Satellite units and simultaneous flows
Introduction
In collective housing projects (apartment buildings, dormitories, service flats, etc...) the heat distribution for central heating (CH) and domestic hot water (DHW) can be implemented using satellite units. The system is made up of a central boiler room with a boiler and pump, heat distribution through a shared circulation pipe, and finally, a satellite unit that manages the central heating and domestic hot water demands of each apartment.
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 using a plate heat exchanger (2). The heat exchanger can be described in more detail by entering the specifications of manufacturers and thus capturing the performance of the sanitary heat exchanger. The domestic hot water temperature is regulated using modulating a 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.
When carrying over these norms to the central heating system (satellite boilers or heat exchangers), the propagation of simultaneous power also becomes important! This effect is amplified by the fact that central heating often operates at lower power but higher flow rates / smaller temperature delta and domestic hot water heat exchangers operate at higher power but lower flow rates.
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 a 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 the case of a building with several units the simultaneous flow rate can go to 10% of the total flow rate which has a big impact on the pipe selection.
Based on 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 is 0.86 m³/h and the secondary flow rate is 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 influences 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 one satellite unit. 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.
Flow rate aggregation method
In the paragraph above the design flow rates from CH and DHW in the common pipe, sections are calculated separately, to calculate the installation components (pipe sections, primary pump, storage tank, boiler, ...) it is necessary to have one design flow rate and one thermal power. Therefore Hysopt has developed two methods to aggregate the flow rates and thermal power in the common pipes as explained below. In the setting overrides the aggregation method can be selected.
Maximum of central heating and simultaneous hot water flow
A weighted average of central heating and total domestic hot water flow
Maximum of central heating and simultaneous hot water flow
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). The DHW flow rate increases non-linearly, as a consequence, the DHW flow rate at the upper apartments will be determining the flow rate, as the number of apartments increases the CH flow rate takes over. The maximum of both will be used for component calculation (pipe sections, primary pump, storage tank, boiler, ...). The design flow rates, thermal power, and temperature regimes (CH and DHW) are shown on the labels below.
A weighted average of central heating and total domestic hot water flow
In the second method, the weighted average of central heating and total domestic hot water flow is taken into account. In contrast to the first method, this method takes 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 some cases a flow rate that is too low. Hysopt uses the simultaneous factor f = DHW,S / DHW,T and then compensates 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 (see chapter Simultaneous flows for DHW) is used to explain how the combined volume flow rate is calculated. Based on 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 use DHW (simultaneity) the satellite units that are left use 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 manufacturer’s specifications, by clicking on the pencil a pop-up will appear where the user can fill in the manufacturer’s specifications (see below, parametrize satellite unit). In the graph below it is shown that the UA-value (600 to 1800) is strongly dependent on the primary and secondary flow rates, so to use the correct UA-value according to the design flows (primary and secondary) the constant Cspecs is incorporated, which is calculated with the manufacturer’s 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.