitive cooling, the air passes through the refrigeration module, which consists of a coil through which water from a chiller circulates. In the passage, the air decreases its temperature from to and its enthalpy decreases without modifying the specific humidity"). For the process to occur in this way, it is necessary that the temperature of the battery, or of the water circulating through it, is above the dew temperature corresponding to state 1. Otherwise, there will be condensation "Condensation (change of state)") and therefore a decrease in the specific humidity").
Otherwise, everything is the opposite of sensible heating. The moisture absorption capacity decreases and therefore the relative humidity increases.
will be negative, since . The sign has no meaning other than indicating the direction of flow of thermal energy, which in this case is extracted instead of contributed.
Humidification without dry temperature change
It is the transformation in which the vapor content of humid air increases without modifying its dry temperature. This process is achieved by adding a small amount of water vapor to the air. The amount of steam added per kg of dry air will be:.
and is usually done by a steam generator.
When steam is added at a temperature equal to or greater than 100 °C, the resulting process determines humidification and slight heating of the air. In any case, the variation in dry temperature is very small, so in practice it is considered an isothermal process.
In this process there is a mixture of two masses, air and water vapor, so the sum of them will be:
Taking a balance of the amount of water you have to:.
Replacing:.
Therefore:.
In a less precise way it can be calculated with volumetric flow rates, that is:.
Dehumidification without change of dry temperature
It is the reverse transformation of the previous one and can be carried out with some type of desiccant or absorbent product that eliminates humidity from the air without changing its temperature, such as silica gel.
Cooling and dehumidification
When a stream of humid air is passed over a cold surface that is maintained at a temperature below its dew point, it will condense part of the water it contains and therefore its humidity will decrease. Ideally, air would leave the system with a dry temperature equal to the battery temperature and with a relative humidity of 100%.
The dew temperature of the battery, , is the temperature of the treated air at its exit and coincides very approximately with the temperature of the cold fluid that circulates inside the tubes, due to the small thermal resistance of their wall.
Of the enthalpy lost by the air, part will be sensible heat (decrease in dry temperature) and another part will be latent heat (decrease in water vapor).
The total heat removed in the battery is that extracted from the air, plus the heat carried by the condensed water. This second term is neglected because it is usually very small:.
Adiabatic cooling and humidification
A process is adiabatic when it is carried out without heat exchange with the external environment. It occurs when an air flow passes through a curtain of water. The water, driven by a pump, is sprayed into the air and vaporizes part of it, increasing the specific humidity of the air. If the chamber were long enough, the air would come out saturated. The temperature of the water should be equal to the humid temperature of the air.
The air evolves, keeping its humid temperature constant. This implies that the air leaves with practically the same enthalpy as it entered, although with a higher specific humidity content. The air cools because it gives up the heat necessary for the vaporization of water, which is incorporated into the air as latent heat.
In practice, the chambers are never so long that the air leaves completely saturated, so humidification will end at some point before reaching saturation. Considering that the air enters the chamber in condition 1 and leaves in condition 2, the humidification efficiency will be:.
with the humid air temperature e equal to the water temperature and y , the conditions at saturation.
This equation provides the exit conditions of the air as it passes through a curtain of water that is at a temperature and has an efficiency.
Humidification of an air stream using sprayed liquid water is achieved with modules called scrubbers. It is also achieved, although with less efficiency, through a module with an adiabatic panel.