GB2147400A

GB2147400A - Drying plant

Info

Publication number: GB2147400A
Application number: GB08424739A
Authority: GB
Inventors: Gerald Vaughan Mcmillan
Original assignee: Jack Ltd W & R
Current assignee: Jack Ltd W & R
Priority date: 1983-09-29
Filing date: 1984-10-01
Publication date: 1985-05-09
Also published as: IT1207565B; NZ205803A; GB2147400B; AU579327B2; AU3367584A; GB8424739D0; IT8422921A0

Abstract

A plant for drying materials such as timber comprises a refrigerating unit 11 coupled to a drying chamber 10. Fans exchange air between the refrigerating unit 11, the ambient environment, and the chamber 10. The refrigerating unit 11 includes a compressor 25, a condenser 12 located in the airstream entering the chamber 10, an evaporator 15, and valving means for controlling proportions of ambient air and air from the chamber passing through the condenser 12 and the evaporator 15. The refrigerating unit 11 operates primarily as a heat pump and can simultaneously function as a heat pump while venting moist air from the chamber 10 and recovering a substantial proportion of the energy contained within that moist air. <IMAGE>

Description

SPECIFICATION Drying plant This invention relates to a drying plant for the drying of both vegetable and animal materials but more particularly the drying of vegetable material such as timber.

Plants for the drying of timber are known and generally consist of a drying chamber in which the timber to be dried is stacked.

Heated air is circulated throughout the chamber and for this purpose there is generally an air inlet into the chamber and an air outlet. In the case of dehumidifier or heat pump plants, the heated air is derived from a refrigeration unit which may or may not include auxiliary heaters of some type (e.g. electric resistance heaters). The refrigeration unit is coupled to the drying chamber and its condenser heats the air being introduced to the chamber.

In the case of dehumidification driers the air entering the chamber is substantially the same air as has left the chamber, this air having passed through the evaporator of the refrigerator plant to be cooled and dehumidified before passing through the condenser to be reheated and finally reintroduced to the chamber. The removal of evaporated moisture from the chamber by such plants is achieved substantially by means of condensation on the relatively cold evaporator surface.

While heat pump driers utilise a refrigeration plant which is similar to that used in dehumidifiers, the routing of airflows to and from evaporator and condenser surfaces are quite different.

A heat pump drier can be instructed by its operator or programmed to: (a) heat the kiln, by drawing atmospheric air through its evaporator to extract heat from this air, and pumping this heat to the condenser whence it is released to the kiln atmosphere; (b) lower the kiln humidity, by venting moist air from the kiln chamber through the evaporator to atmosphere. In this way the evaporator will recover energy from the vented air, and this heat is returned to the chamber by the condenser, which releases the heat to a stream of air from the ambient environment being introduced to the kiln to replace the vented air.

In some plants, both operations (a) and (b) can be carried out simultaneously.

It will be appreciated by those skilled in the art that removal of evaporated moisture from the chamber by such plants is achieved substantially by means of venting hot humid air from the chamber.

The total output of a heat pump suffers a progressive decline as the temperature differential across which it is required to pump heat increases and/or the evaporating temperature drops.

In the case of timber drying plants the heat pump is required to extract heat from air at (varying) ambient temperatures and deliver it to a kiln environment which will commence at ambient temperatures and progressively increase throughout the drying cycle. It is typically a design requirement of such plants that they be capable of achieving condensing temperatures of 80"C towards the end of the drying cycle. While these temperatures can be attained, the efficiency of the heat pump being required to pump across a range from evaporating temperatures (for New Zealand conditions averaging say 1 2 C but often colder) to condensing temperatures up to 80"C is severely impaired.

Utilisation of the dehumidifier and heat pump systems withone one plant is also known and such a plant is for example disclosed in Australian Patent Specification 52061/79 of C.E.A.F. S.p.A. The plant described in the aforesaid patent specification incorporates a refrigerating unit which operates as a dehumidifier, but by the incorporation of an auxiliary evaporator and means for directing ambient air flow and the air in circulation within the chamber and refrigerating unit, the refrigerating unit can operate as a heat pump (a) in the initiial stages of drying to lower relative humidity and (b) towards the completion of the drying cycle to carry out heating at elevated temperature.

There are certain limitatiions in this system, notably that the heat pump can only be utilised at the beginning and end of the drying cycle. For the initial stage of drying, the heat pump will work to lower relative humidity by venting a constant volume of air, but it will not be capable of raising kiln temperature more than approximately 1 0,C above ambient, because of the constant air volume being vented. This is a severe limitation considering that most timbers can be satisfactorily dried using a starting temperature far in excess of 1 0,C above ambient.

Moreover, the heat pump can no longer dry efficiently once relative humidity in the chamber has been lowered to equate with ambient relative humidity (typically approximately 80% in New Zealand's climatic conditions). At this point the plant commences operation as a dehumidifier so that more efficient drying may continue with moisture removal now being achieved only by condensation.

While a plant of this type may be operated as a dehumidifier, this will only be possible up to a certain maximum kiln chamber temperature, typically about 45to, hence the need to introduce an auxiliary condenser to dissipate heat to atmosphere so that drying can continue. This limitation exists because there is no provision for decreasing the volume of air passing through the evaporator coil as chamber temperature increases.

Most timbers can tolerate temperatures far in excess of 45"C without problems and will dry much faster under these conditions hence this plant limitation does restrict drying speed.

Temperature in the chamber could be raised by reverting to heat pump operation, but this is not practical since there is no provision for simultaneous venting, and therefore no way of removing evaporated moisture from the chamber while maintaining an upward trend in temperature.

The plant can revert to operation as a heat pump only when there is no longer a requirement to remove evaporated moisture from the chamber-i.e. at the closing stages of the cycle when elevated temperatures are practically essential to ensure acceptable drying times are achieved. It will be recognised, however, that this is the period when the output of the heat pump is most severely affected because it is pumping across the maximum temperature differential. The efficiency of the heat pump will be further impaired if cold weather is experienced at this stage of the cycle.

At this iast stage of drying when elevated temperatures are highly desirable, operation as a dehumidifier would be significantly more efficient, if this were possible. However, plants of this type cannot operate as dehumidifiers at temperatures significantly above 45"C, because there is no provision for decreasing the volume of air passing through the evaporator as chamber temperature increases.

By contrast with known plants described above, the present invention is directed to plants for the drying of materials, especially timber, comprising a refrigerating unit coupled to a drying chamber, fan(s) which in use can exchange air between the refrigerating unit, the ambient environment and the chamber, said refrigerating unit including a compressor, a condenser located in the airstream entering the chamber, an evaporator, and valving means for controling proportions of ambient air and air from the chamber passing through the said condenser and the said evaporator, whereby the refrigerating unit operates primarily as a heat pump and can simultaneously function as a heat pump while venting moist air from the chamber and recovering a substantial proportion of the energy contained within that moist air.

In a preferred version, the plant is also equipped with means to increase the relative humidity in the drying chamber by means of injecting thereto moisutre in the form of steam or atomised water, should circumstances render this desirable.

The present invention is thus directed to drying plant which comprises a drying chamber and a refrigerating unit whereby the refrigerating unit can operate as a heat pump whilst simultaneously venting moist air. In this way relative humidity within the chamber can be controlled whilst the temperature within the chamber rises and then whilst the temperature is controlled the relative humidity within the chamber can be reduced.

Accordingly in a further preferred form of the invention there is included in the refrigerating unit a dehumidification evaporator together with valving means to allow said dehumidification evaporator to be connected to the refrigerating circuit and means whereby when so connected an air flow drawn from the drying chamber can be passed through the said dehumidification evaporator and subjected to a controlled reduction of enthalpy selected to restrict evaporating temperature to a predetermined upper level Accordingly the plant is able to operate a dehumidifier whenever the specific enthalpy differential between chamber and ambient is such that the dehumidification evaporator will provide more heat to the condenser than will the main heat pump evaporat.r Specific enthalpy differential may be measured procesely for this purpose by known control equipment or approximated by use of temperature or wet bulb temperature readings.

According to a second broad aspect the present invention provides a method of drying materials such as timber where the timber is placed within a drying chamber and a refrigerating unit is coupled to the chamber such that air can be exchanged between the refrigerating unit and chamber, the method comprising the steps of raising the relative humidity within the chamber to a predetermined level, controlling the relative humidity at substantially the said predetermined level whilst raising The temperature within the chamber to a predetermined level, and maintaining the temperature within the chamber at substantially the aforesaid predetermined level whilst reducing the relative humidity within the chamber to a predetermined lower level.

In the following more detailed description of the invention according to its preferred from reference will be made to the accompanying drawings in which: Figure 1 illustrates schematically a refrigerating unit of the type according to the present invention connected to a kiln chamber containing timber to be dried; Figure 2 is a schematic illustration of the refrigerating unit of the present invention viewed from within the kiln chamber and showing air flow between kiln chamber, refrigerating unit and the ambient environment when the unit is operative as a heat pump; Figure 3 is similar to Fig. 2 but shows the air flow pattern when the unit is operating as a heat pump with venting; Figure 4 is similar to Figs. 2 and 3 but shows the airflow pattern when the unit is operative as a dehumidifier; ; and Figure 5 is a sectioned elevational view of one form of the refrigerating unit.

Referring firstly to Fig. 1 of the drawings there is schematically illustrated a drying kiln 10 and a refrigerating unit 11. The material to be dried 24 (in this case timber) is completely enclosed by the kiln 10 except for an air inlet 1 7 from and an air outlet 18 to the refrigerating unit. The refrigerating unit 11 is further provided with an outlet 1 9 to, and two inlets 16 and 21 (not shown in Fig. 1) from, the ambient environment. Outlet 1 9 may be ducted vertically or horizontally but must discharge in a position from whence discharged air cannot return in significant quantity to either of the inlets 16 and 21.

Fig. 2 schematically illustrates the refrigerating unit, viewed from within the kiln chamber during operation as a heat pump. The refrigerating unit comprises a compressor (not shown) and a condenser 1 2 which is situated upstream of a fan 1 3. Separated from condenser 1 2 by a wall 14, which divides the refrigerating unit into chambers 11 a and 11 b, is an evaporator 1 5. The evaporator 1 5 is exposed to ambient air via an inlet port 16.

Fan 1 3 is employed to force air into the drying chamber 10 via an air inlet 1 7. Air from the chamber 10 can pass through an outlet 1 8 to chamber 11 b which houses the condenser as shown in Fig. 2.

It will be appreciated by those skilled in the art that the drying chamber 10 is of dimensions sufficient to handle a load of the material to be dried which in the case of timber could, for example be a stack of timber placed on a suitable transportation trolley, bogey or the like. The interior of the chamber 10 is, as previously explained, closed from external surroundings and thus air into and out of the chamber must pass through inlet 1 7 and outlet 18.

Whilst the drawings are of a schematic nature it will once again be appreciated by those skilled in the art that the refrigeration unit is constructed according to known methods of construction with suitable pipe work coupling the various elements of the refrigerator unit and a control system for controlling the operation of the various elements.

For this reason it is not proposed that this specification contain a detailed description of such known art.

There is, however, shown in Fig. 5 an illustration of a refrigerating unit 11 according to the present invention. The unit 11 includes a housing constructed according to known techniques and is internally divided by wall 1 4 to form the separate chambers 11 a and 11 b. Located within chamber 11 b is a compressor 25. The condenser 1 2 situated within chamber 11 b is formed by a condenser 1 2a, heating element banks 1 2b located within a condenser coil hood 1 2c to which is mounted the housing 1 2d for condenser fan 1 3. Also located within chamber 11 b is a dehumidifying evaporator 22 with its associated hood 22a and housing 23a for fan 23 (as hereinafter described).Air intake opening 1 8 into chamber 1 b opens through the wall of the housing.

Within chamber 1 la is an evaporator 15 and housing for evaporator fan 1 spa. Ambient air is drawn by fan 1 spa through inlets 1 6 in the walls and roof of the housing.

A control box 26 is conveniently mounted to one wall of the housing of the refrigerating unit.

With the arrangement shown in Fig. 2 of the drawings the refrigerating unit operates as a heat pump with air within the chamber flowing in a closed circuit to and fro.m the drying chamber 10 via condenser 1 2 and fan 1 3. Ambient air is drawn by fan 1 5a through inlet 1 6 to evaporator 1 5 and then back to atmosphere via outlet 1 9.

Fig. 2 therefore shows the refrigerating unit acting as a heat pump, extracting heat from the ambient air passing through opening 1 6 and transferring it to the air entering the kiln through opening 1 7. Kiln temperature will rise while the plant operates in this manner.

Referring now to Fig. 3 of the drawings there is shown an outlet 1 8a between the drying chamber 10 and the evaporator chamber 1 la. A modulation control unit which is schematically shown at 20 controls venting means in the form of for example dampers whereby outlet 18a and inlet 21 to chamber 11 b can be completely closed or opened by controlable degrees. Inlet 21 into chamber 11 b opens to admit ambient air.

Fig. 3 shows the refrigerating unit still acting as a heat pump, but simultaneously venting a proportion of the air leaving the kiln (i.e.

the airstream passing through opening 1 8A) to the atmosphere, first passing the evaporator 1 5 to recover energy from that airstream.

The heat pump effect will tend to raise kiln temperature while the venting will tend to lower kiln relative humidity.

Referring now to Fig. 4 of the drawings further elements of the refrigerating unit are illustrated. These further elments consist of an auxiliary dehumidifying evaporator 22 and the means for drawing an airflow of selected volume from the drying chamber which in the preferred form of the invention is fan 23. Fan 23 is of a variable speed or selective speed type and is operative to draw from the air stream, indicated at A, issuing through inlet 18 a secondary air stream A,, which passes through the dehumidifying evaporator 22 before rejoining the main airstream passing condenser 12, fan 1 3 and re-entering the drying chamber via inlet port 1 7. In this way the airstream A1 is subjected to a controlled reduction of enthalpy selected to restrict evaporating temperature to a predetermined upper or maximum level.

Fig. 4 shows the refrigerating unit acting as a closed circuit dehumidifier. Operation in this manner will tend to raise kiln temperature by the addition of compressor and fan energy while simultaneously lowering kiln relative humidity by means of condensation of moisture on the dehumidification evaporator 22.

To more fully describe the invention reference will be now made to its mode of operation. The drying chamber 10 is provided with sensors (not shown) which are located near the roof of the chamber and close to the side of the chamber from which the air approaches the stack of material to be dried (e.g. close to the loading door side of the chamber). These sensors are thus able to sense the relative humidity and temperature of the air being presented to the stack of material to be dried. The plant is provided with a suitable control panel located in control box 26 which can be readily utilised by the plant operator. To commence the drying operation the operator will, on the control panel, select a maximum humidity which can, for example, be 80% relative humidity (RH). The operator will also select a final relative humidity which, by way of example, can be 50% RH.Finally the operator will select a temperature setting which, once again by way of example, can be 65"C.

The plant will commence operation purely as a heat pump (transferring atmospheric heat to the drying chamber 10) as shown in Fig. 2 of the drawings. This mode of operation will raise the temperature within the drying chamber 10 causing evaporation of moisture from the material to be dried until the relative humidity rises to the 80% RH setting. This first phase would normally take only a few minutes, and will not occur at all if ambient RH is already above the maximum RH setting.

If for any reason, RH in the chamber does not rise to the maximum setting, means to achieve this by injection of steam or atomised water can be utilised. Immediately RH in the chamber exceeds maximum RH, the plant will then pass into a mode of operation which is illustrated in Fig. 3 of the drawings where it continues to operate as a heat pump but with venting. This venting is carried out automatically so that the plant operates as a true heat pump whilst simultaneously venting moist air (after recovering energy in the evaporator 1 5 prior to exhausting).This selective venting is controlled by the modulation control 20 operating the dampers associated with outlet 1 boa and inlet 21 to ensure that maximum possible heat is pumped into chamber 10 consistent with ensuring that sufficient venting occurs to prevent RH in chamber 10 from rising substantially above the (80%) setting.

In this manner the automatic venting will occur whilst the heat pump continues to raise the temperature in the chamber 10 at substantially the pre-selected 80% relative humidity. When the selected temperature setting is reached the system becomes automatically temperature controlled and from this point the dampers associated with outlet 1 boa and inlet 21 will modulate to vent as much air as possible consistent with ensuring that temperature in the chamber does not fall substantially below the 65"C) temperature setting. In this way, temperature in the chamber is maintained while RH is progressively lowered.

When the relative humidity has dropped to the pre-selected final RH setting (50%) the machine will automatically hold RH at this level. The final relative humidity figure will be set at a level sufficient to prevent over-drying of the material within the drying chamber 10.

When ambient temperatures fall towards zero, operation of the heat pump may be impaired by the formation of ice on the evaporator 1 5. For heat pump operation in ambient conditions close to zero, an automatic defrost system (not shown) is provided to de-ice the evaporator and permit heat pump operation to resume.

Whether the plant will operate as a heat pump or dehumidifier depends upon the difference between ambient and kiln chamber temperatures. A switching device is provided so that the plant automatically commences to operate as a dehumidifier (as in Fig. 4) wherever this differential exceeds a preselected level.

In the dehumidification mode, operation will continue by use of the auxiliary dehumidification evaporator 22 and fan 23, and control is automatically passed to a humidity sensing device on which the final RH set point (50%) has been set. If RH is above the 50% set point the drying by dehumidification will continue. If during dehumidification, temperature rises above the pre-selected temperature setting (65"C) the machine will switch to heat pump mode and the vents 18a and 21 will open (as in Fig. 3) until temperature has fallen to the set point. Control will then return to the humidity sensing device, and the plant will revert to operation in dehumidification mode (as in Fig. 4). When RH has dropped to the final humidity set point (50%) the machine will hold it at this level.

The drying plant according to the present invention is effective and economic in operation. The plant does not suffer from the marked fall-off in output normally associated with known drying equipment when a great difference exists between ambient and kiln chamber temperature.

The plant primarily operates as a heat pump and is capable of performing complete drying cycles without the use of the dehumidification evaporator 22 and fan 23.

The dehumidification mode is provided only to significantly improve total plant output and efficiency under operating conditions which would otherwise severely affect them. Such conditions may occur when cold ambient conditions prevail, where elevated kiln temperatures are required, or where a combination of both conditions exists.

Claims

1. A plant for the drying of timber or other material, comprising a refrigerating unit coupled to a drying chamber, a fan or fans which in use can exchange air between the refrigerating unit, the ambient environment, and the drying chamber, the refrigerating unit including a compressor, a condenser located in the airstream entering the drying chamber, an evaporator, and valving means for controlling proportions of ambient air and air from the drying chamber passing through the said condenser and the said evaporator, whereby the refrigerating unit operates primarily as a heat pump and can simultaneously function as a heat pump while venting moist air from the drying chamber and recovering a substantial proportion of the energy contained within that moist air.

2. Drying plant as claimed in claim 1, further including means to increase the relative humidity in the drying chamber by means of injecting thereto moisture in the form of steam or atomised water.

3. Drying plant as claimed in claim 1 or 2, wherein the refrigerating unit includes a dehumidification evaporator together with valving means to allow the said dehumidification evaporator to be connected to the refrigerating circuit and means whereby when so connected an air flow drawn from the drying chamber can be passed through the said dehumidification evaporator and subjected to a controlled reduction of enthalpy selected to restrict evaporating temperature to a predetermined upper level.

4. Drying plant as claimed in any preceding claim, wherein the refrigerating unit includes two compartments, one of which houses the said condenser and has an inlet and outlet for passage of air to and from the said drying chamber and an inlet for flow of ambient air into said one compartment, and the other compartment houses the said evaporator and has inlets for passage of ambient air and an air flow from the said drying chamber and an outlet for exhausting air therefrom after passing through the said evaporator.

5. Drying plant as claimed in claim 4, wherein venting means control air flows through the inlets for ambient air into the said one compartment and drying chamber air into the said other compartment.

6. Drying plant as claimed in claim 5, wherein the said venting means comprise dampers controlled by a modulation control unit.

7. Drying plant as claimed in any preceding claim, further including sensors within the said drying chamber which sense relative humidity and temperature of the air being presented to the material to be dried.

8. Drying plant substantially as herein described with reference to the accompanying drawings.

9. A method of drying timber or other material, where the material to be dried is placed within a drying chamber and a refrigerating unit is coupled to the drying chamber such that air can be exchanged between the refrigerating unit and drying chamber, the method comprising the steps of raising the relative humidity within the drying chamber to a predetermined level, controlling the relative humidity at substantially the said predetermined level whilst raising the temperature within the drying chamber to a predetermined level, and maintaining the temperature within the drying chamber at substantially the aforesaid predetermined level whilst reducing the relative humidity within the drying chamber to a predetermined lower level.

10. A method as claimed in claim 9, wherein control of relative humidity whilst raising the temperature within the drying chamber is effected by operating the refrigerating unit as a heat pump whilst selectively venting an air flow from the drying chamber to atmosphere and an air flow of ambient air through the condenser of the refrigerating unit.

11. A method as claimed in claim 9 or 10 wherein control of temperature whilst lowering the relative humidity in the drying chamber is effected by operating the refrigerating unit as a heat pump whilst selectively venting an air flow from the drying chamber to atmosphere and an air flow of ambient air through the condenser of the refrigerating unit.

12. A method as claimed in claim 10 or 11, wherein the said air flow from the drying chamber is passed through an evaporator.

13. A method as claimed in claim 10, 11, or 12, during the difference between the specific enthalpy of ambient air and air in the drying chamber is monitored, and if the differential exceeds a determined upper level an airflow from the drying chamber is directed through a dehumidification evaporator, this airflow being combined with the flow of air passing through the condenser of the refrigerating unit before passing back into the drying chamber.

14. A method of drying materials substantially as herein described.