Condensing Boilers: operating principles, performance, convenience, advantages and disadvantages
The condensing boiler is first of all a normal gas boiler (typically LPG or CNG) in which there are circuits for the production of sanitary hot water (if provided) and for domestic heating.
To understand the particularity of this solution, it is important to first understand the operation of a traditional boiler.
During combustion, the temperatures reach high values allowing the transfer of the generated heat to the fluid of the heating system by means of a heat exchanger of suitable dimensions, situated inside the body of the boiler.
Not all the heat developed by the combustion is capable of being transferred to the heat transfer fluid, in particular for limits in the extension of the exchanger and for the need to hesitate acid condensate to the chimney, in fact, the fumes are particularly acids and aggressive and for this reason they are evacuated to a temperature at which it is assured of the absence of condensate.
The temperatures at which the fumes are evacuated are normally of the order of 200 ÷ 250 ° C for conventional boilers and low efficiency of 140 ÷ 160 ° C for conventional boilers with high efficiency.
Introducing a particular exchanger capable of condensing the fumes is possible to reduce the temperature of the same values of the order of 40 ° Crecuperando addition to the portion of "heat sensitive" between the flue gas temperature and the condensing temperature of the same, also the part on the so-called "latent heat" due to the condensation of the fumes.
Such heat recovery is made possible by the use of materials able to resist the attack of acids condensates, such as stainless steel and plastic materials resistant to heat, and allows to operate the preheating circuit water system return heating.
Following the path of the fumes and the path water you can 'see how they are countercurrent to maximize the heat exchange.
The Fallacy EFFICIENCY - PCI-E PCS
Pointing with PCI and PCS respectively the lower calorific value and higher fuel, and evaluating their performance according to the usual equation:
ηcomb Qcomb = / (mcomb • PCI)
you get a yield value of greater than unity (or 100%) as evaluating the chemical energy it refers to the primary PCI, calorific value that is measured without condense the products of combustion, while it would be desirable to use the PCS, measured also taking account of condensation.
In practice it is as if you are referring to the PCI and was added energy "free" (approximately 11%) and on the basis of this assessment as total efficiency (taking into account the recovery more stringent).
All this does not mean that condensing boilers, heat recovery by virtue of greater than an equivalent conventional boiler, allow us to obtain better energy performance and emissions and lower fuel consumption.
The reality is always more complex than the theory, it is not enough to use a condensing boiler to be sure to get these improvements.
HUMIDITY '- CURVE OF BALANCE AND DEW POINT
The ability of a boiler to recover heat by cooling the products of combustion beyond the dew point of the flue gas, it allows a potential increase in efficiency that leads in some cases to a misunderstanding on the real value of the performance, the origins of which have been exposed and always motivated in the previous post.
For this to happen it is necessary that the boiler operates under certain optimal conditions, conditions which are strongly influenced by the plant in which it is installed and the thermal management of the users.
To be able to understand better what happens it is necessary to introduce some concepts, such as the curve of equilibrium between gas and liquid phase, curve that identifies, for a given pressure, the percentage of moisture limit that a gas can contain without encountering its condensation.
From combustion of a given mass of fuel, such as methane or natural gas, it is possible to derive theoretical way, with a simple chemical balance, the composition of the combustion products, with reference to the excess of air (or of residual oxygen in the dry gas) compared to stoichiometric conditions.
Note these data it is possible to identify, for each adjustment of the burners (which defines precisely the percentage of excess air), the temperature at which condensation starts.
Assuming we have a 3% O2 in the flue dried, operating the heat recovery from the flue gases can be seen as cooling the flue gas to a temperature of about 56 ° is being extracting only the sensible heat contained in the same, and the temperature is reached balance begin the flue gas condensation.
Once reached the equilibrium conditions must be made following the curve up to the temperature at which no more recovers the heat of the fumes (in this case, being in conditions of flue gas condensation, latent heat is subtracted).
The temperature limit is essentially determined by the surface of the heat exchangers, and obviously will not be going too below certain temperatures as the size of the boiler and the costs would be disproportionate to the benefit obtained.
Downloading the fumes at 40 ° C leaving a residual moisture content of 7.5%, having started from a value of 16.5%.
In addition to the heat exchange surface, another factor that influences the rate of condensation is represented by the oxygen content in the flue gas, which increase corresponds to a decrease of the temperature of dew.
It also appears that the fumes will continue to cool down the chimney, but since there is no heat recovery device, such condensation will occur without any effects to the performance of the boiler.
PERFORMANCE OF CONDENSING BOILER
In the evaluation of the efficiency of the boiler several factors, in addition to thermal energy due to the condensation, must be taken into account, in particular the heat dissipation to the outside, a factor that contributes in a more or less sensitive to the difference between the yield related to combustion and the real performance "seasonal average" of the boiler.
Under this aspect the condensing solution has several advantages compared to a traditional solution, both in the aspect of the lower heat exchange with the external flame extinguished due to lower losses through the casing (mainly due to lower temperatures of ' casing itself thanks to lower flue gas temperatures downloaded), both with the ability to control who can usually operate in modulation scheme, adjusting the heat output to the load required by reducing the intermittent.
The condensing boiler, has the water circuit and the path of the fumes in countercurrent to maximize the heat exchange.
The TD between the inlet temperature of the water (corresponding to the return temperature of the circuit) and the exhaust temperature of the flue gas tends to zero for a heat exchanger of infinite surface, but contrary to what one might think, once defined the boiler and therefore the heat exchange surface, this difference depends on the power to the hearth, or from the load at which the boiler operates, in fact the temperature of the return water essentially depends on the heating elements installed in the environments and their heat exchange surfaces, in relation the temperature of the environments themselves (and a high temperature of the return water indicates a low heat exchange with the environment).
Increasing the power of the hearth the curve of the fumes moves upward increasing the Dt, with consequent reduction of the recovery of yield by condensation as the fumes are discharged at a higher temperature.
With a return water temperature of 40 ° C and a Dt of 5 ° C is obtained an increase of the yield of the order of 6.3%, increase that tapers with increasing T (4.8% for AT = 10 ° C , 0.77% for T = 20 ° C).
The cause of such "non-increase" should now be clear, in fact, the increase of the exhaust temperature of the fumes there moves to the values of temperatures for which you are unable to obtain the condensation of the same inside the boiler, with the consequent impossibility of extract the latent heat.
An immediate consequence of this, for the choice of a condensing boiler and of its power, is the correct evaluation of the flue gas temperatures in conditions of nominal load and minimum load.
A correct choice of the boiler and an equally proper management of the system, priority must be given a device capable of operating correctly in the setting conditions (which occurs simultaneously adjusting the amount of fuel and air to obtain a premixed combustion) without appreciable change in the oxygen residue, and to use a minimum load so as to maintain a temperature difference between the flue gas and return water in the lowest possible, avoiding as much as possible the intermittent, obviously without underestimating the need for appropriate surfaces of the heat exchangers in the environments.
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