Hiroyuki Iyota

DRYING OF SLICED RAW POTATOES IN SUPERHEATED STEAM AND HOT AIR

Drying experiments were conducted on raw potato slices, using atmospheric pressure superheated steam and hot air as drying media at 170 and 240°C. Mass changes of the material were continuously measured, the conditions of cross section near the surfaces were observed with an electron microscope, also color changes of their surface were measured during drying. The respective drying methods and temperature conditions were compared and it was found that, in the case of superheated steam drying, moisture content temporarily increases due to steam condensation in the initial stage of drying, therewith, as well as starch gelatinization rapidly develops. Meanwhile, in case of hot air drying, starch gelatinization occurs more slowly than with superheated steam drying and that non-gelatinized starch granules remain on the surface when drying was completed. Furthermore, surface color measurements showed that samples dried by superheated steam were more reddish than ones dried by hot air and the surfaces were more glossy, because no starch granules remain on the surface in case of superheated steam drying.

SIMULATION OF SUPERHEATED STEAM DRYING CONSIDERING INITIAL STEAM CONDENSATION

A heat and mass transfer model was proposed for the superheated steam drying, focusing on phenomena which occur during the initial stage of drying, i.e., condensation of superheated steam on material surfaces and subsequent shift from condensation to evaporation leading to the beginning of the actual drying (Reverse Process Model). Next, drying equations considering the reverse process were formulated for a shrinking/swelling infinite flat plate to calculate moisture content and temperature distributions in a material, changes in mass of a material with time, and a characteristic drying curve. Then, the influence of the initial thickness of a material and the heat transfer coefficient were investigated. In addition, calculation results were compared with experimental ones with regard to the change in mass of material with time and the characteristic drying curve, and good conformance was obtained for the initial stage of drying.

NOVEL WATER FACILITIES FOR CREATION OF COMFORTABLE URBAN MICROMETEOROLOGY

The cooling effects of artificial water facilities – fountains, waterfalls and urban canals – have not received much attention. To improve the meso-space thermal environment in an urban area, especially that during the midsummer, we propose using novel artificial water facilities to alter the temperature and humidity, so as to improve pedestrian comfort. To study this, we have carried out field measurements and wind tunnel tests, and we report the results in this paper. At first we made measurements of wind velocity, temperature and humidity around such artificial water facilities in Tennoji Park in Osaka city (a typical Japanese large city). The results showed reduced temperatures on the leeward side, and demonstrated the usefulness of water facilities in improving the thermal environment. Also, we constructed an insulated 1/400 scale wind tunnel model of the situation and measured, by means of X-type hot wire anemometer, the air currents on the wind tunnel surfaces in urban, suburban and rural terrains. We also measured, using fine-gauge T-type thermocouples, the air temperature distribution in front of and behind the model of the water facility. The results confirmed the qualitative ranges of the cooling effect provided by the actual water facilities. In particular, the cooling effect produced by a 1.0 mm diameter nozzle fountain was found to extend a distance downstream equal to ten times the height of spout.

Effect of Time-Dependent Humidity Profiles from Air to Superheated Steam on Drying of a Wetted Starch Sphere

The effects of air humidity when drying with superheated steam (SHS) were examined considering displacement by SHS. A wetted raw starch sphere (ball) was used as the model material. It was found that the humidity profile strongly affects the mass change and properties of the products. The material became porous when it was dried with pure SHS from start to finish, which differs both from products dried with high-humidity air alone, and from products dried with air followed by SHS within a few minutes. A prediction method for adiabatic saturation temperature over a wide range of humidity is also proposed.

Effect of Humidity on Drying of Porous Materials in Fluidized Bed under Reduced Pressure

For fluidized bed drying under a reduced pressure, the effect of the humidity of the drying gas on the drying characteristics of porous materials immersed in the bed was examined experimentally and theoretically. The temperature at the sample center increased with the humidity at relatively high pressures in the drying chamber (101.3 and 50 kPa), and the degree of the increment in the temperature with the humidity increased with the chamber pressure. The effect of the humidity on the temperature at the sample center and the drying time was insignificant at a relatively low chamber pressure (20 kPa).

Effect of Moisture Content on the Expansion Volume of Popped Amaranth Seeds by Hot Air and Superheated Steam Using a Fluidized Bed System

The effect of the initial moisture content (X0) of amaranth seeds on expansion volume after popping was examined in hot air and superheated steam (SHS), using a fluidized bed system (FBS). The moisturized seeds were prepared under various vapor pressures due to various saturated salt solutions. In hot air, the maximum expansion volume was shown by seeds having X0 of 0.16 at 260 °C for 15 sec, reaching 8.7-fold of the pre-popped seeds. Heating by SHS decreased the volume slightly. Thus, popping of amaranth seeds is influenced not only by the moisture content of the seeds, but also by moisture in the heat media.

Popping of Amaranth Seeds in Hot Air and Superheated Steam

Abstract: Amaranth seeds can be popped by evaporation of contained moisture during heating. The relationships among the expansion ratio by popping and the type of heating media, gas temperature, initial moisture content of the seeds and heating time were clarified based on empirical study. And using a simple calculation model, the popping mechanism and effect of initial condensation of superheated steam were investigated. According to the results, the maximum volume of the seeds after popping was 8.7 times greater than that of raw seeds at optimal conditions. Compared with the case of hot air, the expansion ratio by superheated steam was slightly lower.

Prediction Method for Drying Time of Wet Porous Material in Humid Hot Air and Superheated Steam

The effects of dry-bulb and wet-bulb temperatures on the dryingtime of a wet spherical porous material in humid hot air and superheated steam were investigated. A wet spherical brick was used as the sample porous material. The experimental results revealed that the normalized drying characteristics curves in the falling drying rate period obtained under different experimental conditions were all in good agreement. In addition, the time required to reduce the moisture content below the critical moisture content was almost the same under all wet-bulb conditions at the same constant drying rate, regardless of steam condensation.

A reverse process of superheated steam drying from condensation to evaporation

To clarify the mechanism of heat and mass transfer in the early stages of superheated steam drying which accompanies condensation and evaporation, an experiment in which a water surface was used as the dried material was conducted under atmospheric pressure. Temperature profiles in both the gas phase and the liquid phase near the water surface and the liquid level were measured precisely. From the results, heat transfer rates at the water surface and the amount of steam condensed into water were determined; in addition, the relationship between these two was investigated both experimentally and theoretically. Furthermore, a characteristic curve of drying accompanying condensation and evaporation in the early stages of superheated steam drying was derived semiempirically. In this drying characteristic curve, there is a point at which neither condensation nor evaporation occurs. This is defined as the “reverse point.” Introduction of this reverse point and the time ratio for condensation and evaporation during the early stages of superheated steam drying are clarified.

Optimal Conditions for Popping Amaranth Seeds

Amaranth seeds can be popped under suitable heating conditions. On the basis of experimental results obtained in our laboratory, we have developed a prototype of a continuous processing system for commercial application. In addition, the effects of gas temperature, flow rate, and feed speed on the popping quality of seeds, such as their volume expansion ratio and yield, were examined. The experimental results showed that the undersized yield ratio increased with the flow speed, whereas it decreased with an increase in the gas temperature. In addition, to achieve a high expansion ratio and maximum output, the feed speed was increased with the gas temperature. Furthermore, measuring the differential pressure in the test section of the experimental apparatus enabled the estimation of the quantity of seeds therein during the popping experiment.