Heat records

The buyer of any technique or any other item, especially at the stage of purchase, is constantly faced with a dilemma “which is better”? To buy something simple and inexpensive, or more interesting at the moment , but also more expensive? There is no exact answer to this question it depends on a number of other factors, of which economic considerations are not always the most important.

For the market of durable goods another factor comes into play: the cost of ownership, maintenance and operating costs in general. But in order to determine the choice, it is necessary to know which proposals exist, and how one differs from the other. Sometimes there is a difference, and quite serious.

This is one of the most important factors for heating equipment. It is taken for a long time, is not cheap and requires considerable expenditures of energy carriers in other words, own funds , as a result these costs will be many times higher than the cost of equipment itself. And there is even a choice. A simple heating boiler is inexpensive, but a condensing boiler is more expensive. And there are buyers for all of them. The first ones can work with an efficiency of about 90 %, the second ones with up to 110 %.

110% EFFICIENCY? NO ERROR!

It has been known since school that the coefficient of efficiency of any system cannot exceed one hundred percent. Nor can it be equal to this figure: all sorts of losses are inevitable. Nevertheless, in the case of condensing boilers, we often find efficiency figures of the order of 106-109%, sometimes a little more or less. There is no mistake about it, they think a little differently. To understand this phenomenon it is necessary to understand what is possible to get from the boiler and what are the pitfalls.

Combustion of any organic fuel produces water vapor, carbon dioxide, and heat. If you remember chemistry lessons at school, the mantra comes to mind: “plus tse-two, plus ash-two-o”. Then, in the next chemistry class, they add the words “plus cu” to this formula. “Ku”, t. e. Q is heat emitted. To this Q we can say our “koo” and sit down in front of it. Warm up.

But this formula, whatever coefficients and numbers it includes, is valid only until the products of combustion including heat are divided into two parts. We are not interested in carbon dioxide, and everything is more interesting with water vapor. When its temperature drops, the process of condensation begins – the transition of steam into liquid. And at the same time, without any chemistry, according to the laws of physics, additional heat is released. This is the so-called latent heat of condensation, also called the higher heat of combustion in these two definitions some words can be combined, the meaning will not change , which is not taken into account in simple calculations and is not used in simple convection traction boilers. And yet its significance is not insignificant. For natural gas methane the higher heating value is about 11 % of the heat produced by the combustion of the fuel alone the lower heating value . For diesel fuel, which is often used in heating systems, this adds up to about 6 %, for liquefied gas liquefied butane gas about 9 %. All fossil fuels have this heat, but other fuels, both liquid and solid, give even less gain. Finding data for both the higher and the lower heating value is easy, at least for fuels of uniform chemical composition. So, taking into account the higher heat of combustion, the combustion efficiency of fossil fuel plant can be quite higher than 100%. If, of course, the installation is able to “collect” this heat and use it efficiently.

WHERE IT WORKS?

In order to make use of the latent heat of combustion in any installation, we first need to know why we might need it. Here the principle “the more powerful device, the more it makes sense to complicate the system” is mostly applicable. And fuels are burnt almost exclusively for three main purposes: transport, electricity generation or heating. In the first two it makes sense to collect this heat only when we are talking about very large installations, and the third is quite suitable for the “private person”.

In the transport sphere, say, in auto transport which also uses combustible fossil fuel the theoretical gain is miserable: the internal combustion engine efficiency is far from 100%, most of the energy is spent on heating the engine which also needs to be cooled. In such conditions it makes no sense to try to utilize condensation heat, even theoretical gain is not needed. ICE condensation heat recovery system makes sense only for some very large engines, for example in ship installations: fuel consumption is high, lots of heat is released, including with the exhaust gases. Collect it and

use for some additional purposes is quite real, although additional devices will be needed.

In large power plants like cogeneration plants or other types of power stations it is the same: the point is to gather and use as much energy of all kinds as possible on a larger and larger scale, i.e. to extract it from the ground or to exploit it for the needs of the environment. e. power. Even if the main objective is the production of electricity, and this heat, as in the case of generator sets, is a by-product. With a variety of ways it can be used.

But things are a little different with heating systems. If the fuel is burned in order to “get warm”, it is logical that it can be used “to the maximum. Everything goes. Even if we’re talking about heating on a very small scale, such as a private house. There are some limitations, but using condensing boilers for this purpose is quite realistic and economically advantageous. Of course, the greater the power and fuel consumption , the greater the benefits. However, it is cost-effective to make a home heating system only if gas or liquid fuel is used for heating. For solid fuel boilers the use of the calorific value is problematic – there is simply very little of it. However, there is one little trick to using solid fuel. We’ll mention it later.

FUEL QUALITY

The real efficiency of any boiler will depend on many factors, and the fuel quality is the parameter that the user can not control. These impurities are not much in the slab – only a few percent in total – but we have to take them into account. In natural gas the main component is methane, in smaller amounts there is propane and butane, in liquefied gas the main component is a mixture of propane and butane, in diesel gas – a mixture of heavier hydrocarbons. Besides any fuel contains some molecular nitrogen, oxygen, water. These components have no effect on the combustion, they are considered as “ballast”. Among the harmful impurities are mainly compounds of sulfur, nitrogen, phosphorus. Other substances can be found in trace amounts. By the way, there are also them in the combustion air, although in insignificant quantities. These compounds mostly do not burn, there is no need to expect heat from them, but they can react chemically during combustion. If we are talking about a traditional boiler – with normal quality fuel the concentration of “active chemistry” on the air is so low that there is no need to talk about it. Another thing is if the boiler is a condensation boiler: These substances will be accumulated in the condensate along with the water in the flue gases. As a result, instead of water we get a chemically active mixture. Two problems arise from this: In a conventional boiler and its chimney condensate formation is inadmissible, and in a condensing boiler all elements on which condensate forms and is removed must be resistant to its long-term effects.

As for solid fuels that are made from vegetative raw materials, they necessarily contain water: moisture can reach tens of percent. During combustion a lot of energy is spent on heating and evaporation of this water. Theoretically, if it is condensed, we can get additional energy. But in practice, at least in home heating systems, it is too complicated. It is impossible to automatically dosage the solid fuel, the effect would not be great. The exception are pellet boilers that use wood pellets as fuel. But even among them there are practically no condensing models. In addition these boilers should be more correctly called recovery boilers: in such condensate there is practically no water formed during the combustion of fuel, the main contribution is made by the water that “has already been”. Of course, in large systems recuperation is used, but these are not boilers, but devices separate from them.

HEAT LOSS IN THE BOILER

Consider any convection heating boiler. It does not matter what kind. If we assume that the amount of heat released during the combustion of the fuel in the boiler is 100%, the heat balance looks something like this.

Most of the heat energy goes where it is needed – to heat the liquid in the heating system. Some will go “down the drain” and be irretrievably lost. Another part of the energy is consumed for heating the boiler drum. It is not always possible to count its losses, because the boiler itself is located in the boiler room, kitchen or living room. This heat is still used for heating, except that we cannot control it. Ultimately, in rural areas and is nowadays not uncommon steel or cast iron boilers without any cladding, a kind of symbiosis of wood stove and liquid heating system. But even in case of modern gas heating boiler its efficiency will be around 90%. To increase the efficiency is possible, but only by some percent.

In principle, the more the flue gas is cooled in the boiler, the more energy is used for its intended purpose. But the “colder” the exhaust gases are, the harder it is to “take away” heat from them. The system becomes more complex, and the addition is small. And we also need to consider that the boiler can operate at different air temperatures, in different modes, but the fact is that neither in the chimney,

or even more so in the boiler itself there should be no condensation process. Recall that condensate is chemically quite active, and the materials of the convection boiler and the more so the chimney are not designed to interact with it. Temperature of gases at the outlet of the boiler can be around 150-200 °C, in older models it is higher, in some modern low-temperature models it is lower, around 100 °C. The rest of the heat literally goes out the window. Of course, the condensation occurs somewhere “down the chimney”, but this does not do us any good. However, there is no harm either.

In condensing boilers to this heat balance is added energy of the higher heat of combustion. Of course, you can not collect it all, too, some losses will be here too. It is impossible to completely “dry” the flue gases. But some though small amount of heat will be added from stronger cooling of flue gases. Losses through the body of the boiler itself should also be reduced by better insulation at least no worse than on conventional boilers . The fact that the condensing boiler usually has more “noisy” elements than a conventional boiler. Noise from the burner, pumps and fans can easily be reduced with an insulating jacket.

In total, the efficiency of such a boiler may well be at the level of 108 – 109% when working on natural gas , because the temperature of flue gases at the outlet is quite low. The difference in heat utilization compared to a conventional boiler can be approx. 15%. But this is only in theory and under certain conditions. When working with a boiler in the heating system it is necessary to consider them together.

CONDENSING BOILER AND HEATING

A little tricky

Here, to begin with, let’s imagine that the boiler consists of two separate heat collection units in fact this is not always the case, at least in individual heating systems . The first block has the same functions as a traditional boiler: burner, combustion chamber and a kind of heat exchanger. Generally speaking, there is only one requirement here – heat resistance. Condensation is assuredly not forming, so there’s no need to worry about the corrosion of the assembly. The hot gases flow into the second block – the heat exchanger – where they are intensively cooled and where the condensate. Here, firstly, the temperature is still quite high, and secondly, the material has to be acid resistant, because the condensate is a weak but still acid solution, and it’s also quite hot.

The more heat is removed in this, the second heat exchanger, the more effectively the boiler works as a whole. And for this, at least “on the fingers”, we need to make another balance. The task of the heat exchanger or rather two, it is necessary to take into account the one that is in the first block is to take away a certain amount of heat. Its value is quite determinable, it corresponds to the current demand for heating and hot water preparation, if such a task is set .

At the inlet to the heat exchanger, we have hot gas at the outlet, it must cool down. In the water circuit – on the contrary: in the input cold water or antifreeze will take away this heat. We can only manipulate the amount of heat, t. e. by the supply of fuel burned by the burner. No more. The design of the heat exchanger or the heating system “on the fly” we obviously cannot change, even the pump or pump system that pumps the liquid, usually has a fixed capacity.

The only way we can cool flue gases is by taking their heat and giving it to the boiler water, entering the heat exchanger. And the lower is its temperature, the more heat can be collected. But this water came to us from the heating system, it can’t be very cold by definition.

You have to remember about low- and high-temperature heating systems. The main representatives of the first – the floor heating, the second – the usual radiators. For the first ones the typical temperature of the return line in the boiler it will be the “inlet” is around 30 °C. The second has a temperature of 50 °C or more. Flue gas condensation temperature of 55-60 °C. It is clear that in the first case the condensation will be much more efficient, in theory – up to 109-110%. Well, if the temperature of the liquid in the return line is the same or even slightly higher than the condensation temperature, you should not expect miracles. In this case the same boiler will be more efficient than a traditional boiler, but the game will not be theoretically possible 15%, but about 5%, and the efficiency will be around 96% to 99%. Quite a lot, if we don’t take into account the complexity of the system. But if they are, it is worth calculating whether such a gain is economically justified.

By the way, in passing we can make another conclusion: since the efficiency of the condensing boiler is very dependent on conditions, and we can change, in general, only the fuel supply – compared to the convection boiler, it makes sense to use more complex burners and control systems of their work.

THE CONDENSING BOILER STRUCTURE

Boilers with two heat exchangers, main and condensing, are not often used. This is more typical for some fairly large and powerful models: the convection part is taken from the corresponding boiler, and “screwing” to it the condensing heat exchanger is a matter of technology.

If for traditional boilers of low power most often use flat heat exchangers took the burner from the oven of a gas stove, put a radiator on it, “covered” with top gas removal system – that’s basically the whole boiler , the condensing boilers are characterized by a cylindrical heat exchanger: The burner is placed at the end of the cylinder. Of course, the design also includes a condensate collecting device.

Open combustion chambers are not typical for these boilers, closed combustion chambers are required. Burners – with modulation of both fuel and air supply technical features depend on the design of the burner . The material of the heat exchanger is mostly silicon-aluminum alloy silumin or acid-proof stainless steel the spindles are stainless steel.

In all other respects, except for a more complex control and monitoring system, the boilers are not much different from the convectional boilers. Dimensions and appearance are similar within the same output range. The main external distinction is the condensate outlet: Small wall mounted models often have an all-inclusive design that includes an expansion tank, a circulation pump, sensors and a main control panel in the enclosure.

If the boiler is dual circuit, which is often the case with comparatively small models design type , the heat exchanger can be either bi-thermal or split. In the bi-thermal heat exchanger of both circuits is made as a single unit, the heating and DHW pipes are coaxial, one inside the other the inner pipe refers to the DHW circuit . A separate secondary heat exchanger for hot water is made separately, it is heated from the primary heat exchanger.

Boilers with bi-thermal heat exchangers are cheaper, simpler, but require high quality water passing through them, otherwise the cross section of the tubes will quickly overgrow with limescale and the efficiency will decrease. Separate heat exchangers are less sensitive to salts in the water, allow you to get a little more hot water per unit time, but require the introduction of additional elements in the system direct heat exchanger, three-way valve and its control devices , they cost a little more expensive. Normally the secondary heat exchanger material is stainless steel.

Many manufacturers offer wall hung boilers with integrated boiler as a variant though, in this case boilers often become floor standing .

As the output of boilers increases, they are less often equipped with additional fittings: guessing the parameters of these elements in complex heating systems becomes impossible. First of all, the built-in expansion tank and pump group disappear from the boiler delivery set even more powerful models are not delivered with control panels: Of course, all this can be bought individually, choosing the components that are best suited for a particular object: If necessary, many boilers allow for the use with other heat generators: in a cascade with similar boilers, together with solar collectors, etc. e: Here everything is exactly the same as with other types of boilers.

Circulation pumps with adjustable shaft speed and therefore performance have recently appeared on the market. Previously, the speed could only be changed during service setting of the boiler, and not always. Pump – a detail not very big, but quite expensive in any design. New gadgets are more expensive than conventional ones, and require more complex algorithms than just “on-off” which means that the controller must support their operation . Their advantages are lower noise level and energy consumption and the possibility of more precise setting of the necessary fluid current. We can assume that these pumps will soon be installed on most boilers, especially on condensing boilers.

CHIMNEYS

But chimneys for condensing boilers must be different from the traditionally used. Recall that even when the boiler is operating at maximum energy recovery, when the efficiency is close to the theoretically achievable, some part of the condensate will not be recovered and will pass on. And then we have the chimney, which is probably colder. This means that condensation will continue in the chimney: Conclusion – the chimney must be made of acid-proof materials: The usual materials for the “condensation” chimney are acid-proof stainless steel or plastic: Coaxial designs are common, where one chimney is inserted into another. They are usually made of plastic: The gas temperature is not too high, plastic can stand more than that. Condensate does not harm the plastic chimney either, at the same time installation costs are reduced. Limitation – the length of the coaxial chimney must not exceed 3-5 meters: Usually it is led directly into the wall. By the way, here everything is the same as with other types of boilers: plastic flues can be installed on traditional boilers as well: But if the flue system has a horizontal part, you can still determine the type of boiler by looking at it: convection boilers have a slight slope “from the boiler”, condensing boilers – “to the boiler”. The explanation is simple: if condensate forms in the chimney, we must give it the opportunity to drain. There is no point in flooding a conventional boiler with condensate, while there is no obstacle for a condensation boiler – it will still drain through the condensate drain.

SCOPE OF APPLICATION OF CONDENSING BOILERS

Condensing boilers for private use appeared on the market not so long ago. Most of them are produced in Europe and sold there: we are lagging behind. And that is a very good thing.

In the not too distant past, when fuel cost kopecks and cents , there was no point in having condensing boilers for users – it was hard to pay them back. Since then the situation has changed a little: fuel has become more expensive. And in Europe, where it is much warmer than here, on a mass scale they start to install condensing boilers. It’s a matter of heating costs. In Europe, the gas for the end user costs about 5-10 times more depending on the country than here. The costs are solid, no difference in wages not so big, by the way will not compensate it. At this gas price even a 15% discount on the use of the “condensate boiler” or even a 5% discount “in the worst case” quickly results in a considerable sum which covers the costs of the initial purchase of the more expensive boiler. In our country, of course, you have to wait longer to save money, so both condensing and traction boilers are popular.

The economic effect of the purchase of a condensing boiler can be expected in several basic cases. And here again the principle “the more powerful the more heat required – the more sense” is valid. It is best to install it in a new home designed for permanent living, and the further north you go, the greater the effect. But you need to look at the average January temperature in this region, in this regard you can only compare it with the European part of America in Sweden, Finland and Canada, the other countries are warmer. To get the maximum effect, it is worth arranging low-temperature heating systems in the house: under-floor heating. It is also much easier to plan a suitable chimney for a condensing boiler in a new building. It doesn’t make economic sense to make specialized repairs of floors and chimneys in a house that is already well-equipped.

There is a recent tendency to use condensing boilers in cascade systems, when instead of one large boiler is placed several smaller boilers of different capacities. Such boilers are very compact. It is also convenient that one boiler should work the whole heating season, and not many – you can connect one by one, as the frost gets stronger. And the system becomes more reliable: if one boiler fails, it can be shut down for the time of repair, and the load can be transferred to the remaining boilers. For individual boilers there are no special restrictions on the geographic location. It is more complicated with large capacity boilers, designed for collective use. In very cold weather, the water even in the underground heating system can be very cold before it reaches the user, so low-temperature “collective” heating is not applicable everywhere, and in high-temperature mode the condensing boilers are not very effective. That is why in the northern regions, common boiler houses are equipped with traditional boilers with high supply temperature.

A good opportunity to save money would be operation of boilers with additional control and monitoring systems. These are systems of weather-dependent regulation, remote control, adjustment and programming, devices for remote monitoring, access and control.