Wheels, desiccant

The use of rotary desiccant wheels for dehydration has been practiced for several decades. Carl Munters [13] created the first rotary adsorbers by impregnating corrugated paper with LiCl and then ran the rotor using direct heat exchange to regenerate the sorbent. More recently Munters, NTI and others have developed improved adsorbents for rotary dehydrators. 

The work of Collier at the University of Florida [14] produced the finding that a modified Brunauer type I isotherm, with a more modest degree of curvature to the isotherm, was the theoretical optimum for deep dehydration cycles that were expected to be used in open cycle desiccant cooling cycles. The adsorbent was dubbed a type IM (M for moderate). To understand this designation zeolite type X with its incredible steep isotherm is designated a type IE (E for extreme). 

This work prompted a flurry of activity in the mid- to late 1980s to find the type IM isotherm. A number of inventions can be found in which alumina, or silica gel are blended with zeolites type X or Y to mimic the shape of the isotherm that Collier defined. Mol Sieve type DDZ-70(g) is in fact one of only a few true type IM isotherms. This product and Engelhard s type ETS-10 both have the required isotherm shape for water and deliver the benefits expected, to wit excellent capacity for water, self-sharpening mass transfer zone and low energy investment required to regenerate. Mol Sieve type DDZ-70(g) is used commercially in rotors 

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Heat recover) from exhaust air—If a heat recoveiy system allows an increase in the rate of outside air supply, lAQ will usually be improved. Proper precautions must be taken to ensure that moisture and contaminants from the exhaust air stream are not transferred to the incoming air stream. An innovative way of recovering heat and reducing the dehumidification cost is to use the waste heat to recharge the desiccant wheels that are then used to remove moisture from the supply air. In this method, the energy savings have to be substantial to offset the high cost of the desiccant wheels. 

The design employs filters for particulate removal, a single desiccant wheel coated with both low temperature VOCs oxidation catalyst based on nanostructured catalyst and regenerable VOCs adsorbent made from modified mesoporous silica. The distribution of the active elements along the wheel thickness was optimized and shown in Fig. 12.8-6. The adsorbents are coated along two-thirds of the wheel thickness and all catalysts are concentrated on one... 

Wheels Desiccant wheel Catalyst-adsorbent wheel 17.00 11.00 11.00 8.00 ... 

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. 

A small 3 cm x 3.5 cm section of the catalyst-coated desiccant wheel (25 cm diameter) was cut and placed in specially made holder shown in Fig. 12.9-6a. The piece of sample was tested in a 0.2 m3 environmental chamber at Chiaphua Industries Ltd. (Fig. 12.9-6b) for reduction of airborne VOC. The chamber was filled with the target VOCs through two stage saturators shown in Fig. 32b. Once the VOC level in the chamber stabilized, the fan was turned on to circulate the air through the sample. Three sets of sensors were located at the inlet and outlet of the holder, as well as in the center of the chamber. The chamber temperature and relative humidity were kept constant during the test. Figure 12.9-6c shows the results for VOC levels of 4000, 2000 and 1000 ppb at room temperature. The reduction rate was slower because of the low VOC concentration and the poor air circulation in the chamber. Also unlike the Prototype Unit, the catalyst was kept at room temperature throughout the test. 

After the six-month field test, the Prototype Unit was disassembled and the component parts were individually swabbed with sterilized cotton wools (4 cm2). Each samples were stored in 1 ml sterilized distilled water and 50 pi of samples were transferred to TSA and MEA plates. The TSA plates were incubated at 37 °C for 24 h and bacterial colonies were counted. The number of fungal colonies was determined from the MEA plates after incubating at 30 °C for 5 days. The results of the test are shown in Table 12.9-7. The results show that bacteria and fungi thrived near the air intake where they deposited on the grills, panel and around the intake slots. However once the microorganisms are drawn through the catalyst-coated desiccant wheel, the number of viable... 

Kodama A, Hirayama T, Goto M, Hirose T, and Critoph RE. The use of psychrometric charts for the optimisation of a thermal swing desiccant wheel. Appl Therm Eng 2001 21 1657. 

Fig. 5 (A) Desiccant wheel. (B) Corrugated and hexagonal channel shapes. (From Refs. " l)...

The regeneration air stream starts at approximately 75°F db, 40% rh. From state 5 to state 6, this air stream acquires heat from the process air stream through a heat exchanger, and from state 6 to state 7, the regeneration stream is further heated to an appropriate temperature for desiccant regeneration. Finally, the regeneration air is cooled and humidified as it extracts moisture from the desiccant wheel (state 7 to state 8). At state 8, the regeneration stream is exhausted to the outside. 

Air should be dried with a chemical drier (e.g. a rotating desiccant wheel which is continuously regenerated by means of passing hot air through one segment of the wheel). 

The details of a desiccant wheel are shown in Figure 8. Generally, the desiccant material is coated onto a ceramic honeycomb structure formed into the shape of a wheel. The wheel constantly rotates between the process air and reactivation or regeneration air streams. Generally, a complete cycle (rotation) of the wheel takes about 6 minutes. However, it may vary between 10 to 20 rph. A section of the wheel is always in the adsorption mode where the moist air is dried, while the rest of the wheel is in the regeneration mode. A cycle may... 

Process air is generally hot (305 K) and humid prior to entering the desiccant wheel. 

Air leaving the desiccant wheel is relatively hotter (325 K) than point 1, but it is much drier than when it entered the wheel. 

Hot, moist outside air (305 K) is generally used for regeneration of the desiccant wheel. 

Several investigators [15, 17-19] studied the effect of balanced and unbalanced flows in both the heat exchangers and in the desiccant wheel. In the balanced flow mode of the operation, entire regeneration flow is heated to a maximum regeneration temperature. When an unbalanced flow is used, a portion of the preheated regeneration air stream from the sensible heat exchanger is vented to the atmosphere. An increase of 10 to 15 percent in the COP value was reported when unbalanced flow was used compared to a balanced flow system. 

NTU for desiccant wheel 28.5 Sensible heat exchanger effectiveness 93%... 

The rotational speed of the wheel plays an important role in determining the design and optimum performance of the desiccant wheel as it indirectly... 

Major components of a desiccant coohng system are a) desiccant wheel assemblies, b) reactivation heaters, c) heat exchangers for post coohng, d) indirect evaporative post coolers, e) filtration systems, and f) control systems. An excellent description of these units is given by Harimann [47]. 

A desiccant wheel assembly consists of i) a core that contains the desiccant materials, ii) a core/wheel support structure, iii) a drive system to rotate the wheel at a very low speed, and iv) a set of air seals to separate the process air stream from the regeneration air stream. The core has a monolith honeycomb type structure. Both sinusoidal and hexagonal channels are used in commercial equipment. Various types of materials have been used for support matrix that includes ceramic, glass fibers, and corrugated aluminum sheets. The desiccant materials may be washcoated, impregnated, or formed in situ on the support matrix. 

Often an evaporative cooler is used in conjunction with the desiccant wheel to further adjust the humidity and temperature of the process air. The process air is flowed through a wetted medium. Since it is an adiabatic process and the air... 

Percent removal of some selected pollutants by an IM desiccant wheel obtained from a field test... 

D.G. Waugaman and C.F. Kettleborough, Combining Direct and Indirect Evaporative Cooling with a Rotating Desiccant Wheel in Residential Applications, ASME/JSME/ISES Solar Energy Conf., Honolulu, HI, 1987, 848. 

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