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Heat pump dehumidifier drying technology - status, potential and prospects

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... These effects are reduced by drying at the lower temperatures presently used in heat pump dryers (Perré and Keey, 2006). In spite of these features, heat pump kilns have limited penetration in the New Zealand market, producing some 2.8% of the volume of dried lumber in 2001 (Bannister et al., 2002), which is similar to other countries with significant softwood processing industries. Alexiadis (2003) found that 8.8% of timber kilns in Canada were of the heat pump type. ...
... " One was worried about mould formation during drying, which is likely to be due to kiln overloading, essentially a management issue. These comments indicated the firms using heat pump kilns were not satisfied with the operating costs, the drying speed and the fact the control was different from vented kilns (see Bannister et al., 2002). They also felt that the heat pump suppliers did not provide enough support. ...
Article
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This study of industrial energy behaviours identifies barriers to the use of energy-efficient drying technology in the New Zealand timber industry, and explores these barriers through the “energy cultures” lens. Vented kiln dryers were preferred by larger firms and heat pump kiln dryers were used by smaller firms. Although few firms could specify all their costs, we found no significant differences in the average operating costs, drying costs or commercial success of the larger and smaller firms. We found that socio-technical barriers create “energy cultures” at the level of both the firm and the sector, supporting the dominance of vented kiln dryers. The prevailing technologies, practices and norms at the sector level strongly support vented kilns, the status quo being embedded in the socio-technical context, hindering technological learning, improved energy efficiency and innovation. Influential stakeholders in the industry were thus part of, and locked into, the industry-wide energy culture, and were not in a position to effect change.We conclude that actors external to the prevailing industry energy culture need to leverage change in the industry norms, practices and/or technologies in order to reap the advantages of energy-efficient drying technology, assist its continued evolution and avoid the risks of path dependency.
... In the re-circulating HPD, both sensible and latent heat can be recovered from dryer exhaust air, improving the overall thermal performance . There are a lot of studies in literature about heat pump assisted drying systems [1][2][3][4][5][6][7]. In the drying process, the moisture of the air must be regulated. ...
Article
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A re-circulating heat pump dryer (HPD) system was designed, constructed and tested at steady-state and transient conditions. Refrigerant 134a was used as a refrigerant in this system. The tests were performed to observe behavior of HPD system. So, changes of temperature and relative humidity of drying air through the dryer and heat pump operating temperatures were observed during the drying process and effects of by-pass air ratio (BAR) on the system's parameters as system performance and specific moisture extracted ratio (SMER) at steady-state were investigated. The HPD system was also tested to investigate temperatures and relative humidity changes of drying air during drying process on the system's parameters depend on time. Air flow rate circulated through the HPD system was 554 m3/h during the all tests. According to test results, the system's parameters did not change up to 40% of BAR. Then the coefficient at performance (COP) and SMER values were decreased after 40% of BAR. While SMER values changed between 1.2 and 1.4, COPsys changed between 2.8 and 3.3 depend on BAR. As well as during the drying process, the COP and SMER values were also affected and decreased depend on time.
... [4] Dehumidifier drying is a niche player in the softwood timber drying industry. [5] Compared with conventional vented dryers, many timber processing firms view dehumidifier dryers as expensive to run, unacceptably slow, and suitable only for small operations. [6] On the other hand, dehumidifiers have a number of advantages for drying high-quality timber, [7] their energy efficiency carries benefits, [8] and they can reduce atmospheric emissions. ...
Article
This article reviews the operational performance of a batch-mode dehumidifier timber dryer from a range of perspectives, covering a period of 10 years. The drying rate, energy efficiency, and dehumidifier operating environment are analyzed for an illustrative drying run and for a series of 111 runs. Factors affecting the energy consumption and the productivity of the dryer are assessed. The success of the equipment, the system design, and the methods used for control, air-flow reversal, and heat rejection are discussed. Recommendations arising from the authors' experiences with the dryer are presented.
... Heat pump dehumidifiers/dryers (HPDs) have been in widespread commercial use since the 1970s, particularly in the timber and food processing industries (Bannister et al., 2002). It has been found that the colour and aroma qualities of dried agricultural products using HPD were better than those when using conventional hot air dryer (Prasertsan and Saen-saby, 1998;Soponronnarit et al., 1998;Strommen, 1994). ...
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Response surface methodology (RSM) was employed to optimize the drying conditions of horse mackerel dried in a heat pump dehumidifier (HPD). The independent variables were drying air temperature (20–30°C), drying air velocity (1.5–2.5m/s) and sodium chloride (NaCl) content in the osmotic solution (5.0–15.0%), respectively. The coefficient of determination, R2 values for specific moisture evaporation ratio (SMER), drying rate (DR), total colour difference (ΔE) and total volatile base nitrogen (TVBN) were greater than 0.89. The combined effects of these variables on SMER, DR, ΔE and TVBN were investigated. The results showed that SMER, ΔE and TVBN varied curvilinearly with increasing of temperature, velocity and NaCl content in the osmotic solution and DR increased linearly. Based on response surface and desirability functions, the optimum conditions for horse mackerel drying by HPD were: drying air temperature 30°C, drying air velocity 1.5m/s and NaCl content in the osmotic solution 9.9%.
... Dehumidifier dryers have the potential to be more energy-efficient than conventional heat-and-vent drying processes, by recycling the latent heat of drying within the kiln. It is increasingly accepted that dehumidifier dryers can provide economic solutions for a number industrial drying processes (Bannister et al., 2002). Carrington et al. (2000) have established the importance of minimising the combined dehumidifier and airflow system losses in order to optimise the overall drying speed and energy efficiency. ...
Article
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We present a first order model of airflow within the wood stack of a timber dryer during the constant-rate drying period, allowing the exergy destruction and entropy generation rates to be determined for heat, mass and momentum transfer. The effects of varying timber stack and airflow variables on the entropy generation rate are determined. We find that heat transfer is the dominant contribution to entropy generation and observe a well defined minimum in the specific exergy loss rate as a function of relative humidity. Stack design parameters that minimise the overall exergy destruction rate are determined.
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Quality characteristics of Shiitake mushroom using two types of dryers, energy-efficient heat pump hot-air dryer and electric heater hot-air dryer were compared. Energy consumed during drying by heat pump hot-air dryer and electric heater hot-air dryer were 22.8 kWh and 28.9 kWh, respectively. Total polyphenol content of heat pump hot-air dryer and electric heater hot-air dryer after drying were 290.55?10.56ppm and 192.99?6.53ppm, respectively. No differences were observed between dryers in reconstitution rate and browning ratio after drying. Also, there were no differences between dryers in color value and {\Delta}E value after drying. Shiitake mushroom drying at 45^{\circ}C by heat pump hot-air dryer was proved to be more efficient in energy consumption than by electric heater hot-air dryer.
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When drying frozen materials using a heat pump dryer, frost is formed on the evaporator, resulting in the liquid flood back of the compressor. Therefore, in this paper, a numerical analysis was conducted of a heat pump dryer with attached solar collector and air collector in order to enhance the low temperature performance of the heat pump dryer. According to the analysis results, when the solar collector was used in the given conditions, the low temperature performance was improved through the increase of evaporating pressure by approximately 61 kPa. Also, the COP increased by a maximum of 1.5 compared to the standard cycle. Meanwhile, when using the air collector, the evaporating pressure increased by 209 kPa and the COP increased by a maximum of 5.5 over the standard cycle. Thus, it is found that the air collector is more effective than the solar collector in the considered conditions
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This book is primarily a general text covering the whole sweep of the forest industries. The over-riding emphasis is on a clear, simple interpretation of the underlying science, demonstrating how such principles apply to processing operations. The book starts by considering the broad question "what is wood? " by looking at the biology, chemistry and physics of wood structure (first 4 chapters). This sets the scene. Next key chapters examine "wood quality"- explaining how and why wood quality can be so variable and implications for processing. Finally, in a series of chapters, various "industrial processes" are reviewed and interpreted. The 2nd Edition is a total revision. A few chapters remain relatively unchanged (no change for the sake of change). Many have been totally rewritten. All chapters have been written by specialists, but the presentation targets a generalist audience.
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Heat pump dryers are used in various industries, but heat pump operation is impossible when the drying chamber has a low temperature. To facilitate operation of the heat pump, the drying chamber must be adequately heated. This study seeks to improve drying chamber efficiency by applying a heat recovery system to send waste heat to a second drying chamber. EES, a commercial software, was used to analyze the heat pump cycle for 12 tons of frozen red pepper based on a 60HP compressor. Data for dehumidification level and relative humidity was used to estimate the actual amount of heat recovery. In addition, numerical analysis was performed to determine the optimal capacity of the heat exchanger. The proposed heat recovery system succeeded in improving energy efficiency by reducing the dryer heating time by approximately 28.8%.
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Data obtained from laboratory measurements on a dehumidifier using R134a in a scroll compressor are analysed to establish empirical sub-models for the principal drier components. A model for the dehumidifier system is set up by coupling the sub-models using energy and mass balances. The performance data obtained using the system mdoel is in good agreement with the measured data obtained with four system configurations under a wide range of operating conditions. The model is used to demonstrate how the performance for the dehumidifier relates to that for the coupled kiln-dehumidifier system. By resizing the heat exchangers it is shown that the energy efficiency of the prototype system can be increased significantly, the dehumidifier specific moisture extraction rate approaching 10 kgkWh−1 at high humidity.
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The performance of a laboratory scale dehumidifier has been measured under controlled laboratory conditions, yielding a maximum dehumidifier specific moisture extraction rate (SMERd) of 7·94 kg kW−1 h−1 at 50°C and 85% relative humidity. Although we are not aware of any published data on dehumidifier systems having higher SMERd, there appears to be further scope for improvement, as the exergetic efficiency of the unit was in the region of only 4–12%. © 1997 by John Wiley & Sons, Ltd.
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The validation of a dehumidifier wood drying kiln model established previously has been conducted by using the performance data for a commercial scale kiln. The good agreement between the modelled and measured performance results shows that the model can be used for the design and analysis of dehumidifier wood drying kilns.
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A critical issue for the plantation forest industry is the preservation and control of genetic material in order to facilitate the fertilisation process. This is best achieved by drying at low temperature and low humidity prior to extraction of seeds and pollen. For example, freshly picked green cones of Pinus radiata are best dried at 25°C, 20% relative humidity to avoid thermal damage and to prevent mould formation. Similar conditions are required for drying pollen catkins. These conditions can be met efficiently and reliably by the use of a dehumidifier dryer. This paper describes the design of such a drying system, including a review of the dehumidifier and the kiln design parameters necessary to achieve the required drying conditions. Performance data and commercial operating experience are also presented and analysed. The system is effective and reliable, and has increased both the speed of drying and the quality of the product.
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The performance of batch-mode dehumidifier dryers undergoes significant changes during the drying process due to the range of temperature and humidity conditions experienced. Consequently steady-state methods are inapplicable for determining the performance of such kilns. In this paper a dynamic dehumidifier kiln model is used to estimate the drying speed, energy use and annual income for a batch-mode dehumidifier kiln. The model has been validated at both the dryer and dehumidifier levels, and the product is Pinus radiata, an easy-to-dry plantation soft-wood. The results show that control limits for the evaporating and condensing temperatures must be well matched to maximise the drying speed in the constant rate period. In the falling rate period, it is important not to place a lower limit the relative humidity unnecessarily. The use of supplementary heat to accelerate the kiln start-up is shown to have only a small influence on the kiln performance.
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A whole dryer model has been used to investigate the influence of the system design on the efficiency and productivity of a batch-type dehumidifier dryer. The product is an easy-to-dry timber, Pinus radiata. The model, which has been validated at both the dryer and dehumidifier levels, includes sub-models for the whole dryer energy balance, control of preheating, temperature and relative humidity, and the airflow system. The dynamic response of the system is illustrated and the influence of the dehumidifier capacity and the kiln airflow rate on the dryer performance is established. The effect of varying the airflow system losses is also determined. On the whole, drying speed and operating income increase with the dehumidifier capacity and the kiln airflow rate. The energy used by the dryer in a complete drying cycle is strongly influenced by the fan power requirements, and the airflow system losses have a significant adverse effect on the operating income. The results demonstrate the importance of balancing the dehumidifier and the airflow system losses in order to obtain an optimum combination of drying speed and energy efficiency. Copyright © 2000 John Wiley & Sons, Ltd.
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The performance of a dehumidifier dryer is influenced strongly by the operating temperature and humidity. This paper shows how heat conduction and air leakage losses can cause the temperature of a dehumidifier kiln to collapse in a batch drying process, resulting in increased drying time and energy use. By means of a dynamic simulation model it is shown that heat loss due to an uninsulated floor alone may be sufficient to prevent a kiln reaching its normal operating temperature. It is shown that the effect of heat losses is exacerbated when the dehumidifier capacity is modulated for humidity control. Auxiliary heating can prevent temperature collapse in a poorly insulated kiln. This maintains the drying speed but adds considerably to the energy used. To avoid these difficulties the insulation and air seals of a dehumidifier dryer should be appropriate to the power dissipated by the dehumidifier and fans. An example is presented in which sealing and insulating the kiln yields a reduction of 44 per cent in the drying time, a reduction of 32 per cent in energy use, and an increase of 168 per cent in net operating revenue. Copyright © 2000 John Wiley & Sons, Ltd.
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Heat pump dryer is a complex system because of the interaction of heat and mass transfer of the working fluids. Since the system cannot be completely close, ambient conditions (temperature and humidity) influence the performance of the system. To investigate the performance of the heat pump dryer thoroughly, simulation models of heat pump dryer components have been developed. The finite-difference method was employed in the simulation to examine the state of the working fluids and heat and mass transfer. The simulation of each component can be used to construct different system configurations the results of which are reported in Part 2.
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The addition of a heat pump to a conventional hot air drying system can enhance the efficiency of drying. To investigate the performance of heat pump assisted continuous drying system a detailed simulation model has been developed. The system modelled consists of a vapour compression heat pump coupled to a continuous cross-flow dryer. The model takes account of detailed heat and mass transfer phenomena taking place in each component of the system and can be used to investigate many different system configurations.
Guidelines for Operating Dehumidifier Timber Kilns
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Economic reduction of greenhouse gas emissions through enhanced dehumidifier timber drying
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Bannister, P., Chen, G., Grey, A., Carrington, C. G. and Sun, Z. F., 1997. Economic reduction of greenhouse gas emissions through enhanced dehumidifier timber drying. In Proceedings, 19th International Congress of Refrigeration, International Institute of Refrigeration, Linz, Austria, 241-249.
Development of the ECNZ electric heat pump dehumidifier drier pilot plant
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Barneveld, N. J., Bannister, P. and Carrington, C. G., 1996. Development of the ECNZ electric heat pump dehumidifier drier pilot plant. Proceedings the annual conference of the Institute of Professional Engineers New Zealand, Vol 2, Part 1, 68-72, Dunedin, NZ.
Dehumidifier Dryers for Hard-to-Dry Timbers
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Carrington, C. G., Sun, Z. F., Sun, Q., Bannister, P. and Chen, G., 1999. Dehumidifier Dryers for Hard-to-Dry Timbers, In: Refrigeration into the Third Millennium, 20th International Congress of Refrigeration, Sydney, Australia. International Institute of Refrigeration, 19-24 Sept 1999. Paper 548, 2718-2725.
Heat Pump Dehumidifier (HPD) Timber Drying Update
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Carrington, C. G., Sun, Z. F., Bannister, P., Chen, G., Lecamwasam, L., and Anderson, J. A., 2001. Heat Pump Dehumidifier (HPD) Timber Drying Update, Forest Research Wood Drying Seminar, Christchurch, New Zealand.
Modelling heat pump grain drying systems
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Charters W. W. W. S. and Aye, Lu, 1993, Modelling heat pump grain drying systems. Proceedings Australasian Heat and Mass Transfer Conference, University of Queensland, 1993, (31-1) -(31-6).
Economic performance of enhanced dehumidifier kilns
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Chen, G., Bannister, P., Carrington, C. G. and Sun, Z. F., 1997. Economic performance of enhanced dehumidifier kilns. Proceedings of the IPENZ Annual Conference 1997, Vol.2, 144-148, Institute of Professional Engineers New Zealand, Wellington, NZ.
A Survey on Dehumidifier Wood Drying Operation in New Zealand
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Chen, G., Bannister, P., Carrington, C. G., 2002b. A Survey on Dehumidifier Wood Drying Operation in New Zealand, Research Report, Energy Group Limited.
Design and applications of controlled-atmosphere dehumidifier fruit dryers, IPENZ Transactions
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Chen, G., McHuge, J., Bannister, P., Carrington, C. G. and Sun, Z. F., 2000. Design and applications of controlled-atmosphere dehumidifier fruit dryers, IPENZ Transactions, Institute of Professional Engineers New Zealand, Vol.27, No.1/Gen, 31-34.
Performance of a heat-pump assisted dryer
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Chou, S. K., Howlader, M. N. A., Ho, J. C., Wijeysundera, N. E., and Rajasekar, S., 1994. Performance of a heat-pump assisted dryer, Int. J. Energy Research, 18, 605-622.
Drying rate curves for dehumidifier drying of Pinus radiata boards
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