District Heating Substations. Performance, Operation and Design

Sammanfattning: This thesis work is concerned with the efficient layout and operation of substations, i.e. those district heating (DH) system units which connect the network and internal building heating systems, such as radiator heating systems and domestic hot water distribution networks. The ambition was to make realistic investigations closely related to practical technologies within the field. The themes of the papers included were chosen according to their importance from a scientific point of view, but also by taking into account preferences expressed by the research partners. As an introduction, the general state of the art and important results obtained by other authors are presented. Next, seven papers are included. The main topic of papers I-III is the performance of the substations during altered operation, as: · Varying forward temperature and differential pressure of the DH water · Forced building warm-up in the morning · Reduced water flow rate in the radiator circuit · The stability of the domestic hot water temperature control at varying load · Varying circulation water flow rate in the domestic hot water circuit. Paper IV is a report dealing with the optimum choice of the temperature level in domestic hot water heaters, involving topics like water quality, corrosion, fouling and bacterial growth. Starting with an international literature survey, the report also presents the results of advanced laboratory tests and field experiments. Based on the findings, various types of fouling mecha-nisms as well as preventive steps are discussed, mainly applied to plate heat exchangers in DH substations. General recommendations concerning factors affecting particle fouling, scaling and microbial fouling are given, bearing in mind medium parameters such as temperature, flow velocity, pH and the concentration of the matter forming the deposit. Paper VI describes laboratory tests on heat transfer reduction in a small plate heat ex-changer while using a new type of drag reducing additive, a zwitterionic surfactant. An im-portant property of the surfactant is said to be that its mycelle chains, which reduce the turbu-lence of the solution stream near the pipe wall, are very stable. A static mixer situated imme-diately before a heat exchanger inlet was found to significantly increase the overall heat trans-fer coefficient in the plate heat exchangers tested. Papers V and VII describe the question of extreme loads in DH substations, and of design criteria for the substations. A sophisticated method for cutting the total load peaks in the sub-station at minimum inconvenience to the consumer is addressed. Extreme-value theory is in-troduced as a tool to handle the prediction of large hot water loads occurring at given, low probability. It is shown that extreme-value Gumbel distribution theory could be used for the sizing of new hot water heaters, or for the validation of the sizing of heaters already in opera-tion. Furthermore, simulation of domestic hot water consumption, and the design criteria based on quantile approach are also presented. Simulated design flows obtained according to the criteria are compared with empirical values, and with flows recommended by the Swedish District Heating Association.