Introduction
In collective housing (apartment buildings, dormitories, service flats, etc...) projects the heat distribution for space heating (SH) 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.
When carrying over these norms to the space heating system (satellite boilers or heat exchangers), the propagation of simulateous power also becomes important! This effect is amplified by the fact that space 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 space 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.
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 of space heating remains unchanged. The example below shows that there is a big difference between thermal power and flow rate according to SH 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 (SH 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 SH flow rates are summed (0.33 m³/h + 0,33 m³/h = 0.66 m³/h). The DHW simultaneous flow rate is calculated as explained in the box below: the simultaneous 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 rate.
