WO2021130784A1

WO2021130784A1 - A novel high-efficiency two-chamber boiler using turbulent reverse flow of combustion gases

Info

Publication number: WO2021130784A1
Application number: PCT/IR2020/050015
Authority: WO
Inventors: Seyed Mohamad Reza KARIMI POUR; Alireza ZAHERI
Original assignee: Rayramesh Hossein
Current assignee: Rayramesh Hossein
Priority date: 2019-12-24
Filing date: 2020-04-24
Publication date: 2021-07-01
Anticipated expiration: 2022-06-24

Abstract

A novel two-chamber design for thermal boilers is presented in this research. The use of a direct flame burner causes exhaust gas turbulence and increases the gas pressure in the main chamber. The high-pressure gases, which have lost their kinetic energy due to collision with spirals, leave the main chamber and enter into the secondary chamber, where their energy is used to preheat inlet water. The control of distance between spirals, the reverse flow of exhaust gases in the chambers, and the specific geometry of the spirals maximize boiler efficiency.

Description

A novel high-efficiency two-chamber boiler using turbulent reverse flow of combustion gases

Invention field

Thermodynamics, combustion, heat transfer, materials selection

Technical problem

Thermal boilers are pressure vessels containing some parallel blades, which warm up or transform water into steam. The boilers come in various shapes and sizes based on their capacity ranging from 10,000 to 500,000 kcal/hr. These boilers are used to generate hot water in low-pressure systems with a maximum of 5 bar pressure such as domestic and light industrial applications. The thermal efficiency of the boilers is about 60% for solid fuels and 70% for fluid or gas fuels. The following problems are identified in using the boilers:

( 1 ) Low efficiency: A variety of parameters including exit-gas temperature, gas recirculation times, heat surface area, excess air, fuel composition, ambient temperature, and boiler/burner compatibility affect boiler efficiency. Lack of a well-controlled system results in a waste of exhaust gas energy, and the rise in the temperature of exhaust gases up to 300°C — thereby reducing the thermal efficiency of the boilers.

(2 ) Limited heat capacity: The design of traditional boilers using parallel cast- iron blades is such that more blades are required to increase the heat capacity of the boilers. On the other hand, increasing the number of blades results in thermal heterogeneity in the first and last blades, which causes high temperature gradients and thermal shocks in the blades. (3) Environmental pollution: An improper fuel-air mixture in traditional boilers produces hazardous greenhouse gases such as CO and NO_x, which causes air pollution. In addition, traditional boilers are not able to use the exhaust gases. Consequently, these emissions exceed the standard limit and intensify environmental degradation.

(4 ) Restrictions on fluid working pressure: Maximum fluid working pressure for a boiler depends on the boiler’s nominal capacity and inlet flow-rate. The higher-capacity boiler is, the more flow-rate and consequently more pressure it requires. Due to the specific design of blades — i.e., low thickness to boost heat transfer — the maximum nominal pressure in traditional boilers cannot be more than about 6 bar, which is a potential problem for the production of high- capacity boilers.

(5) Sediments: The water used in the boilers is composed of various soluble and insoluble materials that could settle in the boiler tubes and cause a number of problems such as walls corrosion, energy losses, and boiler perforations. Depending on material, thickness, and heat transfer coefficient of the sediments, the deposition of these particles in the watercourse reduces the heat transfer rate and boiler efficiency. Lack of proper heat transfer in the pipes of the boiler increases the operating temperature of blades, and consequently reduces the blades’ service life. Removal of sediments from boiler components is an expensive process especially in the case of hard sediments.

(6) Product development challenges: The four stages of a technology lifecycle include research and development, ascent, maturity, and decline (or decay). In the decline phase, financial profits from selling the product start dipping, and customers turn to buy other products. As a solution, producers could benefit from utilizing inexpensive production methods or targeting other markets. After the decay phase, there are usually two methods to return the product to competitive markets; (1) product development exploiting new knowledge and technologies, (2) design of new high-quality products that can be developed. Traditional boilers have reached a dead end in terms of research and development, and hence a new technology is required to boost the quality of the equipment.

(7 ) Application of expensive burners: As the traditional boilers evolve, some modifications have been made to the previous generation to increase their efficiency. The alterations include the incorporation of a condenser into the traditional boilers to absorb heat from combustion products and warm up boiler inlet water to increase boiler efficiency. These boilers, known as condensing boilers, use very expensive premix burners.

(8) Low corrosion/oxidation resistance:

(a) Corrosion of internal surfaces affected by flammable gas

- Low-temperature corrosion: This corrosion occurs during boiler shutdown when exhaust gas temperature reaches below the dew point and is often associated with dew point corrosion.

- High-temperature corrosion: This corrosion occurs due to the presence of corrosive gases in combustion products.

(b) Corrosion of external surfaces

- Oxygen corrosion: Insoluble oxygen in the feed water causes pitting corrosion.

- Hydrogen diffusion: Hydrogen diffuses in steel grains and reacts with carbon, which causes a transgranular fracture.

Prior developments The first generation of hot-water boilers was made using cast-iron blades and has been marketed for several years. However, due to the low efficiency and problems mentioned in the previous section, the boilers were rendered obsolete in developed countries. In an attempt to improve the efficiency of the boilers, pressurized steel boilers and multi-pass pressurized steel boilers were then suggested. These boilers, also known as traditional boilers, did not develop further.

As the development progressed, the use of condensation technology, in which a condenser is employed to absorb latent heat from combustion products, was highly regarded. This approach resulted in a significant increase in boiler efficiency. Condensing boilers are usually built with aluminum- silicon and stainless steel alloys. Because of the high cost, low heat transfer coefficient, and high weight of stainless steel, aluminum- silicon boilers received more attention. These boilers, however, have a number of disadvantages such as low nominal capacity (up to 1.6 million calories), limited working pressure (up to 6 bar), application of expensive premix burners, and low resistance against acid condensation.

Technical solution

The objective of this invention was to design and manufacture a novel boiler that could solve the problems of previous generation. The boiler, with its new capabilities, was designed to fulfill the needs of various industries. A high-pressure turbulent flow in the combustion chamber was used to stabilize the combustion, reduce the kinetic energy and the flow-rate of combustion products, and extend heat exchange time between water in the spirals and combustion gases. It is important to say that the combustion in the chamber was achieved by the use of an inexpensive direct burner. A gas-condensing chamber was also used to receive residual energy of ignition gases. Due to the low energy and flow-rate of exhaust gases, the chamber was designed to provide the most exposure between exhaust gases and water transmission pipes, thereby intensifying energy exchange between them. In the condensing chamber, the exhaust gases flow against the water flow, which is a critical factor in increasing the efficiency of the machine up to 98%.

In water heating boilers, the larger the nominal capacity of the boiler is, the higher the operating pressure of the boiler would be. At high capacities, boilers must withstand high pressures safely. The use of pipes with high pressure tolerance of up to 60 bar, as well as the specific way of connecting and securing the spirals, result in a growth in nominal capacity of the suggested boiler up to 10 million kcal.

Well-controlled combustion process is one of the most important features of the fabricated boiler. Incomplete burning of combustible materials causes air pollution through the emission of excessive greenhouse gases such as CO and NO_x that severely damage the ozone layer and the environment. By controlling the combustion process using thermodynamics relations, the unbalanced production of these gases exceeding standard values could be avoided. The combustion process is accomplished by employing a burner in a chamber, where the distance between the spirals is adjustable to increase the space required for the complete burning of combustible materials.

The turbulence of combustion gases produces local vibrations in the spirals, and consequently prevents the sediments from settling and adhering to the pipe wall. According to the Navier-Stokes and Reynolds equations, these vibrations alter water flow to a permissible level, resulting in flow separation and sediment deposition prevention. As a result of the reduction of sediments which cause corrosion, the service life of the pipes increases. In order to boost the life of the boiler, appropriate materials to the working environment were used in the manufacturing of the tubes and other boiler components. For instance, the tubes are made of stainless steel (SS316L), which has excellent corrosion resistance and high ductility needed for forming low-diameter spirals. This material selection method extends the boiler’s life by up to 40 years.

Description of the drawings

Figure 1 is a view that shows configuration of the presented boiler. The two-chamber boiler comprises a separating wall 31 which is connected to the body with two angles. The left chamber is referred to as the main chamber and the right one is indicated as the secondary or condensing chamber within the text hereafter.

Main chamber

The configurational diagram showing section A-A of the main chamber is presented in Figure 2. The main chamber contains four SS316L spirals (1 to 4) having a distance D from each other. The outer spirals 1 and 4 are conical in shape, while the inner spirals 2 and 3 have a flat configuration. Hub 9 serves three primary goals. First, the side surface of the element has a helical groove that gives a taper shape to the outer spirals. Second, it consists of two holes by which the hot water of the confined spirals is transferred to the exit collector 16. The hot water from outer spirals are directly transferred to the exit collector via vertical pipes; however, due to space limitation, the hot water from the internal spirals are taken out using the holes created in the hub 9. Third, to use the effective length of the flame produced by the burner, a spacer 11 is mounted on hub 9 so as to push the burner tip back, and produce enough space for the flame. The space created at the center of the spirals is a cylindrical volume which forms a combustion chamber. Hub 10, behind the end spiral 4, does not fulfill the second and third tasks of the hub 9. Instead, a plate of SS 310 is welded to the end hub 10, with which the cylindrical space is closed. The flame tip spreads in the cylindrical space after hitting the plate. There are several clamps (e.g., 15) welded to the spirals. These clamps have holes for two purposes: holes in which retainers (e.g., 22) insert, and holes through which screws (e.g., 23) pass. Retainers are fixed into the holes to bear the weight of the spirals and keep them upright. These retainers are mounted on some legs (e.g., 33 and 34). Adjusting the distance between the spirals is achieved by the screws that pass the threaded holes of the clamps. Using connector 32 (Figures 3), the exhaust gases and the respective heat, are directed from the main chamber into the secondary compartment, where they are utilized for preheating inlet feed water.

Secondary chamber

Figure 4 is a section view demonstrating the secondary chamber in more detail. In the secondary chamber, there is a set of four flat spirals (5 through 8) made of stainless steel. These spirals have a distance d from each other, as shown in Figure 5. To adjust the distance of the spirals and maintain their weight in the secondary chamber, a system similar to the one used in the main chamber was employed. Each spiral is connected to the inlet water collector (17) using a stainless steel knee (e.g., 18). A circular SS310 plate (13) is welded to the outer spiral to prevent waste of heat transferred to the secondary chamber from the main one.

Water path

Water from the inlet collector 17 enters the secondary spirals from their inner radius, then moves toward spirals installed in the main chamber, therein it exits from the inner radius to reach the output collector 16. The illustration of water path is given in Figure 6. Water flow in the spirals of the secondary chamber is outward, whereas it is inward in the spirals of the main chamber.

Geometric features of the spirals The ratio r/R in the taper spirals (Figure 7) is of vital importance in the turbulence of the combustion products. The smaller the ratio, the greater the gas turbulence. The optimal ratio is achieved when it does not result in high thermal and compressive shocks. Besides, chamber volume should be appropriate for the flame. The ratio varies for boilers with different heat capacities and is directly related to the flame volume. This ratio plays a critical role in creating a complete burning. Another factor contributing to the complete burning is the distance D (Figure S), which controls the pressure inside the combustion chamber. By increasing the distance, the internal pressure of the chamber decreases. The use of an optimum spacing could increase the internal pressure of the chamber, as well as the collision and turbulence of the combustion products. Thus, the kinetic energy and flow-rate of the exhaust gases reduce, which gives them enough time for heat exchange upon crossing thermal passages. As the chamber pressure increases, some hot gases that are not allowed to exit thermal passages come back to the chamber center with their high pressure. The high temperature of these gases contributes to creating a complete combustion. As a result, thermal efficiency improves, and air pollutant emissions such as CO and NO_x are reduced.

The combustion products, which contain acidic gases and water vapor, are continuously evacuated from the main chamber and inserted into the secondary chamber through knee 32. The secondary chamber, which preheats inlet water, receives energy from these combustion products. Consequently, an acidic film with a PH of 3 through 5 is left, which flows into the tray 26 embedded below the boiler. The design of the condensing chamber is such that the flammable gases lose their energy in a new pass, and the water vapor in the products — together with some acidic gases — is condensed by contacting with the cold pipes. The cooled gases, with very low pollution, are then released from the chimney (24). In the design of the condensing chamber, optimal values of the inner radius R’ (Figure 9) and distance d (Figure 5) are required to provide the ideal condition for turbulence, prevention of main chamber pressure drop, prevention of the return of flammable gases from condensing chamber to the main chamber, maximum heat transfer, and maximum condensing. An important feature of the boiler is the concurrent flow of the fluids, which increases the thermal efficiency of the system. The cold water flows outward from the center. Due to the contact of hot exhaust gases with cold inlet water at the center, significant energy exchange occurs in this region, which causes an increase and decrease in inlet water and exhaust gases temperature, respectively.

Invention benefits

(1) High efficiency: The efficient design of the chambers, which allows maximum heat transfer between water and combustion products, enhances the efficiency up to 98%.

(2) Economical design: Despite the use of stainless steel in boiler structure and spiral-shaped tubes, the specific design of the boiler (optimal use of combustion, control of turbulence and gas pressure) has significantly reduced pipe and material requirements. The equipment is symmetrical in size and weight and has a price equivalent to one-third of boilers made of aluminum- silicon.

(3) Inexpensive burner: The use of cheaper direct flame burner instead of expensive premix burner is one of the most important properties of this work.

(4) High service life: The use of SS316L in the spirals of the boiler increases its resistance to acid and heat, which extends the boiler’s service life to 40 years. Therefore, its longevity is promoted unlike the conventional aluminum- silicon boilers that have a lifespan of 15 years. (5) No need to remove sediments: The turbulence in the gas flow, and the respective vibration in the tubes, prevent sediments from settling, which makes the boiler last longer without the need to repair.

(6) Higher working pressure and heat capacity: Conventional boilers were able to withstand the pressure of up to 6 bar, and produce heat capacity of 1,500,000 kcal or at best 2,500,000 kcal. In the new boiler, the heat capacity is increased to 10,000,000 kcal. Also, due to the use of seamless pipes and the correct welding method according to WPS, the tubes can tolerate the pressure of up to 60 bar. Tungsten Inert Gas (TIG) technique was exploited for joining tubes.

(7) Low pollution: Due to the optimization of the combustion process, which results in the complete burning of flammable materials, the new boiler is much less polluting than other boilers.

(8) Development feasibility: The new boiler can be upgraded through development plans, such as removal of the chimney from the boiler, or automating the adjustment of spiral distance to create an ideal ignition space.

Manufacturing of the boiler components

To produce helical tubes from initial flat ones, the initial length of the flat tube must be sufficient. Such tubes were not available in the market. To provide such a long tube, several shorter-length tubes were bonded head-to-head using the TIG welding technique. The long flat tubes were then used to produce helical tubes via roll bending operation. Hydrostatic test ensured that the tubes were not leaking. Hubs 9 and 10 are produced using the casting process. The grooves in these hubs, on which conical parts of spirals sit, are then produced with a CNC machine. Industrial applications of the invention

Heaters and hot-water boilers are the most common applications of this invention. Another area where this equipment could be used is the high-capacity production of distilled water which is still under investigation.

Claims

Claims This invention is not limited to the aforementioned descriptions. One who is expert in the field appreciates that many alterations and modifications without departing the spirit of this invention are possible. What is claimed is:

(1)The high-efficiency two-chamber boiler using the turbulent reverse flow of combustion gases is a novel boiler design comprising:

- A separating wall

- A set of at least four spiral-shaped tubes made of stainless steel in the main chamber

- A set of at least four spiral-shaped tubes made of stainless steel in the secondary chamber

- A knee-shaped connector that connects the two chambers

(2) The main chamber spirals of claim 1 comprises:

- A conical outer spiral, to which a hub and a spacer are connected.

- A conical inner spiral, to which a hub and a steel plate is welded. The plate closes the chamber interior space and distributes the flame.

- At least two spirals placed in between the inner and outer conical spirals.

(3) The secondary chamber spirals of claim 1 includes

- A flat outermost spiral with a plate attached to it.

- A flat innermost spiral that is blocked from behind.

- At least two flat spirals disposed between the innermost and outermost spirals. (4) A direct flame burner is joined to the hub attached to the outer spiral placed in the main chamber. This burner combusts a mixture of fuel and air within the cylindrical space created in the center of the spirals.