# Design of an experimental set up for convective drying: experimental studies at different drying temperature

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DOI: 10.1007/s00231-012-1060-4

- Cite this article as:
- Mohan, V.P.C. & Talukdar, P. Heat Mass Transfer (2013) 49: 31. doi:10.1007/s00231-012-1060-4

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## Abstract

An experimental setup is designed to investigate the convective drying of moist object experimentally. All the design data, components of setup, materials and specifications are presented. Transient moisture content of a rectangular shaped potato slice (4 × 2 × 2 cm) is measured at different air temperatures of 40, 50, 60 and 70 °C with an air velocity of 2 m/s. Two different drying rate periods are observed. Results are compared with available results from literature.

### List of symbols

- a, b
Constants

- A
_{1}, A_{2}, A_{3} Constants

- B
Breadth of the moist object (cm)

- C
Width of the moist object (cm)

- db
Dry basis

- L
Length of the moist object (cm)

- m
Mass of the object (g)

- M
Moisture content (kg/kg of db)

- RH
Relative Humidity (%)

- T
Temperature (°C)

- V
Volume (cm

^{3})

### Greek symbols

- θ
Angle (°)

- ϕ
Diameter (cm)

- Φ
Non dimensional moisture content

- ρ
Density of object (g/cm

^{3})- ε
Porosity

### Subscripts

- 0
Initial condition

- a
Air

- s
Solid or dry matter

- w
Water

- eq
Equilibrium

## 1 Introduction

A variety of drying methods exist in the literature. These can be classified as convective or direct drying, indirect or contact drying, dielectric drying, freeze drying, natural air drying etc. The convective drying is mostly used in industries like agricultural and food industry, bio-oil industry, building materials, chemical/ceramic industry, paper industry, textile industry, nuclear waste disposal etc.

Experiments on convective drying of moist object need extra effort and attention. Convective drying depends on many factors like air velocity [1], air temperatures [2], air humidity, steady uniform air flow, turbulence level etc. Maintaining these external factors according to the desired conditions is a challenging task during an experiment. A carefully designed experimental facility is very much required to perform a well-controlled experiment with a greater accuracy.

*Experimental studies*: A plenty of experimental studies have been carried out for convective drying of potato [2, 3, 4, 5, 6, 7, 8, 9, 10], apple [1, 2, 9, 11] and other food products [5, 6, 12, 13, 14]. A large fraction of the experimental works focuses on the measurement of the rate of moisture transfer and thereby finding the properties like moisture diffusivity [4, 5, 13], heat transfer coefficient [7, 9] etc. A correlation describing the diffusivity of the spherical grain with moisture content and temperature during drying was established by Dutta et al. [13]. Evaluation of transient convective heat transfer coefficient on rectangular shaped potato and apple slices during drying was analyzed experimentally by Akpinar [9] and found that the variation of heat transfer coefficient with respect to time is negligible.

Different methods of convective drying can be seen in the literature. A cyclone type convective dryer was used by Akpinar et al. [7, 9]. A convective vertical flow dryer was used by Hassini et al. [4]. In both these works, experimental set up was described in brief.

A considerable number of literature are dedicated to determine the effect of shrinkage [3, 8, 9, 10, 11, 14] in potato and similar food products. The structural changes of potato by light microscopy and effect of shrinkage during drying were analyzed experimentally by Wang and Brennan [3]. They found that the percentage changes in thickness, length and width of the potato samples during drying increased linearly with decreasing moisture content. The effect of shrinkage of mango and cassava were analyzed by Hernandez et al. [14] and they produced an analytical model of moisture content distribution with shrinkage and without shrinkage. The effect of shrinkage of broccoli stems during convective drying was analyzed by Simal et al. [12]. They presented correlations between shrinkage and effective diffusion coefficient during convective drying. A review paper was presented by Mayor and Sereno [11] describing the modeling of shrinkage during convective drying for different food products. They have classified all the models into two different categories: empirical and fundamental.

Higher temperature and lower humidity enhances more rapid rate of drying. If drying takes place too fast, case hardening [15] occurs. This means that the surface becomes hard, preventing the escape of moisture from the inner part of the object. Fernando et al. [15] analyzed the surface hardened or case hardened layers leading to restricted moisture movement through resulting surface hardened layers and retarded drying rates. Oztop and Akpinar [2] used a cyclone type convective dryer for measuring the transient moisture distribution of both potato and apple. Velic et al. [1] investigated experimentally the convective drying of apple in laboratory conditions. They concluded that as the airflow velocity was increased, the heat transfer coefficient was also increased. Baroni and Hubinger [16] found that drying time drastically reduced when the onion sample was soaked with NaCl solution.

*Drying/mass transfer through other materials*: Murugesan et al. [17] conducted an experimental and numerical simulation of natural convective drying with building materials. The drying time increased nearly 30 days to dry the brick in natural convection drying. Talukdar et al. [18, 19] studied experimentally and numerically combined heat and mass transfer processes through building materials like cellulose insulation and spruce plywood.

*Numerical studies*: While some of the works in the literature consider both experimental and numerical study [2, 6], there are works which are solely dedicated to numerical model [20, 21, 22].

The models have been developed for 1-D Cartesian [7], 1-D cylindrical [21], 2D [2, 20, 21] and 3-D [22] geometries. While calculating the transfer coefficients, some of the works used a CFD model [20, 22], others were based on experimental correlation [21].

In the literature, experimental works are found to be lesser compared to numerical works. Very few literatures discussed about density, porosity, volume content of water, air and solid of the moist object. These properties could give more understanding about the drying physics. There is no design data available for convective dryer in the literatures and these data could be very useful to research community and industry. This motivates the present work with well designed experimental setup and experiments in same.

In the present work, the design details of an experimental set up for convective drying are presented. The experiments are performed for an air velocity of 2 m/s and at different drying temperatures of 40, 50, 60 and 70 °C. While the results from the experimental set up would be valuable for the drying research, it will also serve as a bench mark data for the different numerical model to be developed in the near future. The details of the experimental conditions presented in this work would help the modeler to simulate the drying of potato as close as possible to the experimental condition and thereby validating their model more efficiently.

## 2 Experimental setup

### 2.1 Materials and methods

Fresh potato is peeled and cut into slabs with a length (L) of 4 cm, breadth (B) of 2 cm and a width (C) of 2 cm. Sample is placed on a tray placed in the test section. A similar dimension of potato piece is prepared for finding the initial moisture content.

### 2.2 Experimental design

Pressure drop due to friction in test section: 172.8 Pa

Pressure drop due to enlargement in diffuser section: 70.2 Pa

Pressure drop due to sudden contraction: 68.5 Pa

Pressure drop in settling chamber: 125.9 Pa

Pressure drop at inlet of the centrifugal blower: 29.9 Pa

Pressure drop at outlet section: 60 Pa

Pressure drop due to obstruction: 34 Pa

Pressure drop due to honeycomb: 20.3 Pa

Total Pressure drop: 581.8 Pa

Maximum volume flow rate: 0.1 m

^{3}/sActual pressure drop: 1346.7 Pa

Power requirements for centrifugal blower and motor: 259.8 W or 0.353 HP

Area ratio of diffuser section (outlet to inlet area): 3

Angle of diffuser: 2θ = 7°

Area ratio of contraction cone: 6

Contraction angle: θ = 8°

Required heat load: 4.406 kW

Specification of experimental setup and accessories

Sl. no. | Components | Dimensions | Specifications | Materials |
---|---|---|---|---|

1 | Centrifugal blower inlet | ϕ = 9.5 cm | – | Mild steel |

2 | Centrifugal blower | – | 1,800 rpm, single phase, 220 V, 0.5 HP, 0.1 m | Mild steel |

3 | Centrifugal blower outlet | Cross section = 5″ × 5″ (=12.7 × 12.7 cm | – | Mild steel |

4 | Diffuser | Length = 72 cm, 2θ = 9, exit cross section = 24 × 24 cm | 2θ = 9, A = 3.6 | Bakelite |

5 | Settling chamber | Length = 88 cm, cross section = 24 × 24 cm | – | Bakelite |

6 | Heating coil | Totally 31.8 cm, insertion length should be 22 cm, 1 cm clearance in top and bottom | U type, 500 W, Single phase, 220 V | Mild steel |

7 | Honeycomb (a) | ϕ = 1 cm, length = 10 cm | – | Aluminium |

Honeycomb (b) | ϕ = 1 cm, length = 5 cm | – | Aluminium | |

8 | Contraction cone | Length = 50 cm, cross section: inlet = 24 × 24 cm exit = 10 × 10 cm | θ = 8, contraction ratio = 6 | Bakelite |

9 | Test section | Length = 100 cm, cross section = 10 × 10 cm | – | Bakelite |

10 | Tray to keep the object | 10 cm long thin plate | – | Steel plate |

11 | Outlet section | Cross section = 10 × 10 cm | – | Bakelite |

12 | TESTO Multi function probe, Part No: 0635 1535 | ϕ = 12 mm, handle length = 745 mm | −20 to +70 °C, 0 to +100 %RH and 0 to +20 m/s, ±0.3 °C, ±2 %RH, ±0.03 m/s | – |

13 | TESTO multi function instrument | – | Part no: 435-1 | – |

14 | OHAUS’s adventurer basic level single pan balance | – | Capacity : 310 g Readability : 1 mg Linearity : ± 2 mg Pan size : 100 mm | – |

15 | Auto transformer dimmerstat portable type, 15D-3P | – | 415 V/0–470 V, 3 phase, capacity: 15 A, 50 Hz | – |

The present convective dryer experimental setup is similar to a wind tunnel, but has additional features required to produce uniform flow within the test section. It essentially consists of a centrifugal blower, entrance section, diffuser section, settling chamber with heaters, contraction section and test section. Honeycombs are used in settling chamber and entrance of test section for flow straightening purpose. Ten numbers of U type of heaters (each 500 W) are fixed in the settling chamber in a staggered manner. Auto Transformer Dimmerstat portable type (15D-3P) is connected for maintaining/controlling the voltage supply of heating coils, thus maintains the air temperature. Contraction cone is used to accelerate the air flow smoothly from a larger cross section to a smaller one to achieve uniform velocity profile and low turbulence level in the test section. Two holes of 13 mm diameter are provided on the top of test section for inserting the multi function hygrometer probes (TESTO, Part no: 0635 1535) which is used to measure the upstream and downstream temperature, %RH and velocity of air. Both hygrometer probes are connected to TESTO indicators, Part no: 435-1. The air velocity is controlled by a throttling valve at the inlet of the centrifugal blower.

The hot air oven (YSI—431, IS—3119) used for finding the initial moisture content is constructed with double walled construction. Anodized Aluminium or highly polished stainless steel is used for inner chamber and outer chamber is made of mild steel sheet. Special grade glass wool is used to fill the gap between the oven walls for proper insulation. Heating elements in hot air oven is made of high grade imported nichrome wire. Temperature is controlled by imported capillary type Thermostat.

## 3 Experimental procedure

The air velocity chosen for the present study is 2 m/s. The temperature is maintained in range of 40–70 °C in the test section. The centrifugal blower and heaters are first switched ON for 20 min. After achieving the required velocity and temperature conditions, a known initial mass of rectangular shaped moist object (4 cm × 2 cm × 2 cm) is placed on the thin tray at the middle of the test section. Mass of the moist object, upstream and downstream temperature and humidity is measured for every 20 min of 16 h of drying. The length, breadth and width of the moist object are measured every 20 min by vernier caliper. Mass of the object is measured by OHAUS’s model Adventurer Basic level Electronic Single Pan Balance (Model AR 3130) with a readability of 0.001 g.

Initial moisture content of the moist object

Sl. no. | Initial mass of the object (gm) | Size of the object (cm) | Temperature maintained (°C) | Total time (h) | Final mass of the object (gm) | Moisture content (kg/kg of db) |
---|---|---|---|---|---|---|

| ||||||

1 | 17.382 | 4 × 2 × 2 | 105 | 24 | 2.951 | 4.8901 |

2 | 17.376 | 2.948 | 4.8962 | |||

3 | 17.39 | 2.960 | 4.8723 | |||

Average | 4.8862 | |||||

| ||||||

1 | 17.386 | 4 × 2 × 2 | 60 | 35 | 2.956 | 4.8802 |

### 3.1 Experimental uncertainty and repeatability test

Uncertainties of the parameters during convective drying

Parameters | Uncertainty |
---|---|

Upstream temperature | ±0.15 °C |

Downstream temperature | ±0.15 °C |

Ambient air temperature | ±0.15 °C |

Upstream %RH | ±1 % |

Downstream %RH | ±1 % |

Upstream velocity | ±0.015 m/s |

Downstream velocity | ±0.015 m/s |

Uncertainty in the mass loss measurement | ±0.001 g |

Uncertainty in moisture content | ±0.035 % |

Uncertainty measurement of length of the object | ±0.01 mm |

Uncertainty measurement of volume of the object | ±0.23 % |

Uncertainty measurement of density of the object | ±0.23 % |

Uncertainty measurement of porosity and shrinkage | ±0.23 % |

the same observer

the same measurement procedure

the same measuring instrument, used under the same conditions (2 m/s and 60 °C)

the same location

repetition over a same time duration (20 min).

Estimated mean and 95 % bounds of the data for upstream temperature, velocity and %RH during repeatability tests at drying temperature of 60 °C and velocity of 2 m/s

Sl. no. | Test | Upstream temperature (°C) | Upstream velocity (m/s) | Upstream %RH | |||
---|---|---|---|---|---|---|---|

Mean | 95 % bounds of the data | Mean | 95 % bounds of the data | Mean | 95 % bounds of the data | ||

1 | Test1 | 60.1 | ±0.4 | 2.10 | ±0.22 | 13.2 | ±1.4 |

2 | Test2 | 60.1 | ±0.4 | 2.11 | ±0.24 | 14.3 | ±1.4 |

3 | Test3 | 60.1 | ±0.6 | 2.04 | ±0.28 | 13.8 | ±1.6 |

## 4 Results and discussion

_{wet}) and final mass of the dry potato (m

_{dry}) are measured with the weighing balance. The moisture content in dry basis is calculated from the expression,

_{0}= 4.8862 kg/kg of dry basis (db). In a similar way using Eq. (1), the transient moisture content of the moist object is calculated at different times. Experimental density is calculated by mass of the moist object divided by volume of the moist object. Figure 3a shows the plot between experimental densities of the moist object and moisture content at 40 °C. It is compared with density calculated from Wang and Brennan’s [3] correlation.

_{1}, A

_{2}and A

_{3}are constants at 40 °C (1.08, 0.3 and −0.325 respectively) and found from non linear regression procedure and M is the moisture content. Good agreement between experimental and Wang and Brennan’s [3] values of the density can be seen from the figure.

_{0}is the initial moisture content and M

_{eq}is the equilibrium moisture content.

Figure 3b shows the comparison of current experimental results with analytical solution of Rosello et al. [25]. The plot has drawn at 40 and 60 °C between natural logarithmic of non dimensional moisture content and the drying time in hour. It is noted that the experimental results are in good agreement with analytical solution. As can be observed, increasing the air drying temperature caused an important increase in the drying rate.

^{3}after 13 h of drying at 40 °C. This maximum value of density is almost same with 1.38 g/cm

^{3}that proposed by Wang and Brennan [3]. The corresponding moisture content at this maximum density is 0.867 kg/kg of db as seen from Fig. 6a. The maximum value of density varies from 1.25 to 1.34 g/cm

^{3}for different temperatures. Shrinkage rate is responsible for the increased apparent density at non dimensional moisture contents from 0.1 to 0.6. Further reduction in moisture content (or increase in drying time) resulted in a decrease in density of the moist object. During the final stage of drying (non dimensional moisture content approximately less than 0.1 or after 13 h of drying) the volume of the sample does not change with the further loss of water (or further increase of drying time). Therefore, the density of the sample decreases with decreasing moisture content at low moisture contents or higher drying time.

It can also be seen from Fig. 6a, b that the air temperature has an effect on the density of object. The density at a given moisture content (or drying time) is decreased with increasing drying air temperature. While drying with high temperature air, the outer layers of the material become rigid and their final volume get fixed comparatively early in the drying process. As a result, there is a lower density at high temperature than that at low temperature for a given moisture content (or drying time). This was represented by case hardening layer and briefly explained by Fernando et al. [15].

Estimated mean and 95 % bounds for upstream temperature and humidity

Sl. no. | Drying air temperature (°C) | Upstream temperature (°C) | Upstream %RH | ||
---|---|---|---|---|---|

Mean | 95 % bounds of the data | Mean | 95 % bounds of the data | ||

1 | 40 | 40.3 | ±0.6 | 34.6 | ±3.7 |

2 | 50 | 50.2 | ±0.6 | 17.9 | ±2.6 |

3 | 60 | 60.4 | ±0.6 | 13.3 | ±2 |

4 | 70 | 70.3 | ±0.6 | 12.1 | ±1.6 |

_{0}, ρ

_{0}and M

_{0}are initial volume, density and moisture content of the moist object respectively. Assume that, if there are no water-fibre interactions, then the volume of the moist object is,

_{s}, V

_{w}and V

_{a}are volume of solid (or dry matter), water and air respectively. Volume of water in the moist object at moisture content M is

_{0}) of a moist object during drying decreases almost linearly with decrease in moisture content (Fig. 8a). The volume shrinkage does not show much variation with air temperature. It is observed that the volume loss (\( {\text{volume}}\,{\text{loss}} = \frac{{{\text{V}}_{\text{initial}} \, - \, {\text{V}}_{\text{final}} }}{{{\text{V}}_{\text{initial}} }} \times 100\,\% \)) experienced by the sample is from 74.13 to 81.56 % for temperatures range of 40–70 °C.

Where, Φ is non dimensional moisture content, a and b are constants. A curve fitting procedure is used to find the constants which are given as a = 0.72 and b = 0.284. It is noticed that the variation of shrinkage is almost independent of air temperatures and the r^{2} values are 0.985, 0.987, 0.993 and 0.994 for different air drying temperatures 40, 50, 60 and 70 °C. The shrinkage of moist object during convective drying can be estimated by Eq. (9), even the initial moisture content is not known.

The volume fraction of air in the moist object is represented as V_{a}/V_{0} and is calculated using Eq. (7). The variations of V_{a}/V_{0} with non dimensional moisture content at different air temperatures are shown in Fig. 8b. It is observed that there is a gradual rise in V_{a}/V_{0} in moist object when the moisture content is decreased (right to left in Fig. 8b). This is because of more amount of air occupy the pores of the moist object when moisture is removed. Air temperatures also have significant effect on V_{a}/V_{0}. The average percentage of volume of air pores are varied as 6.92, 8.44, 8.98 and 9.99 % with different drying air temperatures 40, 50, 60 and 70 °C respectively. At a given moisture content, the volume of air increases with air temperature. The higher air temperature takes off a large amount of moisture from the moist object as explained in Fig. 4, and hence, these pores are filled with the air.

Figure 8c shows the variation of porosity with respect to non dimensional moisture content at different air temperatures. The porosity (Eq. 8) increases gradually when the moisture content decreases. It is observed that both volume ratio of air pores, V_{a}/V_{0} (Fig. 8b) and porosity, V_{a}/V show the same drying mechanism as explained before. The maximum porosity varies from 24 to 46.72 % for different air temperatures ranging from 40 to 70 °C.

_{w}= 2.953 M. This means that the volume of water (in cm

^{3}) is equal to 2.953 times of moisture content of the moist object, irrespective of temperature. It is observed that there is no effect on the variations of volume of water with moisture content at different drying air temperatures. Using this relation V

_{w}= 2.953 M, it is possible to find the moisture content (kg/kg of db) or volume of water (cm

^{3}) if either one of them are known.

## 5 Conclusions

The design and development of an experimental set up was described to study the drying mechanisms of convective drying. This facility is designed to achieve a maximum velocity of 8 m/s and maximum temperature of 70 °C in the test section.

The initial moisture content of potato is observed to be 83 % in terms of mass and 90 % in terms of volume.

Two stages of drying viz. initial drying period and falling rate period are identified from the drying curve. No constant drying rate is found from experiments. In the early stage or first 2 h of drying, the density of moist object is almost constant and then it starts increasing gradually with decrease of moisture content.

The maximum density of potato is measured to be 1.340 g/cm

^{3}at 40 °C, showing a good agreement with the work of Wang and Brennan [3]. Density is seen to be decreased with the increase of drying air temperature at a given moisture content.The test facility was able to maintain a reasonably uniform upstream temperature. The 95 % bound was seen to be in a range of (±0.4 °C)–(±0.6 °C) about the mean for the four temperature considered. The %RH value at the upstream location was possible to maintain with a 95 % bound of ±1.6 to ±3.7 % for all the test cases considered.

Volume shrinkage is reduced linearly with decrease in moisture content. The sample loses volume from 74.13 to 81.56 % for different air drying temperatures of 40–70 °C, during the 16 h of drying time. The experimental data is correlated to find the sample volume at any moisture content.

The volume fraction of air (V

_{a}/V_{0}) and porosity (V_{a}/V) are increased with decreasing moisture content and increasing drying air temperature. It is found that the volume of water (in cm^{3}) in the moist potato is 2.953 times of its moisture content (in kg/kg of db).