Introduction
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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
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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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On the basis of the DHW design flow rate at the level of the heat exchanger and the performance of the heat exchanger (UA-value - W/K, from the technical specifications of the manufacturer), the primary DHW flow rate is calculated which is necessary for the transfer of the required thermal power. In this example, the primary flow rate 0.86 m³/h and the secondary flow rate 0.6 m³/h. The primary and secondary flow rate for central heating remains unchanged. The example below shows that there is a big difference between thermal power and flow rate according to CH and DHW, also the UA- value of the heat exchanger has a influence on the primary return temperature and flow rate of DHW.
Now the design flow rates and thermal power (CH and DHW) are known on the primary side of the satellite unit. In order to determine the design flow rate and thermal power into the common pipe sections, the CH flow rates are summed (0.33 m³/h + 0.33 m³/h = 0.66 m³/h). The DHW simultaneity flow rate is calculated as explained in the box below: the simultaneity flow rate per unit is converted to liters/sec (1), then to a total flow rate in liters/second (2) using the inverse simultaneity formula, the total flow rates of the different satellite units are summed and calculated back to a simultaneity flow rate. According to the standard the calculation needs to be done in liters/second.
In the text above the design flow rates from CH and DHW in the common pipe sections are calculated separately, therefor it has to be taken into account that some of the satellite units use DHW simultaneous and others will use CH. Simply taking the maximum of CH and DHW flow rate would result in a flow rate which is too low. Hysopt uses the simulateous simultaneous factor f = DHW,S / DHW,T and then compensate for the central heating volume flow rate and thermal power on units not in DHW mode, by computing the combined volume flow (V_dotCH,DHW). In the calculation below the above example is used to explain how the combined volume flow rate is calculated. On the basis of the known supply temperature, flow rate and thermal power, the return temperature is calculated.
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).
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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.
Furthermore the design flow rate of DHW is determined as the maximum flow rate from the Eco-design draw-off pattern (S, M, L, XL - XXL) that can be set by the pull-down menu (see usage pattern). For simulations the draw-off pattern between the apartments can be shifted in time so not all apartments use DHW at the same time. The requested design temperatures can be filled in the upper fields.
