NZ524470A

NZ524470A - Heat pump drier with improved control

Info

Publication number: NZ524470A
Application number: NZ524470A
Authority: NZ
Inventors: Eric William Scharpf; Cedric Gerald Carrington
Original assignee: Delta S Technologies Ltd
Priority date: 2003-02-28
Filing date: 2003-02-28
Publication date: 2005-12-23

Abstract

An apparatus and process including a heat sink exchanger (26) to cool and condense liquid out of a drying gas with a heat transfer surface arranged to exchange heat with a first sub- stream of the drying gas and a heat source heat exchanger (27) arranged to exchange heat with a second sub-stream of a drying gas and arranged in a functionally parallel configuration with said heat sink heat exchanger (26) so that each of said drying gas sub- streams exchanges heat with one of the two said heat transfer surface per cycle through the heat exchange system and a gas movement device (35) for propelling the drying gas through the heat exchanger system in either a forward or reverse flow path direction. The apparatus and process can also include controlling the amount of heat rejected from apparatus (26) based on maintaining the wet bulb of the drying gas nominally constant and controlling the amount of refrigerant in the heat exchanger circuit based on maintaining the dry bulb temperature of the drying gas within certain limits.

Description

<div class="application article clearfix" id="description"> 524470 No: 524470 Date: 28 February 2003 NEW ZEALAND PATENTS ACT, 1953 intellectual PROPER OFFICE OF HI 0 1 HAR 2004 RECEIVED COMPLETE SPECIFICATION HEAT PUMP DRIER WITH IMPROVED CONTROL We, ERIC WILLIAM SCHARPF a US citizen of 278 Blueskin Road, RDl, Port Chalmers 9005, New Zealand and CEDRIC GERALD CARRINGTON a New Zealand citizen of 17 Torr Street, Dunedin 9001, 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: 2 FIELD OF THE INVENTION The present invention relates to the drying of materials using an improved heat pump system to move much of energy needed to evaporate water and/or other liquids from the wet material. It has particular application to the drying of timber but is also well suited 5 for numerous other drying processes. BACKGROUND TO THE INVENTION Heat pumps and heat pump based drying systems have been generally known in industrial applications including timber drying for a number of years. References to the use of heat pump refrigeration cycles in clothes drying date back to the 1940s in US 10 2,418,239. Because of the complexities of both the drying process itself and the operation of a nominally closed loop drying system driven by a heat pump dehumidifier, there has been a need to provide active control of the process to both maintain its peak efficiency throughout the drying process and to ensure the quality of the dried product. This need is complicated by the fact that the control of a heat pump dehumidifier system 15 and the drying process parameters themselves are linked by multiple feed-back processes that are fundamentally different from the more commonly practiced but less efficient heat-and-vent drying systems. There have been several attempts to address this control problem in the heat-and-vent technology that are generally relevant to this invention. It is important to note that they 20 only relate to drying processes where the drying gas enters the process, is heated to increase its moisture uptake capacity, passes across the material being dried to take up some of the moisture, and a substantial portion of the moisture laden gas is then vented from the process, while the remaining balance is typically reheated and re-circulated into the material inlet air-stream. This heat-and-vent process is fundamentally different 25 from a nominally closed loop system where the drying gas is cooled and its vapour-phase moisture content partially condensed to increase its moisture uptake capacity, heated to provide energy for further moisture evaporation, and passed across the material to be dried where it takes up more moisture before it is recycled through the process again and again with only minor purge and make-up streams removed and 30 added to control various gas compositions. As such, these other attempts to control heat- 3 and-vent processes do not address the problem for high efficiency heat pump driven systems with nominally closed loop drying gas recycle streams. One such attempt to improve heat-and-vent drying control has been put forward by Rosenau in US 4,356,641. This patent describes a heat-and-vent drier focussed on 5 maintaining an acceptable constant rate of drying for lumber. This system provides control by sampling the moisture content of the lumber to determine the rate of change of moisture content in the lumber stack, and then adjusts the rate of drying if required by varying the drying gas wet bulb temperature set point. The difference between the actual wet bulb and the desired set point controls the rate of drying gas venting or a 10 steam spray injection. The dry bulb temperature is also used to help control the system in that the difference between the measured value and the set point is used to control the heat input rate from the kiln heater. Aside from not considering the option for a recirculating drying gas system driven by a heat pump dehumidifier, the method changes the wet bulb temperature of the drying gas to control the drying rate which can 15 involve other difficulties discussed further in the detailed description of the invention. An example of control technology applied to traditional heat-and-vent timber kiln drying efficiency was put forward by Gelineau and Kinney in US 4,599,808. Their system improves drying efficiency by maintaining a constant rate of drying through maintaining a constant dry bulb temperature difference in the drying gas flow before 20 and after it passes over the material being dried. Again, this method does not address the operational limits and opportunities associated with a heat pump driven system. Another example of control technology applied to the traditional heat-and-vent timber kiln process was put forward by Moren and Ab in US 5,940,984. They use the temperature difference across the timber stack as the measured variable to control the 25 process. Although they follow a schedule with a nominally constant wet bulb temperature in the drying gas during the main part of the drying process, they decrease the wet bulb temperature at the end of the process which would cause difficulties with a heat pump dehumidification process. Since there is no heat pump dehumidification addressed in this work, it is also not able to address the efficiency issues associated with 30 the simultaneous control of a heat pump dehumidifying system. In addition, since heat and vent systems typically run with higher heat transfer per pass of drying gas, the 4 drying gas temperature change as it passes over or through the material being dried is higher and more easily used as a control variable. With the lower temperature differences present in heat pump dehumidification systems, this is not a practical measured variable for control and thus the problem remains. 5 One example of previous methods to address the problem of heat pump drier control is described by Lewis in US 4,250,629. As with most efficient heat pump systems, this is a nominally closed loop process which heats the air before it enters a drying chamber and then removes some of the moisture from the air after it leaves the drying chamber before it is largely recirculated and goes through the process again. This control system 10 is focussed on increasing the range of drying gas dry bulb temperatures over which the system can operate and provides this control by measuring the dry bulb temperature of the drying gas in the vicinity of the heat pump evaporator and then acts to vary the amount of air drying gas bypass around the heat pump evaporator according to this dry bulb temperature. The dry bulb temperature of the drying gas is the measured variable 15 used to manipulate the amount of heat rejected from the kiln chamber and thus is related to the control of the overall drying rate of the system. This method has the disadvantage that, as the product dries and the wet bulb temperature falls in response, the dehumidifier drying capacity also falls, typically reducing the drying rate and the drying efficiency unnecessarily. Such systems require active intervention to repeatedly adjust 20 the dry bulb set point as the drying process advances to sustain the productivity and efficiency of the system. The timing of these adjustments is critical. If the temperature is increased too early, the product may be damaged and lose value. If the adjustment takes place too late, the drying time will be unnecessarily extended, with accompanying loss of productivity and increased drying costs. 25 Another example of heat pump drier control has been put forward by Thompson in NZ 213728. He describes a heat pump timber drying process and apparatus which uses multiple chambers mid a heat reservoir with a generalised control system based on dry bulb and wet bulb temperatures of the drying gas stream. This control system is focussed on maintaining a desired dry bulb temperature and relative humidity in the 30 drying chamber rather than focussing on optimising the performance of the heat pump in the context of the drying process. The control strategy suffers from similar limitations 5 to those of Lewis in US 4,250,629 and as a result does not fully address the problem of integrated control of the drying process and the heat pump system. In heat pump dehumidifier drying applications, the drying gas flow is primarily or fully in recirculation and there is a key difference with respect to the prior art relevant to the 5 present invention. This difference relates to the fact that the heat flow required from the condenser drops off significantly as the material approaches its final dry state under batch drying conditions as practiced in the prior art. The corresponding method of heat pump and process control must therefore consider the best way to address this situation in the context of both the limitations and capabilities of the heat pump as well as the 10 characteristics of the moisture release from the material being dried. This makes such single focus control designs present in the prior art open to the improvements proposed in the present invention. It is therefore, the object of the present invention to provide an improved control method and means to increase the efficiency and performance of a heat pump dehumidifier suitable for use in the variable demand conditions of a material 15 batch drying system. SUMMARY OF THE INVENTION In one aspect the present invention may be said to consist in a method of operating a heat pump for drying a material including: sensing a wet-bulb temperature and dry-bulb temperature in a drying gas flow, in a first drying stage after initial heat-up, controlling 20 the rate of heat rejection from the drying gas flow to maintain the wet-bulb temperature substantially constant and allow the dry-bulb temperature to rise to increase the driving force for moisture removal and thus maintain the rate of moisture removal from the system for a longer part of the process, and in a second drying stage when the dry-bulb temperature reaches a limit, also controlling refrigerant flow through the heat pump to 25 vary the rise in or maintain the dry-bulb temperature and optionally vary the wet bulb temperature to adjust the driving force for moisture removal from the material being dried to control the quality of the material being dried and to improve the efficiency of the overall process. 30 In another aspect the present invention may be said to consist in an apparatus for drying a material including: a chamber for a material, a heat pump for drying the material using 6 a drying gas flow, sensors for detecting wet-bulb and dry-bulb temperatures of the drying gas flow, and a controller for controlling operation of the heat pump based on wet-bulb and dry-bulb temperatures, wherein the controller operates the heat pump to: a) in a first drying stage after initial heat-up, control the rate of heat rejection from the drying gas flow, to maintain the wet-bulb temperature substantially constant and allow the dry-bulb temperature to rise to increase the driving force for moisture removal and thus maintain the rate of moisture removal from the system for a longer part of the process, and in a second drying stage when the dry-bulb temperature reaches a limit, control refrigerant flow through the heat pump to vary the rise in or maintain the dry-bulb temperature and optionally vary the wet bulb temperature to adjust the driving force for moisture removal from the material being dried to control the quality of the material being dried and to improve the efficiency of the overall process. Optionally, the moisture removal rate is also sensed to assist in controlling the heat rejection rate and refrigerant flow through the heat pump to optimise drying. 15 BRIEF DESCRIPTION OF THE DRAWINGS Preferred embodiments of the invention will be described with reference to the accompanying drawings, of which: Figure 1 shows an example system flow and control diagram, and Figure 2 shows an example temperature profile during timber drying. 20 DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS A preferred embodiment of the invention is an improved efficiency heat pump drying process and apparatus. An improvement is to control the heat pump and the drying process in concert through rejecting heat from the process using input from the drying gas dry bulb and wet bulb temperature sensors and optionally the amount of total liquid 25 removed from the system such that the wet bulb temperature is kept constant through the main drying period while the dry bulb temperature increases to provide the optimum driving force for moisture extraction from the material being dried as measured by the amount of liquid removed from the system through the drain line or the difference between the wet and dry bulb temperatures within the limits of the heat pump system 7 capabilities. Then when the drying process has progressed to the point where the drying gas dry bulb temperature reaches a maximum value based on the limits of the heat pump compressor system, the control adjusts the total refrigerant flow through the compression system down while keeping the drying gas wet bulb temperature largely 5 constant. The following description of the process and apparatus of this invention, by way of example only and with reference to the accompanying drawing and graph in Figure 1 and Figure 2, indicates the presently preferred embodiments of the invention. A preferred embodiment of the present invention consists of a heat pump dehumidifier drying process and apparatus to control both the heat pump part of the system and the drying process parameters to provide the maximum possible drying rate at the most efficient heat pump operating conditions based on a combination of wet bulb and dry bulb temperatures in the drying gas flow stream and based on the moisture removal rates from the overall system. The specific hierarchy of control in the preferred embodiment initially runs the process at the maximum drying capacity and rate of heat rejection. To maintain the overall stability of the process and heat pump operation at the highest drying rate and most efficient heat pump conditions, the preferred embodiment then increases the dry bulb temperature as the drying progresses while maintaining the wet bulb temperature roughly constant. Then when the dry bulb temperature reaches a pre-determined maximum, the heat pump refrigerant flow is reduced to limit the further rise in dry bulb temperature and reduce the power consumption of the heat pump. As this maximum is approached, the wet bulb temperature may then optionally be varied to limit the overall driving force for drying the material to prevent internal stresses from damaging the material being dried based on a combination of the difference between the wet and dry bulb temperatures and the rate of overall moisture extraction from the system. This new control scheme has the benefit of keeping the evaporating and condensing temperatures within the allowed ranges for the heat pump compressor system as well as driving the heat pump system and the drying process at their maximum efficient states 30 according to the natural drying rate reduction as the drying process progresses. This comes out of increasing the drying gas dry bulb temperature at a substantially constant 10 15 20 25 8 rate of dehumidification in order to increase the driving force for drying, while maintaining a slower variation in the wet bulb temperature over the length of the drying process to smoothly remain within the heat pump compressor operating limits and properly manage the stresses present in the material being dried, as the inherent drying 5 rate naturally drops off as the drying process progresses. Thus in response to the falling drying rate of the material being dried, the new control system automatically increases the drying force applied to the product in order to substantially maintain the drying rate. Because the adjustment in the drying force can be linked to the moisture content of the product, the driving force can be controlled to ensure it is consistent with the capacity of 10 the material to tolerate the progressively more aggressive drying conditions. The result is that the maximum drying rate is maintained longer than with the prior art, and the drying end-point is achieved more quickly while avoiding drying conditions that could damage the product. Thus, the performance of the drier can be optimised during the start of the drying 15 process to ensure the heat pump is highly loaded when the dry bulb temperature is lowest, and the humidity highest using a high refrigerant flow in the heat pump. The preferred embodiment will also control the drier to maintain the maximum possible drying rate as long as possible. Then, when it is no longer possible to maintain the maximum drying rate because of drying material stress and transport limitations, the 20 control will manage the heat pump so that it operates effectively and efficiently at higher dry bulb temperatures and lower humidity under lower loads, as required to complete the drying process as fast and efficiently as possible using a lower refrigerant flow and higher active heat pump condenser area per unit refrigerant flow. This control will also maximise heat transfer at the condenser to enable the higher dry bulb 25 temperatures for the drying gas flow to be achieved more efficiently. Furthermore, all of this is accomplished without disrupting the drying gas flow or negatively affecting the pressure drop in the drying gas circuit. Referring to the simplified process flow diagram in Figure 1 from a heat pump perspective, the heat pump refrigerant is compressed from low pressure by a 30 compression system 101. The flow of refrigerant and compressor power is controlled by a control signal from an integrated control system 115 which in turn takes input from 9 sensors 112, 113 and optionally 114 in the drying gas medium and the stream of moisture extracted from the drying material. It is important to note that although the temperature sensors 112 and 113 have been shown where the drying gas enters the material drying chamber, there are numerous other functionally equivalent locations 5 where the temperature sensors can be located in the drying gas flow stream without materially changing the invention. Furthermore, additional system protection sensors can also be included in the heat pump refrigeration circuit without materially changing the invention but they would not provide primary operational control for the process. The configuration shown in Figure 1 illustrates the example of control of the total refrigerant flow in the heat pump using a single variable flow compressor compression system. It is also acknowledged that various other compression systems, such as a multi-compressor array with multiple refrigerant flow valves, can also accomplish this efficient control of total refrigerant flow in the heat pump circuit as part of the present invention. The high pressure refrigerant is then condensed in heat exchanger 103 which provides at least part of this heat of condensation to the drying gas stream. The condensed high pressure refrigerant then passes through an expansion device (not shown) and then to an evaporator 102. Note that the evaporator may consist of a number of heat exchange modules in a functionally parallel configuration to each other and/or the condenser or be in a functionally series configuration relative to each other or the condenser from the perspective of the drying gas flow. Figure 1 shows two evaporator modules 102 in functionally parallel configuration with each other and in a functionally series configuration relative to the condenser from the perspective of the drying gas flow. The evaporator heat exchanger is specifically positioned to remove at least some heat from the drying gas stream such that moisture from that drying gas stream is condensed out and removed from that drying gas stream through a collector 104, measured by sensor 114 and sent to an external drain system. 30 After passing through the evaporator heat exchanger system, the low pressure evaporated refrigerant returns to the compressor system in a standard recirculation flow configuration. 10 Referring to the process from the perspective of the drying gas flowing through the drier in an anti-clockwise direction, the heat pump compressor system 101 operates to move heat from the lower temperature evaporator heat exchangers) 102 to the higher temperature condenser heat exchanger 103. As the drying gas passes over the 5 evaporator heat exchangers) it will lose heat and decrease in temperature and as the drying gas passes over the condenser heat exchanger it will take up heat and increase in temperature. The control scheme of the invention will insure that as the drying gas cools down in passing over the heat pump evaporator heat exchangers, it will drop to a temperature below its moisture dew point and some liquid moisture will condense out of 10 the vapour phase and be caught in drain 104 to be removed from the system based on the dew point calculated for the drying gas from the wet bulb and dry bulb temperature sensors 112 and 113. The liquid moisture removed from the system may optionally be measured by sensor 114 from which both the rate of removal and the total integrated amount of liquid moisture removed can be determined for additional optional control 15 purposes. The drying gas will also pass over the heat rejection exchanger 116 which acts to reject heat from the process to the ambient or some other suitable heat sink where the waste heat can be put to beneficial use as part of the control process described in further detail later in this section. This heat rejection coil can be located at several points in the 20 process and is shown in the figure as being upstream of the heat pump evaporator in the drying gas flow stream. As will be apparent to those skilled in the art, this heat rejection coil could also be placed in a location functionally parallel to the heat pump evaporator exchanger or downstream of the evaporator relative to the drying gas flow without materially affecting the present invention. 25 The drying gas then passes over the heat pump condenser exchanger 103 where it is reheated before passing through a fan or equivalent system 105 which provides the motive force to circulate the drying gas through the overall system. The drying gas stream is then guided through the system superstructure 111 in the section of the superstructure 106 by various flow conditioning devices 110 which act to minimise 30 pressure drop in the system. An additional device 107 is shown to guide the drying gas flow around the system and through the material to be dried 108 in a single pass 11 configuration. It should be apparent to those skilled in the art that this drying gas flow guide 107 could be configured in many various ways to achieve different paths for the drying gas to flow through the material to be dried 108. Once the drying gas has passed over and/or through the material to be dried 108 and 5 picked up moisture evaporating from the material, it returns to the heat pump though partition 109. It can be appreciated by those skilled in the art, that additional components specific to timber drying such as auxiliary heaters for sterilization, mid water spray systems for reconditioning can be readily added to the process and apparatus of the invention without materially changing the invention. Similarly there are various methods and apparatus that can be added to the process and apparatus of this invention to reject excess heat from the overall process to the ambient environment without materially changing the invention. These include but are not limited to venting a sub-stream of drying gas, pre-cooling the drying gas entering the evaporator, pre-cooling any make-up or purge drying gas entering or leaving the apparatus, sub-cooling the liquid heat pump refrigerant, de-superheating the heat pump refrigerant leaving the compressor, or partially or wholly condensing the high-pressure refrigerant for purposes of more stable control. As with other heat pump drying systems, additional methods of heat recovery may be optionally applied to the invention without material change to the invention. For instance, it is possible to include the capacity for reclaiming sensible cooling at the evaporator using, for example, either a pair of liquid coupled heat exchangers, or by means of heat-pipe coupled heat exchangers. Also, it is within the scope of this invention to include auxiliary heat sources and sinks 25 separate from the heat pump circuit to enhance and augment the heating of the drying gas by the heat pump condenser and the cooling and partial condensation of the drying gas by the heat pump evaporator without materially altering the invention itself. Although the figures show a preferred embodiment for timber processing, it can readily be appreciated that changes to the drying chamber configuration can be made to £ 20 12 facilitate the drying of other materials, in other drying gas mediums and for removing liquids other than water. These materials include but are not limited to timber, boards, paper, leather, bricks, milk, gypsum, plaster board, textiles, china clay, fertilizer, dye stuffs, tiles, pottery, grain, nuts, seeds, fruits, bio-processing waste, etc. Also, although 5 the system is preferentially focussed on water removal, it can also be configured to remove other vaporisable and condensable liquids from the material to be dried such as various organic solvents to be recovered from solvent based processing steps including painting. The process and apparatus of this invention are also amenable to various drying gas 10 mediums. Although the preferred embodiment for the invention is with air as the drying gas, the process and apparatus can be configured to use 02-free air, nitrogen, argon, oxygen, or any other gaseous medium to take up the moisture from the materials to be dried and condense that moisture out of the system through the heat pump evaporator (Chen, Bannister, McHugh, Carrington and Sun, 2000). 15 In the preferred embodiment for timber drying for a typical charge of green timber with 150% moisture content to start drying to a 9% moisture content before any optional spray reconditioning, the nominal conditions are summarised in Table 1: Table 1 Timber Drying Example Parameter Range Dry bulb temperature of drying gas (average over the system) 35-70C Wet bulb temperature of drying gas (average over the system) 20-65C Drying gas velocity through drying chamber 2-5 m/s Approach temperature in heat pump condenser 2-25C Approach temperature in heat pump evaporator 2-45C 13 Drying gas temperature rise across condenser heat exchanger 3-25C Drying gas temperature drop across evaporator heat exchanger 3-45C Condenser temperature heat pump fluid side 40-85C Evaporator temperature heat pump fluid side 20-65C The performance of the invention would be superior to the existing technology based on the following arguments. The proposed control strategy allows the dehumidifier to operate at its maximum potential capacity throughout the entire drying cycle by using 5 the wet-bulb temperature as the primary measured variable for controlling the rate of heat rejection. Normally with existing control systems, the rate of heat rejection in heat pump drying kilns is controlled to maintain a given dry-bulb temperature. However with that form of control the dehumidifier capacity undergoes large changes in capacity as the relative humidity varies during the drying process. For dehumidifier dryers the 10 drying capacity typically increases by 7% for 1°C increase in the wet-bulb temperature with a fixed dry-bulb temperature. It decreases by less than 3% for 1°C increase in dry-bulb temperature, at a fixed wet-bulb temperature. In effect the system is approximately 2.3 times more sensitive to the wet-bulb setting than the dry-bulb. This is why it makes more sense to adjust the heat rejection to maintain the wet-bulb rather than the dry-bulb, 15 provided both are within acceptable limits. When the rate of heat rejection is controlled by measuring and maintaining the wet-bulb temperature, the dry-bulb temperature rises as the product dries. This is normally acceptable for the product, and is consistent with many accepted heat-and-vent drying schedules. In effect the natural drying trajectory of the heat pump dehumidifier kiln 20 system which uses the wet bulb as the primary measured control variable is already close to accepted schedules, and this proposed invention makes use of this feature. Eventually the dry bulb temperature will reach the safe limit for the product being dried, or the dehumidifier will reach the normal operating limits for the condensing 14 temperature of the compressors. As the limit temperatures are reached as indicated by the drying gas dry bulb temperatures with optional confirmation from the refrigeration circuit sensors, the heat pump refrigerant flow is reduced by reducing the compressor capacity which increases the efficiency of the process. Rather than using the measured 5 refrigerant pressure to specify the limit conditions, this scheme uses the dry-bulb temperature as an indicator. This is cheaper to do, and it integrates well with the overall drying cycle control. A preferred example of the function of the control system based on detailed computer process simulation is shown in Figure 2. The wet and dry bulb temperatures start at an 10 ambient of roughly 12°C as read from the left side of the graph and the drying run begins with a "Heat Up" phase. It is acknowledged that additional auxiliary heaters may be used to accelerate this phase without materially affecting the invention. When the system reaches the preset wet bulb temperature, in this case 41°C, the control system acts to run the heat pump to extract the maximum rate of moisture removal and run the 15 heat rejection coil to maintain that wet bulb temperature through adjusting the amount of heat rejected from the system. Detailed computer simulation has shown that the drying gas temperature can be increased progressively to 60 - 65°C as the drying rate naturally falls within the limits of commonly available compressors as shown in the "Constant Wet Bulb with Increasing Dry Bulb" phase in Figure 2. This gives a higher 20 driving force for moisture removal from the timber during the later parts of the drying process, as indicated by the difference between the wet and dry bulb also illustrated in Figure 2. This is also reflected in the ability of the control system to keep the moisture extraction rate closer to its maximum value for longer as measured on the right side of the graph. In cases where the driving force must be moderated because of limits present 25 in the material being dried, the wet bulb temperature can be increased to control the driving force within these material based limits as shown in the "Increasing Wet Bulb" phase in Figure 2. It will be appreciated that the invention is not restricted to the particular embodiments and modifications described above and that numerous modifications and variations can 30 be made without departing from the scope of the invention. 15 REFERENCES CITED US 2,418,239 1 Apr. 1947 Smith US 4,250,629 17 Feb. 1981 Lewis US 4,356,641 2 Nov. 1982 Rosenau 5 NZ 213728 7 Oct. 1985 Thompson US 4,599,808 15 July 1986 Gelineau and Kinney US 5,940,984 24 Aug. 1999 Moren and Ab Chen, G., Bannister, P., McHugh, J., Carrington, C. G., Sun, Z. F. Design of Controlled Atmosphere Dehumidifier Fruit Driers. IPENZ Transactions, (Institution 10 of Professional Engineers New Zealand) 27:31-34 (2000) </div>

Claims

<div class="application article clearfix printTableText" id="claims"> - 16- WHAT WE CLAIM IS: 1. A heat pump for a drying apparatus including: a means to evaporate a heat pump refrigerant in which at least a portion of the heat of evaporation of the refrigerant is transferred by heat exchange from a drying gas medium, and a means to condense the heat pump refrigerant after it has been compressed in which at least a portion of the heat of condensation is transferred by heat exchange to a drying gas medium, and a means for sensing the wet bulb and dry bulb temperatures of the drying gas, and a means for rejecting heat from the drying apparatus, and a means for controlling the amount of heat rejected from the drying apparatus based on the wet bulb temperature of the drying gas such that the wet bulb temperature is kept nominally constant for an extended period during the drying process, and a means for controlling the total flow of refrigerant in the heat pump circuit based on the dry bulb temperature of the drying gas such that the dry bulb temperature is kept within certain limits between 25 and 90C throughout the drying process. 2. A heat pump according to claim 1 where the means for rejecting heat from the drying apparatus to the external environment involves full time or periodic drying gas venting, pre-cooling the drying gas entering an evaporator, pre-cooling any make-up or purge drying gas entering or leaving the apparatus, sub-cooling the refrigerant after it leaves a condenser and before it enters the evaporator, de-superheating the refrigerant leaving a compressor, and/or partially or wholly condensing the refrigerant. 3. A heat pump according to either one of claims 1 and 2 arranged to use air as the drying gas. 4. A heat pump according to any one of claims 1 to 3 wherein said means for controlling is arranged to change the rate of heat rejection from the drying INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 - SEP 2005 RECEIVED -17- apparatus based on drying time and/or other indicator or indicators such as refrigerant temperature or pressure, moisture content of the material being dried, drying rate, wet or dry bulb temperature and/or relative humidity of the drying gas, or integrated amount of moisture removed. A heat pump according to any one of claims 1 to 4 wherein said means for controlling is arranged to change total system refrigerant flow based on drying time and/or other indicator or indicators such as refrigerant temperature or pressure, moisture content of the material being dried, drying rate, wet or dry bulb temperature and/or relative humidity of the drying gas, or integrated amount of moisture removed. A heat pump according to claim 5 wherein said means for controlling is arranged to change the refrigerant flow by reducing or increasing the heat pump compressor capacity or turning one or more heat pump compressors on or off. A heat pump based process for drying a material comprising: evaporating a heat pump refrigerant wherein at least a portion of the heat of evaporation of the refrigerant is transferred by heat exchange from a drying gas medium, and condensing the heat pump refrigerant after it has been compressed wherein at least a portion of the heat of condensation is transferred by heat exchange to a drying gas medium, and sensing the wet bulb and dry bulb temperatures of the drying gas, and rejecting heat from the drying process, and controlling the amount of heat rejected based on the wet bulb temperature of the drying gas wherein the wet bulb temperature is kept nominally constant for an extended period during the drying process, and controlling a total flow of refrigerant in the heat pump circuit based on the dry bulb temperature of the drying gas wherein the dry bulb temperature is kept within certain limits between 25 and 90C throughout the drying process. INTELLECTUAL PROPERTY OFFICE OF N.Z. 1 - SEP 2005 RECEIVED -18- 8. A heat pump based process for drying a material according to claim 7 including rejecting heat from the process to the external environment by full time or periodic drying gas venting, pre-cooling the drying gas entering the evaporator, pre-cooling any make-up or purge drying gas, sub-cooling the refrigerant after it 5 leaves the condenser and before it enters an evaporator, de-superheating the refrigerant leaving a compressor, and/or partially or wholly condensing the refrigerant. 9. A heat pump based process for drying a material according to either one of claims 7 and 8 including using air as the drying gas. 10 10. A heat pump based process for drying a material according to any one of claims 7 to 9 including controlling the rate of heat rejection based on drying time and/or other indicator or indicators such as refrigerant temperature or pressure, moisture content of the material being dried, drying rate, wet or dry bulb temperature and/or relative humidity of the drying gas, or integrated amount of 15 moisture removed. 11. A heat pump based process for drying a material according to any one of claims 7 to 10 including changing total system refrigerant flow based on drying time and/or other indicator or indicators such as refrigerant temperature or pressure, moisture content of the material being dried, drying rate, wet or dry bulb 20 temperature and/or relative humidity of the drying gas, or integrated amount of moisture removed. 12. A heat pump based process for drying a material according to claim 11 including controlling the refrigerant flow by reducing or increasing the heat pump compressor capacity or turning one or more heat pump compressors on or off. 25 13. A drying system substantially as herein described with reference to the accompanying drawings. 14. A drying process substantially as herein described with reference to the accompanying drawings. INTELLECTUAL PROPERTY OFFICE OF N.Z.

1 - SEP 2005 RECEIVED </div>