Description

Technical Field

[1] The invention relates to heat cells and methods of operation of heat cells. Such heat cells comprise a premix gas burner and a condensing heat exchanger.

Background Art

[2] Heat cells are known that comprise a premix gas burner and a condensing heat

exchanger. Such heat cells can operate at high efficiency thanks to the extraction of latent heat from the flue gas when condensation of water vapor from the flue gas occurs in the heat exchanger. A disadvantage of condensing heat exchangers is that the heat exchanger surface of the heat exchanger in contact with the combustion products gets polluted and filthy due to contact with the combustion gas. In particular, when the combustion products condense on this heat exchanger surface, corrosion of this surface occurs. Corrosion results in disintegration and degradation of the heat exchanger surface due to chemical reactions with its surroundings. In particular, the condensed gaseous medium comprises compounds that chemically react with the heat exchange surface.

[3] EP2600077A1 relates to a heating device for providing a heated liquid medium,

comprising a burner for generating a heated combustion gas. The burner is configured for receiving a burner signal. The heating device further comprises a heat exchanger comprising a tube having an inlet opening and an outlet opening. The outside of the tube defines a first heat exchange surface for contacting the combustion gas and the inside of the tube defines a second heat exchange surface for contacting the liquid medium. The heating device comprises a fan for providing an airflow over the heat exchanger. The fan is configured for receiving a fan signal and a pump for pumping the liquid medium into the inlet opening and out of the outlet opening. The pump is configured for receiving a pump signal. The heating device comprises a control unit configured for emitting the burner signal, the fan signal and the pump signal. The control unit is configured for controlling the burner, fan and pump by emitting the respective burner signal, fan signal and pump signal such that the combustion gas condenses on the first heat exchange surface for cleaning the first heat exchange surface by washing using the condensate.

[4] EP3260804A1 discloses a heat exchanger management system and a method of

operating the heat exchanger management system. The heat exchanger management system includes a memory and an electronic processor electrically connected to the memory and configured to operate one or more burners to transmit heat to a heat exchanger for a first period of time that deposits corrosive condensates on a passivation layer of the heat exchanger, deactivate the one or more burners for a second period of time, operate one or more blowers to move air across the heat exchanger at a temperature that evaporates the corrosive condensates on the passivation layer of the heat exchanger and increases an oxide thickness of the passivation layer on the heat exchanger, and reactivate the one or more burners after the second period of time.

Disclosure of Invention

[5] It is an objective of the invention to increase the functional lifetime of heat cells through an inventive method of operation of the heat cell.

[6] The first aspect of the invention is a method for operating a heat cell. The heat cell comprises a premix gas burner and a heat exchanger. The premix gas burner is provided for the generation of flue gas by surface stabilized combustion of premixed combustible gas. The heat exchanger is provided for transfer of heat from the flue gas through the walls of the heat exchanger to fluid - e.g. water - flowing continuously through the heat exchanger. The method comprises the steps of

- Operating the heat cell in alternating burner-on and burner-off mode. During the burner- on-mode flue gas is generated by the premix gas burner. During the burner-off mode no flue gas is generated by the premix gas burner.

- During at least 50% (and preferably during at least 75%, and more preferably during at least 90%) of the burner-on mode, the heat cell is operated cell such that in the heat exchanger liquid is condensed from the flue gas.

- During the final period of the burner-on mode before switching to burner-off mode, the heat cell is operated such that fluid in the heat exchanger is heated to higher temperatures than the average temperature to which the fluid is heated in the heat exchanger during the burner-on mode; such that during the final period of the burner-on mode the heat cell is operated in non-condensing mode and such that condensation liquid present in the heat exchanger is removed by evaporation, preferably such that when the burner is switched from burner-on mode to burner-off mode no or hardly any condensation liquid is present in the heat exchanger.

[7] Heat cells operating in condensing mode are energy efficient. The lower the temperature of the flue gas at the exit of the heat exchanger, the higher the efficiency of the heat cell because more latent heat is extracted from the flue gas. More extraction of latent heat means more condensation liquid. The condensation liquid comprises water and inevitable impurities derived from impurities or odorant (such as sulphur or sulphur containing molecules or reaction products from NOx) present in the combustible gas. Water - and the condensed impurities - can have negative effects on the lifetime and on the efficiency of the heat exchanger. In stainless steel heat exchangers, pit corrosion and/or fouling can occur. In aluminium heat exchangers, complex reactions between condensation products and the aluminium of the heat exchanger can create fouling of the heat exchanging walls of the heat exchanger, reducing heat exchange efficiency and partly (or even fully) blocking of the flue gas channel in the heat exchanger. The inventive method provides an important reduction of the negative effects of the condensation products on the heat exchanger. By the inventive way of operating the heat cell during the final period of the burner-on mode before switching to burner-off mode, condensation liquid present in the heat exchanger is dried and removed out of the heat exchanger in gaseous form via the chimney. This way, when the heat cell is switched to burner-off mode, less, hardly any or no condensation liquid is present in the heat exchanger. Thus, during the burner-off mode, less or no negative reactions can occur between condensation liquid and the material of the heat exchanger. This way, longer functional lifetime of the heat exchanger is obtained, as well as lower pressure drop in the flue gas channels of the heat exchanger.

[8] A second aspect of the invention is a heat cell, comprising a premix gas burner, a heat exchanger and a controller. The premix gas burner is provided for the generation of flue gas by surface stabilized combustion of premixed combustible gas. Preferably the premix gas burner is a surface stabilized fully premixed gas burner. The heat exchanger is provided for transfer of heat from the flue gas through the walls of the heat exchanger to fluid flowing continuously through the heat exchanger. The controller is programmed to operate the heat cell according to a method as in any embodiment of the first aspect of the invention.

[9] In a preferred heat cell, the heat exchanger is an aluminium heat exchanger, e.g. a

monocast aluminium heat exchanger or a sectional aluminium heat exchanger. Such heat cells have a much longer lifetime as well as lower pressure drop in the flue gas channels of the heat exchanger when operating the heat cell during a longer time, as fouling is drastically reduced.

[10] In a preferred heat cell the heat exchanger is a stainless steel heat exchanger. Such heat cells have the benefit that pit corrosion and/or metal dusting is drastically slowed down. Examples of stainless steel heat exchangers than can be used in the inventions are stainless steel heat exchangers comprising wound stainless steel tubes, or comprising stainless steel plate heat exchangers, or heat exchangers comprising a plurality of - preferably finned - stainless steel tubes provided parallel to each other.

[1 1] In preferred embodiments of the invention, the heat cell comprises a pump for circulation of the fluid to be heated through the heat exchanger. The pump is switched off during the final period of the burner-on mode. This way, there is no longer circulation of fluid through the heat exchanger. Therefore, the fluid present in the heat exchanger will be heated to higher temperatures during the final period of the burner-on mode, resulting in evaporation of condensed liquid in the heat exchanger; and its removal through the chimney.

[12] In preferred embodiments of the invention, the heat cell comprises a pump for circulation of the fluid to be heated through the heat exchanger. The flow rate of the pump during the final period of the burner-on mode is set to be lower than the average flow rate of the pump during the burner-on mode, and preferably at least 50% lower. Because of the lower flow rate of the pump during the final period of the burner-on mode, the fluid will be heated to higher temperatures. As a consequence, the temperature in the flue gas channel of the heat exchanger will also rise up to a level and resulting in evaporation of condensed liquid in the heat exchanger; and its removal through the chimney.

[13] In preferred embodiments of the invention, the heat cell is provided to supply fluid heated in the heat exchanger to a first circuit to heat tap water or to a second circuit for heating. The heat cell comprises a three way valve, provided for switching the supply of the fluid heated in the heat exchanger from the second circuit to the first circuit and vice versa. A preferred embodiment of the inventive method comprises the step of switching the three way valve such that during the final period of the burner-on mode, fluid heated in the heat exchanger is supplied to the first circuit. No longer is fluid supplied to the second circuit. As the first circuit is shorter - meaning that it comprises less fluid - it will lead to faster heating and to higher temperature heating of the fluid flowing through the heat exchanger, resulting in increased temperature in the flue gas channel and consequently in evaporation of condensed liquid in the heat exchanger; and its removal through the chimney.

[14] In preferred embodiments of the invention, the burner load is increased during the final period of the burner-on mode compared to the average burner load during the burner-on mode. As a consequence, the liquid in the heat exchanger - whether continuously flowing or stand-still - is heated to higher temperatures, resulting in evaporation of condensed liquid in the heat exchanger; and its removal through the chimney. In such embodiments, the rate of fluid flow through the heat exchanger can be kept the same as before the final period of the burner-on mode; or can be set lower; or the flow of fluid through the heat exchanger can be interrupted. In embodiments wherein the burner load in increased during the final period of the burner-on mode, the burner load can be or is increased up to a level such that the flue gas temperature is increased such that no condensation of liquid from the flue gas occurs.

[15] In preferred embodiments of the invention, the final period of the burner-on mode before switching to burner-off mode lasts less than 1.5 minute, preferably less than 1 minute, preferably less than 30 seconds. It is preferred to keep the final period of the burner-on mode short, certainly relative to the total time of the burner-on mode, in order not to affect the efficiency of the heat cell too much by the non-condensing operation of the heat cell during the final period of the burner-on mode before switching to burner-off mode.

[16] In preferred embodiments of the invention, the final period of the burner-on mode before switching to burner-off mode is ended and the burner is switched to burner-off mode when the fluid to be heated reaches a set temperature. To this end, a temperature sensor can be provided to measure the temperature of the fluid to be heated, e.g. in the heat exchanger. When water is the fluid to be heated, the set temperature can e.g. be set to a value between 80 and 95°C, e.g. to a value between 80 and 90°C. In such embodiments, proper evaporation of condensed liquid can be obtained without risk of overheating the fluid to be heated, as otherwise boiling could occur of the fluid to be heated. [17] In preferred embodiments of the invention, the heat exchanger is an aluminium heat exchanger, e.g. a monocast aluminium heat exchanger or a sectional aluminium heat exchanger.

[18] In preferred embodiments of the invention, the heat exchanger is a stainless steel heat exchanger. Examples of stainless steel heat exchangers than can be used in the inventions are stainless steel heat exchanger comprising wound stainless steel tubes, or comprising stainless steel plate heat exchangers, or heat exchangers comprising a plurality of - preferably finned - stainless steel tubes provided parallel to each other.

Brief Description of Figures in the Drawings

[19] Figure 1 shows a heat cell according to the invention.

Figure 2 shows comparative test results.

Mode(s) for Carrying Out the Invention

[20] Figure 1 shows schematically an example of a heat cell 100 according to the second aspect of the invention. The heat cell comprises a premix gas burner 105, a heat exchanger 110 and a controller 115. The premix gas burner is provided for the generation of flue gas by surface stabilized combustion of premixed combustible gas. The heat exchanger is provided for transfer of heat from the flue gas through the walls of the heat exchanger to water flowing continuously through the heat exchanger through water channels 120. To this end, a water inlet 122 and a water outlet 124 are provided. The heat exchanger comprises a flue gas channel 126 for the flow of the flue gas generated by the premix gas burner and for the exchange of heat between the flue gas and water flowing through the water channels of the heat exchanger. The heat exchanger is an aluminium heat exchanger (e.g. a monocast aluminium heat exchanger or a sectional aluminium heat exchanger). The controller is programmed to operate the heat cell according to a method as in the first aspect of the invention.

[21] The heat cell comprises a pump 130 for circulation of the water through the heat

exchanger. The heat cell is provided to supply water heated in the heat exchanger to a first circuit 135 to heat tap water or to a second circuit 137 for heating. The first circuit comprises a heat exchanger 138 to heat tap water. The second circuit comprises one or more than one heat exchangers 140. The first circuit is shorter than the second circuit; and comprises less water. The heat cell comprises a three way valve 165, for switching the supply of the water heated in the heat exchanger from the second circuit to the first circuit and vice versa.

[22] A temperature sensor 170 is provided in the heat exchanger to measure the temperature of the water to be heated.

[23] The controller of the heat cell is programmed to control the operation of the heat cell. The controller switches the burner to alternating burner-on and burner-off mode according to heat requirement. During the burner-on-mode flue gas is generated by the premix gas burner. During at least 50% (and preferably during at least 75%, and more preferably during at least 90%) of the burner-on mode, the heat cell is operated such that in the heat exchanger liquid is condensed from the flue gas. During the burner-off mode no flue gas is generated by the premix gas burner. In advanced versions of controllers, the burner can even be modulated during the burner-on mode.

[24] The controller is further programmed to control the operation of the heat cell such that during the final period of the burner-on mode before switching to burner-off mode, water in the heat exchanger is heated to higher temperatures than the average temperature to which the water is heated in the heat exchanger during the burner-on mode; such that during the final period of the burner on-mode the heat cell is operated in non-condensing mode and such that condensation liquid present in the heat exchanger is removed by evaporation, and such that when the burner is switched from burner-on mode to burner- off mode no or hardly any condensation liquid is present in the heat exchanger. To this end, one or a combination of the following approaches can be taken:

1 ) The pump is switched off during the final period of the burner-on mode. This results in an increase of the temperature of the water in the heat exchanger. As a consequence, heat exchange between flue gas and water reduces, and the flue gas temperature in the flue gas channel of the heat exchanger increases, up to a level that no condensation of liquid from the flue gas occurs, and that previously condensed liquid evaporates and is removed through the chimney together with the flue gas.

2) The flow rate of the pump is set during the final period of the burner-on mode to a flow rate lower than the average flow rate of the pump during the burner-on mode, e.g.

preferably at least 70% lower. This results in an increase of the temperature of the water in the heat exchanger. As a consequence, heat exchange between flue gas and water reduces, and the flue gas temperature in the flue gas channel of the heat exchanger increases, up to a level that no condensation of liquid from the flue gas occurs, and that previously condensed liquid evaporates and is removed through the chimney together with the flue gas.

3) The three way valve is switched such that during the final period of the burner-on mode, fluid heated in the heat exchanger is supplied to the first circuit. As the first circuit comprises less water, the temperature of the water in the heat exchanger will increase. . As a consequence, heat exchange between flue gas and water reduces, and the flue gas temperature in the flue gas channel of the heat exchanger increases, up to a level that no condensation of liquid from the flue gas occurs, and that previously condensed liquid evaporates and is removed through the chimney together with the flue gas.

4) The burner load is increased during the final period of the burner-on mode compared to the average burner load during the burner-on mode. This results in increased flue gas temperatures and therefore higher water temperatures in the heat exchanger. The flue gas temperature rises up to a level that no condensation of liquid from the flue gas occurs, and that previously condensed liquid evaporates and is removed through the chimney together with the flue gas. [25] It is possible to program the controller such that the final period of the burner-on mode before switching to burner-off mode is ended and the burner is switched to burner-off mode when the temperature sensor in the heat exchanger detects that the water temperature reaches e.g. 85°C.

[26] Figure 2 shows comparable test results of operating a heat cell comprising a cast

aluminium heat exchanger and a surface stabilized fully premixed gas burner. At given input and output water temperatures of the heat exchanger, the standard method of operation of a heat cell is to run the fan supplying the air required for the combustion of the combustible gas at a constant speed. An amount of combustible gas is added via a venturi to the air flow, thereby creating a premix of combustible gas and air which is supplied to the gas premix burner. The occurrence of corrosive fouling in the flue gas channel of the heat exchanger will create an increase in the pressure drop in the flue gas channel of the heat exchanger. The consequence is a reduction of air flow as the fan is revolving at the same speed, and a corresponding reduction in heat cell power (amount of kW heat generated). Figure 2 shows in X-axis the time during which the comparative trial has been performed, starting from a new heat cell. The Y-axis shows the percentage of heat cell power as a function of test time of the heat cell. The result in the Y-axis is expressed as a percentage of the heat cell power at the start of the test. Curve I represent the results of operating the heat cell according to a prior art method: the heat cell is run in cycles: the heat cell is operated during 6 minutes in condensing mode, after which the burner is switched off during 3 minutes, after which a new 6 minute cycle is started operating the heat cell in condensing mode, and so on.

[27] Curve II is the result of operating the heat cell according to the invention: the heat cell is operated in cycles: during 6 minutes the heat cell is operated in condensing mode, after which the heat cell is operated during 15 seconds with the burner switched on while the water supply to the heat exchanger is switched off. As the water flow through the heat exchanger is stopped, the water standing in the heat exchanger is getting 80 to 90°C.

The temperature in the flue gas channel increases to a level that no condensation occurs in the flue gas channel and previously condensed liquid evaporates and is removed together with the flue gas. Thereafter, the burner is switched off during 3 minutes, after which a new 6 minute cycle is started. It was noticed that virtually all condensation liquid evaporated during the 15 seconds that the heat cell was operated with burner switched on and with shut off water flow. Comparison of the two curves I (prior art) and II (according to the invention) clearly indicate that the inventive method effectively improves the heat cell: corrosive fouling was drastically reduced; and the heat cell power over time stays at much higher level.