Improvements in thermal efficiency of onion slice drying by exhaust air recycling

Figures & data

Figure 1. Schematic of the onion slice drying system: 1) Blower, 2) Temperature–relative humidity sensor, 3) Velocity–temperature sensor, 4) Heater, 5) Drying chamber, 6) Valve

Figure 2. The dimension of onion slice before and after drying

Figure 3. The distribution of hot air along the cross sectional area of the tray dryer

Figure 4. Experimental and modeled moisture ratios as a function of drying time at a drying capacity of 0.1 kg of onion slices (a) without recycled exhaust air, R = 0 and (b) recycled exhaust air to fresh air ratio of 1

Table 1. Parameter estimations of the Newton model for onion slice drying

Drying temperature (°C)	$X_{emc}$	$k\times {10}^{-2},$(1/minute)	Drying time (minutes)	R^2	RMSE
Without recycled exhaust air (R = 0)
70	0.044	3.367	82.136	0.996	0.022
60	0.056	2.544	109.913	0.996	0.019
50	0.075	1.672	170.106	0.995	0.022
recycled exhaust air to fresh air ratio of 1 (50% fresh air + 50% exhaust air)
70	0.047	3.371	82.224	0.998	0.021
60	0.058	2.546	109.999	0.998	0.019
50	0.075	1.555	182.957	0.997	0.022

*$X_{emc}$: equilibrium moisture content (kg water/kg dry material), RMSE: Root mean square deviation

Figure 5. Thermal efficiency as a function of drying time under different drying temperatures and drying capacities without recycled exhaust air (R = 0)

Figure 6. Temperatures of exhaust air at a drying capacity of 0.1 kg of onion slices under different inlet air temperatures without recycled exhaust air (R = 0)

Figure 7. Average thermal efficiency as a function of drying capacity at various drying temperatures without recycled exhaust air (R = 0)

Figure 8. Effect of recycled exhaust air-to-fresh air ratio on the thermal efficiency of onion slice drying at different drying capacities and a drying temperature of 70°C

Figure 9. Thermal efficiency of onion slice drying at 70°C at various drying capacities and recycled exhaust air-to-fresh air ratios (R)

Table 2. Process variables and levels of the orthogonal central composite design

Independent Variables	Symbols	Factor Level
		Star Point (−1.0781)	Low Value (−1)	Center Value (0)	High Value (+1)	Star Point (+1.0781)
Drying temperature (°C)	A	59.61	60	65	70	70.39
Drying capacity (kg)	B	0.021	0.025	0.075	0.125	0.129

Table 3. Experimental variables used for optimization via the orthogonal central composite design and observed responses of thermal efficiency at a recycled exhaust air-to-fresh air ratio of 4.0

Run	Drying temperature (°C)	Drying capacity (kg)	Thermal efficiency (experiment)	Thermal efficiency (predicted)
1	60	0.025	47.45	46.95
2	60	0.125	53.69	54.09
3	70	0.025	49.83	47.52
4	70	0.125	56.84	55.43
5	59.61	0.075	57.11	57.08
6	70.39	0.075	54.79	58.12
7	65	0.021	43.83	46.27
8	65	0.129	53.60	54.40
9	65	0.075	58.27	58.13
10	65	0.075	58.04	58.13

Table 4. Results of the quadratic model of the orthogonal central composite design

Parameter	Regression coefficients	Standard error	P-value	Note
${\gamma }_0$	−41.76	1.72	4.62 × 10^−6	S
${\gamma }_1$	2.40	2.18	0.68	NS
${\gamma }_2$	426.13	2.18	0.03	S
${\gamma }_{12}$	0.77	2.74	0.90	NS
${\gamma }_{11}$	−0.02	3.33	0.80	NS
${\gamma }_{22}$	−2672.06	3.33	0.02	S
R^2	0.87

S = Significant; NS = Not significant

Figure 10. Three-dimensional (a) and two-dimensional (b) plots of the thermal efficiency of onion slice drying