NZ205803A - Drying timber using heat pump unit: temperature raised before humidity is lowered - Google Patents

Description

205803 N.Z.No. 205803 29 September 1983 NEW ZEALAND Patents Act 1953 COMPLETE SPECIFICATION "DRYING PLANT" We, W & R JACK LIMITED, a New Zealand company of 16-20 Home Street, Wellington, New Zealand, do hereby declare the invention, for which we pray that a Patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement : - - 1 - (followed by 1A) N Z. PATENT OfiFinp 28SEPI984 I'i-CEIVEO 205303 - 1A- 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 r, this air having passed through the evaporator of the ,/^EN V* O * refrigerator plant to be cooled and dehumidified before 1 [z passing the condenser to be reheated and finally reintroduced 2 8 MAR 1988 to the0<^iamber. The removal of evaporated moisture from the El' 205803 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 juired to pump heat increases and/or the evaporating >c^5^jgfe^perature drops. 203803 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 12°C but often colder) to condensing temperatures up to 80°C is severely impaired.

Utilisation of the dehumidifier and heat pump systems within one plant is also known and such a plant is for example 4 rl^l-1381 disclosed in |Auoferol i-an Patent Specif ication ] 52QG1/79 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 towards the completion of the drying cycle to carry out heating at elevated temperature.

There are certain limitations in this system, notably :hat the heat pump can only be utilised at the beginning and ^of the drying cycle. For the initial stage of drying, the pump will work to lower relative humidity by venting a 2 8 MAR 1988 constant volume of air, but it will not be capable of raising /- r 205803 kiln temperature more than approximately 10°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 drying schedules calling for a starting temperature far in excess of 10°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 45°C 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 .^Eftflri&ace there is no provision for simultaneous venting, and ' o\ Vy' thec-^forfe no way of removing evaporated moisture from the 2 8 MAR gmber while maintaining an upward trend in temperature V & i 205803 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 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 last stage of drying when most moisture has been removed from the chamber and 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 controlling ^ £ N r"* vp|sppor tions of ambient air and air from the chamber passing 'jM 2 through the said condenser and the said evaporator, whereby 2 8 MAR 1988 205S03 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 moisture 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 in substitution for the main heat pump evaporator, and means whereby an airflow of selected volume drawn from the drying chamber can be passed through the said dehumidification evaporator when it is connected to the refrigerating circuit in substitution for the main heat pump 2 8 MAR If^frumidif ier whenever the temperature differential between Accordingly the plant is able to operate as a / 205803 chamber and ambient is such that the dehumidification evaporator will provide more heat to the condenser than will the main heat pump evaporator.

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 form 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 , ^—~=^Qhamber and showing air flow between kiln chamber, refrigerating unit and the ambient environment when the unit is operative as a heat pump, 2 8 MAR \m A 0 Figure 3 is similar to Figure 2 but shows the air flow E *""" "'attern when the unit is operative as a heat pump with 205803 venting, Figure 4 is similar to Figures 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 Figure 1 of the drawings there is schematically illustrated a drying kiln 10 and a r-efrigerating unit 11. The material to be dried 24 (in this case timber) is completely enclosed by the kiln 10 except for an air inlet 17 from and an air outlet 18 to the refrigerating unit. The refrigerating unit 11 is further provided with an outlet 19 to, and two inlets 16 and 21 (not shown in Figure 1) from, the ambient environment. Outlet 19 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.

Figure 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 12 which is situated upstream of a fan 13. Separated from condenser 12 by a wall 14, which divides the refrigerating unit into chambers 11a and lib, is an evaporator 15. The evaporator 15 is exposed to ambient air an inlet oort 16.

Fan 13 is employed to force air into the drying chamber chamber lib which houses the condenser Air from the chamber 10 can pass 205*03 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 17 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 Figure 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 14 to form the separate chambers 11a and lib. Located within chamber lib is a compressor 25. The condenser 12 situated within chamber lib is formed by a condenser 12a, heating element banks 12b located within a condenser coil hood 12c to which is mounted 6N XM\housing 12d for condenser fan 13. Also located within //<K // ijisj chamber lib is a dehumidifying evaporator 22 with its 28MARiM>c :iat«d hood 22a and housing 23a for fan 23 (as hereinafter pfl&scr ibed). Air intake opening 18 into chamber lib opens through the wall of the housing. 205803 Within chamber 11a is an evaporator 15 and housing for evaporator fan 15a. Ambient air is drawn by fan 15a through a damper controlled inlet 16 in the wall 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 Figure 2 of the drawings the refrigerating unit operates as a heat pump with air within the chamber flowing in a closed circuit to and from the drying chamber 10 via condenser 12 and fan 13. Ambient air is drawn by fan 15a through inlet 16 to evaporator 15 and then back to atmosphere via outlet 19.

Figure 2 therefore shows the refrigerating unit acting as a heat pump, extracting heat from the ambient air passing through opening 16 and transferring it to the air entering the kiln through opening 17. Kiln temperature will rise while the plant operates in this manner.

Referring now to Figure 3 of the drawings there is shown an outlet 18a between the drying chamber 10 and the evaporator chamber 11a. 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 lib can be completely closed or opened by controlable degrees. Inlet 21 into chamber lib opens to admit ambient air.

Figure 3 shows the refrigerating unit still acting as a heat pump, but simultaneously venting a proportion of the air ving the kiln (i.e. the airstream passing through opening 18A) to the atmosphere, first passing the evaporator 15 to cover energy from that airstream. The heat pump effect will .. . 205803 tend to raise kiln temperature while the venting will tend to lower kiln relative humidity.

Referring now to Figure 4 of the drawings further elements of the refrigerating unit are illustrated. These further elements 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 13 and re-entering the drying chamber via inlet port 17.

Figure 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 2QSSQ3 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 Figure 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 illustated in Figure 3 of the drawings where it continues to operate as a heat pump but with venting. This venting is carried out automatically so that plant operates as a true heat pump whilst simultaneously ^ venting moist air (after recovering energy in the evaporator vT 2 8 MAR V ^gprior* to exhausting). This selective venting is controlled by the modulation control 20 operating the dampers associated 205803 with outlet 18a 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%) maximum 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 18a 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 15. 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. 7% Whether the plant will operate as a heat pump or ? dehumidifier depends upon the difference between ambient and 2 8 MAR me A 0Jtiln chamber temperatures. A switching device is provided so ■ -. • 205&Q3 that the plant automatically commences to operate as a dehumidifier (as in Figure 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 Figure 3) until temperature has fallen to below the set point. Control will then return to the humidity sensing device, ,-and the plant will revert to operation in dehumidification mode (as in Figure 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 the dehumidif ication evaporator 22 and fan 23.

The dehumidif ication mode is provided only to 'v 1 sigralficantly improve total plant output and efficiency under 2 8 MAR 1988r oper/ating conditions which would otherwise severely affect 205803 them. Such conditions may occur when cold ambient conditions prevail, where elevated kiln temperatures are required, or where a combination of both conditions exists. "2 8 MAR 1988

Claims (13)

16 205*803 eia-ifflg WHAT WE CLAIM IS:-

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 5 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 . 10 from the drying chamber passing through the said condenser and the said evaporator, sensors within the drying chamber, and control means responsive to temperature and relative humidity sensed by the sensors and causing the plant to operate such that the relative 15 humidity within the chamber is raised to a pre-selected level and substantially maintained whilst the temperature is raised to a pre-selected level following which the said pre-selected temperature is substantially maintained whilst the relative humidity is lowered to a 20 pre-selected lower level.

2. Drying plant as claimed in claim 1, 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. 25

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 205803 17 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 5 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 10 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 15 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 20 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. b 2 8 MAR 1988 18 205803

7. Drying plarrt siobstantially as herein described with reference to the accompanying drawings.

8. A method of drying timber or other material, where the material to be dried is placed within a drying 5 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 including sensing temperature and relative humidity within the drying chamber and comprising the steps of raising 10 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 15 within the drying chamber at substantially the aforesaid predetermined level whilst reducing the relative humidity within the drying chamber to a predetermined lower level.

9. A method as claimed in claim 8, wherein control of relative humidity whilst raising the temperature within 20 the drying chamber is effected by operating the refrigerating unit as a heat pump whilst selectively « 205S63 venting an air flow from the drying chamber to atmosphere and an air flow of ambient air through the condenser of the refrigerating unit.

10. A method as claimed in claim 8 or 9, 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.

11. A method as claimed in claim 9 or 10, wherein the said air flow from the drying chamber is passed through an evaporator.

12. A method as claimed in claim 9, 10, or 11, wherein 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. 'O <j iJ <0> 20

13. A method of drying materials substantially as herein described. W & R Jack Limited By hts/their Attorney DON HOPKINS Registered Patent Attorney ■Wettington-