pve tube. It is auapended by an electronlo load cell which is used to continuously weigh the walnuta a8 they dry. Air flov is supplied by a centrifu-

Transcription

1 WALRUT DRYING Toa Rwasey Toa "orth Chris Tauzer ABSTRACT A laboratory scale fixed bed walnut dryer has been constructed. It va8 built to test procedures 8uch as partial recirculation of drying air and periodic reversal of drying air. The walnuts are dried in a cylindrical container 14 inche8 in diaaeter and 4 feet high. It has the capacity of about 100 pounds. The dryer is in8trumented to monitor the drying as it progre88e8. Twelve wet and dry bulb teaperature 8enaors sea8ure air teaperature8. The walnuts are weighed continuou8ly with an electronic load cell. Air flov8 are aea8ured with orifice meters. All data are recorded automatically with an Apple II per80nal coaputer. Due to slov delivery on aoae parts. the dryer was not completed until aid-october. Only four test drying runs were made this year. The dryer will be run continuously next season. The walnut drying aiaulation coaputer progr.. has been adapted to fit on a per80nal computer. the North Star Advantage. More siaulation runs have been made to examine the effects of dryer operating procedures and dryer design on drying time and energy use. It appears that several generalization8 can be ude concerniq dryer operation. Drying air teaperature should be kept as close to the aaxiaum of 110." as po8sible. There are no saving8 in energy use per ton by drying at lower heated air temperature. just an increase in drying tiae. Drying air flov rate should be limited to' belov 150 ft/ain. Increasing it above this value does little to decrease drying tiae and only increase8 coata for heat and electricity. OBJECTIVES The objectives for this project were: 1. Con8truct a laboratory 8cale fixed bed walnut dryer with capability to recirculate varying..ounts of drying air. 2. Use the Apple II coaputer for data acquisition and to tryout control schemes to vary dryer air recirculation. PROCEDURE The dryer was constructed priaarlly with rigid PVC plastic pipe. It is mounted on a plywood stand for support purpose8. The walnuta were placed in a sheet setal cylinder. 14 inchea in diameter by 4 feet high. It holda about 100 pound8 of walnuts. The cylinder 18 suspended inside a 15 inch diameter pve tube. It is auapended by an electronlo load cell which is used to continuously weigh the walnuta a8 they dry. Air flov is supplied by a centrifu

2 gal blower. The air is routed from the fan through two thermostatically controlled electric heaters. From there it flows through a network of PVC pipes where it can be directed to enter either at the top or bottom of the walnut bed. After passing through the bed, it is directed back to the fan. The primary data monitored are temperatures, air flow rates and walnut moisture content. A total of twelve air temperatures are monitored and recorded continuously wi th an Apple II computer. Wet and dry bulb temperatures are measured into and out of the fan, hea~er, and bed of walnuts. Dry bulb temperatures are also measured at points inside the bed. In all cases, solid state temperature transducers are uaed to do the measuring. They put out a millivolt signal that is directly proportional to the absolute temperature. Wet bulbs are made by covering the temperature transducer with a cotton shoe string that is kept wet with water from small bottles of water. Air flows are measured with differential pressure orifice meters which are placed in the main 3-inch PVC lines. The average moisture content of the bed is determined by taking an oven dried sample of the wet walnuts and continuously weighing the bed with an electronic load cell during the drying. The weight is also continuously recorded on the Apple II. The drying tests typically take from 12 to 36 hours depending on initial nut moisture, drying air temperature and flow rate. Drying tests are made on 100 pound samples from the same batch of walnuts. The walnut drying simulation computer program developed to model ambient air drying has been modified to model heated air drying. The program is currently running on the campus Burroughs B-7800 computer, the Agricultural Engineering department's LSI-ll computer, and a North Star personal computer purchased for this project. Hundreds of simulation runs have been made to study the effects of varying drying operating parameters and design. Specifically, for dryer operation, the effects of air temperature and flow rate on drying time and energy use were examined. For drying design, air recirculation and flow reversal were examined. RESULTS The laboratory scale dryer was completed in mid-october. At this time of the season, most of the walnuts needed little or no drying. Four drying tests were conducted, there primary purpose being to shake down dryer operation and the measurement systems. The major problem found was an air leak between the walnut container and the outer dryer PVC shell. The leak allowed hot air to bypass the walnuts and escape out the dryer exhaust. All of the measurement sensors performed well after some initial problems and testing. The computer program was uaed to generate data predicting dryer performance. The baseline conditionsused in all cases were: location,fresno, CA; month, September; bed dejth, 4 ft; initial walnut moisture,23% (wet basis); air flow rate, 100 cfm/ft ; final moisture content, 8% at top of bed; start time for drying, noon. Data for the effects of drying air temperature are given in Table I. When the drying temperature is increased, drying time is decreased. In going from 90 F to 110 F, the drying time is nearly cut in half, from to hours. In

3 general, the heat energy required per ton decreased slightly with increasing drying air temperature. The lowest value actually occurred at 90 F, though, because the ambient air temperature exceeded 90 F during part of the drying period and the burner was shut off. Fan energy was calculated from a known relationship between pressure drop and air flow through beds of walnuts. It decreased with increasing air temperature because of the lower drying times. Several test cases were run with different air flow rates. Drying air flow rates of 50, 100, 150, 200, 250 and 300 cfm/ft2 were used at a constant air temperature of HO F. Results of these runs are given in Table II. Drying time decreases with increasing air flow rate. It is most dramatic at flow rates under 200 ft/min, when doubling the air flow decreases the drying time by about one third. Heat energy use increases with air flow rate. As seen in Table II, after 150 ft/min, energy use increases at a much greater rate than the drying time decreases. Fan energy use also increases rapidly above air flows of 150 ft/min. At a flow rate of 300 ft/min, the fan energy cost is 30% of the total energy cost. Overdrying can be a serious problem in fixed bed dryers. It is costly in terms of lost revenues due to decreased product weight as well as extra energy requirements for heating and the fan. Overdrying occurs because of the moisture gradient that sets up in the bed as drying progresses. The walnuts closer to the hot air inlet will dry to a lower moisture content because they are exposed to higher temperature air for a longer period of time. The effects of overdrying are shown in Table III. The value of excess weight loss is calculated from the difference in weight between a bed at 8% average and at the calculated average final moisture content. As can be seen, the cost of overdrying is very significant. Partial recirculation of drying air is a procedure practiced in agricultural dryers such as prune tunnel dryers. If the air exiting the dryer is warmer than ambient air, and if its humidity is not too high, recirculation can save energy. Data for recirculation of 0, 25, 50, 75, and 90% of the drying air are shown in Table IV. By recirculation of 75% of the drying air, the energy savings were 31% and 45% for September and November respectively. The drying time was increased by about 3 hours for each case. Recirculation of more than 75% of the air appears to be impractical because of the long drying times. Reversing the air flow periodically in a fixed bed dryer is one way to prevent overdrying. Simulation runs were made in which the air flow was reversed every 2 and 4 hours. Data for these runs are given in Table V. There are considerable savings by reversing the air flow every 2 hours. Overdrying is reduced greatly when compared to the case of straight drying until the walnuts reach 8% moisture on top of the bed. CONCLUSIONS The laboratory dryer is a useful tool to explore dryer operation and control. It will provide valuable data and operating experience with new computer controlled equipment. As the cost of this type of equipment continues to decrease, it becomes feasible to implement it on farm. Based on the simulation results, the following conclusions can be made concerning fixed bed drying of walnuts: -- 26

4 1. The heated air temperature should be kept at 110 F to minimize drying time and energy use. 2. Air flow rates greater than 150 ft/min have little effect on drying time and increase both heat and fan energy costs. 3. Overdrying can be costly, primarily in terms of the value of the excess weight that is lost in the finished product. 4. Recirculation of drying air can save energy. The savings depend on the amount recirculated and also on the ambient temperature. Recirculation of more than 75% appears to be impractical. 5. Periodically reversing the air flow through the dryer prevents overdrying and does not increase drying time

5 Table I Effects of Changing Drying Air Temperature Inlet Temperature Drying Time Heat Energy Fan Energy.L!l (hrs) (therms/ton) ($/ton) (kwh/ton) ($/ton) a b l l ; Table II Effects of Changing Air Flow Rate Air Flow Drying Time Heat Energy Fan Energy (cfm/ft22. (hrs) (therms/ton) ($/ton) (kwh/ton) ($/ton) l Table III Effects of Final Moisture Content Final Top Moisture (% w.b.) Drying Time (hrs) Heat Energy (therms/ton) ($/ton) Value Excess Weight Loss ($/ton) c

6 Table IV Effects of Air Recirculation Recirculation Drying Times (%) (hrs) o (13.75)d 15.0(14.0) 15.75(14.25) 17.75(16.75) 24.25(23.0) Heat Energy (therms/ton) ($/ton) 10.6(17.6) 9.55(14.8) 8.42(12.2) 7.28(9.55) 6.48(7.97) 5.30(8.80) 4.78(7.40) 4.21(6.10) 3.64(4.78) 3.24(3.99) Fan Energy (kwh/ton) ($/ton) 3.24(3.08) 3.36(3.13) 3.52(3.30) 3.47(3.75) 5.42(5.14) 0.23(0.22) 0.24(0.22) 0.25(0.23) 0.28(0.26) 0.38(0.36) Table V Effects of Air Reversal Reversal Time (hrs) Drying Time (hrs) Heat Energy (therms/ton) ($/ton) Value Excess Weight Loss ($/ton) none acost of heat energy 50t/therm bcost of electrical energy 7t/kWh cvalue of crop 50t/lb dnumbers in parenthesis are values for November