Dehumidification >> Calculate Dehumidifier Size Requirements

Desalination produces freshwater by removing dissolved minerals from seawater. The process has a long history as an effective means to meet agricultural, domestic, and industrial freshwater needs in coastal areas. Technologically mature conventional desalination processes that have been widely used to produce freshwater at industrial-scale include multieffect distillation, multi-stage flashing, Calculate Dehumidifier Size Requirements and reverse osmosis. 

Multi-effect distillation and multi-stage flashing are based on liquid-vapor phase-change processes where seawater evaporates to water vapor either at atmospheric pressure by adding heat (multi-effect distillation) or Calculate Dehumidifier Size Requirements at greatly reduced pressure by lowering water's boiling point (multi-stage flashing). This water vapor then condenses to yield freshwater—leaving any previously dissolved minerals as waste byproducts. 

Reverse osmosis, alternatively, is based on membrane technology. Using a high-pressure pump, Calculate Dehumidifier Size Requirements seawater is forced to flow through a membrane. The membrane only allows freshwater to pass while filtering out the dissolved minerals. Freshwater is produced as a result of this filtering with any previously dissolved minerals retained on the input side of the filter. 

The primary restriction on the use of conventional multi-effect distillation, multi-stage flashing, Calculate Dehumidifier Size Requirements and reverse-osmosis technologies is that they are highly energy intensive. The cost of freshwater produced by these three desalination technologies is directly dependent upon the cost of energy, primarily electricity and/or high-grade (high-temperature) thermal energy. 

While these technologies may be considered cost-effective in regions, such as the Middle East, having abundant Calculate Dehumidifier Size Requirements and economical local petrochemical energy supplies they are not well suited to regions such as Hawai'i because of the high energy cost. The primary energy source used in Hawai'i has long been unrefined oil shipped in from Southeast Asia. 

Additionally, these three conventional technologies operate under specialized temperature Calculate Dehumidifier Size Requirements or pressure conditions, e. g., multi-effect distillation requires working temperatures above 100°C, multi-stage flashing requires greatly reduced pressures. Reverse osmosis requires high initial flow pressures and produces a significant reduction of these flow pressures. 

All of these technological requirements lead to high infrastructure and operating costs. In contrast, Calculate Dehumidifier Size Requirements humidification-dehumidification (HDH) seawater desalination represents a relatively new desalination method based on heat and mass-transfer processes. Normal atmospheric air is employed as the medium to convert seawater to freshwater. 

HDH seawater desalination involves two processes. Seawater is first converted to water vapor by evaporation into dry air in an evaporator (humidification). This water vapor is then condensed out from the air in a condenser to produce freshwater (dehumidification). HDH seawater desalination operates under more moderate working temperatures (<80°C) and near-ambient system pressures Calculate Dehumidifier Size Requirements and requires only moderate flow pressures. 

Given these more moderate system specifications, Calculate Dehumidifier Size Requirements low-cost materials such as conventional plastics may be used for system construction. These relatively easy-to-achieve construction requirements are expected to lead to a lower infrastructure cost. Because of the more moderate operating-temperature requirement, HDH seawater desalination can easily be driven by sustainable solar energy. 

This makes HDH seawater desalination particularly attractive to Hawai'i. While its geographical location makes electricity Calculate Dehumidifier Size Requirements and high-grade petrochemical-based thermal energy expensive in Hawai'i, there is abundant solar radiation throughout the islands. Several literature studies are available that explore HDH as an effective means for seawater desalination. 

The early Calculate Dehumidifier Size Requirements work includes those by Bourouni et al. (2001); Al-Hallaj et al. (1998); Assouad and Lavan (1988); Muller-Holst et al. (1999); Abdel-Salam et al. (1993); Xiong, S.C. Wang, Xie, Z. Wang, and Li (2005); Shaobo et al. (2005); Xiong, S.C. Wang, Z. Wang, Xie, Li, and Zhu (2005); El-Dessouky (1989); Goosen et al. (2003); and Al-Hallaj and Selman (2002). 

In these studies conventional shell-and-tube heat exchangers were used as condensers for the Calculate Dehumidifier Size Requirements dehumidification process. Film condensation over tubes is extremely inefficient when air presents in water vapor. Klausner and co-workers at the University of Florida recently described (Klausner et al. 2004; Klausner et al. 2006; Li, Klausner, Mei, and Knight 2006; Li, Klausner, and Mei 2006) an innovative diffusion-driven desalination technology to overcome the aforementioned shortcoming. 

To enhance the condensation in the presence of air, Calculate Dehumidifier Size Requirements a direct-contact condenser was used in diffusion-driven desalination. The diffusion-driven desalination was powered by waste heat derived from low-pressure condensing steam from a power plant and is viable for industrial-scale freshwater production. 

Effective design and optimization of solar-energy-driven HDH seawater desalination systems requires a fundamental understanding Calculate Dehumidifier Size Requirements and accurate prediction of thermal/fluid transport phenomena in virtually all system components. Our research team launched a research program to establish such a knowledge base through combined theoretical modeling and experimental study. 

The project supported by the 2008 U.S. Geological Survey State Water Resources Research Institute Program (WRRIP2008) constitutes the Phase I of the research program. The focus of the Phase I research is on developing theoretical models to describe thermal Calculate Dehumidifier Size Requirements and hydraulic characteristics of the two most critical system components—the evaporator and the condenser. 

Methodology Figure 1 shows a simplified schematic diagram of the proposed HDH seawater desalination system driven by solar energy. The system is composed of three main fluid-circulation subsystems, Calculate Dehumidifier Size Requirements identified as seawater, air/vapor, and freshwater. In the seawater subsystem, seawater is introduced into the system from a seawater reservoir using a pump (a). 

The seawater is pumped through a solar water heater (b) where its temperature is brought to a higher level. The seawater is then sprayed into the top of an evaporator (c) and Calculate Dehumidifier Size Requirements is in direct contact with air that is pumped into the evaporator from the bottom. A portion of the seawater evaporates and thus humidifies the air. 

The seawater not evaporated in the evaporator is collected at the bottom of the evaporator and discharged back to the seawater reservoir (d) as high-salt brine. In the air/vapor subsystem, Calculate Dehumidifier Size Requirements air is pumped into the bottom of the evaporator (c) using a forced-draft blower. After leaving the evaporator, the air is drawn into a condenser (e) where it is in direct contact with freshwater that is sprayed into the top of the condenser. 

Water vapor condenses out from the air into the freshwater. The resulting lowered-humidity air is directed back to the evaporator (c) and Calculate Dehumidifier Size Requirements used repeatedly. In the freshwater subsystem, the water gains heat and mass in the condenser (e). After being discharged from the condenser (e) it is cooled in a freshwater cooler (f) by the incoming cold seawater. 

The freshwater cooler (f) also serves as a heat exchanger for the purpose of preheating the seawater, Calculate Dehumidifier Size Requirements helping to reduce the amount of solar radiation needed in the solar water heater (b) to bring the seawater to the desired evaporator inlet temperature. Finally, a portion of the freshwater is directed back to the condenser (e) to condense water vapor from the humid air. The remaining portion of the freshwater is the production.


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