No full-text available
To read the full-text of this research,
you can request a copy directly from the authors.
Optimum air drying strategy for dehumidifier timber dryers
Drying Technology
Abstract
This paper presents an economic model for analyzing the cost behavior of dehumidifier timber drying process. Various costs, including those of air drying, are identified and quantified. Reasonable assumptions are made to simplify the calculation process. For species of both softwoods and hardwoods, the optimum air drying strategies are investigated based on two scenarios of existing commercial dehumifier dryers and well-run high performance dehumidifier dryers. This paper finds that when the timber maximum air drying degrade rate is less than 5%, air drying adds significant value to the operation of dehumidifier dryers. This benefit accelerates with the extent of air drying. With the use of high performance dehumidifier dryers, the importance of air drying tends to decrease. The cost of air drying is very sensitive to both the assumed interest rate and timber maximum air drying degrade rate.
... The formation of ash deposition is a dynamic process varying with time. In practice drying cases, appropriate control strategies are necessary to optimize the operation of dryers and to achieve optimum throughput, based on the studies by Vasconcelos, [40] Bannister, [41] Unadi, [42] and Correa. [43] In waste heat Subscripts f inlet of mainstream recovery heat exchangers, periodic sweeping measures are adopted to remove ash deposition for ash fouls heat transfer surfaces. ...
In this study, air/water vapor mixture with ash particles was used to simulate actual dryer exhaust gas. An experimental study of the convection–condensation heat transfer characteristics of air/vapor mixtures with ash particles across horizontal tube-bundles was conducted under various water vapor mass fractions and concentrations of ash particles. The variation of convection–condensation heat transfer coefficient with time presented four different types, as follows: almost did not change with time; decreased first, then reached stable values; decreased continuously in the experimental time; and decreased rapidly, decreased slowly, and decreased rapidly, with potential to increase again. In general, the convection–condensation heat transfer coefficient decreased with increasing concentration of ash particles and increased with increasing water vapor mass fraction. The reducing effect on the convection–condensation heat transfer coefficient by ash deposition would be weakened by increasing the water vapor mass fraction, but the heat exchanger would be blocked further easily by increasing the concentration of ash particles. Then, a distribution diagram was drawn to present different types of convection–condensation heat transfer curves under various working parameters. Sootblowing operation strategies were proposed under different working conditions to provide reference for the actual operation of the waste heat recovery heat exchanger for dryer exhaust.
... [9][10][11] To support better dehumidifier design and control in the New Zealand timber industry, the authors previously developed process models for the timber, the dryer chamber, the air-flow system, and the heat pump. [12][13][14][15] The aim was to improve the application of heat pump timber drying through improved system integration and active process control, an important goal, as highlighted by Minea. [16,17] As part of this work, the authors had key roles in a firm that supplied a dehumidifier dryer to a timber processing company in Whangarei, NZ, in 2003. ...
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.
... Chen et al. [1] developed an economic model for evaluating the role of air drying in the overall timber air-=dehumidification drying process for both softwood and hardwood species. The results indicated that air drying added a significant value to the operation of all dehumidifier kilns. ...
To monitor the air-drying time of dimensional lumber, load beams equipped with a data logger were used to collect lumber weight loss data. A weather station on the same site as the air-drying experiment was used to obtain the information regarding air temperature and relative humidity and wind velocity. A multiple regression model for estimating the drying rates for 2-in. × 4-in. × 8-ft (38.5-mm × 90-mm × 250-cm) green spruce/pine dimensional lumber was developed. Data analysis revealed that wind direction did not have a significant effect on the rate of moisture loss. Thus, the resulting regression equation takes temperature, relative humidity, initial moisture content, and wind speed into account. Based on historical weather information for a particular location, the model developed can be used as a practical guideline for estimating air drying time and therefore offers the user a valuable tool for planning green yard inventory.
The worldwide use of biomass has increased drastically during the last decade. At Swedish sawmills about half of the entering timber becomes lumber, with the remainder considered as by-product (biomass). A significant part of this biomass is used for internal heat production, mainly for forced drying of lumber in drying kilns. Large heat losses in kilns arise due to difficulties in recovering evaporative heat in moist air at low temperatures. This paper addresses the impact of available state-of-the-art technologies of heat recycling on the most common drying schemes used in Swedish sawmills. Simulations of different technologies were performed on an hourly basis to compare the heat and electricity demand with the different technologies. This was executed for a total sawmill and finally to the national level to assess the potential effects upon energy efficiency and biomass consumption. Since some techniques produce a surplus of heat the comparison has to include the whole sawmill. The impact on a national level shows the potential of the different investigated techniques. The results show that if air heat exchangers were introduced across all sawmills in Sweden, the heat demand would decrease by 0.3 TWh/year. The mechanical heat pump technology would decrease the heat demand by 5.6 TWh/year and would also produce a surplus for external heat sinks, though electricity demand would increase by 1 TWh/year. The open absorption system decreases the heat demand by 3.4 TWh/year on a national level, though at the same time there is a moderate increase in electricity demand of 0.05 TWh/year. Introducing actual energy prices in Sweden gives an annual profit (investment cost excluded) on national level for the open absorption system of almost 580 million SEK. For the mechanical heat pump technology the profit is 204 million SEK and for the traditional heat exchanger the profit is significant lower. It has been found that a widespread implementation of available energy recovery technologies across Swedish sawmills would result in substantial savings of biomass for other purposes in the society.
This work compares a conventional continuous drying schedule with a solar cyclic drying schedule for the seasoning of an Australian hardwood timber, Eucalyptus grandis, focusing on the simulated stresses and strains developed during drying as a measure of timber quality. The cyclic drying schedule has been found to give lower instantaneous strains, due to the effect of mechanosorptive strains in relieving stresses both in the initial stages of drying and over the entire drying period. The gentler initial drying conditions during cyclic drying are also beneficial compared to the harsher and unmodulated nature of conventional drying schedules. Without the modulation of the external drying conditions in intermittent or cyclic drying, the mechanosorptive strains are unable to relax or mitigate the stresses that are caused naturally by timber drying. There is some support for these conclusions by comparison with industrial experience and previous laboratory practice for intermittent and cyclic drying.
The use of biomass in the European Union has increased since the middle of the 1990s, mostly because of high subsidies and CO2 emission regulation through the Kyoto protocol. The sawmills are huge biomass suppliers to the market; out of the Swedish annual lumber production of 16.4Mm3, 95% is produced by medium to large-volume sawmills with a lumber quotient of 47%. The remaining part is produced as biomass. An essential part (12%) of the entering timber is used for supply of heat in their production processes, mostly in the substantial drying process. The drying process is the most time and heat consuming process in the sawmill. This study was undertaken to determine the sawmills’ national use of energy and potential magnitude of improvements. If the drying process can be made more effective, sawmills’ own use of biomass can be decreased and allow a considerably larger supply to the biomass market through processed or unprocessed biomass, heat or electricity production. The national electricity and heat usage when drying the lumber have been analysed by theoretical evaluation and experimental validation at a batch kiln. The main conclusion is that the heat consumption for drying lumber among the Swedish sawmills is 4.9TWh/year, and with available state-of-the-art techniques it is possible to decrease the national heat consumption by approximately 2.9TWh. This additional amount of energy corresponds to the market’s desire for larger energy supply.
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.
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.
Lumber Drying: Today and Tomorrow
- G Wengert
- J Denig
Wengert, G.; Denig, J. Lumber Drying: Today and Tomorrow. For. Prod. J. 1995, 45 (5), 22–30.
Modelling of FEC Wood Drying Kiln
- Q Sun
- C G Carrington
- Z Sun
- P Bannister
- G Chen
Sun, Q.; Carrington, C.G.; Sun, Z., Bannister, P.; Chen, G. Modelling of FEC
Wood Drying Kiln, Research Report; Department of Physics, University of
Otago: Dunedin, New Zealand, 1998.
Dehumidifier Dryers for Hard-to-Dry Timbers
- C G Carrington
- Z Sun
- P Bannister
- G Chen
Carrington, C.G.; Sun, Z.; Bannister, P.; Chen, G. Dehumidifier Dryers for
Hard-to-Dry Timbers. In Refrigeration into the Third Millennium, Conference Proceedings, 20th International Congress of Refrigeration, International
Institute of Refrigeration, Sydney, Australia, Sept. 19-24, 1999.
Heat Pump Dehumidifier Timber Drying: Further FEC Trials, Energy Group Limited Research Report EGL-RR-03
- M Dakin
Dakin, M. Heat Pump Dehumidifier Timber Drying: Further FEC Trials, Energy Group Limited Research Report EGL-RR-03, 1997.
Economic Reduction of Greenhouse Gas Emissions through Enhanced Dehumidifier Timber Drying
- P Bannister
- G Chen
- A Grey
- C G Carrington
- Z Sun
Bannister, P.; Chen, G.; Grey, A.; Carrington, C.G.; Sun, Z. Economic Reduction of Greenhouse Gas Emissions through Enhanced Dehumidifier Timber Drying. In Heat Pump Systems, Energy Efficiency and Global Warming,
Conference Proceedings, 19th International Congress of Refrigeration, International Institute of Refrigeration, Linz, Austria, 1997; 241-249.
Economic Performance of Enhanced Dehumidifier Kilns
- G Chen
- P Bannister
- C G Carrington
- Z Sun
Chen, G.; Bannister, P.; Carrington, C.G.; Sun, Z. Economic Performance
of Enhanced Dehumidifier Kilns. In Proceedings of the IPENZ Annual
126
CHEN, BANNISTER, AND CARRINGTON
Conference 1997; Institute of Professional Engineers: Wellington, New
Zealand, 1997; Vol. 2, 144-148.
- G Wengert
- J Lumber Denig
- Drying
Wengert, G.; Denig, J. Lumber Drying: Today and Tomorrow. For. Prod. J.
1995, 45 (5), 22-30.