Technical paper Selection of district heating house stations

Transcription

1 MAKING MODERN LIVING POSSIBLE Technical paper Selection of district heating house s Herman Boysen, Product Application Manager, Danfoss A/S Published in EuroHeat & Power English Edition III 2004 districtenergy.danfoss.com

2 TECHNICAL PAPER Selection of district heating house s Market demant today for district heating house s result on an increasing focus on the following items. Author(s) Herman Boysen, Product Application Manager, Danfoss A/S Danfoss District Energy, Nordborg, Denmark, boy@danfoss.com The fact that Danfoss is now a district heating sub builder gives some more benefits for the customers: More compact s. Higher rate of flexibility in control functions. Short developing time. Optimized adaption of control equipment in the. This article informs about different types of house s concerning: Function of the system. Control function in the system. Comments to the system concerning its control and performance. power distribution big house District heating house s A district heating house system is the connecting part between the district heating utility and the consumer of district heating. The house sub can be owned either by the district heating utility or the consumer. The task of the house is to adapt the supplied heat condition into a heat condition useful for the consumer s building. flat small The demand of the can be split up in two groups, utility demand and consumer demand. The utility demand for the could be: Type of system. Low return temperature. Maximum flow limitation. Heat exchanger between the district heating network and the house network. The consumer demand to the utility company could be: Sufficient supply temperature. Sufficient differential pressure. Low energy consumption. Accurate heat metering. Besides these demands, there are some needs that have to be fulfilled before the right house can be chosen. The demands that have infl uence on choosing the right concept could be: Easy maintenance. Low risk of bacteria. Low noise level. Minimum space demand. Safe function. Accurate and stable temperature control. Low cost. Nice appearance. Long life. FIGURE 1: Different types of district heating subs 2 Danfoss District Energy

3 Group of s in general The district heating subs are normally split up into groups such as: Distribution s. Big house s. Small house s. Flat s. Distribution s are normally connected to the transmission line from the power. The purpose for the distribution is to adapt the supply condition to the district heating network. Big house s are normally directly connected to the district heating network and is supplying a number of flats. Small house s are also directly connected to the district heating network and are normally supplying one-family houses. Flat house systems are normally indirectly connected to the district heating systems and therefore built as low conditioning (PN 10) systems. FIGURE 2: Plate heat exchanger with 4 and 6 connections Big house s Big house subs can be split up into the following main groups: 1. Parallel system with instantaneous heat exchangers for domestic hot-water (DHW) heating. 2. Two-step system with instantaneous heat exchangers for DHW heating. 3. Parallel system with DHW charging system. 4. Indirectly connected DHW charging system. The trend goes in the direction of system types 1 and 2 becoming the commonly used types in future. System types 3 and 4 are still most known in Germany, Austria, Italy and the Czech Republic but there is a trend towards system types 1 and 2 in these countries. This article describes how the systems function in and control parallel and two-step systems with room heating and DHW circuit connection. Heat exchangers The heat exchanger types used in house subs depend on: FIGURE 3: Parallel system with instantaneous DHW system Type of circuit, room heating or DHW. Type of systems, parallel system, two-step system with one or two heat exchanger in the DHW circuit. In room heating systems the used type of heat exchanger is a 4-connection type. In DHW circuits the number of connections depend on type of systems. In parallel systems normally 5 connections are used. In two-step systems with one heat exchanger a 6-connection heat exchanger is used. In two-step systems with two heat exchangers one 5 connection heat exchanger and one 4 connection heat exchanger are used. Fig. 2 shows heat exchangers with 4 and 6 connections. The connections on the heat exchanger are: Primary side of the heat exchanger: T11 T12 T112 T21 T22 T212 inlet for district heating water. outlet for district heating water. inlet for the primary return water room heating system (6 connections heat exchanger). secondary inlet for domestic cold water. secondary outlet of DHW. secondary inlet of return water from DHW circulation water. Parallel system with instantaneous heat exchanger In parallel systems (see fig. 3) the district heating supply water flow rate is split up into room heating and DHW supply system. The district heating water will return directly to the district heating supply system after it has been cooled down. Danfoss District Energy 3

4 The DHW system is an instantaneous system without a buff er or a storage tank to equalize the controlled temperature. Therefore the requirement to the control equipment is very high. In parallel systems the following system conditions infl uence the control of the position of the control valve during operation:. Temperature of the water in the supply net. Differential pressure in the supply net. Rate of capacity. Two-step systems with instantaneous heat exchanger In two-step systems as shown in fig. 4, the district heating supply water is also split up to supply the room heating and DHW supply systems. The primary return flow from the heat exchanger for the room heating is led into the domestic heat exchanger in order to preheat the incoming cold water in this exchanger. In case of hot water tapping it will give a better total cooling of the primary water from the room heating system. However, in case of low return temperature from the room heating system, the return water can be after heated some few C from the return water in the DHW circulation circuit. The system conditions that influence the control of the DHW system are the same as for parallel systems, however the temperature of the water in the supply net in two-step systems is even more critical than in parallel systems because of the preheating of the domestic cold water. As for parallel systems the conditions for sizing of DHW systems are summer conditions. In summer a preheating of the domestic cold water can t be expected and can therefore not be taken into consideration when calculating the system. Therefore the control valve and the heat exchanger for the DHW system have to be calculated for max. capacity at lowest supply temperature. In the wintertime where the capacity of the room heating system is high, a high rate of preheating can be expected. Very often the incoming domestic cold water can be preheated to a level of Two-step system with one DHW heat exchanger Two-step system with two DHW heat exchangers FIGURE 4: Two step heat exchanger system with one and two heat exchangers in the DHW circuit 39 C. This means that the task for the control valve only is to increase the domestic water temperature from 39 C to 55 C. In this case the control valve will operate with a very low lift at max. capacity with the risk of an unstable temperature at part load. Room heating system The room heating system can be directly connected or indirectly connected via a heat exchanger. Both of these types normally have weather compensated supply temperature of the supply water to the room heating system. In the indirectly connected system a heat exchanger is used for transferring the needed heat energy to the room heating system. Here the motorized control valve controls the primary flow rate in the heat exchanger and thereby adjusts the secondary supply temperature according to the outdoor temperature. Directly connected room heating systems normally are equipped with weather compensated mixing loop. Here the district heating supply 4 Danfoss District Energy

5 temperature is mixed up with the return temperature water from the room heating system to the supply temperature needed to heat up the room in the house. As the capacity of the room heating system change very slowly and as smaller variation in the flow temperature is not critical, a room heating system is normally not so complicated to control. Domestic hot-water (DHW) system The DHW part of the system is more complicated to control because of the stringent requirements as to a stable temperatures compared to room heating systems. Apart from this the capacity in this systems also change rapidly. The room heating system and the DHW system normally run independently of each other, unless a hot-water priority function in the controller is active. The hot-water priority is only in function if there is insufficient capacity for heating both the DHW at the same time as the room heating system. If this is the case, the hot water heating demand will have higher priority than the demand for room heating. If the DHW temperature is too low because of lacking capacity, the flow rate to the room heating system will be reduced or totally closed. Temperature of the water in the supply net Very often the supply temperature in the network varies over the year. In winter the supply temperature is high. In summer the supply temperature will normally be reduced to a level of C, sufficient enough to produce hot water at temperatures of C. As the hot water consumption is expected to be of the same size all around the year, the capacity of the control valve in the DHW system has to be sized according to the low supply temperature in summer. This will affect the fact that the control valve in the winter with the high supply temperature will operate with a valve cone lift lower than 100%. Constant permanent deviation from set point after a temperature change, ± 2 K Differential pressure in the network The valve sizing is based on a chosen or the min. available differential pressure across the control valve. If a differential pressure controller is not installed, an increasing differential pressure in the network will therefore result in that the control valve closes accordingly and operates at a lower opening degree. This can be critical for a stable temperature control. A differential pressure controller in the system will be able to minimize variations in the differential pressure across the control valve independently of the variation of the differential pressure in the network. Range of capacity Maximum permanent temperature oscillations, ± 2 K Maximum temperatuer deviation after a momentary laod change, 10 K Transient time after a load change <± 2 K within 120 sec FIGURE 5: Relevant temperature control performance in a DHW system with an instantaneous heat exchanger. A hot-water system has to be able to control a stable temperature under all relevant capacities. The lowest expected capacity for a DHW system can be the water flow that corresponds to one tapping. The capacity of the system then ranges from one tapping up to full capacity of the system. In big systems with one big valve dimension, the control of the flow rate which corresponds to one tapping will affect the operation near the closing point with the risk of unstable controlled tap water temperature. In this case it is recommended to choose two control valves in different sizes connected in parallel. The system can then be operated so that the small valve operates at low flow rates and both valves operate at increasing capacity. Control equipment Correctly chosen control equipment is an important factor for a well functioning system. Danfoss has especially developed control equipment for controlling instantaneous DHW systems. Research and design of this equipment are based on computer simulations, laboratory tests and field tests. Danfoss District Energy 5

6 A relevant example of the quality of the controlled temperature is shown in fig. 5. Experience from this R & D work is listed in the guidelines below (table 1) for a well functioning DHW system: Choose motorized control valves with a short running time, i.e. max seconds from fully closed to fully open valve. The time constant of the sensor must be less than or equal 3 seconds. The sensor must be placed as close to the heat exchanger as possible. Make sure that the required control ratio is met by choosing the right valves and properly pre-adjusted systems. Adjust the systems to operate with fully open valves under a 100 % load. Choose valves with a sufficient authority in the systems. This authority is especially critical in systems with a low differential pressure. Avoid large pressure variation in the systems by using differential pressure controllers. Differential pressure controllers also have a favorable effect on the control ratio of the control valve as well as the valve authority. Valve sizing Valve sizing of a DHW control valve in an instantaneous system can be done as shown in table 2. By means of the k vs value of the chosen control valve the needed Δp v can be calculated. The differential pressure controller can then be set to the sized Δp value so that the control valve operates with a 100 % opening degree at a 100 % load. The calculation shows the capacity of the control valve under different conditions and in different system types. In fig. 6 a split characteristic for a motorized control valve is illustration. The control valve has a k v of 6.3 m 3 /h and a max. valve cone lift of 5 mm at an opening degree of 100 %. In this graph, the valve cone lift can be seen under different conditions and in different systems. The opening degree for the valve at sizing point will be approx. 4.4 mm with the given specifications FIGURE 6: Valve characteristic with a split characteristic. From the figure the valve cone lift can be seen for different systems in winter and summertime After setting the ΔP controller, the valve will be fully open and the valve cone lift will be 5 mm. Under winter conditions the supply temperature is high, in this case 100 C. The control valve in a parallel system will therefore operate with a lower opening degree at 100 % load. Parallel systems has no preheating of the DHW system, thus the fully DHW capacity has to be heated from the primary flow rate in the DHW circuit. Here the valve cone opening degree will be approx. 3.5 mm. Two-step systems have the highest DHW preheating rate in winter where the load on the room heating system is 100 %. Consequently, the degree of after heating of the DHW is low. As can be seen from the graph, the valve opening degree at 100 % load during winter is approx. 2.9 mm. The lower the valve cone lift at 100 % load, the lower the valve cone lift is at part load. The consequence of this is an increased risk of unstable temperature control. Conclusion As mentioned parallel systems and two-step systems without charging tank seem to be the future sub solution. And the reasons are: These systems have no charging tank. System types with charging tanks are Valve lift at different load and systemtypes Valve lift after setting of the dp controller Calculated valve capacity Parallel system. Winter condition Valve opening mm Two step systems. Winter condition 0 0,0 2,1 2,5 2,9 3,2 3,6 3,9 4,3 4,7 normally more expensive than parallel and two step systems without charging tanks. However, the heat exchanger capacity is higher in systems with out charging or buffer tanks. A charging tank in a system will increase the risk of bacteria growth. The control equipment today is designed specifically for the purpose and obtainable for instantaneous DHW systems. Choosing between a parallel system and a two-step system can be difficult. It is established that two-step systems have a better cooling of the district heating water during tapping of DHW. The fact that the return temperature from the room heating system in some cases can be after heated from the DHW circulation return temperature is a disadvantage. However, as the heating costs normally are based on the energy calculation this will normally not have any influence. It has to be taken into consideration that the DHW heat exchanger in a two-step system often is bigger than in a parallel system because it has to be sized for the primary DHW flow rate as well for the room heating flow rate. As the variation of the supply differential pressure has influence on a stable control temperature in this type of systems, it is very important to install a differential pressure controller in the sub. 6 Danfoss District Energy

7 DHW specifications DHW flow rate Q DHW 3 m 3 /h DHW temperature T C DCW temperature T C Capacity P = Q DHW (T 22 T 21) / 0,86 P = 3 (55 10) / 0, kw Basics for sizing a control valve, Summer, parallel and two-step system Capacity P 157 kw Primary supply temperature T C Primary return temperature T C Disposal Δp for the control valve Δp v 0,5 Bar Primary flow rate Q 11 = P 0,86 /(T 11 T 12) Q 11 = /(65 25) 3,4 m 3 /h Calculated k v k v = Q 11 / Δp v k v = 3.4/ 0,5 4,8 m 3 /h Chosen valve capacity k vs 6,3 m 3 /h Real Δp v Δp Vmin = (Q 11 / k vs ) 2 Δp Vmin = (3.4/ 6.3) 2 0,29 Bar Control ratio r 50 Min kv for stable temperature control k Vr = k Vs / r k Vr = 6.3 / 50 0,13 m 3 /h Winter, parallel system T supply winter T sw 100 C T return T rw 18 C dp valve 0,29 Bar flow rate Q 11 = P 0.86 /(T 11 T 12) Q 11 = /(100 18) 1,65 m 3 /h k v at max. capacity k v = 1.65 / 0,29 3,06 m 3 /h Winter, two-step system Relevant preheat temperature DHW T pre 39 C After heating ΔT = T22 T pre ΔT = C Preheat capacity P pre = Q DHW (T pre T21) / 0,86 P pre = 3.0 (39 10) / 0, kw Afterheat capacity P after = P P pre P after = kw T supply summer T C T return relevant T C dp valve 0,29 Bar After heat fl ow rate Q after = P after 0.86 /(T 11 T 12) Q after = /(100 45) 0,87 m 3 /h k v k v = Q after / Δp v k v = 0,87 / ,62 m 3 /h Danfoss District Energy 7

8 More information Find more information on Danfoss District Energy products and applications on our homepage: VF.LA.N2.02 Produced by Danfoss A/S, DH-SM/PL 09/2011