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Development and testing of back-up stove for solar dryers and solar air heaters

1.0       INTRODUCTION

The low solar radiation during winter and rainy season give rise to a need for backup heating source for solar dryers and solar air heaters. The backup charcoal stove consists of three main components, the stove, heat-exchanger and chimney. The stove is made up of the burner and ash tray. The burner is pyramidal with holes along the slanted height for air delivery to the heat-exchanger. Holes at the bottom of the burner allows for easy air circulation and the ash tray for collecting ashes. The heat exchanger is shell and tube type. The chimney has rectangular cross section; it is connected to one end of the shell side of the heat exchanger. The chimney has a rectangular hole at its upper end which serves as an exit for the flue gases.

A fan located at the ash tray blows air through holes at the bottoms of the burner which forced flue gases to pass through a duct located at the slanted side of the burner into the shell side of the heat exchanger. In the heat exchanger, heat is exchanged between the hot flue gases in the shell and ambient air in the tube. The flue gases lost heat whereas ambient air gain heat and transport it to the drying chamber for meat drying.

SIGNIFICANCE

To be use during winter and rainy season when there is low solar radiation

2.0       METHODOLOGY

The development and testing of backup charcoal stove involves the following steps:

1. Collecting information about calorific value of charcoal and its suitability as the heat source.
2. Designing each component of the backup stove (i.e stove, heat exchanger and chimney).
3. Selective of suitable materials for the construction of each component.
4. Construction of individual component.
5. Assembly and testing of the system.

3.0       DESIGN OF A BACKUP CHARCOAL STOVE

The three components of the backup stove are:

- Stove, Heat-exchanger and Chimney.

3.1 STOVE

The stove supply heat to the shell side of the heat-exchanger. It is made up of;

-          Burner and ash tray

3.2 HEAT EXCHANGER

The heat source to the exchanger is burning charcoal. Charcoal burning temperature is around 600oC. From the chimney calculations, the exhaust temperature is 100oC. Ambient air enters the tube side of the exchanger at 25oC and is expected to leave at 65oC.

3.3 CHIMNEY

The stoichiometric air fuel ratio for efficient burning of coal is 11:1(Jidda, 1998). That is m = 11Kg air/kg of fuel. M is the mass of air in Kg used for each Kg of fuel.

DESIGN CALCULATION

TABLE 1

Parameter

Equations

Values

Reference

Average calorific value of charcoal

(C V)

30MJ/kg

Kulla D, 2003

Heat required (Q)

10155.47kJ

Actual heat supply

20310.94kJ

Mass of Charcoal required (M)

0.68kg

Rate of fuel consumption (R)

0.08kg/hr

Volume of Charcoal required (Vc)

Height of stove (h)

0.163m

Length of the burner plate

Pythagoras theorem

0.17m

Overall heat-transfer coefficient (Uo)

58.97W/m2 oC

Linear velocity (Um)

20.24m/s

Heat gain by the cold side (Qc)

115.19kJ

Mass flowrate of hot fluid (mh)

Surface area of pipes(A)

0.0101m2

Length of pipe (l)

0.25m

Total heat transfer rate (Q)

115.06W

The actual exit temperature through the chimney (Th,out)

99.74oC

Actual temperature into the drying chamber

65oC

Draught (h)

2.99N/m2

Brennan et al, 1981

Actual height (H’)

0.295m

4.0    CONSTRUCTION AND ASSEMBLY

The design specifications above were used to construct each component of the back-up stove. Theses components were assemble together

FIGURE 1.  BACK-UP STOVE DIAGRAM

5.0       RESULTS AND DISCUSSIONS

5.1 NO LOAD TEST USING SOLID FUEL STOVE ONLY

This was also carried out in the new campus of Bayero University, Kano on 22nd October 2007 and the following readings were obtained.

APPARATUS

Solarimeter and wet and dry bulb liquid in glass thermometers

PROCEDURE

1. 1Kg of charcoal was added into the stove and fire was lighted.
2. After 15minutes the stove fan was on.
3. Wet and dry bulb thermometers were attached to the chamber inlet and drying chamber exit.
4. Solar radiation and temperatures were measured and recorded every 30minutes

TABLE 2 NO LOAD TEST USING CHARCOAL STOVE ONLY

Time

Ta(oC)

Chamber

Exit air

Tc(oC)

RH1(%)

Te(oC)

RH2(%)

9:00am

32

40

46

36

51

10:00am

32

50

30

38

48

10:30am

32

54

23

39

36

11:00am

33

58

20

42

29

11:30am

34

62

14

45

23

12:00noon

34

62

14

48

22

12:30pm

34

62

14

49

21

1:00pm

34

62

14

50

20

1:30pm

34

62

14

50

20

2:00pm

34

62

14

50

20

2:30pm

34

62

14

52

19

3:00pm

34

62

14

52

19

5.2 Load testing using fossil fuel stove with charcoal as fuel.

The test was carried out at new campus of Bayero University, Kano on 25th October 2007, and the following readings were obtained. The test was carried out on 3mm thick slice of Kilishi. The ratio of water to mix-powder by volume is 30:70.

AIM

To dry Kilishi.

OBJECTIVES

1. To find the drying time.
2. To find percentage moisture removed.
3. To calculate drying efficiency.

APPARATUS

Solarimeter, wet and dry bulb liquid in glass thermometers, stirrer, weight balance and 1Kg of Charcoal

PROCEDURE

1. 1Kg of charcoal was added into the stove and fire was set.
2. After 15minutes the stove fan was on.
3. The meat was arranged on the drying trays in the drying chamber.
4. Wet and dry bulb thermometers were attached to the chamber inlet and drying chamber exit.
5. The drying chamber door was closed and the weight lost was monitored by measuring the weight of the sample after 2hours interval
6. The percentage moisture loss was calculated at each interval.
7. When the moisture content of the sample is around 14%, it was removed from the chamber and dipped into prepared mix-powder solution.
8. The mixture was returned to the drying chamber and the drying process continued.
9. The sample weight was measured after each hour.
10. The percentage moisture loss was calculated at each interval
11. Solar radiation, relative humidity, sample mass and temperatures were measured and recorded every 30minutes

TABLE 3; LOADS TESTING USING CHARCOAL STOVE

Time

Ta(oC)

Chamber

Exit air

Ms(Kg)

Tc(oC)

RH1(%)

Te(oC)

RH2(%)

STAGE 1 DRYING PROCESS

9:00am

33

48

33

40

54

3.02

10:00am

33

51

29

39

56

11:00am

34

54

23

43

44

1.87

12:00noon

34

56

22

48

33

1:00pm

35

56

22

52

27

1.09

STAGE 2 DRYING PROCESS

1:30pm

35

62

14

56

22

2.2

2:00pm

35

60

18

48

33

2.08

3:00pm

35

62

14

49

32

1.9

4:00pm

34

62

14

55

25

1.73

Equilibrium moisture before second stage drying is

Therefore final moisture content of the Kilishi is

Mass of Charcoal left = 0.47Kg

Back-up stove efficiency

TABLE 4

Parameters

Equations

Values

References

Back-up efficiency

0.0714

PMR

Stage 1, PMR = 63.9%

Stage 2, PMR = 21.36%

Final moisture content

7.07%

Mass of Charcoal left

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