Pumps in district heating network

Transcription

1 Department of Energy Technology Pumps in district heating network Haichao Wang Academy Postdoc Tel: Risto Lahdelma Professor in Energy technology for communities Department of Mechanical Engineering School of Engineering Aalto University Otakaari 4, 0250 Espoo, Finland H.C. Wang & R. Lahdelma DH distribution The two pipe system is most commonly used In some systems, three or more pipes are used to distribute heat and/or cooling at different temperatures The water is pressurized to prevent it from boiling at operating temperatures Max temperature in Finland is 20 o C (5 o C in effect) In other countries max temperature from 90 o C to 80 o C A high temperature: Increases transmission capacity as it allows greater temperature difference Decreases pumping costs Allows long transfer distances - But increases heat losses R.Lahdelma H.C. Wang & R. Lahdelma 2

2 DH distribution Controlling the network is in principle possible by changing temperature difference or changing the water flow (or both) The production plant controls only the supply water temperature based on outdoor temperature Pumps in the network maintain sufficient pressure difference between supply and return pipes Customer substations control the water flow through heat exhangers with adjustable valves Return water temperature depends on how well heat exchangers operate R.Lahdelma H.C. Wang & R. Lahdelma 3 Water pumps in DH network A water pump is a device to transmit and distribute water or make it pressurized in the pipelines. It converts the mechanical energy of the prime mover (e.g. an electric motor) to increase the energy of water (e.g. static energy, kinetic energy and pressure energy). In fact, pump is the only network component that supplies energy while other components consume energy in different forms. H.C. Wang & R. Lahdelma 4 2

3 Pump type Pump type can include: Displacement pump Impeller pump, e.g. centrifugal pump The most common pump type in DH networks is the centrifugal pump Properties Pipe size DN50-DN000 Pressure and temperature PN6, 20 o C Capacity 0~00 l/s, 0~260 m 3 /h Typically constant speed Variable speed pumps are becoming more common in real-life networks in order to save pumping energy. A centrifugal pump H.C. Wang & R. Lahdelma 5 Pump functions in DH network Different pumps in the DH network can: circulate water in supply and return pipes of the DH network feed make-up water stabilize system pressure and avoid vaporization Important parameters: flow rate Q (volume or mass, m 3 /s, l/s, kg/s, t/h) lift height H (mh2o, PSI, bar, (k,m)pa, mmhg) pump power P (kw) efficiency (%) Parameter curve of a centrifugal pump H.C. Wang & R. Lahdelma 6 3

4 Excercise There is an centrifugal pump transporting water (density is 960 kg/m 3 ), and the measured volume flow rate is 2.6 l/s, please calculate the mass flow rate in t/h? Answer: 9 t/h H.C. Wang & R. Lahdelma 7 Pressure control in DH network The water in DH pipelines must conquer: the frictional and local resistance losses the elevation difference between heat plant and users The water in the DH network must all times remain pressurized underpressure may cause water to boil (see next page - boiling temperature of water at different pressure) air to leak into the DH network gases in water to dissolve and form air-pockets preventing circulation boiling may typically occur at highly elevated parts of the network on hot surfaces suction-side of pumps H.C. Wang & R. Lahdelma 8 4

5 Pressure control in DH network Boiling temperature of water Also, a sufficient pressure difference between supply and return pipes must be guaranteed for all users usually minimum 50 kpa H.C. Wang & R. Lahdelma 9 NPSH Net Positive Suction Head What is NPSH? NPSH can be expressed as the difference of suction head (total water head at suction inlet) and vapor head of water (water head of vaporization). H.C. Wang & R. Lahdelma 0 5

6 NPSH Net Positive Suction Head NPSHa and NPSHr NPSHa available NPSH of the system the larger the better a measure of how close the fluid at a given point is to boiling and to cavitation NPSHr required NPSH of the pump the smaller the better the head value at a specific point (e.g. the inlet of a pump) required to keep the fluid from cavitating NPSHa should always > NPSHr H.C. Wang & R. Lahdelma Parameter curves of pumps Parameters of different pumps can be shown in a diagram volume flow on x-axis lift height on y-axis Diagram is based on test runs with fixed speed H.C. Wang & R. Lahdelma 2 6

7 Pump power where Pump power Pa = pump axis power (W) = water density (kg/m 3 ) g = gravity constant (9.8m/s 2 ) Q = volume flow (m 3 /s) H = lift height (m) = efficiency ratio ( ), for small pumps Electric motor power gqh Pa K P where m P = electric motor power (W) K = assurance coefficient (>) m = pump motor efficiency H.C. Wang & R. Lahdelma 3 P a Electricity consumption of pump Electricity consumption gqh E Pan n0 (kwh) where n0 = pump operating time (h) 0.00 is the coefficient of W to kw Note that the lift height, volume flow and efficiency vary according to the working point of the pump. H.C. Wang & R. Lahdelma 4 7

8 Affinity laws of pump Q m n Q m n H p n H2 p2 n2 2 Q H P P H Q n P2 H2 Q2 n2 H.C. Wang & R. Lahdelma % Pumps in series Pressure Characteristic pressure-flow response curve of network Max flow rate Water flow Pumps in series increase max p but not max flow rate H.C. Wang & R. Lahdelma 6 8

9 Pumps in parallel Characteristic pressure-flow response curve of network Pressure Max lift head Water flow Pumps in parallel increase max flow rate but not max p H.C. Wang & R. Lahdelma 7 Working point of DH network Valve partly open S2 pressure-flow response curve of DH network 2 S Valve fully open Pa ~ QH norminal revolution reduced revolution lift head property H.C. Wang & R. Lahdelma 8 9

10 Excercise 2 In a DH network, the measured flow rate, lift head and pump power for the circulation water pump at n = 2000 rpm are: 0.7 m 3 /s, 04 m and 84 kw, please calculate the flow rate, lift head and pump power at n=500 rpm (the characteristic pressureflow curve of the DH network remains the same). Answer: m 3 /s, 58.5 m, kw H.C. Wang & R. Lahdelma 9 Pressure diagram of DH network Pressure diagram reflects the distribution of absolute pressure (in mh2o) in the DH network. Pressure curve is a curve connecting all points water heads in the pipelines. Dynamic pressure curve pump is running Static pressure curve pump is stopped Constant pressure point constant no matter the pump is running or not gz P v 2 /2 For water Static water head Pressure water head Kinetic water head Total water head H.C. Wang & R. Lahdelma 20 0

11 Static pressure curve Static pressure curve Heat user Elevation 0 G G HOB Constant pressure point H.C. Wang & R. Lahdelma 2 A B Z o 0 Dynamic pressure curve Determine the pressure figure in the return pipelines by hydraulic calculation; Make sure the available water head in the heat user; Determine the pressure curve in the supply pipelines; Consider the pressure drop in the HOB. Note that the example is for a direct connection low-pressure DH network, in which the pressure is much lower than.6 MPa. It can be also applied to the secondary side of the DH network. HOB 0 G A O B 0 H.C. Wang & R. Lahdelma 22 G O G G A B Z0

12 Use of pressure diagram Enough available pressure difference must be guaranteed for all heat users; Make sure no over-pressure in radiators and other components Steel and aluminium radiator <0.6 Mpa Pipeline and valve <.6Mpa Cast iron < 0.4 Mpa Make sure no vaporization in the DH network, i.e. the pressure at all points should always > the vaporization pressure of water at local temperature; Help to understand the whole pressure level in the DH network and select pumps. H.C. Wang & R. Lahdelma 23 Network design based on techno-economic optimization Pipeline dimensioning is based on techno-economic optimization Pressure loss R=P/L (called specific pressure loss here) should be less than: bar/km (0. kpa/m or 00Pa/m) at main lines 2 bar/km (0.2 kpa/m) at branches near consumer Why? Many factors influence the most preferred value of R, including the initial investment of pipelines, operating cost of pump, supply and return water temperatures, heat losses, etc. but the first two are the most important. Example, a DH network is shown below, our objective is to determine the diameter of the main line. A 200 m B 80 m C 50 m D Flat terrain 3 Heat plant 2 H.C. Wang & R. Lahdelma 24 2

13 Network design based on techno-economic optimization A 200 m B 80 m C 50 m D 3 Flat terrain Heat plant Data input: heat demand: Heat user 2 3 Heat demand (GJ/h) the available pressure in heat users: 50 kpa, pressure drop in plant is 80 kpa static pressure is 20 mh2o (kpa 0.02 mh2o) local resistance pressure loss is 30% of the frictional resistance loss interest rate 7.56%, calculation period 20 year equivalent steel price for pipes: , , Yuan/m 3 steel pump operating time: 40%, 60%, 80% full power operating time in a year water density: kg/m 3 ; g = 9.8 m/s 2 ; pump efficiency: 70% assurance coefficient K =.5; transmission coefficient m = 80% H.C. Wang & R. Lahdelma 25 2 Network design based on techno-economic optimization Optimization process: calculate the mass flow rate in each pipe and in the pump calculate the water velocity and R in each pipe (this part is already introduced by Prof. Risto Lahdelma) dvide pipeline senarios according to the value of R, e.g. here Senario R = P/L (Pa/m) 8.~ ~ ~ ~ ~ ~568.7 calculate the steel consumption in m 3 and the initial investment using variable prices determin the lift head and equivalent full operating time of the pump using variable full operating percentage calculate the pump power and the electric motor power calculate the electricity consumption and pump operating cost for different operating scenario calculate the annuity of initial investment (method in next page) and total annual cost show the optimization results H.C. Wang & R. Lahdelma 26 3

14 Network design based on techno-economic optimization Idea: Assume that investment I is financed by taking a loan for n years with effective interest rate r The loan is paid back with fixed constant annual amounts P The net present value (NPV) of a payment after i years is /( ) i i P r ( r) P To determine the annual payment, the NPV of payments = loan n i I ( r) P P I ai i n i ( r) i Here the annuity factor a is obtained by computing the sum of the geometric series: r a n i n ( r) r i H.C. Wang & R. Lahdelma 27 Network design based on techno-economic optimization Cost (Yuan/a) The relationship of P/L and annuity of initial investment, pump operating cost and total annuity cost ~54.7 Pa/m 24.3~54.7 Pa/m Annunity of initial investment Pump operating cost Total annunity cost 44.9~79.3 Pa/m 7.5~227.2 Pa/m Specific pressure drop R= P/L 78.3~383.5 Pa/m 383.5~568.7 Pa/m H.C. Wang & R. Lahdelma 28 4

15 Network design based on techno-economic optimization Pump operating cost and annunity of initial investment Cost (Yuan/a) Pump operating cost Annunity of initial investment Cost percentage Cost percentage (%) ~54.7 Pa/m 24.3~54.7 Pa/m 44.9~79.3 Pa/m 7.5~227.2 Pa/m 78.3~383.5 Pa/m 383.5~568.7 Pa/m Specific pressure drop R= P/L H.C. Wang & R. Lahdelma 29 Network design based on techno-economic optimization Pump operating cost (Yuan/a) ~54.7 Pa/m 24.3~54.7 Pa/m 44.9~79.3 Pa/m 7.5~227.2 Pa/m 78.3~383.5 Pa/m 383.5~568.7 Pa/m Initial investment of DH pipelines pipeline number design flow rate G' (t/h) length (m) Diameter(mm) water velocity (m/s) R (Pa/m) AB ,72 44,9 BC ,7 54,7 CD ,74 79,3 Optimization results of the problem H.C. Wang & R. Lahdelma 30 5

16 Conclusion of the optimization Conclusion: Answer to the problem there is an optimal scope of R, which should be less than 00 Pa/m but not too small; When R is from 8. Pa/m to Pa/m, the total annual cost first decreases along with the increasing R before the optimum, and then increases dramatically when R > 00 Pa/m; For the example DH network, the optimal R is 44.9 Pa/m 79.3 Pa/m; Similar optimization for DH system, please refer to: Haichao Wang, Wenling Jiao, Risto Lahdelma, Pinghua Zou. (20). Technoeconomic analysis of a coal-fired CHP based combined district heating system with gas-fired boilers for peak load compensation. Energy Policy 39(2), H.C. Wang & R. Lahdelma 3 The End H.C. Wang & R. Lahdelma 32 6