New technologies in boilers

New technologies in hot water boilersEnergy saving: reducing fuel costs and protecting the environment

In the 70s, after the oil crisis, the need to reduce energy expenditure in industrialized countries arose. One of the ways forward was the improvement of thermal processes, based on the reduction of energy losses in industrial combustion processes, applied in the case at hand to heating services and domestic hot water production.

In 1979 Viessmann presented on the market the first boilers capable of modulating the operating temperature without condensation problems, achieving significant energy savings.

In 1981 the first regulation appeared in Spain to regulate the design, assembly and maintenance of air conditioning installations, which contemplates a specific technical instruction for the saving and efficient use of energy (ITIC. 04). It is the first national step towards reducing losses.

In 1982, the first definition of the concept of "low temperature" appeared in Germany, included in DIN 4.702, which in turn served as the basis for the drafting of the Community Directive 92/42/EC. Going a little deeper, let's know how Community Directive 92/42/EC defines the concepts of low temperature and condensation: "Low temperature boilers will be those that can operate continuously with a feed water temperature of between 35 and 40º and that, in certain circumstances, can produce condensation. Condensing boilers shall be those designed to permanently condense an important part of the water vapor contained in the flue gases." Therefore, and following the definitions of the directive, the rest of boilers become standard performance, being defined as those whose average operating temperature can be limited from their design.

To achieve these results, low temperature and condensing boilers must allow the temperature of the water contained to be reduced, without damage.

DETAILED ANALYSIS OF LOW TEMPERATURE BOILER TECHNOLOGY

Conventional boilers require maintaining an average working water temperature, which prevents the appearance of condensation inside the smoke tubes and in the smoke collector. The danger of condensation is further exacerbated, when the fuel used contains sulfur in its composition, since it would cause the appearance of sulfurous and sulfuric acid, highly corrosive to the components of the boiler.

Once the regime temperature is reached, the water vapor evaporates, leaving in the solid state the sulfur residues in the smoke tubes, with a characteristic yellowish color. The continuous repetition of this process leads to the increase of acid deposits, which in turn cause internal corrosion of the walls. This phenomenon occurs with extraordinary speed in conventional boilers. Low-temperature boilers base their technology on the use of heating surfaces (tubular beam) of double wall of 6 mm thickness with air chambers, capable of dosing the heat transmission and avoiding the production of condensation.

This system allows to reduce the temperature of the water inside the boiler to 40º, without condensation. therefore, a smoke outlet temperature of up to approximately 130º can be achieved (always depending on the type of fuel), achieving seasonal yields between 93 and 95%. To all the above we must add the reduction of losses due to convection, radiation and transmission, through the provision of perfect thermal insulation, which prevents unwanted heat leaks. In addition to the reduction of consumption, this type of technique allows the reduction of polluting emissions, by reducing the operating time and by the lower number of burner starts, at which time the pollutant emissions are higher.

ANALYSIS OF THE TECHNIQUE USED BY CONDENSING BOILERS

If previously we saw how the technology used in the construction of low temperature boilers sought to reduce the temperature of the water inside it, without condensation occurring on the smoke side, we will see below how we can take advantage of the latent heat of vaporization with condensing boilers. Condensing boilers use gas as fuel, because they lack sulfur in their composition and because they produce a greater amount of water vapor during combustion, therefore we will talk about gas condensing boilers.

The condensation technique takes advantage of much of the heat that in conventional boilers is lost with the flue gases evacuated by the chimney. In order to take advantage of all the energy put into play in a combustion process, it will be necessary to condense as much of the water vapor produced in that process as possible, recovering the latent heat of vaporization. With them, an instant return of up to 109% could be achieved at best.

For the manufacture of this type of boilers, materials such as AISI 316 stainless steel are used, achieving a durability of the same much higher than that of conventional boilers, despite the condensation. It will be necessary, therefore, to have large contact surfaces between fumes and water, to achieve maximum heat transmission, reaching a smoke temperature at the outlet of the boiler of only 10 ° C above the return temperature.

Thanks to these heat exchange surfaces, the instantaneous performance of these boilers is superior even to that of low-temperature boilers, even at a temperature where no condensation occurs. The degree of thermal insulation of boilers obeys the same criteria as low temperature boilers: maximum reduction of losses due to radiation and convection, reaching a level of 0.3%.

Condensing boilers occupy a privileged position, not only in terms of thermal performance, but also in terms of reducing pollutant emissions, thanks to the design of your home, cooled by water, which in Viessmann boilers, considerably reduce emissions of nitrogen oxides and carbon monoxide to values below the minimum required by the Swiss Environmental Regulation, having even obtained ecological distinctions. We can affirm for all this, that condensing boilers are the maximum current exponent of energy use in combustion processes, for comfort use, together with the maximum reduction of polluting emissions.

THE "SEASONAL PERFORMANCE"

Let's analyze below the concept that really reflects the greater or lesser efficiency of a combustion-based system: the "seasonal performance". Instantaneous performance is what boiler manufacturers usually provide among their technical characteristics. This performance results from the ratio of useful power to rated power, expressed as a percentage. The useful power is that actually transferred to the water and results from subtracting from the nominal the losses suffered during the combustion process. These losses are, in different percentages, the following:

• Losses due to radiation and convection through the boiler envelope, despite its external insulation.• Loss of a small part of the fuel, which does not burn, being wasted in the process (unburned).• Losses of sensitive heat through the smoke duct. The fumes leave the boiler at a temperature higher than that of the water contained in it.• Latent heat losses through the fume circuit. The fumes leave the boiler with a moisture content, the result of combustion, wasting a significant amount of energy (greater or lesser depending on the hydrogen content of the fuel used). To avoid losses due to radiation and convection, it is necessary to provide the boiler with an adequate thickness and insulation quality, both in the envelope, as well as in the front (door) and rear (smoke collector).

Currently, manufacturers of high-performance boilers ensure the highest quality in the level of insulation of their generators, reducing themselves to losses of less than 0.3%. The reduction of unburned requires a perfect regulation of the burner, ensuring that the mixture between fuel and oxidizer is as intimate as possible and the granulometry of the spray (in the case of diesel) is extremely fine.

In this aspect, gaseous fuels benefit from liquids and, above all, presenting a much lower level of unburned. The losses through the smoke circuit are significantly reduced in low temperature generators, being limited to minimum values in condensing boilers, with total losses of this nature, lower in many periods of operation to 3%, being set for standard boilers in a band close to 10%.

Among those listed, the most significant are those due to losses due to the smoke circuit and those of radiation and convection. Seasonal performance will be determined by integrating over time the instantaneous yields obtained over a full period of operation (a heating season, for example). It should be borne in mind that the losses due to the smoke circuit will depend on the hours of operation of the burner coupled to the generator, since as the thermal demand of the installation increases, the total losses inherent in the combustion increase.

The losses due to radiation and convection will depend on the total hours in which the generator is at temperature of regime or, what is equal, to the annual hours in disposition of service. A boiler for the production of domestic hot water will generally be at regime temperature (average temperature approximately 80 °C), 24 hours a day, 365 days a year, when the hours of thermal demand for this service will not exceed, as a general rule, and for residential buildings 5 hours a day. With these approaches, seasonal performance will be harmed in direct proportion to the increase in the average temperature of the generator, during the hours in service.

FUNCTIONAL DIFFERENCES

Let's compare the functional differences in operating modes between standard boilers and low-temperature or condensing boilers. HEATING Conventional boilers: the average water temperature inside the boiler will be between 70 and 80°C, in order to avoid the phenomenon described above of condensation.

The temperature of the water sent to the emitters will be achieved by mixing part of the water from the boiler and part of that from the return of the installation, always depending on the outside temperature. The high average temperature of the boiler will cause high and virtually constant service disposal losses (radiation and convection) during this service arrangement. During the period of operation of the burner, the losses by the smoke circuit will be high, except during the setting to regime temperature, which due to the low temperature of the water, the heat transmission and therefore the thermal use will be greater. However, this transitional period is very short in relation to the number of hours of operation at the regime, so the fumes will come out at a constant temperature, because the average temperature of the water in the boiler is practically constant. Sensitive heat losses will be high (smoke temperature approximately 240°C) and total latent heat losses (condensation is avoided at all costs).

Finally, when the load factor drops, the continuous operation of the burner is interrupted, causing frequent switching on and off of the burner, so, due to the pre-hydrations prior to the ignition, air is introduced at room temperature, in a home at high temperature, stealing heat from the water, until the appearance of the flame. Low temperature and condensation boilers: the boiler allows the decrease of the water temperature, depending on the instantaneous real demand of the installation, so we no longer talk about a constant temperature in the water. This makes it possible to significantly reduce losses due to radiation and convection; moreover, the level of insulation is significantly increased, both in the body (thicker insulating material) and in the door (special refractory material).

As for the losses due to the smoke circuit, it is necessary to take into account the constructive characteristics of these boilers, which allow working at a smoke temperature of 130ºC (in the case of low temperature) and without lower limit, reaching up to 10°C above the return temperature (in the case of condensation), which greatly reduces this type of loss.

DOMESTIC HOT WATER

Conventional boilers: the boiler remains at regime temperature permanently, with full service availability throughout the year, so that losses due to radiation and convection will be continuous during all hours of that period. The losses due to fumes will be similar to those indicated for operation in heating mode, but the losses due to pre-spills will be greater, since the burner will start and stop, to keep the temperature of the primary constant, although there is no demand for domestic hot water. Low temperature and condensation boilers: given the high number of hours in service arrangement, the main difference will lie in the reduction of losses due to radiation and convection, due to the improvement of thermal insulation.

To this we could add the incorporation of temperature regulation systems, of proportional action, which reduce the number of pre-barred significantly, stabilizing the operation of the burner.

We therefore found that, while losses from the fume circuit are more important in determining instantaneous performance, for the calculation of seasonal performance those due to radiation and convection by the boiler body are more significant.

Let's apply the above analysis, to determine the measures to be adopted to improve seasonal performance in heating installations and domestic hot water production:

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