6 District heating Substation District Heating Training Course

Transcription

1 k v = q v p 6 District heating Substation District Heating Training Course B09HVen Mikkeli Polytechnic Siemens Building Technologies Building Automation

2 Siemens Building Technologies Ltd. Building Automation Gubelstrasse 22 CH-6301 Zug Tel Fax Mikkeli Polytechnic 2001 Siemens Building Technologies Ltd. Subject to change 2/12 Building Automation

3 Table of contents 6.1 Design basis Heat demand Domestic hot water Space heating Ventilation Substation component sizing Pump Control valve Pipes Renovation...11 Siemens Building Technologies District Heating Training Course Mikkeli Polytechnic B09HVen Building Automation Table of contents /12

4 6 DH substation Learning goals Principles In this chapter DH substation dimensioning basics are presented according to the Finnish District Heating Association requirements. The aim is that substation is dimensioned and implemented to achieve efficient operation of the consumer equipment and the whole DH system. Heating system of the building connected in DH system is designed, chosen and installed so, that - it creates the best possible thermal comfort in the building - energy consumption is as low as possible - heat demand is as low as possible - control system utilised external heat energy (sun, lights, human beings, machines) - heat capacity of the building utilised for saving energy and heat capacity - temperatures of the space heating are adjustable according to outside temperature - temperatures are at the lowest possible level - control system equipment can operate in variable pressure conditions - space heating networks need as little adjusting (flow balancing) as possible 6.1 Design basis Dimensioning temperatures When starting to design a heating system of a building connected to DH system, it should be studied as a comprehensive way. At the start of designing it is important to clarify needed dimensioning basis (design background) from local authorities at local outdoor dimensioning temperature (e.g. in Finland: t u = -26 C to 30 C) - radiator supply temperature (+ 70 C) - radiator return temperature (+ 40 C) - domestic hot water supply temperature (+ 55 C) - DH supply water temperature (+ 70 C in summer up to 115 C in winter) Table 6-1 Dimensioning temperatures in Finland HEATING PURPOSE PRIMARY C SECONDARY C IN OUT IN OUT DOMESTIC HOT WATER SPACE HEATING And NEW BUILDINGS VENTILATION RENOVATION REMARKS At dimensioning outdoor temperature Max. 5 C Higher than Secondary Return temperature Can be freely chosen within limitations mentioned above Every heating circuit must be dimensioned separately. Dimensioning starts calculating heat losses of the building, which together with needed DHW capacity makes the heat 4/12

5 demand from DH network. Secondary network pressure losses are depended on losses in pipes and fitted equipment. According these calculated parameters heat exchanges, pumps and other equipment are chosen in boundaries of permissible overall pressure losses of the substation. Maximum pressure loss of a heat exchanger is 20 kpa in primary and secondary side, except DHW secondary side where 50 kpa is accepted. Designing the heat exchangers are the following approximations for numerical values used. - Specific heat capacity of water c p 4,2 kj/kg K - Density of water ρ 1,0 kg/dm 3 The heat exchanger manufactures are using more specific values for dimensioning, which are coming from the mean temperature difference of the dimensioning temperatures. Computer programs for dimensioning the heat exchanger shall include the possibility to check the function of the heat exchanger in all possible working conditions Heat demand In buildings heat demand is divided in different heating circuits. In the past when heat exchangers were big, heavy and expensive connected these heating circuits together heated by same heat exchanger. The result of this solution was increased need of control systems in secondary side. It also reduced temperature difference in primary side and increased expenses of the DH company. Nowadays every circuit have its own heat exchanger and control system to regulated primary side DH water flow according to the need of secondary side. The result is better cooling of the DH water and reduced thermal losses in DH network. In CHP system better cooling means less expense and more electricity for the CHP company and better energy efficiency for the customer Domestic hot water Dimensioning of the heat exchanger for DHW heating diverted of the other heat exchangers by the dimensioning temperature of DH supply. It is possible that the DHW load is the same during the summer time as well as in wintertime, but DH supply temperature is lower in summers. In summer conditions the heat exchange surface area need is bigger than in wintertime, caused only by lower supply temperature. That s why the dimensioning of the DHW heat exchanger placed in summer conditions of the DH network. Equation (6.1) Φ = ρ qv cp t When founded out the maximum flow of the DHW, the capacity of the heat exchanger is calculated next. Following equation is generally used in heat demand calculations. Φ capacity, kw ρ density, kg/dm 3 q v flow, dm 3 /s c p t specific heat capacity, kj/kg K temperature difference, K 5/12

6 Example 6.1 ( DHW ) For example, if maximum flow capacity is 1,2 dm 3 /s and dimensioning temperatures are according Finnish requirements, the capacity of the heat exchanger is Φ =1,0 kg/dm 3 1,2 dm 3 /s 4,2 kj/kg K (70-25)K Φ = 227 kw In DHW heat exchanger dimensioning the temperature difference is same in primary and secondary side, thus flow capacity is the same in both sides Space heating Radiator heating Heat exchanger for space heating (radiator network) is dimensioned next. Starting point of designing are temperatures in the secondary side. Next example is calculated according Finnish requirements, where secondary supply water temperature is 70 C and return temperature is 40 C. Renovation make an exception for dimensioning, because earlier designing temperatures 90 C 70 C were used for radiators. In new requirements it is allowed to use temperatures 80 C 60 C when re-designed the building. When calculating the heat losses of a building the outside wall structure, the designing outdoor temperature and required indoor temperature are needed. Heat losses together are equal than heat demand of radiators which is equal to the capacity of the heat exchanger. Flow capacity of secondary side is a sum of the radiator flows. The same result can be achieved by dividing the heat demand by temperature difference and water properties. After these calculations it is able to calculate the primary side flow capacity by using primary side designing temperatures 115 C 45 C, in other words temperature difference 70 K ( C). Pressure losses in pipes less than 100 Pa / m are possible without any risk of pipe damages. In a copper pipe critical velocity of water flow is 1,0 m/s. Higher velocity can caused corrosion damages in a short period. Copper pipe is quit critical for water quality like impurities and ph-value of water. Cavitations accelerates the erosion of valve plug and seat at the differential pressure controller and causes noise. It can be avoided if the differential pressures given in the following chart are not exceeded and if the respective static pressures are adhered to. Example 6.2 (secondary) HVAC-consultant has calculated, that heat losses of a building are 215,0 kw heated by radiators. From heat exchangers selecting diagrams 217 kw heat exchanger is found. Designing temperatures in secondary side are 70 C 40 C as mentioned before, which means temperature difference 30 K. Flow capacity for radiator network in secondary side is calculated according capacity of the heat exchanger using equation (6.1) in form q v = ρ c p t q v = 217 kw / (4,2 kj/kg K 1,0 kg/dm 3 30 K) q v = 1,72 dm 3 /s Now the capacity of the heat exchanger and secondary flow for pump dimensioning is calculated. What is needed is the primary flow for control valve dimensioning. Example 6.3 (primary) In primary side designing temperatures are 115 C 45 C and temperature difference for used in calculations is 70 K. We know the capacity of the heat exchanger, 217 kw, thus only solvable problem is primary flow capacity. 6/12

7 q v =217 kw / (4,2 kj/kg K 1,0 kg/dm 3 70 K) q v = 0,74 dm 3 /s As we can see the primary flow is totally different than the secondary flow and we shouldn't get mixed up with each other. Floor heating In recent years floor heating has increased its popularity a lot in residential buildings. The reasons for that is better thermal comfort in the residences and there is less problems with interior design. Heat exchanger for floor heating is designed as radiator heat exchanger for the maximum heat demand. In floor heating each under floor heating coil is the equivalent of a radiator in heating system. The biggest differences between these systems are the designing temperatures, which are lower in floor heating than in radiator heating. Generally the supply water temperature in floor heating is under +45 C and cooling of the secondary network is about 10 C, thus the secondary flow rate is higher than in the same building heated by radiators. Increased need of secondary flow can make problems with heat exchanger pressure losses in the secondary side. One solution is passing the heat exchanger so, that part of the flow go through the heat exchanger and part of flow is passing it to be connected in secondary supply pipe of the other side of the heat exchanger. This connection is designed to help the regulation process for the low temperatures needed in floor heating. T E T C T E 4 0 C 1 15 C 4 5 C 30 C Fig 6-1 Floor heating pass-by connection Floor heating piping is mainly build by using plastic materials and piping is recommended to protect for over heating. Plastic piping materials for continuing use are planned to use in temperatures under +70 C. In Finland overheating is prevent by using temperature controlled valve to stop the secondary flow from the heat exchanger to floor heating circuits. Pipes to used in floor heating must be O 2 diffusion protected. Example 6.4 (floor heating) In quite big one family house heat demand can be 10 kw heated by floor heating system. Designing temperatures are 115 C 45 C ( t = 70 K) in the primary side and in the secondary side the temperatures are 40 C 30 C ( t = 10 K). Primary and secondary flows are calculated as before. Primary flow: q v = 10 kw / ( 4,2 kj/kg K 1,0 kg/dm 3 70 K ) q v = 0,034 dm 3 /s Secondary flow: q v = 10 kw / ( 4,2 kj/kg K 1,0 kg/dm 3 10 K ) q v = 0,24 dm 3 /s 7/12

8 As seen above, primary flow is really small and if pressure difference in DH is very high, the control system has impossible task to regulate the secondary supply temperature constant. This uneven secondary supply temperature is prevented mostly by passing the heat exchanger and mixed up secondary return flow to supply flow Ventilation Heat exchanger dimensioning in ventilation and in air-conditioning systems is similar by designing temperatures than in space heating (radiators). Like in space heating, the peak heat demand is calculated according the lowest outdoor temperature (designing outdoor temperature). Ventilation heating circuit have own heat exchanger and control system. Ventilation systems are different in Nordic countries than in Central Europe. Air capacity is halved in very cold days and heat exchanger dimensioning is totally different than in ventilation with constant air capacity. Reduced air capacity induced to study heat transfer surface sufficiency depended on lower DH supply temperature. Another important thing is notice dimensioning temperatures of the ventilation coils, in Finland 60 C 40 C. Wrong dimensioning of the heat exchanger for ventilation has been common mistake in Finland and has caused problems to both, the DH company and the customer. DHS DHW VS SHS DCW DHWC P1 HE1 TV 1 TV 2 HE2 P2 SHR TV 3 HE3 P3 VR DHR Fig 6-2 Substation (DHW, SH, Ventilation) 6.2 Substation component sizing Component sizing is very important in many ways. The substation is an aggregate of components and one wrongly selected component can spoil the functional wholeness. Main target is satisfy for both, the customer demands and DH system requirements. Fulfilled customer demands, like constant DHW temperature, are the best marketing resources for the companies working in DH sector. Idea is, that every component and equipment is dimensioned just for the building and purposes of it. Over- or undersize dimensioning leads always to the problems for the customers perspective and extra expenses for DH company or substation supplier point of view. 8/12

9 6.2.1 Pump Pumps are needed to circulate secondary water from the heat exchanger to the radiators, floor heating, ventilation coils or DHW circulation. There is two values needed to select a pump. Water flow and head. Water flow depended on the heat demand and the temperature difference. Head consists from the pressure losses of the heat exchanger, the pipes and the components of whole circuit. Example 6.5 (pump head and flow) Secondary flow 1,72 dm 3 /s was calculated in example 6.2. In building connected DH system pressure loss of the heat exchanger is 15 kpa, losses of the piping are 20 kpa and loss of the radiator valve is 2 kpa. These together make 37 kpa as a head. Now the pump can be selected according to flow 1,72 dm 3 /s and the head 37 kpa. Important is to show separately the losses of the heat exchanger and the circuit, because heat exchanger manufactures have different amount of losses in their heat exchangers, thus selection can change (increase/reduce) the needed head. General Control valve Control valves shall be dimensioned according to the dimensioned values of the heat exchanger and according to the normal pressure difference given by the DH company. The minimum pressure difference is 60 kpa. It is recommended to use two or more parallel connected valves when the heat demand fluctuates rapidly and widely. The authority of the control valve must be at least half of the total pressure loss of the regulated circuit. If the pressure difference of the DH water fluctuates more than 400 kpa, it is recommended to use pressure controller. Control valve is most critical component to noise problems because of cavitations. More detailed description is in chapter 7. Example 6.6 (DHW control valve) In pre-existing example 6.1 was used 1,2 dm 3 /s as water flow in DHW. Local DH company guaranteed pressure difference 150 kpa for dimensioning the substation. The heat exchanger has 20 kpa pressure loss and another 5 kpa is needed for substation piping and equipment. This means that 125 kpa is remaining for the DHW control valve dimensioning. First step is to find out the flow coefficient value of the control valve. Equation 6.2 k v = q v p k v = flow coefficient value q v = DH flow, m 3 /h p= dimensioning pressure difference, bar Flow 1,2 dm 3 /s is 4,32 m 3 /h and 125 kpa is 1,25 bar. kv = 4, 32 1, 25 kv = 3, 9 From diagram the control valve k v = 4,0 is found. Next step is to check real pressure loss of the selected control valve and the authority. 9/12

10 Equation 6.3 Real pressure loss can be calculated by qv pv = 2 v k p v = pressure loss of selected valve, bar q v = DH flow, m 3 /h k v = flow coefficient value of selected valve pv = 4, 32 4, 0 2 pv = 1,17 bar (117 kpa) Equation 6.4 Authority ( β ) of the control valve is β = p p v hc p v = pressure loss of selected valve, bar p hc = pressure loss of heating circuit, bar β = 117, 15, β = 0,78 Last step is to check that authority β = 0,78 is over the required 0,5 of the whole circuit. Type of the control valve can be found from the Siemens catalogue: VVF Example 6.6 (Space heating) In example 6.3 needed DH flow was 0,74 l/s which is 2,66 m 3 /h. Guaranteed pressure difference 150 kpa is got in previous example. Again 5 kpa is reserved for the substation piping and in SH heat exchanger the pressure loss is another 5 kpa, thus 140 kpa is to use for the SH control valve dimensioning. kv = 2, 66 1,4 k v = 2,25 From the catalogue control valve k v = 2,5 is found and again the real pressure loss must calculate according to equation 6.3. pv = 2, 66 2, 5 2 pv = 1,13 bar (113 kpa) Authority checking according to equation /12

11 β = 113, 15, β = 0,75 Authority 0,75 > 0,5 thus selection of the type of the control valve can be found from the Siemens catalogue: VVF , Pipes Primary side pipes are mainly made from steel and joining methods are welding and flanged join. Some manufactures are using copper as well. In the secondary side steel, copper and plastic pipes are in use. Steel and copper are used for radiator network and plastic pipes are used for floor heating systems when temperatures are lower. 6.3 Renovation When a renovation occurs, it is normally most economical to change the substation as a whole. When changing or repairing a part of the substation, all other old part of it shall also be checked and renovated if necessary. If the substation is older than 15 years, it is always recommended to replace it entirely. Important is to take contact to the DH company to check the right dimensioning of the substation at the beginning of the renovation process. The dimensioning is based on real measured values of flows and temperatures. Otherwise the dimensioning of the substation and its equipment in an existing building is similar to the one in new buildings presented earlier. If there is asbestos in the thermal insulation of the heat exchanger or piping, it is better to remove according to national laws and instructions. 11/12

12 Siemens Building Technologies Ltd. Building Automation Gubelstrasse 22 CH-6301 Zug Tel Fax Siemens Building Technologies Ltd. Subject to change 12/12 Building Automation