Direct transfer of heat from the burning fuel to the coolant is impossible. In heating boilers, it is performed due to the operation of a special device. This is a heat exchanger for a gas boiler. The service life of the apparatus and its efficiency depend on its design and material.
The main function of the heat exchanger for the boiler
The boiler burner is supplied with gas and combustion air. The gas burns, releasing heat, the combustion products are removed outside. The heat source in this case is a stationary element.
The heat carrier - water or antifreeze - enters the heat exchanger. It is a device that provides heat exchange between two media with different temperatures. The latter is located in the combustion chamber above the burner. Water moving through the heat exchanger is heated and supplied to the heating pipes. Most often, the device takes the form of a set of plates or tubes. The larger its working surface, the better and faster the water heats up.
Manufacturing material
The heat exchanger for the boiler is made of durable materials, good heat conductors, not prone to corrosion and sufficiently resistant to pressure. Since you have to take into account the cost of the material, there is little choice.
Steel
This is the most accessible material. The steel is very strong, but it lends itself well to processing. The price is not high. The advantage of this option is resistance to high temperatures. Steel is ductile and does not crack when heated, does not deform even in areas in contact with the burner.
A steel heat exchanger for a solid fuel or gas boiler is prone to corrosion. Water inside the pipes and combustion products in the boiler chamber have a destructive effect on the material. This affects durability. The steel model weighs a lot, this leads to additional fuel consumption for heating the element itself.
The stainless steel heat exchanger is corrosion resistant and has a service life of at least 50 years.
Cast iron
The material is much more resistant to corrosion than steel, it is not afraid of rust and the action of acid anhydrides. The service life reaches 50 years. However, cast iron is a brittle alloy and can crack under the influence of temperature. To avoid damage, the cast-iron tubular heat exchanger must be flushed: if ordinary water is used, then once a year; if antifreeze - then once every 2 years; if distilled liquid - once every 4 years.
The weight of the cast iron element is even greater, so more fuel and time has to be spent on heating.
Copper
Copper is a noble metal that does not corrode of any kind. It is chemically inert, it tolerates pressure well. Copper conducts heat better, so less fuel is required to heat the element itself and the flowing liquid. The weight of the copper model is small, the dimensions are compact with a very developed working surface.
The disadvantage is the high price. Also, the copper heat exchanger is too sensitive to high temperatures. More often found in boilers from foreign manufacturers.
Heat exchanger classification
Gas boilers can perform several functions. The main one is home heating.However, dual-circuit models also heat water for various domestic needs, from washing dishes to a bathroom. On this basis, heat exchangers are distinguished.
Primary
Serves the heating system. It is a pipe with a rather large diameter, bent in the form of a coil in one plane. To increase the working surface of the device, plates of different sizes are also placed here.
The primary heat exchanger is subjected to the highest loads. From the outside, combustion products act on it - soot, dirt, acid anhydrides, from the inside - salts dissolved in the coolant. To reduce wear, the part is coated with paint and treated with anti-corrosion compounds.
The best option is a stainless steel or copper heat exchanger, as it is not susceptible to rusting and is not afraid of salt deposits.
Secondary
Such a heat exchanger heats the hot water supply liquid. Its heating temperature is lower, but it is not worth heating water for domestic needs above +60 C. Most often it is a plate structure: it is assembled from many plates with extruded passages through which tap water circulates. Multi-pass models are more effective, since within one plate the liquid changes direction several times, that is, it stays in it longer and warms up better. It is made from steel, copper, aluminum.
Bithermal
Represents 2 pipes inserted into each other. The coolant moves along the inside, and water for hot water supply moves along the outside. The heating fluid is heated in the combustion chamber and partially gives off heat to the domestic water.
The design is much cheaper. But although the water heats up faster here, its volume is limited. In addition, the bithermal heat exchanger is very sensitive to water quality and gets dirty much faster. Cleaning the device is not enough. To prevent quick clogging and failure, it is necessary to install water filters at the inlet.
It is not possible to clean the combined heat exchanger as a normal separate one. In case of large salt deposits or clogging, the element will have to be replaced.
Criterias of choice
When choosing a device, the purpose is taken into account - in this case it is heating the coolant, and the type of medium - steam, water, antifreeze. A gas boiler usually works with water, but there are exceptions.
Other selection criteria:
- Heat carrier temperature at the inlet and outlet - it is necessary to calculate how much heat the consumer should receive. Based on these data, the power of the heat exchanger is calculated.
- Allowable pressure loss - the water pressure decreases as it passes through the heat exchanger. If it falls too low, it is not possible to create a column of hot water of sufficient height.
- The maximum operating temperature - on the burner reaches 600-700 C. This temperature can withstand a cast-iron and steel heat exchanger, copper with some difficulty. The aluminum model must not be used.
- Maximum Working Pressure - Does not limit the choice of design or material.
Dimensions are a significant parameter. With the same efficiency, a shell-and-tube heat exchanger occupies 3-4 times more area than a plate heat exchanger.
Correct operation
Transportation, installation and operation of the heat exchanger are described in detail in the instructions:
- The heat exchanger in the apparatus is placed so that there is free access to it for inspection and repair.
- Start-up is carried out at stable pressure and temperature readings. The temperature must not be increased by more than 10 degrees per minute or the pressure must not be increased by more than 10 bar per hour.
- When filling with water, the air valves and the valves behind the heat exchanger remain open. After starting the pump, they are closed. Thus, a stable pressure is achieved.
- Change the heating parameters smoothly. The slower this happens, the longer the seals and the heat exchanger itself will last.
- The device needs to be cleaned periodically. The plate is cleaned directly in the frame, then the plates are taken out and washed. Another method is possible: first removal and then cleaning of the plates. Shell and tube cleaning is not recommended. In case of complex blockages, the master puts a plug.
- Before restarting, check the condition of all gaskets. The pressure and temperature are set as for the 1st start.
To avoid salt deposits, a filter is placed on the water pipe in front of the boiler inlet.
Possible malfunctions
Most problems require specialist intervention. Some can be eliminated by the user:
- Depressurization - if caused by contamination, it is sufficient to clean the heat exchanger. If the connection to the network is incorrect, check the connection with the drawing in the instructions.
- Decrease in efficiency - in case of mechanical contamination, the device is washed. If the reason is the accumulation of oil, substandard gases, install additional devices to remove them.
- Leakage - Most often caused by decomposition of the seals. They are being replaced.
- Mixing of working media - occurs when plates or tubes are corroded. The plates can be partially replaced; the shell-and-tube heat exchanger will have to be replaced.
As long as the warranty is in effect, it is forbidden to open the heat exchanger by yourself and carry out any repairs.
Popular manufacturers
Heat exchangers are manufactured by many manufacturers. The following companies were the most popular in 2019.
Navien
The largest Korean manufacturer. Produces products for household boilers. The advantage is resistance to low water quality and water hammer. The device is perfectly adapted to poor operating conditions.
Baxi
Italian manufacturer. It presents on the market wall-mounted and floor-standing condensing boilers with a cast-iron heat exchanger, as well as electric heaters.
Primary heat exchangers are made of copper and brass. Stainless steel is used for the secondary lamellar. This increases the cost of the product, but ensures maximum durability.
Other
There are other worthy manufacturers on the market:
- Fondital Victoria Compact is an Italian company. Offers high performance bithermic copper heat exchangers.
- Beleto is a well-known Russian plant that produces a variety of gas equipment. Manufactures various types of steel, cast iron and copper heat exchangers.
- Ariston - offers aluminum and copper heat exchangers. The materials are insensitive to corrosion, and the manufacturing technology guarantees their strength.
If there is a need to increase the efficiency of the boiler when replacing the device, consult a specialist in order to calculate the required parameters.
