Design of Solar Cabinet Drier

5/28/2018 Design of Solar Cabinet Drier

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Design of Cabinet Drier

Introduction

It is evident that agriculture is one of the sectors having high comparative advantagefor Nepal.

Agriculture contributes the largest share in the GDP. Unfortunately, the rate of agriculturalproduction of the country is so low that it is not keeping at pace with the rate of population

growth. The supply of food is in serious shortage. In the rural areas of Nepal, preservation of

agricultural product is one of the central problems. The producer needs to bring their produce to

urban areas as their local markets are too small to accommodate all the fresh vegetables and

fruits. However, because of lack of transportation infrastructure, this is often difficult, and results

in high rate of spoilage of products. A large quantity of agro products is also getting wasted

every year simply because of the lack of appropriate food conservation technique.

Drying reduces the moisture content of the product and also results in reduction inweight and

volume. This eases packaging, storage, handling and transportation. The dehydration of the food

product heaps in food preservation as the biological and biochemical process during storage isgreatly reduced due to reduction in the moisture content. The dehydrated food products can be

preceded for further processing and commercial use, so, it can be the source of income

generation. Also during off or lean season (when the price is high) the product can be of greater

convenience for users.

Principle of Solar Drying

Drying is the applied to the product, the rate at which the products internal moisture is released

from its surface and the rate at which moist air is removed from the area surrounding the product.

Thus, varying the heated airs temperature and humidity controls the drying rate of theproduct.

Warm, dry air can absorb so much moisture at one time and it will become saturated thereby

slowing the drying process considerably, unless the air is frequently replaced. Therefore, best

drying is achieved when the air mass moves constantly over the product being subjected to the

drying removal of moisture from the product to a moisture content level considered safe for

storage. It is a simultaneous heat and mass transfer process that vaporizes liquid water, mixes the

vapor with the drying air, and removes the vapor by carrying away the mixture mechanically.

In order to adequately dry a material, it is first necessary to know the initial moisture content of

the material to be dried and the desired moisture content of the final product. Drying rates are

controlled by the rate at which heat is process.

Problem Statement

Locals of Lubhoo, Lalitpur (around 5 km far from Kathmandu City) used to cultivate tomatoes

for two seasons in a year. They use to collect 50 -60 kg of tomatoes on every 3 4 days for each

production month. Problem indicated by the locals are price fall up to Rs. 2 per kg during pick

production of tomatoes and rise above Rs. 50 per kg during off-season. So, locals are very much


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willing to preserve for sell in off-season. According to them, preservation around 20 kg per 4~5

days is required for off-season and selling of remaining during production.

Objectives

To design Solar Cabinet dryer for Tomatoes drying.

About Tomato

Its scientific name is Lycopersicon esculentum Mill Tomato contain much Vitamin B and C, iron and phosphorus although a ripe tomato Cultivated in Falgun ~ Ashar and Shrawn ~ Magshir in Kathmandu Orange red in Color, Acidic, Average weight 50 g Optimum climatic conditions for cultivation are

o 21 0c - 27 0c temperatureo PHshould be in between 5.8-6.8o Elevation 1000-2000 m

When tomatoes are dried to a low moisture content, so that they are hard (eg 5% water), they can

be pounded or milled to a powder.

o Tomato jamo Green tomato chutneyo

Tomato ketchup for 1kgo Tomato soup for 1kg

Drying Condition: Tomatoes Drying

S.N Parameter Value

1 Wi(Initial Mass of fresh Product) 15 Kg

2 Mi( Moisture Content Initial) 90 %

3 Mf( Moisture Content final) 5 %


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Design Assumption

S.N. Parameter Value

1 Ta (Ambient Temp) 25 C

2 Ti (Initial Temperature of Cabinet ) 60 C

3 Tf(Final Temperature of cabinet) 50 C

4 K (Solar Drying factor) 1.25

5 Solar Irradiance (Kathmandu) 5.19 KW

hr/m2/day

6 Collector Efficiency 70 %

Design of Cabinet Solar Dryer

Designing of solar drying systems is a very complex task consisting of both technical as well as

practical design factors which differs along with the design and type of the solar dryer to beconstructed. The elementary theoretical design based on the static parameters of Solar cabinet

Dryer has been discussed in this section.While designing, following parameters have to be assumed or known before hand.

Reference material/ product to be dried and its physical properties;o Initial moisture content in wet basis, Mi:o Desired final moisture content in wet basis, Mf:o Maximum permissible temperature, Tmax(Ti):

Capacity, wi: in kg of fresh product/ batch Desired drying time, t: in hrs Drying season Ambient conditions;

o Average ambient air temperature, Ta: in oCo Average relative humidity of air, a:in %o Average velocity of air entering the collector, v: in m/seco Average solar isolation on horizontal surface, I: in W/m2

Collector Parameterso Collector efficiency, c: in %o Temperature of absorber plate, Tw: in oCo Equilibrium relative humidity of air at chamber temp, e: in %o Slope of collector to horizontal (for winter season), : in degrees o Air flow through Dryer: Steady flow

4 Total Time required to dry Tomato 5 Day

5 Temperature to be maintained 55 C
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Amount of water to be removed from the drying product

The amount of water to be evaporated, mwfrom the product during drying process can becalculated by using the value of its initial moisture content Mi and desired final moisture content

M fas given below.

mw(kg) = wi(MiMf) / (100 - Mf) (1)Where, wiis initial mass of fresh product to be dried.(Exell, R.H.B., 1980, Basic Design Theory for a Simple Solar Rice Dryer)

Energy Required for Drying

The amount of energy required, E, to evaporate the mwquantity of water can be calculated as;

E = mw x L (2)

Where, L = latent heat of evaporation of water,L (kJ/kg) = (2500 - 2.34T) (3)

(GTZ, Solar Drying)

Where, T is average inside air temperature in oC.The temperature of air leaving the drying material (Tf) can be determined with the help of

psychometrics chart using initial ambient air conditions.The average air temperature inside the drying chamber is given by,

T = (Ti+ Tf) / 2

This amount of energy (E) from eqn.1 is theoretical value required to evaporate the mwamount

of water from a free water surface. But in general, this water is bounded with commodity and

thus, more heat is required to evaporate it. At the same time, in practice, some amount o f air willnot come in contact with the moisture of material and there will be some heat loss by the airthrough sides of drying chamber. Therefore, the total amount of energy required is given by,

Et = k x E

(GTZ, Solar Drying)

Where k is constant (1


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The quantities of air are usually expressed in terms of the volume (V) at atmospheric pressure (P)and temperature (T) instead of in term of the mass, ma. Hence, it can be expressed in volume as,

V (m3) = maRTa/ P (5)

Where, R = 0.291 kPa m3/kg.oK is the gas constant.

P = 101.325 kPa is atmospheric pressure.

Detailed Designs

The detailed design of different parts of dryer is given below.

Air Inlet Passage into the Collector

For natural convection type dryer, the air-circulation inside the dryer is assumed to be steady.The airflow rate Q, inside the collector is given by,

Q (m3/s) = V / (t) (6)

Where, t is drying time in sec.

The air inlet area into the collector is given by;Ai(m

2) = Q / v

Let, h be the height of air inlet passage, the width, b of air passage is given by;

b = Ai / hIn order to prevent the entering of dusts and insects inside the collector and drying chamber, the

air inlet passage of collector is covered with wire net. This wire net covers certain area of inletpassage hindering the airflow into the collector. To compensate this factor, the inlet area should

be slightly increased.

Collector

The collector must provide the energy required to increase temperature of air from the ambienttemperature to specified temperature.

The energy that is be provided by collector to air is

Ec(kJ) = ma Cp (Ti - Ta) (7)

Where, Cp = Specific heat capacity of air at constant pressure (kJ / kg.C)

The energy received by collector from the sun is given by;

Er = I x Ac x t

Where, Ac = collector area (m2)

I = Solar insolation (W/m2)

Assuming collector efficiency (c), the energy supplied by collector to the air will be:


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Ec = cx Er (8)or, Ec = cx I x Ac x t

or , Ac = Ec/( cx I x t)

The effective width of the collector is equal to that of air passage without insulation, and the

effective length of the collector is given by,

lc = Ac / bc

In solar drying systems, insulation is one of the important factors, which greatly affects its

performance. The bottom and sides of the collector and all sides of drying chamber must beproperly insulated with the insulation material. Different types of insulation materials may be

used based on practical experiences. Rubber gasket is used in between the upper face of collectorbox sides and glazing to avoid air leakage.

Absorber Plate

The temperature of absorber plate in collector increases to certain value when solar radiationfalls on its surface. The surface area of the absorber plate may be insufficient to transfer required

amount of energy to the flowing air, if only flat surface is used, the calculated value of thesurface area determines whether the surface area with flat plate is adequate or not.

The detail calculation is performed as follows.

In the collector, absorber plate is placed in between glazing and inner collector box. So, itdivides collector air passage into two parts- upper part and lower part. Thus, both the faces ofabsorber plate must be checked for surface area requirements. It can be assumed that both of

these parts transfer equal amount of energy to air.

The calculation can be performed by assuming the values for absorber plate temperature (Tw),temperature of inner face of glazing (Tg) and temperature of inner face of collector box (Tb)

Calculation of Surface Area for Upper Face of Absorber Plate

Temperature difference between absorber plate and glazing;

T1 = TwTgMean temperature, T1 = (Tw+ Tg) / 2

Using the relation for natural convection between enclosed parallel plates,Raleigh number, Ra = (g T1l

3) / (9)

Where, g = acceleration due to gravity,

l = plate spacing, = Kinematic viscosity (m

2/s) of air at T1,


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k = Thermal conductivity (w/mC) of air at T1,= Thermal diffusivity (m

2/s) of air at T1,

= 1 / T1, Volumetric coefficient of expansion,

Nusselt number, Nu = 1 + 1.44 [ 11708/RaCos] x [ 1 {(Sin 1.8)1.6x 1708}/ RaCos ]

+ [(RaCos / 5830)

1/3

)1] (10)

(Duffie, John A. and Beckmann, W.A., 1985)

Heat transfer coefficient, h = Nuk / l (11)

Now, using energy balance equation for absorber plate,Ec/ 2 = h As T1t (12)

Where, T1 = temperature difference = T1 - Ta

Therefore, the required surface area of the upper face of absorber plate is given by,

As = Ec/ (2 h T1t) (13)

The surface area for lower face of absorber plate should also be calculated in the similar way asabove taking temperature difference between absorber plate and bottom base plate (inner plate of

collector box).

The highest value of the As among upper and lower face of absorber plate should be taken as therequired surface area of the absorber plate. If this area is greater than that of effective collector

area, the absorber plate should be corrugated.

Trays

Depending upon the batch capacity of fresh product to be dried and area required for spreadingthe product, required number of trays is to be placed inside the drying chamber. The total lengthof the drying chamber will be equal to the width of collector without insulation. The gap between

each row should be maintained around 20cm.

Chimney

In most real situations, the air leaving the drying chamber is moist and close to ambienttemperature (Sodha et al., 1987; Akachuku, 1986). So, a solar chimney would not have an effect

on dryers performance unless the solar heating of the air within the chimney is significant,capable of inducing upward flow of air through the chimney. The role of chimney can be

suppressed if a constant air flow is induced by an electrical fan. However, for chimney designedfor natural air flow, following considerations have to be made.

The momentum equation along the chimney yields:

P = H(a- ch)g(B/760) - w[2(S+P)H/(S*P)] (14)

where P is the required suction pressure (N/m2; usually ~ 0.5 mm of water for solar chimneys,

Das and Kumar, 1989), g is the acceleration of gravity (m/s2), a is the average density of air,


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ch is the average air density in the chimney, B is the barometric pressure (mm Hg), P is thedepth of the chimney (m), S is the width of the chimney (m) and H is the height of the chimney

(m) and w is the shear stress acting on the air in contact with the chimney surface (N/m2). Thelatter is given by:

w= chuch2

fch/2 (15)

Where uch is the average air velocity in the chimney (Q/[S*P]) while the friction factor fch canbe found for laminar flow as:

fch= 64/Re (16)

with the Reynolds number given as:

Re = Dhchuch/ch (17)

where Dh is the hydraulic mean diameter of the chimney defined as:

Dh= 2S*P/(S+P) (18)

andchis the average air viscosity in the chimney.

Solving the Eqns. (4.144.18) gives the effective dimensions of the chimney.

The chimney should be provided with a roof to prevent the drying chamber from rain or otherforeign particles.

Design Calculations

Design of Solar Cabinet Dryer for Tomatoes Drying

Step 0 Parameter Value

LetWi (Initail Mass of fresh

Product)15 Kg

TomatoMi ( Moisture Content

Initial)90 %

Tomato Mf ( Moisture Contentfinal)

5 %

TomatoTotal Time required to dry

Tomato5 Day

Assume Sunshine Hour per Day 5.19 hr /day


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Step 1 CalculationMass of Water to be

Removed from Drying13.42 Kg

Let Ta (Ambient Temp) 25 C

298 K

Let

Ti (Intial Temperature of

Cabinet ) 60 C

LetTf (Final Temperature of

cabinet)50 C

Calculation T (Average Temperture) 55 C

Step 2 Calculation L (Laten Heat of Water) 2370.75 KJ/ Kg

CalculationE (Energy req for Dryingtotal tomatoes)

31817.96 KJ

Let K (Solar Drying fator 1.25 1 to 3

Calculation

Et (Actual Heat requied

for drying) 39772.45 KJ

Step 3 GivenCp (Specific HeatCapacity of air

1 KJ/kg K

CalculationMass of air required fordrying

3977.25 Kg

Given R 0.291Kpam3/kg K

Given P 101.325 Kpa

CalculationVolume of air required for

drying 3403.88 m3

Step 4Air Inlet to Collector

Design

Assume Velocity of Air requied 1.25 m/s

Assume Irradiance 1000 W/m2

Calculation Q (Flow rate required) 0.036436372 m3/s

Calculation Ai (Area of Inlet Passage) 0.029149098 m2

29149.09774 mm2

Let

Bi (Breath of inlet

passage) 25.00 mm

CalculationLi (Length of inlet

Passage)1165.96 mm

1166 mm

Including edges 1266 mm

Step 5 Collector Area


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Calculation

Assume Collector Efficiency 70 %

CalculationEc (Energy Provided by

Collector to air)139203.5773 KJ

Calculation Area of the Collector 2.128690358 m2

2128690.358 mm2

Calculation Bc (Breath of collector) 1266 mm

Calculation Lc (Length of Collector) 1681.429983 mm

Step 6 Cabinet Design

Also dia. Mm Weight of each tomato 50 gram

No of tomato 300

Calculation Lc (Lenght of Cabinet) 1266 mm

No. of Tomato in Length

Wise Rw25.32 no

18.99

19

CNo. of Tomato in requied

in Breath Wise15.78947368 NO

No. of Tray required 2.631578947

3

Calculation Bc (Breath of Cabinet) 300 mm

400

Calculation H (Height of Cabinet ) 300 mm

Step 6 Tray Size Calculation

Length of Tray 1166

Breath of Tray 300


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References

Mohammed Solar Drying , A practical manual Volume 2, Renewable EnergyProduct,Khumaltar

Exell, R.H.B., 1980, Basic Design Theory for a Simple Solar Rice Dryer GTZ, Solar Drying Effects of Storage Period on Some Nutritional Properties of Orange and Tomato ,Peter

AbahIdah, John Jiya Musa* and Abdullahi

Solar engineering , http://solarcooking.org/drying/Whitfielddrying1.htm
http://solarcooking.org/drying/Whitfielddrying1.htmhttp://solarcooking.org/drying/Whitfielddrying1.htmhttp://solarcooking.org/drying/Whitfielddrying1.htm

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Design of Solar Cabinet Drier