Desiccant dehumidifiers

Rady MA, Huzayyin AS, Arquis E, Monneyron P, Lebot C, Palomo E. Study of heat and mass transfer in a dehumidifying desiccant bed with macro-encapsulated phase change materials. Renew Energy... 

In summer the ventilation air has to be dehumidified to keep the required comfort and to prevent from condensation at cold ceilings. The air dehumidification is done by a liquid desiccant dehumidification and cooling system, sketched in Figure 271. Warm and humid outside air is cooled and dried in a special dehumidifier by a concentrated Lithium Chloride salt solution (LiCl-H20) before it is blown into the atrium of the building. From there several air handling units draw the air into the offices and provide additional cooling on demand. 

The collector array and the components of the desiccant cooling system, such as dehumidifier, regenerator, storage and desiccant handling system, have... 

The desiccant solution has been mixed on-site from salt and water and filled into the tanks in November 2001. The heat and cold recovery system is in automatic operation since December 2001. The dehumidifier is part of this system and is used as heat exchanger in winter. 

The chemical product used in the design project (chapter 12) is a household appliance designed to deliver clean air by removing and killing airborne microorganisms, and converting carbon monoxide and common VOCs found indoor into harmless carbon dioxide and water. It also dehumidifies indoor air and maintains a comfortable humidity level that suppresses fungal proliferation. The appliance is intended to maintain its performance without maintenance for at least two years and is expected to have a functional life of at least five years. The product contains an active formulation of (1) low temperature oxidation catalyst, (2) VOCs adsorbent and (c) desiccant. 

In order to produce a functional prototype unit within the remaining three month of the project, two commercial desiccant-based dehumidifier appliances (Zojirushi s RV-D60-HC) were purchased from Japan in March 2003. The... 

The dehumidification rate for the Prototype Unit was measured and compared with an unmodified dehumidifier. The Prototype Unit and dehumidifier appliance were placed in a 60 m2 room with open windows. The test was conducted at an ambient temperature of 23 °C and relative humidity of 80 %. The amount of water collected after 8 h was weighed and dehumidification rates of 0.14 Kg/h and 0.13 Kg/h were obtained for the Prototype Unit and unmodified dehumidifier, respectively. This indicates that coating the formulated catalysts onto the desiccant wheel does not significantly affect its dehumidification function. 

Lithium chloride is used in the production of lithium metal by electrolysis. It also is used in metallurgy as a eutectic melting composition with potassium chloride (LiCl 41 mol% KCl 59 mol%). Other applications are in low temperature dry-cell batteries as a dehumidifier in air conditioning in welding and soldering flux as a desiccant in fireworks and in mineral waters and soft drinks. 

Nowadays, rotor-type dehumidification and air-conditioning systems are spotlighted and begin to be industrialized, because the systems don t use CFCs which are known to be representative chemicals destroying an ozone layer in earth. Generally, silica gel is used as a dehumidifier in the dehumidification and air-cooling rotor. Therefore, it is important to improve the dehumidification efficiency of silica desiccant in this field. 

Zeolite monoliths have been useful for such apphcations as rotatory adsorbers for use as dehumidifiers and desiccant cooling processes [253] or in VOC treatment systems [269]. Alumina-coated sUicon carbide monoliths have also been employed as supports for B-ZSM-5 membranes [270] providing a larger surface area per unit volume, compared to traditional membrane supports. With these membranes, these authors have reported n/f-butane and H2/f-butane separation selectivities of 35 and 77, respectively [85]. Also, silicalite-1 membranes supported on stainless steel grids (Figure 10.29) have shown a good performance in the separation of n/f-butane mixtures, with separation factors as high as 53 at 63°C [255]. 

Nearly all types of coating processes will benefit from a consistent process air dew point. Humidity control systems may be complex, incorporating dehumidifiers (for dew point reduction to about 5-iO C). desiccant dryers (for products requiring low product temperatures and dew points below 0 C), steam humidifiers (to increase dew point in cold, dry weather), etc. It is suggested that laboratory trials be conducted to evaluate product and process sensitivity to variations in inlet air dew point. 

In the majority of installations, a flapper damper or diverter valve is employed to vary the flow across the heat exchanger to maintain a specific design temperature of the hot water or a specific steam generation rate. In some CHP designs, the exhaust gases are used to activate a thermal enthalpy wheel or a desiccant dehumidifier. A thermal wheel uses the exhaust gas to heat a rotating wheel with a medium that absorbs the heat and then transfers the heat into the incoming airflow into which the wheel is rotated. 

Desiccant dehumidifiers reduce humidity in the air by using solid desiccant materials or liquid desiccant materials to attract and hold moisture. They can operate independent of chillers and can also be operated in series or parallel with chillers. Recoverable heat from the exhaust gases of turbines and engines for power generation or engine-driven chillers is used for regenerating desiccant material in these dehumidifiers. 

Desiccant dehumidification systems remove moisture from the air by forcing the water vapor directly into a desiccant material. The moisture from the air is attracted to the desiccant because an area of low vapor pressure is created at the surface of the desiccant. The pressure exerted by the water in the air is higher, hence the water molecules move from the air to the desiccant and the air is dehumidified. 

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