The development of individual construction, the construction of multi-storey residential buildings, various buildings and structures for commercial purposes have led to the need to use transportable boiler installations or stationary boiler houses for heat supply of these objects, operating mainly on natural gas and using modern heating boilers as heat generating installations. But, along with this, on the territory of Ukraine and the countries of the former USSR, a large number of already existing housing stock and industrial enterprises, built in the 50-80s of the last century, use boiler and pumping equipment of an outdated type for heat supply and preparation of hot water.
According to the Ministry of Energy and Coal Industry of Ukraine, as of March 2021, at least 21% of old boilers should be modernized or boilers replaced. At the same time, at least 25% of boiler equipment in Ukraine has been in operation for more than 30 years. The efficiency of many of these boilers does not exceed 60-70%. Here we note the critical degree of physical deterioration of heating networks (according to the Ministry of Energy and the Association “Ukrteplokommunenergo”, as of March 2021, at least 35% of all trunk pipelines in Ukraine are in emergency condition, and the value of heat losses in them ranges from 15 to 30% ) Of course, the solution to these problems should be carried out in a complex, however, in this article we will consider issues relating specifically to heat-generating boiler equipment.
On the territory of Ukraine, as in the countries of the former USSR, there are 2 main types of industrial gas (and solid fuel) boilers, characterized by the method of washing heating surfaces with fuel combustion products: fire-tube and water-tube boilers. In turn, these boilers are divided into hot water boilers (heating the coolant according to different temperature graphs, for example, 95/70 С, 115/70 С, 150/70 ˚С1) and steam boilers used to produce saturated or superheated steam …
Fire-tube heating boilers are a cylindrical combustion chamber (fire tube), with smoke tubes located around the chamber, through which combustion products circulate, providing convective heat exchange. The “water jacket” of the boiler is located in the space between the fire tubes.
Types of fire tube boilers
There are 2 main types of fire tube boilers:
- 1) Two-pass boilers, or boilers with a reversible combustion chamber, in which the first passage of the combustion products circulation is the combustion chamber itself, and the second passage is a bundle of smoke tubes.
- 2) Three-pass boilers – boilers in which the first pass is the combustion chamber, and the second and third are 2 channels of smoke tubes. Three-pass boilers, due to the larger area of convective heat exchange surfaces, have a higher level of cooling intensity of combustion products.
Fire tube boilers of modern type have a number of indisputable advantages, such as:
- • Compact design due to more intense heat flow.
- • Delivery of a complete boiler in the form of a single monoblock.
- • The minimum hydraulic resistance (measured in millibars), due to which relatively low-power pumping equipment is used to circulate the coolant through the boiler.
- • The tightness of the combustion chamber, which allows the use of blast burners and, as a result, does not make the use of smoke exhausters mandatory.
- • Small inertia – quick response to changes in heat load, which simplifies and optimizes the automation of these boilers.
- • Less, in contrast to water-tube boilers, labor costs for the manufacture of boilers.
- • No need for additional lining.
- • General simplification of construction and installation work.
The experience of developing fire-tube boilers in the USSR showed, along with advantages, a number of significant disadvantages, mainly due to design errors, which became the reason for the decision to abandon their production in favor of water-tube boilers. The first developed fire-tube models had a low value of efficiency, high metal consumption, limitation in heat and steam production. The flame tubes were not very reliable. As practice has shown, the use of the Normative Method of Thermal Calculation of a Boiler Heating Unit, which is relevant for water-tube equipment, for the design of fire-tube boilers in many cases was incorrect.
Water tube steam and hot water boilers were the main type of boilers manufactured in the USSR. These boilers, depending on the modification, were intended for burning natural gas or solid fuel (mainly coal). Structurally, water-tube boilers are a bundle of a large number of pipes that are reduced to one or more drums that serve as reservoirs for water or steam. A tube bundle with a drum is placed above the combustion chamber. A coolant circulates inside the tube bundle. Partitions installed in the combustion chamber provide multiple passage of fuel combustion products through tube bundles, which ensures high-quality heat transfer from combustion products to the coolant.
Also, in many models of water-tube steam boilers, heating surfaces such as radiation tube screens are provided, through which heat exchange is carried out not by convection, but by radiation. The coolant passing through a large number of wall tubes is converted into steam.
Advantages of water tube heating boilers
Water tube boilers have a number of advantages, such as:
- • The circulation rate of the coolant in the tube bundles is at least 1 m/s, which protects the boiler tubes from possible boiling of the coolant and sludge deposition.
- • Lack of stringent requirements for the quality of the coolant.
- • Possibility of normal long-term operation in forced mode.
- • The speed of the boiler operation to the nominal mode.
- • Complete explosion safety.
- • Durability of operation.
However, in addition to the positive characteristics, water-tube boilers also have a number of disadvantages.:
- • The above-mentioned high speed of the heat carrier leads to a high hydraulic resistance of these boilers, which requires significantly higher consumption of electrical energy for the operation of the boiler circulation pumps.
- • The complexity of the design of boilers, a large number of various units that complicate their transportation, installation and repair work.
The mass production of water-tube boilers in the USSR led to the fact that a large number of boilers such as NIISTU, DKVR, PTVM, KVGM and other similar ones, which are morally and physically obsolete and require modernization, function in Ukraine. Of course, these boilers were once manufactured in accordance with the strict requirements of the then GOST and many of the operating boilers are distinguished by the high quality of the boiler pipes and welds. However, their long service life requires constant repair of these boilers, replacement of tube bundles, screens, and lining work.
Modernization of water tube boilers
If we talk specifically about the modernization of water-tube boilers, then here, first of all, the question of the characteristics and safety of operation of gas-burning devices of these boilers is raised, the degree of their automation and, as a result, the environmental parameters for emissions of harmful substances do not correspond to modern standards. A fairly common practice today is to replace obsolete burners with modern European-made gas burners. The group of companies “Ukrainian Energy Industrial Group” has experience in implementing similar solutions based on Italian-made CIBITAL UNIGAS burner equipment.
CIBITAL UNIGAS has developed special models of gas burners for water tube boilers. One of the features of such boilers is the shallow depth and significant volume of the furnace space, which requires a short torch for the burners. Therefore, it was decided to manufacture these burners not with one nozzle, as in standard burners, but with four short flame nozzles. Such a technical solution ensured high-quality preparation and combustion of the gas-air mixture, simplified commissioning, and, most importantly, provided complete automation of the boilers and modulation of their power in a wide range. As a result, the efficiency of the boilers increased by at least 10-15%. Optionally, UNIGAS burner models are available with frequency control of engine speed (to save energy) and control of the oxygen content in flue gases (to further save gas and reduce harmful emissions).
It should be noted that the installation of modern gas burner devices is relevant not only in terms of replacing outdated burners, but also when converting water-tube boilers operating on coal. Depending on the current physical state of the heat exchange surfaces of such boilers, their conversion to gas fuel entails an increase in the efficiency of a gas boiler by 5-15%.
As for fire-tube boilers, this type of boiler equipment began to be massively introduced for heat supply to housing and communal services and industry from the late 1990s – early 2000s as part of the implementation of projects for the decentralization of heat supply systems. Fire tube boilers of both Ukrainian and foreign production, which began to be introduced, no longer had the above disadvantages, since their designs were developed by each manufacturer on the basis of modern computer programs. Almost each of the major manufacturers of fire tube boilers already had their own research organizations, the main task of which was the issue of modernizing the design of boilers at the stage of their design, as well as developing recommendations and requirements for the operation of boilers. This includes:
- 1) Increasing the efficiency of boilers – choosing the optimal area of the heating surfaces of boilers, researching the operation of boilers with additional heat exchangers (economizers) from combustion products, as well as determining the economic efficiency and payback period for the introduction of economizers.
- 2) Reduction of emissions of harmful substances in combustion products – calculation of the design of fire-tube boilers, use of special burners with low NOx emissions.
- 3) Ensuring the maximum service life of boilers and equipment operation, preserving the integrity of the fire-tube monoblock structures – using water treatment plants in boiler plants (removal of hardness salts, iron), sludge and microbubble separators, removal of dissolved oxygen, increasing the pH of the coolant, etc. Plus regular maintenance.
- 4) Optimization of the overall dimensions of boilers as a whole and of individual units, as well as their material consumption. This includes the economic aspect, namely, reducing the cost of manufacturing boilers while maintaining high quality parameters of equipment.
- 5) The choice of the optimal solution for the use of gas burners – selection and determination of standard sizes of burners, their thermophysical characteristics, their cycle of operation and the necessary functions. Modern gas burners, with the right choice, have a long service life, and their maintenance costs are relatively low.
- 6) Maximum automation of equipment operation. This includes, first of all, the use of the most modern means of automation and visualization of the operation of boilers and auxiliary equipment – boiler controllers that control the operation of boilers and burners with control of the operating parameters of the equipment (temperature and pressure of the coolant, control of the rarefaction of flue gases and their temperature, control of the circulation of the coolant, automatic stopping the operation of boilers in emergency situations, etc.) Boiler automation contributes to saving energy resources, since with its help a modern heating boiler releases exactly the amount of thermal energy that is needed in a specific time period.
- 7) Maximum safety of the equipment. This task is closely related to the issue of automating the operation of boilers, since it is the control systems for the operation of boiler equipment that are responsible for the safety of the operation of boiler units, monitoring the parameters given in the previous paragraph. Modern fire tube boilers are equipped with a complete set of functional devices necessary for safe operation.
Thus, today it is fire tube boilers that occupy the main niche in the line of industrial boiler equipment with a capacity of up to 30 MW. In Ukraine, among the national manufacturers of such boilers, the main niche is occupied by KOLVI boiler equipment, on the basis of which many construction projects have been implemented. As an example, the most ambitious project of such a plan was implemented on the basis of KOLVI equipment in the resort town of Truskavets, where in 2001-2011. 21 boiler houses with a total capacity of 51 MW were built or reconstructed, which heat 158 objects for various purposes. Since 2019, the Ukrainian Energy Industrial Group Group of Companies has begun implementing a large project for the construction of 17 modular transportable boiler houses with KOLVI gas fire tube boilers.
After analyzing all of the above, we can draw conclusions about the following three aspects of the relevance of the service modernization of heating boilers:
- • The economic aspect is a reduction in the cost of energy resources (gas, solid fuel, electricity), the cost of maintenance and repair of boilers.
- • Environmental aspect – reduction of emissions of pollutants into the atmosphere.
- • The safety aspect is ensuring uninterrupted and trouble-free heat supply during the heating season.