Implementation of condensing economizers for small boilers

. Reducing the dangerous potential of global warming and its impact on climate change requires reducing carbon dioxide emissions into the atmosphere, which is inextricably linked to reducing the burning of fossil fuels. There are various techniques to accomplish this task. One of such technologies is the maximum use of excess heat of exhaust gases with the subsequent condensation of vapors contained in them and use of the obtained heat for own needs of the heat energy source. Because there are many designs, it is necessary to carry out various studies to determine the optimal design of these apparatuses. In order to fulfil this work, we have made a laboratory model of a condensing economizer with a developed heat exchange surface. We also carried out a series of tests to construct graphical dependences of experimental data, which will help us to determine the heat transfer coefficient from flue gases to water taking into account the condensation of water vapors and to calculate the optimal geometry of channels for the flow of different heat carriers. All these studies will allow us to obtain the optimal design of condensing economizer.


Introduction
Currently, gas boilers are one of the most widespread heating heat generators in the world [1].The most energy efficient of them, providing the highest technical and economic indicators are boilers equipped with condensing heat exchangers.Similarly, in almost all cases of using condensing heat exchange equipment, the energy efficiency of the technical solution is ensured due to the fuel component in operating costs, despite the growth of capital investments in installations [9].Support by many countries at the state level of energy-saving technologies and concern for environmental protection contribute to the active introduction of innovative technology that meets all standards [12].The priority task is to increase the efficiency and improve the environmental performance of heat supply systems [6].
At present, the most appropriate technology for increasing the efficiency of fuel combustion by boilers is viewed as the improvement of the methodology for the full use of the thermal energy of flue gases [21].The basis of this phenomenon can be described as a decrease in the temperature of the combustion products below the dew point temperature at pressure in the convective beam or surface, followed by the utilization of the latent heat potential.The advantages of installing surface condensing economizers include sufficient fuel economy, compactness, reliability, quiet operation, energy independence, and modularity [5].All thermal engineering advantages of these heat exchangers are based on enhancing the efficiency of the gas boiler combined with it by utilizing the inert heat of water vapour suspended in the volume of combustion products in the tail-heating surface outside the heat generator [7].
For this reason, we decided to come up with and manufacture the optimal design of a new economizer, with an effective width of gas channels and easy to operate and install in cramped conditions of modern heat generators of low power.In addition, it was important to obtain fuel savings no lower than in boilers equipped with built-in convection units.This model makes it possible to develop a methodology for selecting and calculating the main design parameters for mass production of these heat exchange surfaces [5].In this paper, an attempt is made to formulate the main provisions for the design and development of condensing economizers, experimental study of operating conditions and features processes that involve heat and weight transfer in condensing heating surfaces [3].

Materials and Methods
The equipment used was a traditional single-circuit gas convection boiler LOGANO G234 WS with a power of 38 kW, mounted on a pedestal, with a partial preliminary mixing gas burner of the BUDERUS design and a channel-type condensing economizer with horizontal finned flue gas passages [14].The apparatus we created is a heat exchanger that transfers heat between flue gases leaving the boiler and water flowing through the channels, designed from aluminum plates with fins to increase heat transfer, by using intense flat fins.
On the flat side of these plates, we applied solutions for the manufacture of flat spaces into which liquid enters.We also installed connectors for a threaded connection to connect to a water source system.This economizer is supposed to be installed behind the boiler along the path of flue gases [6].Due to the specially selected geometry of the channels for gases and water, a countercurrent movement is organized in the heat exchanger, which contributes to increased heat transfer and efficiency [9].There are many ways to connect the heat exchanger to the flue system of the boiler, but in our experiment, we stopped on its installation on a horizontal section as the simplest and safest, excluding the overturning of the draft and pollution of the room.The laboratory model of the heat exchanger is shown in the photo in Figure 1.
In the course of the experiment, we set ourselves the task of determining and deriving, based on graphical dependences of the temperature change of water and flue gases on time, the coefficient of condensation heat transfer from flue gases to moving water [4].Since the theory of condensation of water vapor from flue gases is currently poorly understood, and mainly dependencies are used that are typical for the condensation of water vapor in horizontal channels or the condensation of vapors from a moving air stream.These dependences make it possible to more accurately determine the heat transfer coefficient is based on similarity criteria, which ultimately affects the selection of the geometric characteristics of the corresponding parts of the economizer [1].An attempt was also made to reveal the dependence of geometric criteria on the heat flux values.and choose the optimal geometry for the intensification of this flow [11].The thermodynamically irreversible process of heat transfer in low-temperature convective heating surfaces of condensing heat exchangers is associated with the condensation of water vapor from fuel combustion products and is accompanied by heat transfer during mass transfer [10], which is due to the transfer of sensible heat and the release of latent heat of vaporization [20].Considering the condensing (tail) part of the economizer, we can say that condensation will occur on a cooled surface, the temperature of which should be less than the saturation temperature at constant pressure (i.e., the temperature at which water freezes the fuel combustion products) [18].
The resulting film of condensate flows down the walls of the heat exchanger channels under the influence of gravitational forces [19].At the same time, intensive mass transfer processes take place between the flue gases and the condensate [12].
In general, within the given temperatures, mass transfer processes have not material impact on the amount of heat transfer coefficient, because changes in the value of the condensate depends on the thickness of the condensate film mass transfer coefficient do not affect heat transfer so much, since changes in the flow regime of flue gases and coolant and geometric characteristics have a greater effect cross-sections of channels and spaces of fluid motion.Also, an increase or decrease in film thickness can cause an increase or decrease in its thermal resistance, which ultimately can reduce or increase heat transfer, playing a purely role of a heat exchange process [13].

Results
During the experiment, a series of necessary parameters were obtained for plotting graphical dependencies, in particular, this is the difference in flue gas temperatures at the inlet and outlet of the heat exchanger, the temperature potential of combustion products on the outside of the economizer, the temperature of the liquid at the inlet and outlet of our closed loop, volumetric gas and water consumption [3].As a result, we built a non-linear graphic dependence shown in fig. 2. [2] The experiment showed that heating and lowering the temperature of combustion products and water in the economizer are non-linear.In the primary section A -B, the temperature potential of the exhaust gases decreases more rapidly, until condensation of moisture vapor starts.The transfer of thermal energy in this case occurs mainly due to convection.In parallel, the gradient of water heating in this area is low.[9] Then, in the section B-C, at a temperature potential of gases less than 100 0 С, condensation occurs when they come into contact with a condensing surface with a temperature below 60 0 С, and the temperature gradient is transferred both by convection and condensation.[11]  The irreversible process of the environment is characterized by heat and mass transfer convective heating surfaces is inextricably linked with the condensation of water vapor from combustion gases as they pass through channels with a circulating liquid [4].To implement these processes, it is necessary to ensure the contact of flue gases with the cold surface of the heat exchanger with a temperature below the temperature at which water freezes the combustion products at a given pressure [15].Nevertheless, a water film of condensate of variable thickness is formed on the surface, flowing down under the action of gravitational forces [17].

Discussion
The exchange of thermal energy between the products of combustion of natural gas at elevated temperatures and liquid without the organization of a phase transition from one state to another can only be carried out when a temperature gradient occurs [2].The component of the beginning of mass transfer is the gradient of partial pressures of the substance [3].If the partial pressure of liquid vapor in high-temperature combustion products is equal to the partial pressure of liquid vapor above the condensate film, then there is no mass transfer between flue gases and water [20].
The ongoing heat exchange in such a situation can be described by the laws of convective heat transfer [10]: where: Q -the amount of heat transferred from gases to water by convection, kW; -convection heat transfer coefficient kW/(m 2 •°С); α к -temperature of gases and water °С;  г ,  вод F -total contact surface, m 2 .At that moment, the partial pressure of liquid vapors in the combustion products and on top of the liquid film are not equal, then water vapor will either condense from gaseous products into the water film, or change the state of aggregation.In this case, this process can be described as an equation [8]: where: ΔG -the amount of moisture evaporated or condensed in the process of mass transfer between gas and condensate (water) kg/s; β -mass transfer coefficient, kg/(m 2 •s•Pa); -partial pressures of water vapor in flue gases and on the surface of  г ,  вод condensate (water), Pa; F -total contact heat exchange surface, m 2 [3].The positive sign in front of the operator ΔG denotes the condensation of liquid vapors from the combustion products into liquid, the minus sign indicates when the transfer of vapors from the condensate film (water) to the flue gas flow begins.
The heat flow leaving during the mass transfer process in this case [5]: Q м =rxΔG=rxβx(P г -P вод )xF (3) where: r -latent heat of vaporization of water at a given temperature, kDJ/kg.In general, the total amount of heat that changes from heated flue gases to liquid is calculated from the explicit convective exchange of thermal energy, as well as wet heat transfer and the latent heat of moisture formation, which is obtained as a result of evaporation of water or condensation from combustion products of gaseous: 4)   where: C вод -specific heat capacity of water, kDj/(kg•К).Analysis of the process of transferring heat during the formation of the model allows, for quasi-stationary conditions within an elementary volume, to consider heat and mass transfer only within the gas flow, localizing it on the surface of the condensate film.In the accepted notation, in fact, is the characteristic size of the channel [21].
Thus, the total heat transfer dQ, heat transfer by convection dQ FK and mass transfer by diffusion dQ FM leads to a change in the enthalpy of the elementary volume of wet combustion products dI: dI=dQ=dQ FK +dQ FM (5) then in the accepted notation: (6) where: C p k -isobaric heat capacity of combustion products, kJ/(kg•K); ρ г -density of combustion products, kg/m 3 [4].For total heat transfer by convection α к and mass transfer by diffusion α м , assuming the possibility of using the principle of additivity, we obtain: α = α к + α м (7) then the balance equation takes the form:

Conclusion
1.During a series of several studies, it was possible to identify the most efficient shape the surface that transfers heat allows obtaining the maximum heat transfer from flue gases, as well as the minimum formation of droplets, excluding an increase in the moisture content of the outgoing combustion products [2].
2. The resulting graphical dependencies allow one to obtain an equation for calculating the heat transfer coefficient from flue gases, taking into account the evaporation of water vapor [6].
3. The developed type of economizers can be introduced into existing boiler plants based on traditional gas boilers, which will help increase efficiency by about 7 percent [9].

Fig 1 .
Fig 1. Practical model of a condensing economizer

Fig 2 .
Fig 2. Flue gas temperature change and heated water temperature change in the economizer.