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Introduction
In collective housing (apartment buildings, dormitories, service flats, etc...) projects the heat distribution for central heating (CH) and domestic hot water (DHW) can be implemented using satellite units. The satellite unit system exists of a central boiler room with a boiler and pump, heat distribution happens via a shared circulation pipe. The central heating and domestic hot water needs of each individual apartment is managed by a satellite unit.
The Hysopt software has a generic prototype that can be used to design and simulate systems with different types of satellite units. The model consists of an open / closed priority valve (1) that always gives priority to DHW to guarantee maximum comfort. The DHW is separated from the primary grid by means of a plate heat exchanger (2). The heat exchanger can be described in more detail by entering the specifications of manufacturers and thus capture the performance of the sanitary heat exchanger. The domestic hot water temperature is regulated by means of modulating 2-way valve and a PI controller (3). With a balance valve (4) the domestic hot water design flow rate is balanced.
Simultaneous flows for DHW
Domestic hot water flow rates have always been problematic to calculate, because of issues with simultaneous usage of hot water tapping points. Full load conditions (all showers in use at the same time) will result in very large flow rates and oversized pipes. Many calculation methods for simultaneous flow rates are known and used for domestic hot water piping. Most of these methods only account for simultaneous flow rates, and not for simultaneous power, because domestic hot water networks are mostly operated as single pipe / fixed temperature systems.
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Hysopt incorporates an extension of the DIN 1988-300 (2012) standard into the Hysopt software. We have extended the calculation to cope with simultaneous central heating and domestic hot water usage, and with combination of power needed in mixed systems.
In the example below, there is a shower- and a kitchen tap. When the flow rates are summed the total flow rate is 0.22 l/s, When using the simultaneous factor this becomes 0,17 l/s. For one satellite unit the difference between total and simultaneous flow rate is quite small, in case of a building with several units the simultaneous flow rate can go to 10% of the total flow rate which have a big impact on the pipe selection.
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In the graph below, the design flow rate of DHW and CH is shown as a function of the number of apartments. The CH flow rate, logically increases linearly as the number of apartments increases (blue solid line). In the case of DHW the total flow rate DHW (green dots line) is much higher than the DHW flow rate with simultaneity (green solid line), as more apartments are added the difference increases. As explained above, because not all apartments uses DHW (simultaneity) the satellite units that are left uses CH therefore the combined flow rate is calculated (black dots line).
Parameterization of the satellite unit
The satellite unit must first be "parameterized" according to the manufacturers specifications, by clicking on the pencil a pop up will appear where the user can fill in the manufacturers specifications (see below, parametrize satellite unit). In the graph below it is shown that the UA-value (600 to 1800) is strongly depended on the primary and secondary flow rates, so in order to use the correct UA-value according the design flows (primary and secondary) the constant Cspecs is incorporated, which is calculated with the manufacturers specifications, then the UAdesign is calculated.
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