Energy Engineers, Part A: Journal of Power and

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Performance of a heat pump drier for copra drying M Mohanraj, P Chandrasekar and V V Sreenarayanan Proceedings of the Institution of Mechanical Engineers, Part A: Journal of Power and Energy 2008 222: 283 DOI: 10.1243/09576509JPE548 The online version of this article can be found at: http://pia.sagepub.com/content/222/3/283

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TECHNICAL NOTE

283

Performance of a heat pump drier for copra drying M Mohanraj∗ , P Chandrasekar, and V V Sreenarayanan Dr Mahalingam College of Engineering and Technology, Pollachi, Coimbatore, India The manuscript was received on 25 October 2007 and was accepted after revision for publication on 10 January 2008. DOI: 10.1243/09576509JPE548

Abstract: A heat pump drier is designed and fabricated to investigate its performance for copra drying. The experiments are carried out at 40 ◦ C drying air temperature with a velocity of 1.5 m/s. The results showed that moisture content (wet basis) of the coconut is reduced from 52.6 to 8.5 per cent in 48 h. The average coefficient of performance of the heat pump is estimated to be about 3.5. The specific moisture extraction rate is calculated to be about 0.85 kg/kW-h. The copra obtained is graded as 92.7 per cent milling copra grade 1 (MCG1) and 7.3 per cent MCG2 according to Bureau of Indian standards (BIS: 6220–1971). Keywords: heat pump drier, copra, drying

1

INTRODUCTION

characteristics, drying characteristics, and quality of copra obtained in heat pump drier.

Heat pump drying has been reported to be an energyefficient method for the dehydration of agricultural materials. Heat pump drying is highly energy-efficient due to its high coefficient of performance (COP) in the order of 3–4 and also has ability to work independent of ambient weather conditions unlike solar driers. Several researchers have reported the analytical and experimental studies on heat pump for drying applications [1–3]. In many such studies, the common conclusion was that the heat pump driers offer products of high quality with less energy consumption. When the quality of dried food products is paramount, heat pump drying offers an attractive option to enhance the product quality and reduces the spoilage through better regulation of the drying conditions. It has been reported that color and aroma of products using heat pump driers were better than products obtained from conventional drying methods [4, 5]. No significant work has been reported on use of heat pump driers for copra drying. Hence, the main objective of this research is to study the performance

∗ Corresponding

author: Dr Mahalingam College of Engineering

and Technology, Udumalai Road, Pollachi, Coimbatore, Tamil Nadu 642003, India. email: [email protected] JPE548 © IMechE 2008

2 2.1

MATERIALS AND METHODS Experimental set-up

The schematic and photographic view of the heat pump drier used for copra drying is shown in Figs 1 and 2, respectively. The drier consists of mainly two circuits: drying air and refrigerant. The drying air circuit consists of an air cooled condenser, 1 HP blower, control valve, and a drying chamber. The chamber is made up of mild steel sheet of 2 mm thickness with width, depth and height of 1000 × 600 × 1000 mm3 , respectively. The drier chamber containing nylon trays has about 90 per cent porosity to hold the drying materials and to expose them to the airflow. The airflow ducts and drier cabin are thermally insulated with thermocole of 25 mm thickness. The refrigeration circuit consists of hermetically sealed compressor of rated input power of 1020 W, condenser, receiver, sight glass, refrigerant drier, thermostatic expansion valve, accumulator, and an evaporator. The refrigerant pipes are thermally insulated with glass wool to reduce the losses in refrigeration pipelines. The heat pump is evacuated with the help of vacuum pump and a right quantity of R22 is charged in the circuit. Heat pumps using R22 can operate up Proc. IMechE Vol. 222 Part A: J. Power and Energy


284

M Mohanraj, P Chandrasekar, and V V Sreenarayanan

Fig. 1

Schematic view of experimental set-up

to 60 ◦ C condensing temperature. The ambient air is heated when it flows over the condenser coil, where heat is released by condensing refrigerant. The air at pre-set drying temperature enters the drying chamber and absorbs moisture from the coconuts. In a closed system, it is common that harmful micro-organisms grow and accumulate at the wetted evaporator and adjacent surfaces. The presence of these microorganisms in the recirculation processes is certainly undesirable [4]. Therefore, the air leaving the drying chamber is exhausted to the atmosphere. Eight calibrated thermocouples (Pt-100) with ±0.25 ◦ C accuracy are fixed at different locations to measure the temperature of the air in air circuit and in the refrigeration circuit through digital scanner, having 0.1 ◦ C resolution connected with a rotary selector switch. The relative humidity of the ambient air is calculated from the measured wet and dry bulb temperatures using two mercury thermometers of sensitivity 0.5 ◦ C, one covered with wetted cloth.

The relative humidity of the air at the entry and exit of the dryer cabin are measured by the same method by using four (Pt-100) thermocouples connected with digital scanner having 0.1 ◦ C resolution. Power input to the compressor, blower, and the fan are measured separately with the energy meter having ±0.25 per cent accuracy. Input voltage and current to the compressor are measured by digital voltmeter and ammeter, respectively. An U-tube manometer is connected across the orifice meter to the pipe connecting the blower and drying chamber. A digital electronic balance of 1 kg capacity having ±0.001 g accuracy is used to weigh the samples during the drying processes. Pressure at the inlet and outlet of the compressor are measured by compound gauges with an accuracy of ±0.25 per cent. The velocity of air at inlet of the tray is measured with the help of the vane type anemometer having ±0.01 m/s accuracy. 2.2

Fig. 2

Photographic view of experimental set-up

Experimental procedure

The broken coconuts are loaded over the tray of drying chamber. The air velocity at tray inlet is adjusted to 1.5 m/s by controlling the speed of the blower. The initial moisture content of the copra is measured from five different cups, selected at random. The drying air temperature at the drying chamber inlet is adjusted to 40 ◦ C. During the experiments, temperature at various locations in refrigeration and air circuit, power input to the compressor, fan and blower, pressure at suction and discharge of the compressor, velocity of air, moisture content of copra, wet bulb and dry bulb temperature at ambient, drier inlet and outlet are measured for every 1 h interval. The relative humidity of air is calculated from measured wet and dry bulb temperatures by using Psychometric chart. After attaining about 40 per cent moisture content, the copra kernels are scooped from the shells and dried further. Drying

Proc. IMechE Vol. 222 Part A: J. Power and Energy


JPE548 © IMechE 2008

Heat pump drier for copra drying

characteristics of copra such as moisture content, drying rate (DR), moisture ratio (MR), and specific moisture extraction rate (SMER) are determined by using equations (1) to (4). The performance of the heat pump is calculated by equation (5). 2.3

The quantity of moisture present in a material can be represented on wet basis and expressed in terms of percentage. About 10 g samples are chopped from five cups selected at random and kept in a convective electrical oven, which is maintained at 105 ± 1 ◦ C for 4 h [6]. The initial mass (Wt ) and final mass (Wd ) of the samples are recorded with the help of electronic balance. The moisture content on wet basis is calculated by equation (1). The procedure is repeated at every 1 h interval till the end of drying processes Wt − W d × 100 Wt

(1)

The DR should be proportional to the difference in moisture content between material to be dried and the equilibrium moisture content [7]. The concept of thin layer drying is assumed for the experiments as given by equation (2). Mathematically, it can be expressed as thin layer drying equation and the MR is calculated by using equation (3) dM = −k(Mt − Me ) dt Mt − Me MR = = e −kt M0 − M e

DR =

(2) (3)

SMER is the energy required for removing 1 kg of water content. It is calculated as [8] SMER =

md Pf + Pc + Pbl

(4)

The COP of the heat pump is defined as [8] COP =

thermal energy released by condenser electrical input to the compressor Table 1

(5)

Grading

Grading of copra is done at the end of drying according to the Bureau of Indian standards (BIS: 6220–1971) by selecting 100 cups at random (Table 1). 3

Data analysis

Mwb =

2.4

285

RESULTS AND DISCUSSION

The ambient temperature and relative humidity during the experiment is varied between 28 and 32 ◦ C, and 57 and 72 per cent, respectively. The variations of ambient relative humidity and temperature are shown in Fig. 3. A high relative humidity of 84 per cent at the drier outlet is recorded at the initial stage of drying and gradually reduced to 45 per cent during the later stages. The variation of moisture content (wet basis) with drying time is illustrated in Fig. 4. The moisture content of the coconut is reduced from 52.6 to 8.5 per cent in 48 h. The moisture reduction during the initial stages of drying is found to be high because of free moisture migration from the outer surface layers, and then decreases due to internal migration of moisture from inner layers to the surface. This results in a process of uniform dehydration of kernel. The rate of evaporation from the copra is high compared to sun drying due to its high heat, mass transfer coefficients, and forced circulation of hot dry air through the drier. The DR of copra against MR in a heat pump is shown in Fig. 5. The DR in the initial stages was very high and decreases during later stages. The DR at the initial stage of drying was about 1.72 kg water/kg dry matterhour. Drying process occurs in the falling rate period with a steep fall in moisture content in the initial stages of drying and becomes very slow in the later stages. The variation of COP is presented in Fig. 6, which shows higher COP values during daytime. The observed maximum and minimum COP is about 3.7 and 3.3, respectively. About 63 kg of moisture content is removed from 400 coconuts to produce about 78 kg of copra. The power consumed by compressor and the blower is found to be 74 kW-h. SMER of the heat pump drier is calculated as 0.85 kg/kW-h.

Grading of milling copra according to BIS: 6220-1971 Requirements

S. No

Characteristic

1 2 3 4 5 6 7 8

Impurities, percentage by weight (maximum) Mouldy cups, per cent by count (maximum) Black cups, per cent by cont (maximum) Wrinkled cups, per cent by count Chips, per cent by weight (maximum) Moisture content per cent by weigh (maximum) Oil content (on moisture free basis) per cent by weight (minimum) Acid value of extracted oil (maximum)


MCG1

MCG2

MCG3

0.5 4 5 5 5 6 70 2

1 8 10 10 10 6 68 4

2 10 15 15 15 6 66 10



286

M Mohanraj, P Chandrasekar, and V V Sreenarayanan

Fig. 3 Variation of relative humidity and ambient temperature Fig. 6

Coefficient of performance versus drying time Table 2

Trial I Trial II Trial III Average

Grading of copra

Grade I (%)

Grade II (%)

94 93 91 92.7

6 7 9 7.3

Table 2 shows the different quality of copra obtained in a heat pump drying. Three trials are taken and the copra is graded in each trial and then the average is calculated. The copra obtained is graded as 92.7 per cent milling copra grade 1 (MCG1) and 7.3 per cent MCG2.

Fig. 4

Moisture content versus drying time

4

CONCLUSION

A heat pump drier is designed, fabricated, and tested for copra drying. It is concluded that the quality of copra obtained in a heat pump drier is graded as 92.7 per cent MCG1 and 7.4 per cent MCG2. The drying period is considerably reduced in a heat pump drier. The SMER of the heat pump drier is estimated to be about 0.85 kg of moisture removed per kW-h with an average COP of 3.5. Heat pump drying is more suitable for large-scale copra processing, especially for obtaining good quality oil. REFERENCES

Fig. 5

Drying rate versus moisture ratio

1 Achariyaviriya, S., Soponronnarit, S., and Terdyothin, A. Mathematical model development and simulation of heat pump fruit dryer. Dry. Technol., 2000, 18, 479–491. 2 Alves-Filho, O., Strommen, I., and Thorbergsen, E. Simulation model for heat pump dryer plants for fruits and roots. Dry. Technol., 1997, 15, 1369–1398.




Heat pump drier for copra drying

3 Fatouh, M., Metwally, M. N., Helali A. B., and Shedid, M. H. Herbs drying using a heat pump dryer. Energy, Convers. Manage., 2006, 47, 2629–2643. 4 Prasertsan, S. and Saen-saby, P. Heat pump dryers: research and development needs and opportunities. Dry. Technol., 1998, 16, 235–250. 5 Strommen, I. and Kramer, K. New applications of heat pumps in drying processes. Dry. Technol., 1994, 49, 223–254. 6 Sreenarayanan,V.V.,Viswanathan, R., and Swaminathan, K. R. Studies on mechanical drying of copra. J. Food Sci. Tech., 1989, 16, 347–348. 7 Ei-Sebaii, A. A., Aboul-Enein, S., Ramadan, M. R. I., and El-Gohary, H. G. Empirical correlations for drying kinetics of some fruits and vegetables. Energy, 2002, 27, 845–859. 8 Hawlader, M. N. A. and Jahangeer, K. A. Solar heat pump drying and water heating in the tropics. Sol. Energy, 2006, 80, 492–499.


287

APPENDIX Notation md Me Mt Mwb M0 Pbl Pc Pf Td Tw Wt Wd

mass of water evaporated (kg) equilibrium moisture content (per cent) moisture content at time t moisture content (per cent) moisture content at time t = 0 blower power (kW-h) compressor power (kW-h) fan power (kW-h) dry bulb temperature (◦ C) wet bulb temperature (◦ C) weight of the sample at time t(g) weight of the dried sample (g)

