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Different ways for dehumidification

The motivation and need to tackle the project is due to the fact that most of the facilities in the non-residential building sector have a high latent load.

 

To illustrate with some indicative figures, for an office building located in Málaga, with a useful area of 100 m2, an occupancy of 10 people for 10 hours a day and some enclosures in accordance with the provisions of the technical building code, the latent load represents 21.38% of the building’s total thermal load for a relative humidity setpoint of 50%.

Prototype of the air conditioner with dehumidification using desiccant liquids.

Comfort condition range

The hours, in which a drying process is effective, to achieve comfort conditions range between 1,000 and 4,000 hours in cities such as Almería or Málaga, accounting for practically 50% of the hours of the year.

Fig.1: Importance of latent load for an office building located in Málaga.

In the previous figure it can be seen how, in the case of the city of Málaga, the number of hours per year that can be treated by adiabatic drying covers a large part of the year (3,775 hours), with the shaded area being the comfort area, between 21ºC – 26ºC with 30% and 60% relative humidity.

 

In energy terms, bringing these hourly conditions of temperature and humidity to comfort conditions (blue area), represents a saving of 1.72 kWh/kg of dry air treated during a calendar year, that is, 1.72 kWh for each kg of dry air treated in one hour throughout the year, with an hourly distribution as shown in the following figure.

Fig.2: Hourly reduction in latent load by isenthalpic drying per kilogram of dry air treated in one hour for one year.

Dehumidify the air

At present there are several systems to dehumidify the air. Among them is the dehumidification process carried out by conventional air conditioners under refrigeration treatment. It is necessary to cool the air to reduce the humidity below what is necessary to meet the demand, and then heat it.

 

The evolution of the air goes through having a system that cools the air until saturation, then the humidity is eliminated by condensing the water vapor on the surface of the heat exchanger until it reaches the required humidity, the temperature at this point is low to introduce it in the room, so it is necessary to heat it up to the discharge temperature. In other words, to reach the required impulse conditions it is necessary to cool the air and then reheat it.

Fig.3: Cooling process carried out by conventional dehumidifiers.

Desiccant wheels

There is also the dehumidification process carried out by desiccant wheels. Desiccant-based systems claim to address some of the aforementioned problems. The most widely used desiccant systems are desiccant wheels. These systems are based on the adsorption of water vapor when passing through a matrix made of a hygroscopic material (such as silica-gel for example); the humidity of the air stays in the desiccant wheel obtaining a hot and dry air.

 

The evolution of the air when it is treated with a desiccant wheel causes it to heat up and lose moisture when passing through the desiccant wheel, to later cool it by exchanging sensitive energy with the interior air. Solutions based on desiccant wheels usually require large surfaces for their installation. Despite being a technically and economically viable solution, its implementation has not been extended.

Fig.4: Cooling process carried out by desiccant wheels.

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Liquid desiccants

The low quality of indoor air must also be taken into account in these processes. It is expensive to treat outside air to acclimatize it to the supply conditions so the amount of outside air is limited to a minimum and the inside air is recirculated. This causes the so-called Sick Building Syndrome caused by the stale air inside the building.

 

For this, an innovative dehumidification section has been developed based on the use of liquid desiccants for the combined control of temperature and humidity with better performance. These drying liquids are dehydrated vegetable oils, so they capture moisture from the air. Initially, it was experimented by spraying the desiccant liquid on the air flow, but the dragging of liquid droplets by the air flow and the large number of needles injecting liquid, made this technique rejected.

 

Later we invert the system and create a membrane where we place a layer of desiccant liquid. With the air flow we generate an overpressure that keeps the liquid on the membrane. The air passes through the membrane and the dehydrated liquid captures the value of water contained in the air. The advantage of the proposed drying system lies in the improvement of the available exchange surface, and thus increase the efficiency in mass and energy transfer. All this has made it possible to develop a compact technological solution that allows incorporating the drying solution into conventional air conditioners with a controlled investment.

Fig.5: Dehumidifier design scheme using desiccant liquids.

Several factors were analyzed to optimize the system. The height of the desiccant liquid will determine the amount of water that the desiccant liquid will be able to retain. On the other hand, it will also define the pressure loss in the ventilation system. The liquid head will be controlled with the flow rates driven by the extraction and liquid drive pumps.

Importance of the degree of saturation

The degree of saturation of the desiccant liquid is very important. As the water vapor adsorption occurs in the desiccant liquid, it will begin to saturate according to a relationship defined by the mass and energy transfer equations of the mathematical model. In this case, it is necessary to remove the desiccant liquid in order to regenerate it; The regeneration process consists of eliminating the water vapor retained in the desiccant by increasing the temperature. The characteristic time of the dehumidification process will mark the flow of waste to circulate towards regeneration. The definition of the characteristic time will be made under the assumption of a minimum of 92% purity of the desiccant liquid. And the inlet conditions of the inlet desiccant liquid will have minimum conditions of purity of 96% and with a temperature close to room temperature.

Fig.6: Cooling process carried out by the dehumidifier using desiccant liquids.

A system for capturing and retaining drops of desiccant liquid that can be dragged by the process air stream has also been implemented. In this sense, specific filters have been developed that capture possible drops that have not settled on the collecting surfaces and / or have been transported by the stream of treated air.

Promising results

The data obtained in the initial experimentation is promising. Although they would not yet be substitutes for conventional equipment, as they require a large volume of air to achieve satisfactory results. Of course, we have achieved the same ratio “kilogram of water/hour/electrical kilowatt”. Therefore, we are presented with an important development potential.

Fig.7: Comparison of conventional cycle versus cycle with liquid desiccants.

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