GB2052704A

GB2052704A - Air Temperature Control Apparatus

Info

Publication number: GB2052704A
Application number: GB8008739A
Authority: GB
Original assignee: GEA Luftkuehler GmbH; GEA Luftkuehlergesellschaft Happel GmbH and Co KG
Current assignee: GEA Luftkuehler GmbH; GEA Luftkuehlergesellschaft Happel GmbH and Co KG
Priority date: 1979-05-31
Filing date: 1980-03-14
Publication date: 1981-01-28
Also published as: GB2052704B; DE2922179A1; MX152704A; DE2922179C2

Abstract

An air temperature control apparatus for controlling the temperature of drying air for kilning comprising two heat exchange units W1 and W2 each having two elements W11, W12 and W21 and W22 respectively. One of the units W1 is placed in thermal contact with hot exhaust air FL, the other W2 in contact with cold feed air AL. The units W1, W2 are connected via heat conducting fluid lines such that by altering the position of valves 21 heat can be transferred either directly from unit W1 to unit W2, or indirectly via a heat pump circuit 16 comprising an evaporator unit 17, a compressor 18, a condenser unit 19 and a pressure reducer 20 such that heat is transferred at a higher temperature, or both. A waste heat exchange circuit 37 is connected to a motor 27 which drives the pressure reducer 20 to allow waste heat to be extracted and transferred via piping to a waste heat exchange element 26 which serves to further heat the feed air AL. <IMAGE>

Description

SPECIFICATION Air Temperature Control Apparatus The invention relates to air temperature control apparatus and concerns apparatus for controlling the temperature of drying air flowing through a kiln floor during the kilning of agricultural products.

Some agricultural products, more particularly plant substances such as cereals or fruit have to be dried during processing. This applies particularly to the preparation of malt used for example for the preparation of beer or whisky.

In the preparation of malt, that is, of a germinated cereal such as barley, the cereal is first watered with aerated water to soften it. The softened cereal, which has a water content e.g. of 45% is then germinated under constant aeration, whereafter it is known as green mait. The green malt is then placed in a drying chamber in which the 45% water content of the malt is reduced td, as a rule between 1.5 and 3%. The term "kilning" or "sweating" is used to denote this drying of germinated or green malt and the drying of other agriculturalproducts.

Sweating or kilning proceeds in, for example, a kiln disposed inside a drying chamber, that is, a chamber which is adapted to store high-water content agricultural products and which has a perforate floor up through which warm drying air may be blown. The kiln floor is made of wire mesh or perforate sheet metal or the like. When kilning is done in association with a floor of this kind -process is described as "floor or hurdle kilning".

In floor kilning the floor extends-within the drying chamber. An air intake below the floor allows fresh feed air to enter the drying chamber and exhaust air is discharged through an outlet above the floor or hurdle. The feed air is heated external air and the kiln floor Or hurdle is accordingly associated with means for controlling the temperature of the drying air blown through the hurdle, the means comprises a main air heater disposed before the feed air intake. The main air heater serves to heat the external air taken in from the surrounding atmospheric air; In a known drying process for the kilning of malt for beer production, the temperature ofthe feed air entering the drying chamber is raised by the main heater from 53 to 86 C. Three consecutive drying phases occur during the kilning of the malt.First the withering phase, which lasts for approximately 12 hours and during which time the feed air, that is the drying air, enters the chamber at a temperature which increases from 53 to 60so, second the heating phase which lasts approximately from 3 to 4 hours in which time the temperature of the feed air is increased from 60 to 820C, and third the final phase, which lasts 5+ hours and in which time the temperature of the feed air supplied is increased from 82 to 86 CC.

During the drying phases the water content of the green malt decreases from 45% at the beginning of the kilning period to 10% at the end of the first or withering period and finally to 2% at the end of the-complete kilning period of 21 hours.

Considerable amounts of energy are consumed if the drying air has to enter the drying chamber at, for instance, the feed air temperature hereinbefore specified for the kilning of malt for beer production.

In order to reduce the cost of hurdle kilning a process has been developed in which the apparatus for controling the temperature of the drying air passing through the hurdle is associated with a heat exchanger allowing the exhaust air to yield some of its heat to the feed air by direct heat exchange. However, since the exhaust air and the inlet air ducts are usually very far apart from one another, the constructional outlay on, in particular, the necessary cross-flow heat exchanger is very high.

A A second improved temperature control apparatus which can be installed without any constructional modification has an indirect heat recovery apparatus comprising two recuperative nested tube type heat exchange units connected to a closed pipe system which includes a circulating pump which forces a heat conducting fluid through the pipe system. In this apparatus one of'the heat exchange units is an air cooler and is placed in the flow of the exhaust air adjacent but outside the exhaust air duct. The second unit is placed exterior to the:chamber adjacent-the feed air intake and acts as an air preheater which heats air taken from the ambient atmosphere by means of the heat removed from the exhaust air by the heat conducting fluid flowing through the first heat exchange unit.

Indirect heat recovery has been found to provide a reduction of, for instance, between 30 and 35% of the energy consumption of the main air heater, but in the light of rising energy costs, in particular so far as large plants are concerned this reduction is unsatisfactory.

Accordingly, the present invention provides air temperature control apparatus for controlling the temperature of drying air flowing through a kiln floor during the kilning of agricultural products, having a main stage air heater an-d-an indirect heat recovery apparatus comprising: first and second nested tube heat exchange units; a closed pipe system incorporating a pump; and a heat pump circuit having an evaporator unit, a compressor, a condensor and a pressure reducer connected in circuit via piping.

The addition of the heat pump circuit according to the invention to the air temperature control increases the amount of energy that can be saved over the energy saving produced by the existing control apparatuses. For example, the control apparatus of the present invention may provide an energy saving of between 50 and 55% whereas the known apparatuses provide a-saving with the region of 30 to 35%.

If streamlined oval ribbed tubes are used for the nested tube type heat exchange units of the heat recovery facility, the additional air resistance caused by such units, and therefore the additional power required for the fan which blows the air inside the drying facility-i.e. the power consumption of such fan - can be small.

Water is often satisfactory as the heat vehicle circulating in the pipings; however, antifreeze can be added to it. for instance, if the plant has to be shut down at weekends and there is a likelihood of frost.

Conveniently, the construction of the two nested tube type heat exchange units disposed in the exhaust air flow and the external air flow is more particularly adapted in detail to the external conditions of the plate where the hurdle is installed.

Preferably each heat exchange unit comprises first and second heat exchange elements.

It is desirable that the first and second heat exchange units may be placed in contact through either the heat pump circuit or the closed pipe system or both.

Use of the heat pump circuit allows the heat energy removed from the exhaust gas to be transferred at a higher temperature to the second heat exchange unit to be relinquished to preheat the drying air.

In a preferred first embodiment of the invention the first and second heat elements are connected in series and in a first arrangement of the valve means, said valve means are so positioned that the first and second heat exchange units are brought into thermal contact only via the heat pump circuit whereas in a second arrangement the valve means are positioned so as to disconnect the heat pump circuit and connect the closed pipe system to the heat exchange unit.

If this first embodiment is used during the hurdle kilning of malt for beer or whisky production, the first arrangement of the valves is selected during the withering phase and the first part of the heating phase.

The second arrangement of valves is used for the remainder of the heating phase and for the final kilning phase.

It is convenient in a second preferred embodiment of the invention for a first arrangement of the valve means to thermally connect the heat pump circuit between the respective second heat exchange elements of the first and second heat exchange units and the closed pipe system between the respective first heat exchange elements of the first and second heat exchange units whereas a second arrangement of the valve means causes all the heat exchange elements to be connected to the closed pipe system.

Again if this second embodiment of the control apparatus is used during the hurdle kilning of malt the first arrangement of the valve means is selected during the withering phase and the first part of the heating phase while the second arrangement is used for the second part of the heating phase and for the final kilning phase.

Preferably, in this second embodiment the second arrangement of the valve means causes the first element and the second element of the first heat exchange unit to be connected to the second and first elements respectively of the second heat exchange unit.

In the further third preferred embodiment a first arrangement of the valve means allows the heat pump circuit to be thermally connected between respective second elements of the first and second units and the closed pipe system between the first elements of the units, however, in a second valve arrangement the heat pump circuit is brought into thermal contact with the sedond element of the first unit and the first element of the second unit, the closed pipe system being connected between the remaining elements of the respective units.

If this third embodiment is used to control air temperaturesduring the hurdle kilning of malt, the first arrangement of the valve is adapted during the withering and heating phases and the second arrangement during the final kilning phase.

A fourth embodiment is also preferred in which a first arrangement of the valve means connects the first and second elements of the first heat exchange unit in series and said first element is thermally connected via the heat pump circuit to the second element of the second heat exchange unit while the first element of the second heat exchange unit is connected via one heat conducting fluid line to the first element, and via a second line to the second element of the first heat exchange unit, whereas in a second arrangement the valve means are so positioned that the heat pump circuit is bypassed and the closed pipe system connects the first element of the second unit via the second then the first element of the first unit of the second element of the second unit which is connected to the first element of the second unit in this fourth embodiment.In malt hurdle kilning operation the first valve means arrangement may be used during the withering phase and the first part of the heating phase and the second arrangement during the remainder of the heating phase and the final kilning phase.

Direct heat exchange may be provided in many of the embodiments described above, that is, the second element of the first heat exchange unit may act as the evaporator unit and the second element of the second heat exhange unit may act as the condenser unit of the heat pump circuit.

However, it is generally preferred to provide separate evaporator and condenser units and make thermal contact between the first and/or the second elements of the first unit and the heat pump circuit via a heat conducting fluid circuit and the evaporator and between the first and/or the second elements of the second unit and the heat pump circuit via a second heat conducting fluid circuit and the condenser of the heat pump circuit.

Conveniently the same heat conducting fluid may be used in each separate circuit.

In known, malt hurdle kilning apparatuses the air is directly heated before entering the hurdle by an oil or gas burner having a naked flame.

It is however, a preferred feature of the embodiments of the present invention to provide a main stage air heater comprising a nested tube waste heat exchange element unit to supply heat extracted from a compressor motor which operates the pressure reducer to the drying air.

The main stage heater may also include an oil gas burner similar to that used in the known apparatuses connected in series with the waste heat exchange element.

The compressor motor mentioned above may be, for example, a gas or diesel engine having a lubricating oil circuit, a water cooling circuit and an exhaust gas duct each in thermal contact with a respective heat exchanger.

Heat may also be supplied to the waste heat exchange unit via a boiler heating chamber.

Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings in which: Figure 1 is a diagrammatic view of a complete hurdle arrangement for the kilning of malt comprising a known system for controlling the temperature of the drying air flowing through the hurdle; Figure 2 shows a first embodiment of air temperature control apparatus according to the invention; Figure 3 shows a second embodiment of air temperature control apparatus according to the invention; Figure 4 shows a third embodiment of air temperature control apparatus according to the invention;; Figure 5 shows a fourth embodiment of air temperature control apparatus according to the invention, and Figure 6 is a graph indicating the variation with time in hours of the air temperature measured in OOC throughout a kilning operation.

Referring now to the drawings, Figure 1 shows a complete hurdle kiln drying chamber 1 having a known type of air temperature control system.

The drying chamber 1 is defined by external walls 2. A hurdle support member 2' comprises a base projecting into the chamber from and perpendicuiar to a side one of the walls 2 and an upturned free end face remote from but parallel to the side wall. Thus, the support member 2' forms an L shaped projection across which a floor 3 of the hurdle on which the malt is placed extends.

The floor may be made, for example, of wire mesh.

An intermediate region of the base of the member 2' is integrally formed with a downwardly extending chimney in which is situated a kiln fan 6.

In the known air temperature control system external feed air AL is drawn through an air duct 7 and over a main stage air heater disposed in the - duct 7. The main stage heater 9 is an oil or gas burner having a naked flame 10 which heats the air AL directly.

The heated air ZL then passes up through the inlet chimney and into the hurdle 1' via an air inlet in the base of the support member 2'. The air 2L then flows up through the malt 4 on the hurdle floor 3. Exhaust air FL from the hurdle 1' passes into a vertical outlet 8 adjacent the upturned end face of the support member 2' and is discharged to atmosphere through an exhaust duct 7'.

Outlet 8 and inlet duct 7 are joined by a connecting passage 13 formed between an external wall and the upturned end face of the support member. The passage 13 is provided with a throttle valve to allow an adjustabie controlled amount of the exhaust air FL to mix directly with the incoming feed air AL.

The known air temperature control system also includes an indirect heat recovery apparatus comprising two recuperative nested, tube type heat exchange units W1, W2, connected in a closed pipe system 11, 11' incorporating a circulatory pump 12. The tubes of the heat exchange unit W1 and W2 may be finned and oval.

The first exchange unit W1 is positioned externally of and adjacent to the exhaust duct 7' while the second unit W2 is similarly positioned adjacent the inlet duct 7.

By means of a heat conducting liquid circulating in the tubes 1 5 of the heat exchangers W1, heat is removed from the discharge exhaust air by the liquid which conducts the heat via the pipe system 11, 11' to the second exchange unit W2 where the heated liquid relinquishes the heat obtained from the exhaust air to the feed air being drawn towards inlet duct 7.

Thus the first heat exchange unit W1 acts as an air cooler and the second heat exchange unit W2 forms an air preheater.

The drying chamber 1 used in association with the embodiments of the air temperature control systems of the present invention to be described hereafter is basically similar to that described above and hence only the portions of the drying chamber directly connected to the control systems are shown in Figures 2 to 5.

In the first embodiment of an air temperature control system according to the present invention shown in Figure 2, the two heat exchange units W, and W2 both comprise two elements connected in series.

The elements of heat pump W, are joined by connection of an inflow pipe 11' of one element W", and an outflow pipe 23 of the other element w12W An outflow line 11 of the heat exchange W" having a valve 21 is connected through an evaporator unit 17 of a heat pump circuit 1 6 to an inflow line 23' of the element WX, such that a heat conducting fluid which may be, for example, water flowing through the lines 11 and 23' is brought into thermal contact with a condensed vapour passing through a pipe system 22 of the heat pump circuit.

The pipe system 22 passing through the evaporator unit 1 7 connects first to a compressor 1 8. From the compressor 18 the vapour carried by the pipes passes through a condenser unit 1 9 and the then condensed vapour passes to a pressure reducer 20 connected via pipes 22 back to the evaporator unit 17.

An outflow line 11" of one element21 of the second heat unit W2 is connected via a valve 21d and through the condenser unit 19 to an inflow line 24b incorporating a pump 12b of a second element W22 of the second heat exchange unit W2 An outflow line 24a of element W22 and an inflow line 11"' of element W21 are connected together in a similar manner to that described for the heat exchange unit W,.

Bypass lines 11 a and 23' a branch from a portion of line 11' connecting the unit W, to the valve 21b and from a portion of line 23' connecting the pump 1 2b to the evaporator unit 1 7 respectively.

The bypass lines 23'a and 1 a connect to the second heat exchange unit W2 by joining with a region of outflow line 11" of element W2, between the element and the valve 21 d and a region of the inflow line 24b of element W22 between the pump 1 2b and the condenser 19 respectively.

The pressure reducer 20 of the heat pump circuit 1 6 is connected by a line 27a to a compressor motor 27 which is a gas or diesel engine.

The motor 27 is provided with a lubricating oil circuit 29, a water cooling circuit 31, and an exhaust gas duct 33 through which the motor exhaust gases MA escape, all connected into a waste heat exchange system 37.

The oil circuit 29 cooperates thermally with a heat exchanger 28 connected via line 30' to a second heat exchanger 30 which is in thermal contact with the water cooling circuit 31 and via a heat conducting fluid outflow line 34 incorporating a valve 36a to waste heat exchange element 26 which in this embodiment acts as the main stage air heater 9.

The second heat exchanger 30 is linked by a waste heat conducting fluid line 31 a to a third heat exchanger 32 thermally connected to the gas exhaust duct 33 to remove heat from the exhaust gases MA.

From the third heat exchanger 32 the waste heat conducting fluid (which may be the same substance as the other heat conducting fluids) passes through an inflow line 32a in which is incorporated a boiler heating chamber 35 which may be a gas or oil fired hot water boiler, and a pump 1 2c to ensure circulation of the fluid to a waste heat exchange element 26.

During the withering phase and the first part of the heating phase of a kilning operation, the heat pump 16 is connected to the two units Wr, W2 by opening valves 21b and 21d. Valves 21c and 21a remain shut to prevent bypass of the heat conducting fluid round the bypass lines 11 a and 23'a.

The heat conducting fluid circulating through the heat exchanger unit W, removes heat from the exhaust air FL flowing out of exhaust duct 7.

The heated fluid is circulated by the pump 1 2a through the outflow pipe 11' (in the direction indicated by the arrow on the line), through the open valve 21 b and the evaporator unit 1 7.

In the evaporator unit 1 7 heat is transferred from the heated fluid to the condensed vapour flowing through pipes 22. Thus, the condensed vapourvapourises and is passed along pipes 22 to the compressor 18 which compresses the vapour causing the temperature of the vapour to rise. The cooled fluid is then returned by pump 1 2a via line 23' to the heat exchange unit W, to remove more heat from the exhaust air FL.

Meanwhile the cool fluid from the heat exchange unit W2 is pumped along the line 11" through the valve 21 d by the pump 1 2b and comes into thermal contact with the high temperature vapour in the condensor unit 1 9.

The cool fluid causes the vapour to condense and heat is transferred during condensation from the vapour to the cool fluid. Thus, by using the heat pump circuit 1 6 the temperature of the fluid flowing through lines 11" and 24bis raised above the temperature which would be obtained by direct heat transfer.

The heated fluid is then pumped through the line 24b into the pipes 1 5 of heat element W22 and via lines 11"' and 24a to the other heat element W2,. Heat is therefore transferred from the heat fluid to the feed air AL entering the air inlet duct 7.

The again cool fluid will then be recirculated through the condenser unit 1 9 repeating the heat transfer cycle.

The condensed vapour in the condenser unit 19 returns to the evaporator unit 17 via the pressure reducer 20 which is operated by the motor 27 and which by reducing the pressure lowers the temperature of the condensed vapour still further to enable efficient heat transfer from the fluid circulating through the heat exchange unit W, and vapourization of the condensed vapour to restart the heat pump cycle.

As mentioned above, in this embodiment the mainstage air heater 9 is the waste heat exchange unit 26 which uses the waste heat produced by-the compressor motor 27 to heat the preheated air AL to AL required temperature before it is passed over the malt to be dried.

Waste heat produced by the motor is passed into the lubricating oil circuit 29, the water cooling system 31 and is darried out of the system by the exhaust gases along exhaust gas duct 33.

During the withering phase the position of the valve 36a is such that bypass line 34a is closed and cool fluid from the heat exchange unit 26 passes through outflow line 34 to heat exchanger 28 where it removes heat from the lubricating oil circulating in circuit 29. The heated fluid then travels along the line 30' to a second heat exchanger removing heat from the water cooling system.

Line 31 a connects the second heat exchanger 30 to the third heat exchange 32 where the heat conducting fluid removes heat from the exhaust gases passing through the duct 33 associated with unit 32.

The line 32a connects the unit 32 to a boiler heating chamber 35 which is not in use during the withering phase. The fluid is then pumped into the pipes 26a of the waste heat exchange element 26 by means of pump 12c..Thus the heat conducting liquid relinquishes heat to the preheated feed air AL flowing through it producing hot air with which to dry the malt 4.

The cooled fluid is then recirculated through the system via outflow line 34.

During the second part of the heat phase and the final kilning phase, the heat pump unit 1 6 is bypassed by closing valves 21 b and 21 d and opening valves 21 c and 21 a. Thus, fluid circulating in the pipes 1 5 of heat exchange unit W1 removes heat from the exhaust air FL and leaves the unit W, via outflow line 1 11,.

The heated fluid then flows down line 11 a through the open valve 21 c and via pump 1 2b incorporated in inflow line 24b to the second heat exchange unit W,. Heat is removed from the fluid by the feed air AL entering inlet duct 7 as the fluid circulated first through the element W22 and then via connecting lines"' and 24a through the other element W21 Cool fluid is discharged from the heat exchange unit W2 via line 11" to bypass line 23'a and through the open valve 21 a to line 23'. Pump 1 2a then pumps the fluid into the heat exchange element W12 so that heat may be once again retrieved from the exhaust air FL.

Also during the heating and final kilning phases the boiler heating chamber 35 is operative and valve 36a is in such a position that the fluid flowing from the heat exchange element 26 along line 34 also passes through the bypass line 34a.

Thus, either the heated fluid flowing from the heat exchanger 32 down line 32a will be further heated in the boiler heating chamber 35 or, alternatively, cool fluid exiting from the waste heat exchange element may bypass the heat exchangers passing via bypass line 34a and line 32a,to the boiler heating chamber 35 and being heated merely by the operation of the chamber.

Like the previous embodiment, the embodiment shown in Figure 3 includes a heat pump circuit 1 6 comprising the evaporator 17, compressor 18, condensor 1 9 and pressure reducing valve 20. However each of the heat exchange units W, and W2 comprises two independent heating elements, that is unlike the previous embodiment the elements of the respective units W, and W2 are not connected together in series.

Morever, in this embodiment, the heating element W" is connected via outflow line 11 in which is incorporated a pump 12d, through a valve 21 m directly to the heating element W21.

Similarly, an inflow 11' of the heat element W" connects the heating element W2, back to the heating element W" via two valves 21 r and 36b.

The remaining element W12 of the heat exchange unit W, has an outflow line 23 including a valve 21f, connected through the evaporator 1 7 of the above described heat pump circuit 1 6 to the inflow line 23' of the same element W,2. A pump 1 2e is incorporated in the inflow line 23'.

Similarly the second element W22 of the second heat unit W2 has an outflow line 24a incorporating a valve 21 v and a pump 12f, which is connected through the condenser 19 of the heat pump unit to an inflow line 24b of the element W22. The inflow line 24b includes two valves 219 and 21n.

A portion of the line 23 between the heating element W12 and the valve 21 f is connected by a bridging line 11 'a having a valve 21 e to a part of line 11' between the valve 26b and element W".

Line 11' is connected through the valve 36b by means of a line 11 'a to a part of line 23' between the pump 1 2e and the heat exchange element W12.

A branching line 11 c having a valve 21s connects a point on the line 11' immediately prior to the valve 21 m between the valve and the heat exchange element W" to a point on the inflow line 24b between the valve 21 n and the heat exchange element W22.

Lines 11 and 24b are also connected via a second line 11 d branching from a point on the line 24b intermediate the valves 219 and 2n. The line 11 d extends to connect with the line 11 at a position between the valve 21 m and the heat exchange element W2,.

Similarly lines 11' and 24a are interconnected via lines 11 'c and 11 'd incorporating valves 21 p and 21 q respectively.

Line 11 'c branches from the outflow line 11 of the heat exchange element W" between the element W" and the valve 21 p. The line 11 'c connects with a point on the outflow line 24a between the value 21 p and the heat exchange element W22 The other branching line 11 'd starts from the other side of valve 21 v and joints with a point on the line 11 between the valve 21 rand heat exchange element W2,.

Unlike the first embodiment, the air temperature control system shown in Figure 3 includes a main stage heat heater 9 having two parts. One part is a conventional oil or gas burner 25 having a naked flame 10 which provides direct heating of the air AL after it has passed through the waste heat exchange element 26. The element 26 forms the first or indirect stage of the mainstage air heater.

Thus although the waste heat exchange circuit 37 connected between motor 27 and heat exchange element 26 of the second preferred embodiments is substantially similar to that to Figure 2, there is no need to incorporate a boiler heater chamber 35 in line 32a to further heat the conducting fluid. For this reason there is no connecting line 34a between the line 32a and the outflow line 34.

The remaining parts of the waste heat exchange circuit 37 corresponds exactly to the circuit 37 described in relation to the first embodiment of the invention.

During the withering phase and the'first part of the heating phase of a kilning operation using the embodiment of the air temperature control system shown in Figure 3, valves 21 e, 21 t, 21s, 21 p and 21 q are closed and valve 36b is in a first position in which fluid between lines 11' and 23' is prevented.

Hence by having the remaining valves 21f, 21g,21p,21mand21nopen, the heat pump circuit 1 6 is connected to heat element W12 and W22 while the heat elements W2, and W11 are connected in a closed pipe system.

Thus, the heat conducting fluid flowing in the pipes 15 of the heat element W" extracts heat from the exhaust air FL and is pumped via outflow line 11 by pump 12d through open valve 21m to the heat element W2,.

The heated fluid circulates through the pipes 1 5 of element W21 relinquishing the stored heat to the incoming feed air AL.

Cooled fluid exists via the line 11' and returns through the valve 36b to the heat element W".

The cooled fluid circulating through the pipes of element W" again extracts heat from the exhaust air and the heat transfer cycle is restarted.

At the same time, heat conducting fluid in the pipes of the heat exchange element W,2 also extracts heat from the exhaust air FL passing through the heat exchange unit W,.

This fluid is heated and then pumped by the pump 1 2e along the outflow line 23 via valve 21 f to bring it into thermal contact, in the evaporator unit 17, with condensed vapour flowing through pipes 22.

Heat is transferred to the condensed vapour which then vaporises and passes along pipes 22 to the compressor 18 wherein the temperature of the vapour is raised still further by compressing the vapour. The now cool heat conducting fluid returns via line 23' and pump 1 2e to the heat exchange element Wr2.

Cool fluid from the heat exchange element W22 is pumped through line 24a and enters the condenser unit 1 9. There, the fluid comes into thermal contact with the heated vapour and heat is transferred from the vapour to the fluid condensing the vapour.

The now heated fluid then returns through valves 21 g and 21 p along line 24b to the element W22 and relinquishes the heat removed from the vapour to the incoming feed air passing through the unit W2.

Cool fluid is repumped to the condensor unit and the cycle begins again.

The condensed vapour in the condensor unit 19 is returned to the evaporator unit 1 7 through the pressure reducer 20 which is operated by the motor 27 to reduce the temperature of the condensed vapour still further ensuring efficient heat transfer from the heated fluid passing through the evaporator unit via line 23.

Waste heat from the motor 27 is extracted from the oil lubricating circuit 29 via a heat exchanger 28 connected to an outflow line 34 through which flows the heat conducting fluid.

The heat exchanger 28 is connected via line 30' to the second heat exchanger 30 which extracts heat from the water cooling system 31.

The fluid then flows through a line 31 a to a third heat exchange which places the fluid in thermal contact with the hot exhaust gases in the gas duct 33.

The heated fluid is then pumped through line 32a by means of a pump 1 2c to the heat exchange element 26 and circulates through the pipes 1 5 of the heat exchange element 26 relinquishing its heat to the already preheated air providing the indirect portion of the main stage air heater 9.

The direct part, that is the oil or gas burner 25, of the main stage air heater remains off during the withering stage of the kilning operation and the operation of the waste heat exchange system 37 is analogous to that described in relation to the first embodiment.

During the second part of the heating phase and the final kilning phase the heat pump circuit 1 6 is disconnected from the heat exchanger elements W,2 and W22 and the closed pipe system is connected to ail four heat exchange elements W", Wr2, W2, and W22.

To obtain this arrangement, valves 21f, 21g, 21n, 21t, 21q and 21v are closed while valves 21e, 21m,21p,2lrand 21sare open. Also the value 36b- is arranged so as to connect line 11' to line 23' via line 11 'b while still keeping line 11' open.

In this case fluid heated by the exhaust air flowing through the heat exchange element W" is pumped back along line 11 through valve 21 m to the heat exchange element W2, and along line 11 c via valve 21s and line 24b to the heat exchange unit W22 by pump 12d.

Thus, portions of the heated fluid flows simultaneously through the pipes of both heat exchange elements W22 and because the valve 21 p is closed cooled fluid exiting from the element W22 is diverted along line 11 'c through the valve 21 p and rejoins with cool fluid passing from the heat exchange element W2, along line 11'.

Upon reaching valve 36b in the line 11' the now cool fluid is diverted along line 11 'b and is pumped by pump 12e into heat exchange element W,2 via line 23'.

The fluid circulates through the pipes of the element W,2 being heated by the exhaust air FL flowing out of the element along line 23 via valve 21 e and entering the other element W" of the heat exchange unit W, via the remainder of line 11'.

Thus further heat is extracted from the exhaust air FL. The heated fluid then passes from the heat exchange elements via line 11 to restart the heat transfer cycle.

The operation of the waste exchange circuit 37 of this second embodiment during the entire heating phase and the final kilning operation is identical to that during the withering phase (i.e. to the operation of the circuit 37 of the first embodiment during the withering phase).

However, the incoming air AL is heated still further during these latter stages by means of an oil or gas burner 25 having a naked flame 10 which provides the direct heating portion of the main stage air heater 9.

Figure 4 shows a third embodiment of an air temperature control system which is substantially similar to the embodiment described in relation to Figure 3.

This embodiment is identical in construction to that of Figure 3 except that because no direct main stage oil or gas burner is provided the waste heat exchange circuit 37 (unlike the second embodiment) includes the boiler heater chamber arrangement of the first embodiment in which a boiler heater chamber is incorporated in lines 32a between the pump 1 2c and a short circuit line 34a, connects a portion of line 32a between the third heat exchanger 32 and the boiler chamber 35 through a valve 36a to the inflow line 34.

During the withering and entire heat phase of a kilning operation utilising this embodiment of the invention, the first arrangement of valves adopted in the second embodiment (shown in Figure 3) is used, that is, valves2le, 21t,21s, 21p, and 219 are closed, valve 36b is in a position which blocks the connecting line 11 'b and valves 21 f, 21 g, 21p, 21m and 21r are open.

The heat exchange elements W,2 and W22 are therefore connected via lines 11 and 11' and elements W1, and W2, are in heat contact through the heat pump circuit 16. Thus during the withering phase and the entire heating phase the heat transfer occurs between the heat unit W1 and W2 in the manner described for the withering phase and the first part of the heating phase in relation to the second embodiment shown in Figure 2.

Similarly, the operation of the waste heat exchange circuit 37 throughout the kilning operation is identical to that described in relation to the circuit 37 of the first embodiment shown in Figure 2 and will not be discussed here.

However, although the arrangement connecting the heat exchange units W, and W2 together is identical to that of the second embodiment, heat transfer during the final kilning phase occurs in a slightly different manner from that described in relation to the second embodiment.

This is achieved by using a different arrangement of closed and open valves. Thus, in the third embodiment shown in Figure 4, valves 21e,21m,21n,21rand 21vare closedwhilst valves21f,21g,21t,21sand21pareopenand valve 36b is arranged to prevent flow between line 23' and line 11'.

Thus, during the final kilning phase the heat exchange element W,2 is connected by means of the heat pump circuit 16 for heat transfer when the element W2, and elements W" and W22 are connected together through a closed pipe system.

Heated fluid is pumped from the heat exchange element W" along line 11 by means of pump 1 2d and diverted along line 11 c via valve 21 t into heat exchange element W22 through line 24b.

The feed air AL flowing past the element W22 extracts heat from the fluid and the now cool fluid passes out of the heat exchange element W22 along line 11 'c through the valve 21 p and back into the heat exchange element W" to be reheated by means of line 11' and valve 36b.

At the same time, heat is extracted from the exhaust air FL by the fluid flowing through the heat exchange element W,2 and hot fluid flows from the pipes 15 along line 23' via valve 21f and is pumped through the evaporator unit 1 7 and back into the element W,2 by the pump 12e.

Thus, the heated fluid passing through the evaporator unit 1 7 comes into thermal contact with the condensed vapour in the pipes 22 causing it to evaporate and remove heat from the fluid. The vapour is compressed in the compressor 1 8 leaves at a higher temperature and passes into the condenser unit 1 9.

Meanwhile cooled fluid from the heat exchange element W21 passes via valve 21 q and line 11 'd into line 24a. The cool fluid is then pumped by the pump 1 2f through the condenser unit 19 and extracts heat from the vapour causing it to condense.

The condensed vapour is returned to the evaporator unit 1 7 through the pressure reducer 20 which causes a further decrease in the temperature of the condensed vapour.

The now heated fluid passes along line 24b via valve 219 and is diverted through line 11 d and the valve 21 t back to the heat exchange element W21 along line 11 to relinquish the heat to the feed air so that the heat transfer cycle may begin again.

The fourth embodiment shown in Figure 5 includes a waste heat exchange system 37 identical to that shown in Figure 3 described in relation to the second embodiment of the invention. Hence, the details of this portion of the arrangement will not be discussed.

However, the heat units W, and W2 are connected in a different manner.

Like the first embodiment, the elements W" and W12 of the heat exchange unit W, are connected in series via the outflow line 23 of element W12 and the inflow line 11' of the second element W" of the heat exchange unit W,. The inflow line 23' of the heat exchange element W,2 is provided with a pump 1 2a and passes through the evaporator unit 1 7 of the heat pump circuit hereinbefore described.

Upon leaving the evaporator unit 17 line 23' is connected through a valve 36d to line 23'b which branches from a portion of the inflow line 23' between the pump and the evaporator unit 1 7.

Line 23'b is directly connected to heat exchange element W2, and provides an outlet for the fluid flowing therein. An outflow line 11 of the heat exchange element W" is directly connected to the element2, to allow fluid intake.

Outfiow line 24a of element W22 transfers fluid via pump 1 2f to the condensor unit of the heat pump circuit 1 6 while the inflow line 24b returns the fluid to the element W22 via valve 219.

In the same manner as described in relation to the second embodiment of Figure 3 a line 11 a connects a portion of the line 24b between the valve 21 g and element W22 to line 11 by means of a valve 36c.

Lines 24a between the pump 1 2f ahd the element W22 is also connected via a line 11 b having a valve 21 h to a portion of line 11 between the valve 36c and the element W2,.

During the withering phase and the first part of the heating phase, the valve 36d is in such a position that fluid may flow along line 23'b through the valve 36d and then via the evaporator 17 to the inflow line 23' of the heat exchange element W,2.

Also the valve 219 is open whereas the similar valve 21 h is closed and valve 36c is in a position which prevents fluid flow between lines 11 and 24b via connecting line 11 a.

Exhaust air flowing through the heat exchange element W" heats up the circulating in the element. The heated fluid then travels along line 11 via valve 36c to the heat element W21 of the second heat circuit W2.

Some of the heat is obtained from the exhaust gases is relinquished to the incoming feed oil passing through the element W2, by the heat conducting fluid.

Fluid at a slightly lower temperature exits through line 23'b and is directed by valve 36d onto line 23' which passes through the evaporator 1 7 of the heat pump circuit 1 6. The heat remaining in the fluid is sufficient to vapourise the condensed vapour passing through the pipes 22.

The vapour is then transferred via pipes 22 to the compressor 18 which in compressing the vapour raises its temperature and is passed onto the condensor unit 19. Pump 1 2f pumps cool fluid via line 24a to the condensor unit 19. Thus, the cool fluid comes into thermal contact with the pressurised vapour and causes it to condense relinquishing the stored heat.

The now heated fluid is returned via valve 219 along line 24b to the heat exchange element where the stored heat is extracted by the passing feed air AL.

The condensed vapour in the condenser unit 1 9 is fed into a pressure reducer 20 operated via line 27a by a motor 27 which by reducing the pressure lowers the temperature of the condensed vapour still further. Thus, ensuring efficient heat transfer to the cooled condensed vapour from the heated fluid flowing through line 23' with the condensed vapour reaches the evaporator unit 1 7.

During the withering phase the main stage heater 9 comprises only the heat exchange element 26 of the motor heat exchange system which operates in the manner described in relation to the embodiment of Figure 3.

During the second part of the heating phase and the final kilning phase the heat pump circuit 1 6 is disconnected completely by closing valve 21g and by altering the position of valve 36d to prevent fluid flowing from the line 23'b along line 23' and through the evaporator unit 17. Vaive 21 h is opened to provide a fluid link between lines 24a and 11 via line 11 b and valve 26c is placed in position to provide a similar connection from line 11 via line 1 lea to line 26b.

Thus, fluid heated by the exhaust air FL passes through line 11, is diverted via valve 36c along the connecting line 11 a, and enters the heat exchange element W22 via line 24b.

The fluid circulates through the pipes 15 of the element W22 losing heat to the incoming feed air AL and emerges from the element W22 via line 24a. However, the short circuit provided by the open valve 21 h of line 11 b ensures that the fluid passes into the other element W2, of heat exchange unit W2 via the line 11 and circulation through the pipes 1 5 losing still more heat to the incoming air AL passing through the unit W2. The now cool heat conducting fluid exits through line 11' and passes via valve 36d into the inflow line 23'.

Thus the cool fluid returns to the heat exchange unit W, and removes heat from the exhaust air FL in the heat exchange element W,2.

The fluid then passes via connecting lines 23 and 1 to the other element W removing further heat from the exhaust air in order to begin the next heat transfer cycle.

As described above in relation to the second embodiment shown in Figure 3, during the heating and final kilning phases the direct heat portion of the main stage burner 9 is switched on so that the air ZL air heated by the heat exchange element 26 passes through the naked flame 10 of oil or gas burner 25 producing hot air prior to entering the hurdle 1'.

A preferred type of heat pump circuit is described above; however one of the two exchange elements W", W12 of the heat exchange unit W, may serve as the evaporator unit 17. In which case the hot exhaust air FL flowing through the element will cause the heat conducting fluid to evaporate removing heat from the exhaust air FL.

Similarly one of the two heat exchange elements W21, W22 of the heat exchange unit may act as the condensor unit 19. Thus, the cold feed air AL itself would remove heat from the vapour causing it to condense.

Figure 6 illustrates graphically the air temperature variation per unit time during the kilning of malt for beer or whisky production both when a known drying proves employing a kilning period of 21 hours is used and also using the second embodiment of the present invention shown in Figure 3.

Various characteristics of the known drying process are shown. The solid line indicates the temperature of the feed air from the surroundings on each introduction to the hurdle 1' while the dashed line shows the temperature of the drying air immediately after leaving the hurdle floor 2.

Similar characteristics are shown for the second embodiment of the present invention.

That is, the bottom portion of the dot-dash line of Figure 6 indicates the temperature of feed air which has passed through the heat exchange element W2, but not the element W22 while the dotted line inducts the temperature variation of the feed air after it has passed through both the heat exchange element W22 and the waste heat exchange element 26 during the withering phase and the first part of the heating state.

The upper portion of the dot-dash line separated from the lower portion by a discontinuity in the curve represents the temperature variation of the external air leaving the heat exchange element W22 during the second part of the heating phase and during the final kilning phase during which time the heat pump circuit 1 6 is disconnected.

Claims

1. Air temperature control apparatus for controlling the temperature of drying air flowing through a kiln floor during the kilning of agricultural products, having a main stage air heater and an indirect heat recovery apparatus comprising; first and second nested tube heat exchange units; a closed pipe system incorporating a pump; and a heat pump circuit having an evaporator unit, a compressor a condensor and a pressure reducer connected in circuit via piping.

2. Apparatus according to claim 1 , wherein each heat exchange unit comprises a first and second heat exchange elements.

3. Apparatus according to claim 1 or 2, further comprising valve means to allow the first and second heat exchange units to be placed in contact through either the heat pump circuit or the closed pipe system or both.

4. Apparatus according to claim 3, wherein the first and second heat elements of heat exchange unit are connected in series in a first arrangement of the valve means in which said valve means are so positioned that the first and second heat exchange units are brought into thermal contact only via the heat pump circuit, whereas in a second arrangement the valve means are positioned so as to disconnect the heat pump circuit and connect the closed pipe system to the heat exchange units.

5. Apparatus according to claim 3, wherein in a first arrangement of the valve means said valve means are so positioned that the heat pump circuit is thermally connected between the respective second heat exchange elements of the first and second heat exchange units and the closed pipe system is connected between the respective first heat exchange elements of the first and second heat exchange units, whereas in a second arrangement the valve means are so positioned that all the heat exchange elements are connected to the closed pipe system.

6. Apparatus according to claim 5, wherein in the second arrangement the valves are positioned to connect the first element of the first heat exchange unit to the second element and the first element respectively of the second circuit.

7. Apparatus according to claim 3, wherein in a first arrangement of the valve means said valves means are so positioned that the heat pump circuit is thermally connected between respective second elements of the first and second units and the closed pipe system between the first elements, whereas in a second arrangement the heat pump circuit is brought into thermal contact with the second element of the first unit and the first element of the second unit, the closed pipe system being connected between the remaining elements of the respective units.

8. Apparatus according to claim 3, wherein in a first arrangement of the valve means said valve means are so positioned that the first and second elements of the first heat exchange unit are connected in series and first element is thermally connected via the heat pump circuit to the second element of the second heat exchange unit while the first element of the second heat exchange unit is connected via one heat conducting fluid line to the first element and via a second line to the second element of the first heat exchange unit, whereas in a second arrangement the valve means are so positioned that the heat pump circuit is bypassed and the closed pipe system connects the first element of the second unit via the second then the first elements of the first unit to the second element of the second unit which is connected to the first element of the second unit.

9. Apparatus according to any one of claims 3 to 8, wherein thermal contact between the first and/or the second element of the first heat exchange unit and the heat pump circuit is made through a heat conducting fluid circuit and the evaporator unit and thermal contact between the first and/or the second element of the second heat exchange unit-and the heat pump circuit is made via a second heat conducting fluid circuit and the condensor of the heat pump circuits.

10. Apparatus according to claim 9, wherein the same heat conducting fluid is used in each separate circuit.

11. Apparatus according to any one of claims 2 to 8, wherein the second element of the first heat exchange unit acts as the evaporator unit and the second element of the second heat exchange unit acts as the condenser unit of the heat pump circuit.

12. Apparatus according to any preceding claim, wherein the main stage air heater comprises a nested tube waste heat exchange unit supplying heat removed from a compressor motor which operates the pressure reducer to the drying air.

13. Apparatus according to claim 12, wherein the main stage air heater further comprises an oil or gas burner having a naked flame placed in series with the waste heat exchange unit.

14. Apparatus according to claim 12 or 13, wherein the compressor motor is a gas or diesel engine having a lubricating oil circuit, a water cooling circuit and an exhaust gas duct each in thermal contact with a respective neat exchanger.

1 5. Apparatus according to claim 12, 13 or 14 wherein heat produced by a boiler heating chamber is supplied to the waste heat exchanger unit.

1 6. Air temperature control apparatus for controlling the temperature of drying air flowing through a kiln floor during the kilning of agricultural products, substantially as hereinbefore described with reference to, and as illustrated in, Figure 2-6 of the accompanying drawings.

1 7. Any novel feature or combination of features herein disclosed.