GB2263968A

GB2263968A - Drying timber

Info

Publication number: GB2263968A
Application number: GB9225280A
Authority: GB
Inventors: Rodolfo Jorge Neumann
Original assignee: Individual
Current assignee: Individual
Priority date: 1991-12-02
Filing date: 1992-12-02
Publication date: 1993-08-11
Also published as: DE4228698C2; DE4228699C2; DE4228699A1; GB9225280D0; DE4228698A1

Description

2263968 1 Condensation Dryin This invention relates to methods and systems

for condensation drying, and is particularly, but not exclusively, concerned with methods and systems for condensation drying of cut timber.

Condensation drying of cut timber is currently carried out within a tank or kiln using heated air or super- heated steam circulated through the timber. The moisture released by the timber is carried away in the circulating drying medium (that is to say in this case, the air or the steam) and is condensed out from it by cooling the moisture-laden drying medium on the evaporator of a heat pump located within the tank or kiln. The dehumidified drying medium from the evaporator is then passed via the condenser of the heat pump to be re- heated before recirculation, using the heat given up at the evaporator.

Although methods and systems operating in the above way have been used successfully, the energy consumption is high and drying takes a long time, and it is an object of the present invention to provide a condensationdrying method and system of improved efficiency compared with the known methods and systems.

According to one aspect of the present invention there is provided a condensation-drying method in which only part of the body of circulating drying medium is cooled by a heat-pump evaporator.

According to another aspect of the invention there is provided a system for condensation drying wherein an evaporator of a heat pump of the system is arranged to cool only part of the body of drying medium circulated within the system.

2 It may be arranged that only a small part of the drying medium flows over the evaporator in the method and system of the invention, whereas all, or at least a larger part, of the main stream flows over one or more condensers of the heat pump. The part of the drying medium that flows over the evaporator may be less than 40%, in particular less than 20%, and preferably less than 10%, of the total mass flow circulating over the material to be dried. In these circumstances, over 70%, in particular more than 90%, of the mass flow circulating over the material to be dried, may then flow over the one or more condensers.

The method and system of the present invention enable significant reductions in energy consumption and drying time to be achieved, and higher drying-medium temperature. The method and system are especially advantageous where the drying medium used is super-heated steam. Condensation drying with super-heated steam is particularly economical, because the heat taken up by the evaporator is mainly latent heat from the condensate produced. Also, very good heat transfer with phase commutation is achieved either side of the evaporator, which reduces super-cooling of the refrigerating agent used in the heat pump.

Methods and systems for condensation drying, all in accordance with the present invention, will now be described, by way of example, with reference to the accompanying drawings, in which:

Figure 1 is a sectional side elevation of a first condensation-drying system in accordance with the present invention; Figure 2 is a sectional end view of a second form of condensation-drying system in accordance with the present invention; 3 Figure 3 is a sectional plan of a third form of condensation-drying system in accordance with the present invention; Figure 4 is a psychrometric chart illustrating operation of the system of Figure 3; Figure 5 is a modified form of the condensation-drying system of Figure 3; and Figure 6 is a further form of condensation-drying system according to the present invention.

The condensation drying system shown in Figure 1 is a longitudinallyventilated vacuum drier, and will now be described.

Referring to Figure 1, an evaporator 1 and condenser 2 of a heat pump of the condensation-drying system in this case, are located within an elongate, closed pressure chamber or tank 3; the compressor (not shown) of the heat pump is located outside the tank 3. The tank 3 which is supplied with super-heated steam as the drying medium, also contains an auxiliary dehumidifier 4 and a fan 5. The fan 5 is mounted adjacent to the condenser 2 for circulating the steam within the tank 3.

A trolley 6 entered into the tank 3, carries stacked planks 7 of timber to be dried, and the main stream of steam circulated through the planks 7 is drawn by the fan 5 through the condenser 2 to be re-heated. A small proportion of the steam, perhaps as little as 10%, or less, of the total body of the drying medium, however, passes through the evaporator 1 located towards the bottom of the tank 3 beneath the condenser 2. This part of the circulating steam is cooled by passage over the evaporator 1 so that moisture that was taken up from the 4 planks 7, condenses out from it into a reservoir 8 at the bottom of the tank 3. The cooled steam returns from below to the circulation path through the stack of planks 7.; but because it is only a small proportion of the main stream of drying medium, it does not have any material effect on the overall drying conditions of that stream.

is The temperature of the circulating stream of steam may be regulated by operating the auxiliary dehumidifier 4 which is located alongside the evaporator 1 and, like it, drains into the reservoir 8. The dehumidifier 4 may in this respect be operated to augment the proportion of steam cooled, so that in conjunction with what passes over the evaporator 1, an increased or reduced amount of cooled steam is returned to join the main stream in the circulation path. The heat taken up from the steam by the dehumidifier 4 is released outside the tank 3, whereas that taken up by the evaporator 1 is used to re heat the main stream of steam in the condenser 2.

Corrosion of the inner wall of the tank 3 may be reduced by using the wall to convey heat to the drying medium. It may be possible in this respect to incorporate the condenser of the heat pump in the wall itself or to use the condenser to heat a fluid that flows within the wall.

The circulation of the drying medium in the drier of Figure 1 is generally lengthwise of the chamber or tank 3. A form of vacuum drier in accordance with the invention, in which the circulation is generally transverse of the chamber, is shown in Figure 2 and will now be described (the same references as used in Figure 1, are used in Figure 2 to refer to corresponding parts).

Referring to Figure 2, the condenser 2 in this case is located above the stack of planks 7. The evaporator 1 and auxiliary dehumidifier 4 are located out of the main circulation stream, below the trolley 6 and on opposite sides of the tank 3 from one another above the reservoir 8 for the condensate water.

A condensation-drying system in which the same essential principles as utilised in the systems of Figures 1 and 2, are used, but where the circulation path for the drying medium extends both longitudinally and transversely of the drying chamber, is illustrated in Figure 3 (the same references as used in Figures 1 and 2, are used in Figure 3 to refer to corresponding parts).

Referring to Figure 3, the evaporator 1 and condenser 2 of the heat pump, and the fan 5, are all located to one side of the stack of planks 7 within the tank 3. The drying medium, which in this case is air, is constrained by flexible baffles or deflectors 9, to follow a circuitous path extending horizontally through and along the stack.

The fan 5 draws the air exiting from the stack into a side passageway 10 to be passed over the condenser 2 for re-heating before re-entry to the stack. However, a small proportion of the air from the fan 5 enters a bypass 11 to flow over the evaporator 1 so as to be cooled by it and condense out the moisture taken up in the stack. This cooled, by-passed air returns to the passageway 10 to mix with the main stream drawn through the fan 5.

The advantage of restricting only a small proportion of the drying medium to flow over the evaporator 1, is illustrated in the psychrometric chart of Figure 4. In this respect, Figure 4 depicts air conditions in the system of Figure 3 as well as in a known condensationdrying system working with the same air temperature of 750C after the condenser 2, with the same psychrometric 6 depression of 25'C, and with the same temperature drop of 30C through the timber stack. The chart demonstrates that for these drying conditions and a similar condensing temperature (approximately 80OC) of the refrigerant used in the heat pump, the air temperature at entry to the stack with the system of the invention, can be as high as 750C with a wet-bulb temperature of 5TC, while with the known form of system, the dry-bulb temperature cannot be higher than 600C and the wet-bulb temperature, higher than 350C. Similar advantages are realised with the systems of Figures 1 and 2.

is In the chart of Figure 4, "1" and 11011 represent the air conditions at entry to, and exit from, respectively, the timber stack, "Ell represents the air condition at the outlet of the evaporator 1, and I'M" the condition at an air-mixing point. The conditions 1, 0, E and M applicable to the system of Figure 3 are identified in the chart of Figure 4, and also in Figure 3 itself, with the subscript 111"; the corresponding conditions applicable to the known system are shown on the chart with the subscript 112".

In the system of Figure 3, and as charted in Figure 4, air enters the stack at I, and leaves at 01, and a small fraction of this air, after passing through the fan 5, is cooled down at the evaporator 1 to condition E,. This small f raction of air at E, is then mixed back into the main stream to give condition M,, bef ore the whole stream is heated in the condenser 2 to condition I, f or entry to the stack.

In the known system, circulated air that is cooled down to a condition E2 in the evaporator of the heat pump, is also heated in the condenser to a condition represented in Figure 4 as C2. After that the dehumidified and heated 7 air is mixed with untreated air to give the condition M2, which is also the condition E2 f or entry to the stack.

Condensing temperature is a limiting factor, and for the same condensing temperature, the system of the present invention operates with a higher temperature rating. With the system of the invention, the difference between the condensing and evaporation temperatures is smaller, in this example, some (750C - 430C) i.e. 320C, instead of (750C - 240C) i.e. 510C. This improves significantly the coefficient of performance of the heat pump. Also, the fraction of humid air that is cooled by the evaporator in the system of the invention is much smaller (5% compared with about 30%) than in the known system, so less air will have to be heated, thus reducing the load on the compressor. Because temperatures are higher with the system of the invention, there is furthermore, the advantage that drying times are shorter, and the compressor can be smaller for the same drying capacity.

With the known form of system, the requirement to cool the air down to condition E2 W"' in general make it necessary for the compressor of the heat pump to bring more heat into the system than allowed because of the low coefficient of performance experienced. Thus, the condition C2 would be higher than 75'C and the limiting condenser temperature would have to rise. In order to avoid this, a second condenser would be used to lose heat to the outside atmosphere, or the insulation of the tank or kiln would be reduced to lose heat that way.

In the system of the present invention, control is enhanced in that the temperature of the circulating drying medium can be regulated from outside and its humidity can be regulated through the compressor, for example by lifting inlet valves (a standard features of certain compressors). With this form of control, heat 8 can be more readily conserved and energy consumption by the compressor reduced significantly.

A-modified form of the system of Figure 3, will now be described with reference to Figure 5.

Referring to Figure 5, the by-passed air in this case is mixed with the main stream of air after the main stream has passed over the condenser 2. The condenser 2 is, in particular, located in advance of the fan 5 in the passageway 10, and also extends out of the passageway 11 to have a part 12 located at an intermediate position in the drying path through the timber stack, so as to maintain the drying temperature in the stack. Furthermore, a fan 13 is positioned in the by-pass 11 to force the by-passed air over the evaporator 1.

The system of Figure 5 can be operated to have a reversed direction of air flow. To this end, the direction of the fan 5 is reversed and the fan 13 switched off. This causes a damper 14 to open so that part of the air stream is circulated by the main fan 5 over the refrigeration evaporator 1. The system then operates in a manner similar to that of Figure 3.

Air dampers 15 and 16 are located, respectively, in front of and behind the evaporator 1. The damper 15 operates to vent moist air to the atmosphere outside the tank 3, whereas the damper 16 operates to admit dry air from outside. The selective venting and admission of air enables the air temperature within the tank 3 to be regulated.

Figure 6 shows a high-temperature drier in which the drying-medium flow is transverse of the timber stack, and in which the circulating water vapour is heated by one or more refrigeration condenser disposed above the stack.

9 Referring to Figure 6, a representative condenser 22 is located above the stack of timber, and the drying medium is driven by a fan 25 over the condenser 22 to be heated. The evaporator 21 of the heat pump is located outside the drying chamber 23 in a separate water-collecting chamber 28 and receives only a small proportion of the drying medium circulated in the main chamber 23. If more water evaporates from the timber than condenses on the refrigeration evaporator 21, the excess can flow out of the system through a ventilation damper 29, so that only a limited increase in pressure can occur within the system. The ventilation damper 29 also protects against the occurrence of a dangerous pressure reduction during the cooling phase of the kiln, by allowing air to flow is into the system from outside.

As illustrated in Figure 6, an additional refrigeration evaporator 30 is installed in the lower part of the chamber 28. This is used to recover heat from the collected condensate for heating the drying medium for a new batch of timber loaded in the chamber 23.

claims:

1._ A method of condensation drying wherein only part of the body of circulating drying medium is cooled by a heat-pump evaporator.

2. A method of condensation drying wherein only a small part of the drying medium is caused to flow over the evaporator of a heat pump, whereas all, or at least a larger part, of the main stream is caused to flow over one or more condensers of the heat pump.

Claims

3. A method according to Claim 2 wherein the part of the drying medium

that flows over the evaporator is less than 40%, in particular less than 20%, and preferably less than lo-oo, of the total mass flow circulating over the material to be dried, and wherein over 70%, in particular more than 90%, of the mass flow circulating over the material to be dried, flows over the one or more condensers.

4. A method according to any one of Claims 1 to 3 wherein the humidity of the drying medium is regulated via the compressor of the heat pump.

5. A method according to any one of Claims 1 to 4 wherein drying-inedium temperature is regulated by means of dampers for venting drying medium from, and admitting comparatively-dry drying medium to, the drying chamber.

6. A method according to any one of Claims 1 to 5 wherein drying-medium temperature is regulated by means of an auxiliary dehumidifier that releases the heat taken up, outside the drying chamber.

11 7. A method according to any one of Claims 1 to 6 wherein the material dried is timber.

8. A system for condensation drying wherein an evaporator of a heat pump of the system is arranged to cool only part of the body of drying medium circulated within the system.

9. A system for condensation drying wherein it is arranged that only a small part of the drying medium circulated within the system is caused to flow over the evaporator of a heat pump, whereas all, or at least a larger part, of the main stream is caused to flow over one or more condensers of the heat pump.

10. A system according to Claim 9 wherein it is arranged that the part of the drying medium that flows over the evaporator is less than 40%, in particular less than 20%, and preferably less than 10%, of the total mass flow circulating over the material to be dried, and wherein over 70%, in particular more than 90%, of the mass flow circulating over the material to be dried, flows over the one or more condensers.

11. A system according to Claim 9 or Claim 10 including a fan for causing the main stream of the drying medium to flow over the condenser, or, where there are more than one.. over at least one of the condensers, and the evaporator is located out of the path of the main stream of drying medium to and from the fan.

12. A system according to Claim 11 wherein the evaporator is located such that the proportion of drying medium that is cooled by it, is mixed with the main stream where the main stream is to enter, or has entered, the material to be dried.

12 13. A system according to Claim 11 wherein the evaporator is located such that the proportion of drying medium that is cooled by it, is mixed with the main stream between the fan and the, or said at least one, condenser.

14. A system according to any one of Claims 8 to 13 wherein a fan is arranged to assist flow of drying medium over the evaporator.

15. A system according to any one of Claims 8 to 14 including deflectors for constraining the drying medium to follow a circuitous path through and along material located in the drying chamber, to be dried.

16. A system according to any one of Claims 8 to 14 including provision for heating the drying medium at an intermediate position of its drying path through the material to be dried.

17. A system according to Claim 16 wherein it is arranged that the heat for heating the drying medium at the intermediate position is derived from the evaporator.

18. A system for condensation drying substantially as hereinbefore described with reference to any one of Figures 1, 2, 3, 5 and 6 of the accompanying drawings.

19. A method for condensation drying substantially as hereinbefore described with reference to any one of Figures 1, 2, 3, 5 and 6 of the accompanying drawings.