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Operation of centralized mixed systems with boilers in series or in parallel. Resolution of a practical case in the CE3X program.
In those cases in which the demand for sanitary hot water and heating in our building or home is covered by mixed systems with centralized boilers (Boilers in series or in parallel), their operation and the percentage of the demand covered by each one Of these, it will depend on how the installation has been designed, depending on whether these equipments work in parallel simultaneously, or in series in a staggered manner.
How is demand distributed in these centralized mixed systems?
BOILERS IN PARALLEL:
When it comes to boilers operating in parallel, the total demand is distributed simultaneously adopting an identical firing rate to satisfy the load. In drawing 1 he explains the process to respond to the increase in demand.
Distribution of demand in parallel (Drawing-1)
SERIES BOILERS:
In this case as the load increases, the firing rate of the first boiler increases until the load demands an additional boiler. At this time, the second boiler will start, which becomes the modulating boiler. Drawing 2 explains the process to respond to increased demand.
Distribution of the demand in series (Drawing-2)
In these systems, the first boiler that comes into operation is called primary, while the following are secondary. In this way, the one with the highest priority (primary) that will be the most efficient will always start, before those with the lowest priority (secondary, less efficient).
Practical case for introducing two centralized boilers in series in the CE3X program for energy certification.
We will carry out an example introducing two condensing boilers in series, with a main or primary boiler with a power of 500 Kw, and a secondary boiler of 400 Kw, both with a combustion efficiency of 98%. In this case, the fraction of power provided by each one is obtained by dividing the power of each boiler by the total power that both generate.
Data entry in the primary boiler in the CE3X application
1.-We define the first boiler, which will act as primary in the system, we mark mixed heating and DHW equipment, and we define it as a natural gas condensing boiler, we add.
2.-Obtaining the seasonal performance and the demand covered by the equipment. In the defined primary boiler, enter the power (500 Kw) and the combustion efficiency (98%), its insulation and, if applicable, the volume of the accumulation tank in the lower area of the screen:
3.-Next we will obtain the average real load, which is the average of the load fractions of the generator during its service time, we click on the question button to the right of the real average load box, we place the fraction of total power provided by the 500 Kw boiler, which It will be 500/500 + 400 = 0.56, and since this generator is the primary one, it comes into load with a power fraction of 0, that is, it is the first boiler to start.
If we look at the window we have obtained a value of the average seasonal or real load factor of 0.24, as well as a fraction of energy supplied by this boiler of 0.95, therefore it is covering 95% of the demand for heating and DHW ,. This value helps us to then assign the percentage of the demand covered by it.
4.-Below we accept and it is reflected the value of the average real load as well as the average seasonal efficiency in DHW and heating of 94.5%, considering a well-insulated and maintained boiler:
And in the window of the demand covered by the equipment we will introduce the previously obtained value of 95%.
We return to the modify tab to save the changes, and thus we define the characteristics of the main boiler.
Entering data of the secondary boiler in the CE3X application
1.-We define the second boiler, that will act as secondary in the system, of the two that will cover our demand, we will indicate a new mixed heating and DHW equipment, with a generator type using a natural gas condensing boiler and we will add it.
2.-Obtaining the seasonal performance and the demand covered by the equipment. In the box "Average seasonal efficiency", enter the power (400 Kw) and the combustion efficiency (98%), its insulation and, if applicable, the volume of the accumulation tank in the lower area of the screen:
3.-Next we will obtain the average real load, we click on the question button in the real average load box, and we indicate the fraction of total power provided by this second 400 Kw boiler, which will be 400/500 + 400 = 0.44, and since this generator is the Secondary, it comes into load with a power fraction of 0.56, that is, it starts up when the installation is demanding more than 56% of its total thermal power.
In this window we have obtained a value of the average seasonal or real load factor of 0.44, as well as a fraction of energy provided by this boiler of 5%, therefore it is covering the remaining 5% of the demand for heating and DHW .
4.-Below we accept and it is reflected the value of the average real load as well as the average seasonal efficiency in DHW and heating of 96.2%, considering that the boiler is well insulated and maintained:
And in the window of the demand covered by the equipment we will introduce the previously obtained value of 5%, this being the demand covered by this second boiler.
We return to the modify tab to save the changes and in this way we have already defined our mixed system with two centralized boilers in series for DHW and heating.
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Article prepared by José Luis Morote Salmeron (Technical Architect - Energy Manager - Google plus profile) Access to his website HERE, in collaboration with Follow us on Google+
