Abstract:
Apparatus for recovering heat from a gas flow defines a heat transfer space through which the gas flow can be directed, the heat transfer space having a first region and a second region, a gas inlet and a gas outlet. A first valve arrangement alters the direction of gas flow through the heat transfer space, the first valve arrangement having a first condition in which the first region is upstream of the second region and a second condition in which the second region is upstream of the first region. First and second heat exchangers are disposed in the first and second regions respectively of the heat transfer space. A second valve arrangement connects each heat exchanger selectively either to a first heat transfer medium flow circuit or to a second heat transfer medium flow circuit.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a heat recovery apparatus. A heat recovery apparatus may comprise a heat transfer space having a gas inlet and a gas outlet and in which at least two separate heat exchangers are arranged successively in the gas flow direction, as well as at least two separate heat transfer medium flow circuits. This kind of heat transfer apparatus provides advantageous heat transfer from a gas flow containing impurities. 
     The invention relates also to a method of operating a heat recovery apparatus, which method is effective to minimize fouling in a the heat recovery apparatus. 
     Utilization of heat recovery in order to recover energy from hot gases is commonly known and is beneficial. However, hot gases typically contain various impurities, the nature of which depends on the origin of the hot gases. For example a considerable amount of hot gas is generated in operation of a heat engine and the combustion gases contain various impurities, for example soot and condensable components such as hydrocarbons, depending on the fuel used in the engine. 
     U.S. Pat. No. 4,869,209 shows an automatic programmable cleaning system for heat exchange tubes of a waste heat boiler. For removing deposits from surfaces of the heat exchange tubes, it is suggested to spray water intermittently into the hot gas flow, whereby the soot that is removed is carried along with the exhaust gases out of the boiler. However, this kind of method is not suitable for general use. Further, evaporating the water consumes the energy of the gas, and the water consumption and the need for maintenance also affects the serviceability of the method. 
     It is of great importance that the heat transfer surfaces of the exhaust gas boiler of a combustion engine should be clean, in order to maintain efficiency, but it has been observed that these boilers tend to foul rapidly. In connection with state of the art combustion engines, and specifically with installations in which the combustion gases contain condensable components, for example hydrocarbons, as may be the case with gas engines, it may be necessary to clean the heat transfer surfaces as frequently as every 500 hours. Specifically, when the heat is recovered at different temperature levels, and condensation takes place especially in the last heat transfer stage on heat exchanger surfaces at lower temperatures, the efficiency of heat transfer decreases rapidly. In this kind of installation, the cleaning of the heat exchanger operating at lower temperature is accomplished in practice by emptying the heat exchangers, whereby its temperature may rise. This causes the condensable components in question to become soft (or melt) and drain down away from heat exchanger surfaces. However, this procedure may decrease the efficiency of heat recovery because during the cleaning operation, which may take approximately 8 hours, heat is not recovered by the heat exchanger in question. Further, the need to empty the heat exchanger may in some cases impose demands with respect to its material and construction, specifically due to the fact that being uncooled, its temperature may rise to a higher level than would be desirable. In that case also there is a risk of boiler fire. 
     In order to maintain the efficiency of heat recovery at an adequately high level and generally to maintain the equipment in an optimal operating state, the heat transfer surfaces must be kept clean by regular cleansing. Various methods and devices for this purpose are known from the prior art, but they still include many restrictions and may be inadequate with respect to their operation. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to provide a heat recovery apparatus and method, by means of which it is possible to improve the state of the art and minimize the before mentioned and other problems of the prior art. 
     In an embodiment of the invention, the heat recovery apparatus comprises a heat transfer space having a gas inlet and a gas outlet and in which at least two separate heat exchangers have been provided successively in the gas flow direction, as well as at least two separate heat transfer circuits. The apparatus also comprises means for altering the direction of gas flow in the heat transfer apace and mean; for connecting each heat transfer circuit selectively to either of the at least two separate heat exchangers. This embodiment is specifically suitable for heat recovery from gases containing sticky, adhesive components, such as condensable hydrocarbons. 
     In this case, for example in connection with a gas engine operating in accordance with the Otto cycle, when the heat transfer circuits are operating at different temperature levels, such that thee temperature level of the gas before entering a heat exchanger connected to a first heat transfer circuit is typically 350-450° C. and after a heat exchanger connected to a second heat transfer circuit is typically 180-200° C., and as the heat transfer circuits operate typically at temperature level of, on average 60—100° C., it is possible to provide an arrangement in which, after the heat transfer surfaces have become fouled, specifically through condensation of hydrocarbons, to an excessive level, or otherwise when desired, the mode of operation of the apparatus may be altered as will be described in following. 
     The heat recovery apparatus in accordance with the invention preferably includes a heat transfer space, which comprises two parallel flow ducts, each of which contains at least one heat exchanger and which flow ducts abut at their other ends on a common space. 
     The present invention provides distinct benefits compared to the prior art. Firstly, the method is very effective compared for example to cleaning by water, which does not have much effect on deposits formed by condensed hydrocarbons. Additionally the cleaning of the heat transfer surfaces is done during the normal operation, whereby there is substantially no interference with the heat recovery. Further, there are no heat losses due to evaporation of cleaning water. Thus, by means of the present invention the heat exchangers can be maintained especially clean, since the change of the mode of the operation may be done always according to the need without impairing the operation. Additionally the present invention does not consume any additional agent for cleaning and also wearing of parts is minimized. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the following, the invention is described, by way of example, with reference to the attached drawings, in which: 
     FIG. 1 shows diagrammatically an apparatus in accordance with the present invention, 
     FIG. 2 shows schematically an apparatus in accordance with the invention in its first mode of operation, 
     FIG. 3 shows schematically the apparatus of FIG. 2 in its second mode of operation, and 
     FIG. 4 shows schematically a modified form of the apparatus of FIG. 2 in its second mode of operation. 
    
    
     DETAILED DESCRIPTION 
     In FIG. 1 there is shown a waste heat recovery boiler  10 , including wall structures surrounding a heat transfer space  11 . The heat transfer space  11  is preferably arranged substantially vertical and it comprises preferably two parallel flow ducts  11 ′,  11 ″, which abut at their other ends, here at their lower ends, on a common space. The ducts  11 ′,  11 ″ are separated by a vertical intermediate wall  17 . At least one heat exchanger  12 ,  13  is arranged in each of the parallel ducts. The heat exchangers have heat transfer surfaces of substantially equivalent heat transfer capability. For example, their surface areas may correspond or they may be otherwise arranged so that their operation corresponds to each other. There may also be other heat exchangers and such are preferably located between the heat exchangers  12  and  13  in the gas flow direction. 
     Correspondingly at their other ends the ducts  11 ′,  11 ″ abut on a means for altering the direction of gas flow, which means here comprises two chambers  14 , is separated from each other by a partition wall  16 . It should be noted that the plane of the partition wall  16  is inclined to the plane of the intermediate wall  17  limiting the flow ducts  11 ′,  11 ″. The partition wall  16  prevents direct flow communication between the gas inlet  18  and the gas outlet  19 . The gas inlet  18  is in operational connection with gas production equipment  2  (shown in FIGS. 2,  3  and  4 ). Advantageously the gas production equipment may be a piston engine utilizing gaseous fuel, in view of the consistence of and temperature level of the gases from such engine. The wall structures of the heat transfer space  11  operate also as walls limiting the chambers  14 ,  15 , whereby the apparatus is constructed as an integrated unit. The walls limiting the chambers  14 ,  15 , including the partition wall  16  and the intermediate wall  17 , may be constructed for example as cooled walls, whereby these walls also include heat transfer surfaces. Additionally, being substantially vertical, the ducts  11 ′,  11 ″ may easily he ventilated for removing hot exhaust gases on shut-down, for example for maintenance. 
     The chambers  14  and  15  are selectively connectable to the flow ducts  11 ′ and  11 ″ so that gas is caused to pass always through the two ducts in series. This is accomplished by means of the valves  20 ,  21  provided in the chambers  14 ,  15 . In the figure the valves are illustrated as slide-type valves, by means of which the opening from the chamber  14  to one of the ducts  11 ′,  11 ″ is open and the other opening is closed, and similarly for the chamber  15 . The valves may also be of a different type, such as butterfly-valves. The valves should be positively controlled so that each chamber  14  or  15  may be connected to only one of the ducts  11 ′,  11 ″ at a time, and so that the gas will always pass through the two ducts in series. The flow control of the gas by the slide valves  20 ,  21  shown in the figure takes place by executing a pull or push movement of the shaft of the valves. 
     Operation of the apparatus will be described in the following with reference to FIG.  2 . FIG. 2 illustrates several valves. Valves that are open are illustrated by thicker lines than valves that are closed. The gas is caused to flow from the gas production equipment  2  to the gas inlet  18 . After passing through the gas inlet  18 , the gas flow is guided in a respective desired manner into the heat transfer space  11 . In a first mode of operation shown herein, the valve  21  directs the gas into the duct  11 ′. The gas flows in the duct  11 ′ substantially vertically downwards and passes the heat exchanger  12  transferring heat to heat transfer medium flowing in the heat exchanger. As can be seen from FIG. 2, the heat exchanger  12  is connected to a first heat transfer circuit  311 ,  312 , which operates at higher temperature level than a second heat transfer circuit  301 ,  302 . The first heat transfer circuit will be also referred to as the HT-circuit and the second heat transfer circuit will be referred to as the LT-circuit. The apparatus thus comprises valves  212 ,  214 ,  221 ,  223 ,  222 ,  244 ,  211 ,  213  for connecting the heat transfer circuits selectively to the heat exchangers  12 ,  13 . The heat transfer medium from the first heat transfer circuit  311 ,  312  is guided under control of the valves  212 ,  214  either to the heat exchanger  12  located in the duct  11 ′ or to the heat exchanger  13  located in the duct  11 ″. In the first mode of operation, which is shown in FIG. 2, the valve  212  is open and valve  214  is closed. The heat transfer medium is thus guided to the heat exchanger  12  located in the duct  11 ′, which heat exchanger  12  is upstream in the gas flow. The heat transfer medium is guided back to the first heat transfer circuit under control of valves  221  and  223 . In the first mode of operation, the valve  221  is open and valve  223  is closed. 
     After passing the heat exchanger  12 , the gas enters the common space connecting the flow ducts  11 ′,  11 ″, from which it passes to the second flow duct  11 ″ by changing its flow direction about 180° upwards. The gas flows upwards in the duct  11 ″ through the heat exchanger  13  transferring heat to the heat transfer medium flowing in the heat exchanger. As can be seen from FIG. 2, the heat exchanger  13  is connected to the second heat transfer circuit  301 ,  302 , which operates at a lower temperature level than the first heat transfer circuit  311 ,  312 . The heat transfer medium from the second heat transfer circuit  301 ,  302  is guided under control of valves  222  and  224  either to the heat exchanger  12  located in the duct  11 ′ or to the heat exchanger  13  located in duct  11 ″. In the first mode of operation the valve  222  is closed and the valve  224  is opened, whereby the second heat transfer medium is guided to the heat exchanger  13  in the duct  1111 , which heat exchanger is downstream in the gas flow. The apparatus is operated in this first mode of operation for a desired period of time or advantageously until the mode of operation is altered based on a predetermined event. 
     The predetermined event comprises preferably the following. During the operation of the apparatus, measurement information is collected by a central processing unit  1  at least from the heat transfer process. The collected measurement information is compared with set values, which have been stored in or are available to the central processing unit  1 , and in the event that the measured information differs from the set values by more than a predetermined amount, the mode of operation of the arrangement is altered. Information regarding the heat transfer media is collected by temperature senders  23 ,  24  provided in the heat transfer medium channels and/or information regarding gas pressure difference and/or gas temperature is collected by pressure and/or temperature senders  25 ,  26  provided in the gas flow channels to be used as measurement information. Thus, when for example the pressure difference of gas flow increases beyond a set value or the temperature of heat transfer medium coming from a heat exchanger is not at a predetermined level, the mode of operation is altered. 
     FIG. 3 shows the second mode of operation, which will be described in the following. The gas supplied to the apparatus is now caused to flow from the gas production equipment  2  through the gas inlet  18  to the duct  11 ″. Gas flows in the duct  11 ″ substantially vertically downwards and passes the heat exchanger  13  transferring heat to heat transfer medium flowing in the hear exchanger. As can be seen from FIG. 3, the heat exchanger  13 , which was connected to the LT-circuit  301 ,  302  during the first mode of operation, is now connected to the HT-circuit  311 ,  312  whereby a cleaning effect of heat transfer surfaces according to the invention is provided. The deposits that accumulated on the surfaces of the heat exchanger when operating in the first mode soften. Especially, when the apparatus is used for heat recovery from exhaust gases of a gas engine, components that condense and harden on a relatively cool heat transfer surface of the heat exchanger  13  in the first mode of operation soften and drain down from the heat transfer surface of the heat exchanger, where from they can be removed from the apparatus through a discharge valve  28 , which is preferably controlled by the central processing unit  1  for example so that the valve is opened during a selected interval after each change in mode of operation. 
     The heat transfer medium of the HT-circuit  311 ,  312  is guided through the open valve  214  of the valve pair  212 ,  214  to the heat exchanger  13  in the duct  11 ″, which is now upstream in the gas flow. The heat transfer medium is guided back to the first haet transfer medium circuit under control of the open valve  223  of the valve pair  221 ,  223 . 
     After passing the heat exchanger  13 , the gas enters the common space connecting the flow ducts  11 ′,  11 ″, from which space the gas is guided further upwards through the flow duct  11 ′ by changing its flow direction through 180λ. In this duct, the gas flows upwards through the heat exchanger  12  and transfers heat to the heat transfer medium flowing in the heat exchanger  12 . As can be seen from FIG. 3, the heat exchanger  12  is connected to the second heat transfer circuit i.e. to the LT-circuit  301 ,  302 . The heat transfer medium of the LT-circuit  301 ,  302  is guided through the open valve  222  of the valve pair  222 ,  244  to thee heat exchanger  12  in the duct  11 ′, which heat exchanger is now downstream in the gas flow. The arrangement is now operated in this second mode of operation for a desired period of time or advantageously until the mode of operation is altered again based on a predetermined event. 
     A controllable central processing unit  1  or the like is provided for controlling the operation of the valves. The operation of the valves  20 ,  21  guiding the gas flow is synchronized with valves  211 ,  212 ,  213 ,  214 ,  221 ,  222 ,  223 ,  224  guiding the heat transfer medium so that the valves of each chamber  14 ,  15  are positively controlled so that a flow connection from a chamber may be established with only one of the flow ducts  11 ′,  11 ″ at a time, the gas is always caused to pass through both of the ducts  11 ′,  11 ″, and the HT-circuit is always connected with the heat exchanger that is upstream of the heat exchanger connected to the lt-circuit. 
     FIG. 4 shows a modification of the apparatus shown in FIGS. 2 and 3. In the case of the apparatus shown in FIG. 4, the heat transfer surface of the first heat exchanger  12  is larger than that of the second heat exchanger, so that in the first mode of operation heat is transferred more effectively to the first heat transfer medium than to the second heat transfer and the first heat transfer medium is heated to a higher temperature than the second heat transfer medium. In the second mode of operation, in which the second heat transfer medium flows through the first heat exchanger  12 , some of the heat transfer medium of the LT-circuit may be guided to bypass the first heat exchanger via a conduit  227 . The by-pass flow rejoins the flow through the heat exchanger  12  downstream of the valve  211 . The temperature of the second heat transfer medium, after mixing the by-pass flow and the flow through the heat exchanger  12 , corresponds to the situation when operating in the first mode of operation. The flow of gas in this case corresponds to that shown in FIG.  3 . The heat exchanger  13  is now connected to the first heat transfer circuit i.e. the HT-circuit  311 ,  312 . According to FIG. 4, in order to compensate or offset the greater power obtained by the larger surface area of the heat exchanger  12 , a part of the heat transfer medium of the LT-circuit  301 ,  302  may be guided to bypass the heat exchanger  12  through the conduit  227 . The conduit  227  is advantageously provided with a closing or shut-off valve  225  and control valve  226 , by means of which the flow rate may be adjusted. The arrangement is operated in the second mode of operation for the time required for cleaning of the heat exchanger  13 , after which the mode of operation is changed again. 
     The foregoing disclosure is merely one possibility to implement the arrangement with heat transfer circuits, and in addition the heat transfer circuits may be provided with separate loops operated by circulation pumps. In this manner the inlet and outlet flows of the separate loops may be separately adjusted (not shown). 
     A corresponding effect may also be achieved by guiding the gas-side flow by altering the position of the valve so that some gas bypasses the heat exchanger directly through heat exchanger space from chamber  14  to chamber  15 . This may be accomplished for example so that the pair of openings, which is in principle required to be closed by each mode of operation, is not totally closed, but both of the valve openings in the chamber are left partly open by the valves  20 ,  21  (FIG.  1 ). 
     In some cases, for example in order to ensure monitoring of the apparatus during the operation, it may be advantageous to perform the cleaning, i.e. change the mode of operation, by operating fully manually. 
     The invention is not limited to the illustrated embodiments but several modifications of the invention are reasonable within the scope of the attached claims. The heat exchanger apparatus may be designed to operate in an orientation that is inverted with respect to that shown, and the ducts  11 ′,  11 ″ are above the chamber  14 ,  15 . The heat exchangers may, as to their actual connections to the heat transfer medium circuit, operate either as parallel, counter or cross flow type. The apparatus may also comprise other heat exchangers and heat transfer circuits.