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Wood (timber and lumber), as solid, porous and hygroscopic material, is an interesting material for drying with heat pumps. In many countries, such as New Zealand, Canada, Poland, northern European countries, and the United States, wood drying, a complex and energy-intensive process, is one of the largest markets for heat pump dryers. The fundamental reason for drying wood (such as softwoods and hardwoods) is to provide useful products by enhancing their properties in order to minimize quality losses, conserve natural resources and, at the same time, make economic profits.

Among other advantages of dried wood (lumber), the following can be noted: (1) efficient utilization of recovered sensible and latent heat; (2) controlled drying rates result in drying low energy costs;

(3) equivalent or even reduced drying time; (4) enhanced sawmill productivity; (5) improved wood quality; (6) dried lumber at less than 20% MCs has no risk of developing stain, decay, or mold as a result of fungal activity; (7) dried lumber is typically more than twice as strong and nearly twice as stiff as wet wood; (8) dried lumber weighs 40 to 50% less than wet, undried lumber; (9) products

Steam

Steam

boiler 0

500 1,000 1,500 2,000 2,500 3,000 3,500 4,000

0 10 20 30 40 50 60

Pressure (kPa,r)

Time (h) Compressor pressures Steam supply with large on/off SV

Suction Discharge Large

on/off SV

(a) (b)

MSV

FIGURE 1.17 (a) Backup steam boiler and distribution header with large size solenoid valve (SV) for steam supply during the preheating step and small size modulating SV (MSV) for continuous steam supply during the heat pump running. (b) Abnormal compressor running profile with intermittent backup steam supply using the large on/off steam valve SV. (From Minea, V., High-temperature heat pump-assisted softwood dryer: Sizing and control requirements & energy performances, in 24th International Congress of Refrigeration (ICR2015), August 16–22, Yokohama, Japan, 2015; reprinted with permission from International Refrigeration Institute, IIR/IIF, Paris, France.)

in service made from properly dried lumber will shrink very little; and (10) gluing, machining, and finishing are much easier to accomplish with dried wood.

Depending on the wood species to dry, heat pump-assisted wood dryers can operate at (1) low temperatures (i.e., at temperatures below 54°C), often loaded with lumber of mixed MCs; (2) medium to high-temperatures (i.e., at temperatures between 82°C and 93°C); and (3) very high-temperatures (i.e., at temperatures up to 110°C), mostly used for drying softwood construction lumber; in this last case, the capital investment per unit capacity is higher, but drying times and energy consumption may be reduced by up to 25% and 50%, respectively, compared to conventional air convective dryers. For medium, high, and very high temperature dryers, steam is the most com-mon source of heat supplied by oil-burned or electrical boilers. Most existing heat pump-assisted wood dryers contain mechanical vapor compression heat pumps operating upon the subcritical cycle (see Section 1.2). Even they can provide efficient and cost-effective drying of low-, medium-, or high-grade wood species, relatively few R&D works and/or experimental research/demonstration studies and/or new industrial applications/implementations were reported during the past 15 years (Minea 2014a, b). On the other hand, in the past, the reliability and energy performance of heat pump-assisted wood dryers using fossil fuels as backup energy (e.g., bark, natural gas, propane, or oil) were often disappointing, because of, among other factors, relatively inadequate heat pump sizing and control strategies.

Carrington et al. (1995) analyzed the operating characteristics of a medium-temperature heat pump-assisted drying system using HFC-134a as a refrigerant with passive evaporator-economized and staged liquid subcooling. The authors reported that the heat pump maximum dehumidification perfor-mance (SMER) could reach 5 11. kgwater/kg at 50°C dry-bulb temperature and 90% relative humidity.

Bannister et al. (1998, 1999) concluded that the energy efficiency of a dehumidifier kiln should be greater than an equivalent air vented kiln drier by a factor of the order of two. Heat losses of the drying chamber and performances of a heat pump dehumidifier with HFC-134a as the refrigerant and a 5-kW compressor have been evaluated for a timber dryer working at dry-bulb temperatures up to 60°C (Bannister et al. 1998). The reported results showed that heat losses due to heat conduction and uncontrolled air leaks accounted for approximately 90% of the total energy input to the kiln.

Because of these heat losses, the drying process mean temperature (around 43°C) was much lower compared to the designed operating temperature (60°C), adversely affecting both the productivity and running cost of the kiln. In their review, Chua et al. (2002) reproduced the schematic diagram of the heat pump-assisted dryer used by Bannister et al. (1999) for drying timber. Even though it did not show any means of controlling the heat pump dehumidification rate, as well as the temperature and relative humidity of the supplied drying air, it included an auxiliary heater, which is absent in many other heat pump-assisted drying schematics.

Relatively few new applications of heat pump-assisted drying systems have been reported dur-ing the past 10 years. Among them, there are some laboratory- and industrial-scale heat pump-assisted kiln (timber) drying systems. In a 13m laboratory-scale hardwood heat pump-assisted ³ drying system, the drying process of deciduous trees, such as hard maple, yellow and white birch, oak (hardwoods), with a 5 6. kW low-temperature electrically driven subcritical mechanical vapor compression heat pump has been extensively studied (Figure 1.18a) (Minea 2015). Electricity and natural gas (steam) have been alternatively used as auxiliary and backup energy sources. The heat pump (compressor and blower) electrical energy consumption varied between 25% and 30% of the total equivalent energy consumption of each drying cycle. The dryer central fan represented 8%–9%

and the electrical (or fossil) auxiliary/backup energy, between 62% and 66% of the total equivalent drying energy consumption. For initial MCs above 41%, the total water quantities extracted above the fiber saturation point (FSP = 25%) were up to 2.9 times higher than those removed below FSP.

Consequently, in these cases, the heat pump dehumidification efficiency (SMERheat pump)was up to three  times higher above the FSP than below this value. The heat pump-assisted hybrid drying cycles reduced natural gas consumption by 56% and the equivalent energy costs by 21.5%, com-pared to conventional drying cycles with natural gas as unique heating energy source (Minea 2012,

2014a, b, 2015). Another 335  m³ batch industrial-scale, air-forced convective, high-temperature dryer for drying coniferous (resinous) lumber (e.g., white spruce, balsam fir, also known as soft-woods) (Figure 1.18b) was modified and coupled with two 65 kW (compressor nominal shaft power input) split-type electrically driven subcritical mechanical vapor compression heat pumps using HCF-245fa as the refrigerant (Minea 2004, 2012, 2015). The compressors, evaporators, variable speed blowers, and controls were installed inside an adjacent mechanical room, whereas the remote condensers are located inside the kiln along with the dryer fans. The dryer fan rotation changed direction every 3 h, at the beginning, and every 2 h toward the end of the drying cycles in order to achieve uniform drying conditions through the softwood stacks and efficient moisture removal.

Three air vents opened when the dryer fan rotation changes direction and when the actual air dry-bulb temperature exceeded the set point. An oil-burned boiler supplied high-pressure saturated steam to the dryer’s heating coils in order to preheat the softwood stacks and, when required, to provide supplemental (backup) heat. The average SMERheat pumpof this hybrid system varied from 2 35. kgwater/kWh (for white spruce) to 1 5. kgwater/kWh (for balsam fir), whereas the average COPheating

varied from 3.0 (at the end) up to 4.6 (at the beginning of drying cycles). The cycle duration ranged from 2.5 days (for white spruce) to 6.3 days (for balsam fir), including the initial preheating steps.

The feasibility of a mechanical vapor compression heat pump has been studied for a timber drying process of a prefabricated house manufacture in Germany (Figure 1.19). In this facility, the timber used for prefabricated houses has to be dried up to final average MCs of 15% (dry basis) to avoid cracks. The main heat source for the facility is provided by 8.2  MW biomass power plant using residual wood. About 5 MW of the wasted thermal power by the extraction–condensation turbine are used to cover the factory’s heating demand (e.g., wood presses, drying chambers, and space heating) via a heat extraction heat exchanger. An air-cooled condenser condenses the remaining steam of the biomass power plant. Instead, installing one additional oil-fired boiler to cover the increasing heat demand for wood drying, especially in the winter, a more environmental-friendly and cost-effective solution, that is, a heat pump system, was proposed. In this concept, the heat pump recovers heat from the power plant condensate at around 55°C and supplies 180 kW of thermal power (i.e., about 73%

(b)

FIGURE 1.18 (a) Experimental setup of a low-temperature heat pump-assisted hardwood dryer with com-pact-type heat pump. (b) Configuration of a heat pump-assisted high-temperature softwood dryer with split-type heat pump. CD, heat pump remote condenser; MSV, modulating steam valve; SV, on/off steam valve.

(From Minea, V., High-temperature heat pump-assisted softwood dryer: Sizing and control requirements &

energy performances, in 24th International Congress of Refrigeration, August 16–22, Yokohama, Japan, 2015; reproduced with permission from International Refrigeration Institute, IIR/IIF, Paris, France.)

of the total heating power demand) to the drying chambers at temperatures varying between 65°C and 90°C. Higher temperatures are needed only during the preheating drying steps. Payback periods of 4–5.5 years have been estimated. During the drying process, the air temperatures are increased stepwise from 50°C to 90°C, and the temperature is held constant several days (IEA 2015b).

1.3 SUPERCRITICAL MECHANICAL VAPOR COMPRESSION HEAT PUMPS