group 2 report of dryer

8/6/2019 Group 2 Report of Dryer

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Table of Contents

1. Introduction ............................................................................................................................. 2

2. objectives ...................................................................................................................................

General Objective ........................................................................................................................ 2

Specific objective ........................................................................................................................ 2

3. Solar Dryers ............................................................................................................................. 3

4. Forced convention Indirect Solar dryer ................................................................................... 3

Advantages .................................................................................................................................. 3

Disadvantage ............................................................................................................................... 4

5. Principles of operation of solar Dryers .................................................................................... 4

6. Experimental set-up ................................................................................................................. 4

Capacity of the dryer ................................................................................................................... 4

7. Experimental testing of indirect forced convection Dryer ...................................................... 5

Dryer testing procedures ............................................................................................................. 5

8. Test results ............................................................................................................................... 6

Test conditions ............................................................................................................................ 6

Forced solar dryer specifications................................................................................................. 6

9. Absorber and air flow rate ..................................................................................................... 11

10. Variation of ambient, collector, inlet and outlet temperature versus time And solar

radiation versus time ....................................................................................................................... 7

Amount of moisture removed ..................................................................................................... 9

Humidity taken out .................................................................................................................... 13

11. Performance analysis ......................................................................................................... 11

12. Conclusions ........................................................................................................................ 14

13. Recommendation ............................................................................................................... 15


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1. Introduction

Agricultural and other products have been dried by the sun and wind in the open air for

thousands of years. The purpose is either to preserve them for later use, as is the case with food;

or as an integral part of the production process, as with timber, tobacco and laundering. In

industrialized regions and sectors, open air-drying has now been largely replaced by mechanized

dryers, with boilers to heat incoming air, and fans to force it through at a high rate. Mechanized

drying is faster than open-air drying, uses much less land and usually gives a better quality

product. But the equipment is expensive and requires substantial quantities of fuel or electricity

to operate.

'Solar drying' in the context of this technical brief, refers to methods of using the sun's energy for

drying, but excludes open air 'sun is drying. The justification for solar dryers is that they may be

more effective than sun drying, but have lower operating costs than mechanized dryers. A

number of designs are proven technically and while none are yet in widespread use, there is still

optimism about their potential. The work being reported in this paper is being conducted at the

Department of Mechanical Engineering solar demonstration center, Mekelle University. The

Experiment has involved a total of six students.

The work has been based on conducting experimental testing of performance of Forced type

solar dryer at different air flow conditions for tomato hence, their potential for successful

dissemination as an alternative drying technology.

2. ObjectivesGeneral Objective

The general objective of the current work is to understand the working principles, conduct

experiments, and conduct performance tests of the solar thermal applications.

Specific objective

To preserve tomato and to conduct performance test of forced type solar dryer at Mekelle

University, Mechanical Engineering Department


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3. Solar DryersSolar dryers are one of the food processing machines which are used to dry and preserve seeds,

fruits, and vegetables.

Solar dryers can be classified in to:

1. Based on mode of exposure to radiation: Direct dryers Indirect dryers.

2. Based on air flow: Natural convection dryers Forced convection dryers

3. Based on circulating temperature: Internal ducting system Separate ducting system

4. Forced convention Indirect Solar dryerA dryer is said to be a forced indirect solar dryers in which the load is placed in an enclosed

drying cabinet there by being shielded from direct exposure to solar radiation and if the air

circulation is dependent on pressure differentials generated by a fan.

Forced Convection solar dryers obviously generate much higher air-flow than that generated bynatural convection dryers and this makes them suitable for drying large loads. Hence, forced

convection type solar dryers considerably reduce drying time compared to open air sun drying,

but their relatively small holding capacity is a major limitation, thereby making corresponding

investment questionable.

Advantages

Solar thermal power is trapped, resulting in a higher temperature around the fooditems, these decreases the drying time.

Higher conservation of nutritional value. Animals and dust do not have access to the food items. In rainy periods, rain will not slow down the drying process.


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Disadvantage

Small capacity

5. Principles of operation of solar DryersAir is drawn through the dryer by natural convection. It is heated as it passes through the

collector and then partially cooled as it picks up moisture from the tomato. The tomato is heated

by the heated air.

Warm air can hold more moisture than cold air so the amount required depends on the

temperature to which it is heated in the collector as well as the amount held (absolute humidity)

when it entered the collector.

6. Experimental set-upThe indirect forced convention solar dryer experimentation was conducted on May 4, 5, 7, and

13, 2011 at the intervals of 10 min from 9:30 A.M. to 5:30 P.M.

Figure 1 shows a schematic diagram of the dryer test facility, designed and constructed to

conduct this work. It consists of a flat plate collector of total area 2 m2

tilted at an angle of 340,

drying bed volume 0.0241m3.

Figure 1: schematic diagram of the dryer test facility

Capacity of the dryer

A general rule of thumb is that one m2

of tray area is needed to lay out 10kg of fresh product.

(Speirs, 1986). Hence, the tray size is 0.34x0.59=0.2006 m2

.therefore it was loaded with a load


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2.0 kg of tomato. The necessary measuring instruments are attached to the apparatus and then

connected to the data acquisition system.

The following measuring instruments were used in carrying out the experiments:

(i) Pyranometer: The type of pyranometer used is a portable digital pyranometer Kipp and Zonen

with sensitivity of 7v/wm-2. The instrument is made by a Netherland company and the instrument

measures global radiation.

(ii) Thermocouple: All temperature measurements were made with K- type thermocouples of a

type appropriate to the expected temperature range of the dryer being tested.

(iii) Wet bulb thermometer

(iv) Digital Balance (to measure the weight)LD series electronic balance(LD-1)

(v) Multi- meter (Victor 70c digital multi meter)

7. Experimental testing of indirect forced convection DryerDryer testing procedures

The performance of the drier was evaluated by conducting tests loaded with tomato, by

measuring the following parameters:

(a) radiation incident on the collector/dryer(b) Air temperatures at various locations in the collector and dryer

To measure the temperature of air at various locations of the collector and dryer, K- typethermocouples and wet bulb thermometers were installed at various points along the length and

breadth of the solar Collector and dryer, as shown in figure 2. All temperature data were

registered at an interval of

ten minutes. Drying test was started at 3:00 and stopped at 11:00.

Figure 2: temperatures at various locations of the collector and dryer


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T1dry bulb temperature at collector inlet

T2wet bulb temperature at collector inlet

T3absorber temperature

T4dry bulb temperature at dryer inlet (collector outlet)

T5wet bulb temperature at dryer outlet

8. Test resultsTest conditions

The solar dryer were tested outdoor at Mekelle University solar demonstration center during the

month of May, when the solar radiation is at higher level.

Forced solar dryer specifications

Specifications of the flat plate collector tested and appliances used are presented in Table 1.

Part Description Type

Collector area 2m2 Glass

Absorber area 2m2

Black painted steel

Tilt angle 340

Insulation Thickness 2.5 cm Wood

Dryer volume 0.0241m3

Wood tray with wire mesh

Table 1: Specifications of the flat plate collector tested and appliances


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9. Variation of ambient, collector, inlet and outlet temperature versus time and solarradiation versus time


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Forced Solar Dryer with Full Fan Rotation and slice tomato, May 13, 2011

Fig 3: Variation of ambient, collector, inlet and outlet temperature versus time

And solar radiation versus time

Amount of moisture removed


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Forced Solar Dryer with Full Air Flow or Full Fan Rotation, May 5,2011

Tomato Weight loss, May 7/2011 For Half Fan Rotation

Forced Solar Dryer with Full Air Flow or Full Fan Rotation and slice tomato, May 13, 2011

Fig 4: weight loss


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10. Performance analysis

The performance of a dryer, or drying efficiency, depends on the duration of drying and the

quality of the end product, besides factors such as collector performance and drying temperature.

From experimental results of different combinations of fan flow rates, Full flow rate, sliced

tomato was found to give the highest air temperature rise and satisfactory moisture removal

through the dryer and actual efficiency as shown in table 2 and figure 5.

Figure 5: Efficiency at different conditions

11. Useful heat and air flow rateQ

u= ()I

TA

cQ

LQ

L= U

LA

c(T

cT

a)

Where:

UL

= overall heat transfer coefficient of the absorber (Wm2

K1

)

Tc= temperature of the collectors absorber (K)


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The theoretical useful energy gained by the collector is expressed as:

Qu

= ()ITA

cU

LA

c(T

cT

a).

If the heated air leaving the collector is at collector temperature, the heat gained by the air Qgis:

Qg= Cam

a(T

cT

a)

Where:

ma = mass of air leaving the dryer per unit time (kgs1

)

Ca= specific heat capacity of air (kJkg

1

K1

).

To find ma

Ti (inlet Temperature) =260C (dry bulb), Relative humidity= 70%

=16 0C (wet bulb)

Humidity ratio (HR) =15/1000=0.015 Kg of water/Kg of air ( from psychometric chart)

T0 (out let temperature) =290C (dry bulb) , Relative humidity=90%

=170

C (wet bulb)

Humidity ratio (HR) =23/1000=0.023 Kg/Kg

Change in humidity = 0.023-0.015=0.008 Kg/kg

For humidity ratio increase of 0.008Kg water /Kg of air each kg of water will require

1/0.008=125kg of dry air.

Using Gas Law

PV=ma*R*Tab

Vair= 125 kg * 0.291 K pa m3/Kg k * 298k/101.3 K pa=107 m3, Tabs =25+273 =298k

Hence 2 kg of tomato will require dry air of 2*107 = 214 m3

of dry air

For dry time of 8 hrs = 8 * 3600 = 28800 sec

Therefore, the air flow rate is = 214m3/ 28800 sec=0.0074m

3/sec = 7.4kg/sec


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The detail calculation is done by excel and attached at page 20

Figure 6: psychometric chart and moisture content

The thermal efficiency of the collector is defined as (Itodo et al. 2002):

The system drying efficiency (s) or system efficiency is the ratio of the energy required to

evaporate the moisture of the commodity to the heat supplied to the dryer. This is a measure of

the overall effectiveness of the drying system. For natural convection dryers, typical values are

10-15%, while a system efficiency of 20-30% could be expected for forced convection dryers.

s = (W*L)/(I*Ac)

Where:

WWeight loss

LLatent heat of evaporation

Humidity taken out

Quantitative performance evaluation of the Dryer was conducted based on the procedure outline

in this paper. For ease of comparison of various flow rates, insolation data for each test was

recorded for the entire test duration. Humidity variations for different flow rates were

investigated on May, 2011. The amount of tomato used in the dryer was as shown in Table 2.

The results are presented in Table 2 and Figure 4.


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Item Full flow rate Half flow rate Remark

Initialweight

FinalWight

%Weightloss

Initialweight

FinalWight

%Weightloss

Weight is in Kg

tomato 2.086 2.034 0.025 2.017 2.001 0.0079

Slice

tomato

0.614 0.108 0.8241 - -

Table 2: amount of tomato used and weight loss

The humidity of drying air is a critical factor controlling the drying rate of the product. The

lower the relative humidity, the greater is the absorbing capacity of drying air. An ideal drying

process would ensure 100% relative humidity in the air leaving the dryer, but considerably less

relative humidity at the inlet of the drying chamber. A comparison of the temperature and

humidity profiles along the length of the collector/dryer is presented, for both operations of fan.

The relative humidity of air at the outlet of the dryer shows that the air still has a considerable

drying potential, implying that the rated capacity of the dryer has not been fully utilized. Further

studies on improving the performance are to be considered.

12. Conclusions

Solar radiation can be effectively and efficiently utilized for drying of agricultural produce if

proper design is carried out. This was demonstrated and the solar dryer designed and constructed

exhibited sufficient ability to dry the sample food item which is tomato.

According to Figure 5, the desired moisture content of tomato is around 13 %. And from the

experiment conducted the weight loss for the full flow sliced tomato is satisfactory to preserve

tomato using the solar dryer at Mekelle University.


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Locally available cheap materials were used in construction, making it available and affordable.

This will go a long way in reducing food wastage and at the same time food shortages, since it

can be used extensively for majority of the agricultural food crops. Apart from this, solar energy

is required for its operation which is readily available in the tropics, and it is also a clean form of

energy. It protects the environment and saves cost and time spent on open sun drying of

agricultural produce since it dries food items faster.

However, the performance of existing solar dryer can still be improved upon especially in the

aspect of reducing the drying time and probably storage of heat energy within the system.

13. RecommendationProper instruction and training of the users for successful dissemination will probably guide local

users on how to dry agricultural produce. The food items should be well protected in the solar

dryer than in the open sun, thus minimizing the case of pest and insect attack and also

contamination

Solar forced dryer would be best preferred to other dryers if the fan is driven by solar cell

because when there is cloud the amount of flow rate of air would be decreased so the flow of air

intake would be minimized.

The absorber should be roughened to increase the effective coefficient of heat transfer betweenthe air and the plate. The roughness ensures a high level of turbulence in the boundary layer of

the flowing air steam. For this reason, crumpled or corrugated sheets and wire screens can be

attractive as absorbing materials.

The chimney should be made of wood to decrease the heat loss from the air before reaching the

dryer.

The size of the drying bed should be increased four times of the present size.


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References

Cullen, R.N., (1989), Technology in Australia 1788 1988, Australian Academy of

Technological Sciences and Engineering 1989 pp. 820-821.

Pangavhane D. R.; Sawhney R. L.; Sarsavadia P. N. Comparative Drying Performance Study

of Natural Convection Solar Dryer with Traditional Grape Drying Methods. International

Energy Journal. 3(1), 13-22, 2002.

Rezuwan, K. (2000). Design and Development of Naturally Controlled Tropical Green Houses

for High Value Vegetables and Fruits Production. Proceedings Of National Horticultural

Conference 2000: MARDI

Yeoh, K.C. (1990). Plastic-Roofed Rainshelters For Vegetable Production In The Lowlands.

Teknologi Kejuruteraan Pertanian. Jil 1. Pp 25-31: Mardi

Yeoh, K.C. (1992). Design and construction of rainshelters. Food and Fertiliser Technology

Centre. Teipei City, Republic of China on Taiwan. Extension Bulletin No. 350.

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