CHAPTER-3

Chapter-3Materials and MethodsThis chapter deals with material and methodology adopted for design, development and performance evaluation of hybrid dryer based on solar and electrical energy. The dryer was fabricated at workshop and tested at Department of Renewable Energy Engineering, College of Technology and Engineering, Udaipur. The study area falls between 2742 N latitude, 7533 E longitude and at an altitude of 582.5 m above mean sea level.3.1 Design of Hybrid Dryer

Figure 3.1: Layout of hybrid dryer 3.2 Features of Hybrid DryerSome important features are:

An evacuated tube collector (ETC) is used for water heating. Hot water is stored in storage tank from which air is heated through heat exchanger unit.

Heated air is then supplied to drying chamber through pipes.

There is no provision for entrance of ambient air directly during day time.

A valve is provided to cut the supply when ambient air temperature is slightly more than the temperature of air coming from heat exchanger.

The temperature at inlet of drying chamber is adjustable so that it can be maintained at required amount of temperature.

At the bottom of drying chamber electric heater is provided to heat the incoming air as per the required amount of temperature.

If temperature of incoming hot air is 50C and the required temperature is 70C, then it will add up and drying chamber will get 70C temperature, so that constant drying can take place.

Blower is provided to flow the air into the chamber.

Above the electric heater, three trays are placed. Approx. 20 kg. of onion is used for drying in the dryer.

The exit air from the drying chamber will not go into the atmosphere. It is passed through the bed of silica gel for dehumidification, so that moisture will be absorbed. Then the air is recirculated into the drying chamber so that the energy input is minimized.

3.3 Fabrication and Dimensional Features of Hybrid Dryer

Plate 3.1 Drying chamber view

Plate 3.2 Heat exchanger cum storage tank viewTable 3.1 Dimensions of hybrid dryerS. No.ParticularsLength(m)Width(m)Height(m)

1.Drying chamberInsulation 0.04 m thick0.700.501.00

2.Heat exchanger cum storage tank0.500.500.50

3.Copper coil1.520.019 diameter-

4.Temperature controller unit0.200.380.34

5.Tray0.600.450.05

6.Stand for heat exchanger unit0.550.551.23

3.4 Main Components of Developed System

1. Solar collector ( ETC)

2. Heat exchanger cum storage tank3. Blower4. Electric heater

5. Drying chamber

6. Dehumidifier

7. Automatic temperature controller3.4.1 Description

3.4.1.1 Solar collector (ETC): Evacuated tube collector is an equipment to the heat water by using solar energy. In this type collector through natural process solar energy is absorbed by the black coating on the outside of the inner tube & the same is transferred to the water inside the tube. Water on the upper side of the vaccum tube becomes hot & lighter in weight, so it starts rising up into the tank as per the thermosyphon principle. At the same time, heavier cold water from the tank comes downward from lower side of the tube. This cycle repeats till all the water is heated so long as solar radiation is available. Hot water is collected into the tank and through a pipe is fed to the heat exchanger cum storage tank. Again cold water is return back into the collector.

Plate 3.3 Evacuated tube collector3.4.1.2 Heat exchanger cum storage tank A storage tank is made up of GI sheet having insulation 4 cm thick thermocol insulation. A copper tube is inserted into the tank in which hot air is circulated. Inlet of copper tube is connected with a pipe which is connected to a blower and outlet is connected to the drying chamber which provides hot air. A gate is provided to open the tank. In this tank hot water is collected through ETC. Due to density difference cold water is set at bottom and outlet is provided at bottom which is connected through ETC and thus water is circulated. Due to hot water coil placed in the tank gets heated and inside the coil heated air circulated. This hot air is passed into the drying chamber. At the top, sensor is placed to measure the temperature of hot water.

Plate 3.4 Heat exchanger cum storage tank3.4.1.3 Electric heater

A fin type electric heater of 1 KW capacity is placed at the bottom of the drying chamber which is used when required amount of temperature is not achieved from solar energy then alternate source of electrical energy is provided. A temperature controller is provided with the heater as when the temperature of heater rises above a set limit then it automatically cut off the supply of heat, so that constant temperature can be maintained into the drying chamber.

Plate 3.5 Electric heater3.4.1.4 Blower

A blower of is placed before the heat exchanger unit. Drying chamber outlet is connected with exhaust air pipe which is connected through blower to the heat exchanger unit. The mass flow rate may be adjusted in the blower.

Plate 3.6 Blower3.4.1.5 Drying chamber

Drying chamber is the main component of the system. It is made up of GI sheet. In this chamber trays are provided on which product, which is to be dried is placed. Temperature controller is provided to adjust the heat input. Temperature sensor is provided above the each tray to measure the temperature of each tray.

Plate 3.7 Drying chamber3.4.1.6 Automatic temperature controller and display unitThis unit consists of temperature controller and temperature display unit. It shows the temperature of hot water stored in tank and temperature of hot air coming through heat exchanger. Plate 3.8 Automatic temperature controller and display unit3.4.1.7 Energy meter

It shows the total electrical energy consumed by the system in kWh from the electric heater, blower and the temperature controller.

Plate 3.9 Energy Meter

Plate 3.10 Hybrid dryer

Table 3.2 Design considerations of hybrid dryerS.NoParametersSpecifications

1.Capacity (kg)20 kg fresh onion flakes

2Initial moisture content(mi)87 % (w.b.)

3Final moisture content (mf)7 % (w.b.)

4Loading rate (LR)5 kg / m2

5Solar insolation (It) 540 W/m2 hr (May month )

6Ambient temperature (Ta)30 C

7Drying temperature (Td)60 C

8Ambient relative humidity (RH)50 %

9Drying time (td)8 h

3.5 Design Procedure 3.5.1 Design of drying chamber

The following design parameters were considered for the design of hybrid drying system for drying of fresh onion.1. Mass of water to be removed during drying, Mw (kg)

Where, mi = Initial moisture content, (% w.b.) m f = Final moisture content, (% w.b.) W = Weight of product, (kg)2. Total energy required, Q (kJ)

Where, W = Weight of product, kg

Cp = Specific heat of water, kJ /kg C

Td = Drying temperature, C Ta = Ambient temperature, C Mw = Mass of water to be removed during drying, kg

= Latent heat of vaporization, kJ/kg3. Mass of air required for drying, Ma (kg/h)

Where

Mw = Mass of water to be removed, kg

Hd = Humidity ratio of drying air, kg of water per kg of dry air

He = Humidity ratio of exist air, kg of water per kg of dry air

t = Drying time, h

4. Volumetric air flow rate, Va (m3/h)

Where Ma= Mass flow rate of drying air, kg/hr.

Vh = Humid Volume (m3/kg) taken from psychometric chart, m3/kg

5. Volume of product to be dried, V (m3)

WhereW= weight of product to be dried, kg

= bulk density of product, kg/m36. Dimensions of drying chamber and trayDrying area AD =

Where

AD = Drying areaTdl = thickness of drying layer, m

7. No. of trays, (no.) 8. Exhaust fan power, Fp (W)

Where

Fp= Exhaust fan power (W)

Va = Volumetric air flow rate (m3/h)

(P = Pressure difference between inlet and exist of dryer, (Pa) a and e= Density of air at inlet and exit of dryer, (kg/ m3) g = Acceleration due to gravity, (m/s2)b = Efficiency of blower (%)

H = Elevation difference between inlet and exit of dryer, (m)

9. Thickness of insulation of dryer cabinetThe amount of heat loss by the dryer surfaces was reduced by providing an appropriate insulation of high density thermocol. The thickness of insulation was found out by heat balance as follow

The surface thermal resistance,

Thermal resistance of insulation, (C m2W-1)

Where, tinsulation = Thickness of insulation, mkinsulation = Thermal conductivity of insulating material, W/ m C

The heat flow from dryer surface and the ambient can be expressed as

Heat flow, Watts =

Where, Td = Drying temperature, CTa = Ambient air temperature, C RDS = Thermal resistance of dryer surface, C m2 W-1

Ts = Temperature of drying chamber after insulation, CThickness of insulation of dryer surface can be expressed as

3.5.2 Design of heat exchanger

1. Heat content of hot water, (kJ)

Where, Qreq = Total energy required for drying of onion, kJ

HE= Thermal efficiency of heat exchanger, %2. Log mean temperature difference (LMTD) of heat exchanger

Where, Thi = Temperature of hot water at heat exchanger inlet, C

Tho = Temperature of hot water at heat exchanger outlet, CTci = Temperature of air inlet, CTco = Temperature of air out, CBut this value only valid for counter flow heat exchanger whereas in this case it was proposed to use cross flow heat exchanger. The LMTD for cross flow heat exchanger was calculated as

LMTDcross = F LMTDcounterWhere, F = correction factor for cross flow heat exchanger which is computed by the chart of correction factor for one fluid mixed and other fluid unmixed condition.

Effectiveness (P) of heat exchanger was as calculated as

P =

And capacity ratio was calculated as

3. Heat transfer area requiredThe aluminum fins are used for heat transfer and heat transfer area of heat exchanger was calculated as

Q = U A LMTDcrossWhere, Q = Heat transfer per hour, kJ/h.

U = Overall heat transfer coefficient of aluminum, kJ/m2 C4. Water flow rate required

The water flow rate required for providing sufficient heat across heat exchanger was computed as

Where,

m = Mass of water to be flow through heat exchanger, kg

Cp = Specific heat of hot water, kJ/ kg C

Thi = Temperature of hot water at inlet of heat exchanger, C

Tho = Temperature of hot water at outlet of heat exchanger, C3.5.3 Raw materialIn the present study onions which were available in market was used in this study.

The onions were stored in a gunny bag and were aerated from time to time. A) Sample preparation

For each batch of 20 kg fresh onion flakes approx. were used. The onion bulbs were peeled and washed under tap water before each experiment, to remove dirt, dust and surface adherings. The onions bulbs were sliced with slicer and then crushed with hand to make flakes.

B) Moisture contentThe initial moisture content of raw onion flakes were determined by oven drying method. An amount equal to 100 g fresh onion slices were placed in thoroughly washed, dried and pre-weighed moisture boxes. The initial weight of each sample was recorded. The moisture boxes were put in the oven having temperature of pre-set at 105 C. Samples were dried for 24 hours. The samples were then taken out of oven and cooled in the desiccator and weighed using an electronic balance having capacity 600 g and least count of 0.001 g. Initial and bone dried weights were used to calculate the initial moisture content. The moisture content 87 per cent is taken for calculations.3.6 Performance Evaluation of Solar Collector

The performance of solar collector was evaluated in terms of collector efficiency. The hourly collector efficiency was calculated by following formula for each day. Then, the collector efficiency during without exhaust air recirculation tests and with exhaust air recirculation tests was averaged separately and plotted with respect to time.

Where,

= Hourly collector efficiency, %

mat = mass flow rate, kg/h

Ca = Specific heat of air, kcal / kg C

Tco = Collector outlet temperature, C

Ta = Ambient temperature, C

It = Solar radiation, kW/m2/day

Ac = Area of collector, m23.7 Performance Evaluation of Drying Unit

The performance of drying unit was evaluated in terms of moisture content variation, drying rate, drying efficiency, heat utilization factor etc. For this purpose, the hourly reductions in weight of representative sample were recorded. As, it was not possible to take weight of whole sample; the 200 g sample was taken as representative sample and the performance was calculated.A) Moisture content

Moisture content of onion flakes during experiment at hourly time intervals were determined by calculating mass of dry matter of flakes. Moisture contents (g water/ g dry matter) at various times were calculated by the formula:

Where, W = Weight of the sample at time , g

DM = dry matter content of the sample, g

B) Drying rateThe moisture content data in each of experiments were analyzed to determine the moisture lost by sample of onion flakes in a known time interval. The drying rate was expressed as g water/g dry matter- h. The drying rate can be calculated as -

Where,WML = Initial weight of sample weight after time, g.

DM = Dry matter

C) Hourly drying efficiency The drying efficiency was calculated by the following formula

Where, mw = mass of water evaporated per hour, kg

= Latent heat of vaporization, kcal / kg

It = Solar insolation, kWh/m2/day A = Total collector area, m2D) Heat utilization factor

The performance evaluation of the recirculatory dryer was assessed using the heat utilization factor (H.U.F) (Das et al., 2001). HUF is the ratio of heat utilized to heat supplied. The data acquired from experiment was used for determining the heat utilization factor (H.U.F.). It can be calculated by following formula

Where,

Tdi = Dryer inlet temperature, C

Tdo = Dryer outlet temperature, C

Ta = Ambient temperature, C3.8 Overall Performance of Hybrid Dryer

The overall performance of the system was evaluated in terms of overall thermal efficiency and daily drying efficiency.

A) Overall thermal efficiency

i. For without recirculation

Where,

th= Thermal collector efficiency, %

mat = mass flow rate, Kg/h

Ca = Specific heat of air, kcal/kg C

Tdo = Dryer outlet temperature, C

Ta = Ambient temperature, C It = Solar insolation, kWh/m2/day

A = Total area of collector, m2ii. For with recirculation

Where,

th = Thermal collector efficiency, %

mat = mass flow rate, Kg/hr

Ca = Specific heat of air, kcal/kg C

Tdo = Dryer outlet temperature, CTm = Mean temperature at collector inlet after mixing recirculated air with ambient air, C

It = Solar insolation, kWh/m2/day

A = Total area of collector, m2B) Exhaust air recirculation ratio

The exhaust air recirculation ratio can be determined by the following equation

Where,

Vr = volume of air recirculated, m3Vi = volume of air feed, m3C) Daily drying efficiency

Where, mev (t) = Total mass of water evaporated per day, kg

= Latent heat of vaporization, kcal / kg

It = Solar insolation, kWh/m2/day A = Total collector area, m2D) Dehydration ratio and dry matter recovery

Dry matter recovery is represented in percent. It may define as the ratio of final weight of dried onion flakes to the initial weight of fresh onion flakes and can be calculated as follows.Dry matter recovery (%) =

3.9 Instruments UsedInstruments required for evaluating performance of hybrid dryer are as follows

1. Moisture analyser2. Temperature data logger 3. Digital solarimeter4. Digital hygrometer

5. Solar data logger6. Digital anemometer

7. Colorimeter

8. Electronic weighing balance

9. Infra red thermometer3.9.1 Digital solarimeter

The digital solarimeter manufactured by M/s. Surya Solar Systems, Ahmedabad was used to measure the solar insolation falling on horizontal surface. The instrument was operated on 9 V DC battery and gives reading in W/m2.

3.9.2 Digital anemometer

The digital anemometer manufactured by M/s. Lutron Instruments, Taiwn was used to measure the air velocity. The instrument can measure air velocity from 0.1 to 40 m/s.3.9.3 Digital thermometer

The digital thermometer manufactured by M/s. Mextech Instruments was used to measure the temperature at various points in dryer. The range of instrument was from -50 to + 300 0C.

3.9.4 Digital hygrometer

The digital hygrometer manufactured by M/s. Mextech Instruments was used to measure the relative humidity of air.

3.9.5 Electronic weighing balance

Weight measurement was done by using electronic balance (Make- Adair Dutt and Co. Pvt. Ltd.) the capacity of balance was 600 g.3.9.6 Moisture analyzer

Moisture analyzer was used to detect or measure the moisture content of the product. The main feature of this machine is that it consumes lesser time compared to oven dry method.3.10 Techno Economics of Hybrid Dryer

Different economic indicators were used for economic analysis of solar drying system under this study. The following parameters were considered to carry out cost economics of drying system.

1. The life of hybrid dryer is assumed as 10 years (n).

2. The discount rate is assumed to be 8 %, (i)

3. The Annual repair and maintenance cost is 3 % of cost.

4. The cost of fresh onion is Rs. 6 / kg during season.

5. The cost of dried onion flakes is Rs. 250 / kg.

6. One labour is required per day and labour charge is Rs. 100 per day.

7. The dryer can be operated 300 days in a year.

8. The cost of system is Rs.1, 00,000.

9. The total material cost for drying of onion flakes is Rs.77, 850 per year. It includes the cost from purchase of raw material to the packaging of dried onion flakes.3.10.1 Net present worth

The net preset worth can be computed by subtracting the total discounted present worth of the cost stream from that of the benefit stream.

NPW =

Where, Ct = Cost in each year

Bt = Benefit in each year

t = 1, 2, 3................n

i = Discount rate (%)

3.10.2 Benefit cost ratio

This is the ratio obtained when the present worth of the benefit stream is divided by the present worth of the cost stream. The mathematical benefit-cost ratio can be expressed as:

Where, Ct = Cost in each year

Bt = Benefit in each year

t = 1, 2, 3................n

i = Discount rate, %

3.10.3 Payback period

The pay back period is the length of time from the beginning of the project until the net value of the incremental production stream reaches the total amount of the capital investment. It shows the length of time between cumulative net cash outflow recovered in the form of yearly net cash inflow. 3.10.4. Internal rate of return (IRR)

Another way of using the internal cash flow for measuring the worth of a project is to find the discount rate that makes the net present worth to the incremental cash flow equal to zero. This discount rate is called the internal rate of return.

IRR =

Where, Ct = Cost in each year

Bt = Benefit in each year

t = 1, 2, 3................n

i = Discount rate, (%)Plate 3.11 : Instruments used Moisture Analyser Hot Wire Anemometer Digital Moisture Meter Digital Solarimeter

Digital Temperature controller Temperature monitor

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