A multi-section vertical absorption heat exchanger for district heating systems

international journal of refrigeration 71 (2016) 69–84

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A multi-section vertical absorption heat exchanger for district heating systems Chaoyi Zhu, Xiaoyun Xie *, Yi Jiang ** Building Energy Research Center, Department of Building Science, Tsinghua University, Beijing 10084, China

A R T I C L E

I N F O

A B S T R A C T

Article history:

A novel building scale multi-section absorption heat exchanger (AHE) that is consist of a

Received 14 April 2016

multi-section absorption heat pump and a plate heat exchanger was designed, developed

Received in revised form 22 July

and applied in district heating system to realize heat exchange from primary water to sec-

2016

ondary water. The multi-section generating, condensing, absorbing and evaporating processes

Accepted 19 August 2016

were designed to realize large external fluid inlet/outlet temperature difference in each com-

Available online 24 August 2016

ponent. The heat transfer loss can be reduced, and a low return primary water temperature can be reached to recover low grade waste heat as heat source. The performance of the pro-

Keywords:

totype AHE was tested. The return primary water temperature was stable and was below

Absorption heat exchanger

30 °C in the past two heating seasons, which was 10–15 K lower than inlet water tempera-

Vertical multi-section structure

ture of secondary water. A temperature difference of 10 K–20 K in each component was

Pressure gradient

realized. A pressure gradient from top to bottom was established to ensure stable fluid flow

Large inlet/outlet temperature

from top to bottom. The AHE has higher heat exchange effectiveness and better regulation

difference

performance compared with plate heat exchanger according to field test. And finally, the

Heat exchange effectiveness

simulation model used to design the prototype machine was verified by the test results.

Simulation model validation

© 2016 Elsevier Ltd and IIR. All rights reserved.

Un échangeur de chaleur multi-section vertical à absorption pour les systèmes de chauffage urbain Mots clés : Échangeur de chaleur à absorption ; Structure verticale multi-section ; Gradient de pression ; Grande différence de température intérieure/extérieure ; Efficacité d’échangeur de chaleur ; Validation du modèle de simulation

1.

Introduction

China’s northern cities have a significant heating demand during winter, accounting for 24% of the nation’s total building energy

consumption in 2013 (Building Energy Research Center of Tsinghua University, 2015). District heating is the most commonly used system, and is responsible for more than 70% of the heating demand. A novel district heating system based on the absorption cycle was introduced by Fu et al. (2008) in northern China

* Corresponding author. Building Energy Research Center, Department of Building Science, Tsinghua University, Beijing 10084, China. Fax: +86 1062770544. E-mail address: [email protected] (X. Xie). ** Corresponding author. Building Energy Research Center, Department of Building Science, Tsinghua University, Beijing 10084, China. Fax: +86 1062770544. E-mail address: [email protected] (Y. Jiang). http://dx.doi.org/10.1016/j.ijrefrig.2016.08.010 0140-7007/© 2016 Elsevier Ltd and IIR. All rights reserved.

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international journal of refrigeration 71 (2016) 69–84

Nomenclature

 m X Q Cp h t UA ΔT

mass flow rate [kg s−1] solution concentration [%] heat rate [kW] specific heat at constant pressure [kJ kg−1 K−1] enthalpy [kJ kg−1] temperature [°C] equivalent heat transfer area [kW K−1] temperature difference [K]

Subscripts g generator c condenser a absorber e evaporator s solution v vapor w primary or secondary water sa saturation rw refrigerant water in inlet o outlet

to reduce the return primary water temperature; this is the key step in recovering industrial low-grade waste heat for district heating (Fang and Xia, 2013; Fang et al., 2012) and reducing the energy consumption of the primary water network. The absorption heat exchanger (AHE) applied in heating station is the main piece of equipment in the new heating system, which completes the heat exchange process from the primary water to the secondary water. A comparison of two types of district heating systems is shown in Fig. 1a–b. Fig. 1a is a conventional waste heat recovery district heating system utilizing a plate heat exchanger to realize heat exchange from primary water to secondary water. Fig. 1b is a novel system utilizing an AHE to realize the same process. Compared to a plate heat exchanger, the AHE can decrease the return primary water temperature from 45 °C to 25 °C. This return primary water can be used to collect low-grade industrial waste heat (from 30 °C to 45 °C). Therefore, more waste heat can be collected

for heating, so the energy consumption for heating can be reduced. In high load condition shown in Fig. 1, when the supply primary hot water temperature is 90 °C, a traditional system can collect waste heat to cover 33.3% of heat load (from 45 °C to 60 °C); a novel system with AHE can collect waste heat that covers 53.8% of heat load (from 25 °C to 60 °C). By using AHE in district heating system, we actually use low grade waste heat (e.g., boiler flue gas low grade waste heat or condensing water waste heat in CHP) to replace part of common heat source (e.g., boiler or steam) for heating. Meanwhile, the multisection vertical AHE can increase the inlet/outlet temperature difference on the primary water side from 45 K (using a traditional plate heat exchanger) to 65 K, thus improving the heat transport capacity of the network by 40%. Fu et al. (2010) evaluated the application of absorption heat exchanger in a CHP district heating system. In that application, the supply primary hot water temperature was 130 °C; the return primary water temperature could be lower than 30 °C by using absorption heat exchangers in substation. This return primary water then went back to CHP plant and recovered the waste heat of condensers in thermal power engines. Absorption heat pumps were also installed at thermal power plants to recover the remaining waste heat. As a result, about 30% more heat could be supplied using the same thermal power engine, and the transportation ability of the primary hot water network was increased by 80%. This type of system has been used successfully in real applications in the cities of Chifeng (Li et al., 2011), Datong, and Taiyuan, resulting in significant energy savings. The absorption heat exchanger (AHE) is a combination of an absorption heat pump (AHP) and a plate heat exchanger (HEX), as shown in Fig. 2a. Fig. 2b illustrates the heat exchange process inside a common AHE in a T-Q chart, where the vertical axis is temperature and the horizontal axis represents the quantity of heat exchanged in each process. It can be found out from Fig. 2b that for each component of the AHP (i.e., the generator, condenser, absorber, and evaporator), the inlet/outlet temperature difference of the external heating source or heating sink is about 10–20 K, which is much larger than that of common absorption heat pumps (5 K). If common absorption heat pump designs are used (for internal process structures), the internal heat and mass transfer processes change to “triangle-shaped” processes with extremely nonuniform temperature differences throughout the entire heat

Fig. 1 – Processes of district heating systems recovering industrial waste heat: (a) traditional system utilizing a plate heat exchanger (HEX); and (b) novel absorption system using the AHE.

international journal of refrigeration 71 (2016) 69–84

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Fig. 2 – Conventional AHE consisting of AHP and HEX: (a) external parameters; and (b) internal heat exchange T-Q chart.

transfer processes. This generates a significant unmatched heat transfer loss, and the required low return primary water temperature cannot be realized as a result. Therefore, a suitable structure to realize large inlet/outlet external fluid temperature difference for each component is required for this AHE. According to the process shown in Fig. 2a, the return primary water leaves the AHE from the evaporator. Therefore, a lower evaporating temperature is also required to reach a lower return primary water temperature when the inlet primary hot water temperature and the inlet/outlet secondary water temperatures are given. Many studies focused on multistage absorption cycle to realize lower evaporating temperature. Some researchers added

a middle-pressure generator and a middle-pressure absorber to a single absorption cycle (Ma and Deng, 1996; Venegas et al., 2002; Yao et al., 2013). Guerra (2012) mentioned another doublestage structure with a middle-pressure absorber–evaporator unit and a low-pressure absorber–evaporator unit that were connected through a water flow circuit between middle-pressure evaporator and low-pressure absorber. However, these cycles are not suitable for large temperature difference heat transfer process because only one stage of condenser and evaporator is used to transfer heat with external fluids; the triangleshaped process cannot be avoided. Han (2009) and Li (2008) presented a mixed structure for AHP. They introduced a middle-pressure condenser, a

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middle-pressure generator and a middle-pressure absorber to a single absorption cycle. The heating sink fluid flowed through low-pressure absorber, middle-pressure condenser, middlepressure absorber and high-pressure condenser in series to reach large temperature difference on heating sink side. Liu et al. (2002) and Park and Kang (2010) divided the evaporator and absorber of a double effect absorption chiller into two sections. The chilled water went through two sections of evaporator in series to realize temperature drop from 15 °C to 7 °C. Dong et al. (2004) connected two single stage single effect AHPs in series. The hot driven water went through two generators in series, and the chilled water went through two evaporators in series. This design could realize large temperature difference for hot water in generator and chilled water in evaporator. However, the studies above required more than one solution cycle and one refrigerant cycle that made the system and control method complicated. And most importantly, none of these studies could realize large temperature difference on both heating source and heating sink sides at the same time. A new integral vertical structure for multi-section generatingcondensing or multi-section evaporating–absorbing processes was introduced by Jiang et al. (2013). The new structure could realize large external fluid inlet/outlet temperature difference on both heating source and heating sink side at the same time. Wang et al. (2013, 2014a) did preliminarily analysis of this new multi-section absorption cycle and provided an optimization design approach based on entransy dissipation method. The novel structure was developed and studied through simulation by Zhu et al. (2014). The model was not verified by test data in the study. The multi-section generator–condenser unit was tested in an experimental test bench to verify the feasibility of the structure (Wang et al., 2014b). These researches represent an important first step to the novel multi-section absorption cycle. But the performances of the entire absorption heat exchanger and its application have not been examined in detail. Furthermore, in traditional district heating systems using absorption heat exchanger (Li et al., 2011), large horizontal absorption heat exchangers are often applied in real projects since one thermal station is usually responsible for several communities that have a total heating area of 100,000–200,000 m2. The heat output is 9–10 MW. However, there is usually not enough space for such large-scale horizontal absorption heat pumps in most heating stations near crowded residential communities. Moreover, when one machine supplies heat for 20–50 buildings, there always exists a large and complex secondary hot water network with high electricity consumption, and the heating of each building is difficult to adjust. Small-scale absorption heat exchanger units with building-scale heat output for single building (or 2–3 buildings) need to be developed and applied in real systems. Therefore, a new absorption heat exchanger using a vertical multi-section structure with a small heat output was developed. A simulation model was set up for this new type of absorption heat exchanger, and a real unit was developed and applied in the city of Chifeng to supply heat for an office building. This unit’s field test and its testing performance are discussed. And the simulation model is verified by test data.

2. Process of the multi-section vertical absorption heat exchanger A new multi-section vertical absorption heat exchanger using LiBr–H2O solution as the working pair was proposed in 2010. It is composed of an elementary unit for the generation and condensation processes (or evaporation and absorption processes) to realize a large temperature drop on the primary water side. This new multi-section vertical AHE, shown in Fig. 3, is composed of six main parts: a hot water-driven generator, a condenser, an absorber, an evaporator, a solution heat exchanger (SHX), and a water heat exchanger (HEX). The generator, condenser, absorber, and evaporator are all divided into three sections from top to bottom (the first, second, and third sections, respectively) using stainless steel plates. U-pipes are applied to let the solution or refrigerant water flow from the upper section to the lower section and maintain the pressure gradient between the two sections. In each section of the generator, absorber, and evaporator, an orifice plate is used to distribute the liquid on heat-exchanging copper coil pipes. The primary water is supplied into the generator from the top section; it then flows through the generator, the HEX, and the evaporator in succession, and finally flows out from the bottom of evaporator and returns to the heat supplier. The return secondary water is divided into three currents that flow separately into the HEX, the bottom of the absorber, and the bottom of the condenser. The heated secondary water collects and then flows out to terminal users and equipment. The generator and condenser form an entire chamber above the absorber–evaporator chamber. A solution tank and a refrigerant tank are placed below the absorber and evaporator, respectively. With the previously mentioned flow direction of the primary water and secondary water, strictly decreasing pressure is established from the top section of the generator– condenser chamber to the bottom section of the absorber– evaporator chamber. According to this vertical design, only one solution pump is needed to lift the solution from the solution tank below the absorber to the top section of the generator; the solution flows through the six sections due to the effects of gravity and the pressure gradient. The AHE is much more stable and easier to control than a horizontal absorption heat pump. Fig. 4a presents the external parameters of the multisection vertical absorption heat exchanger (AHE) that was used in the heating stations near terminal users. The supply/ return temperature of the primary water is 90 °C/25 °C; the flow rate is 2.4 m3 h−1; the return/supply temperature of the secondary water is 40/50 °C; and the flow rate is 15.6 m3 h−1. This multi-section vertical AHE can realize an extremely unmatched flow rate heat exchange process, and it also decreases the return primary water temperature to 25 °C, which is much lower than the secondary water temperature. Fig. 4b illustrates the heat exchange process inside the multisection vertical AHE in a T-Q chart. The multi-section vertical AHE creates three condensing pressures and three evaporating pressures in order to effectively change the huge “triangleshaped” heat transfer process shown in Fig. 2b into three small triangle-shaped processes, thus decreasing the heat transfer loss and increasing the temperature drop on the primary water side.

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Fig. 3 – Internal structure and process of the multi-section vertical AHE.

3. Simulation of the multi-section vertical absorption heat exchanger 3.1.

Modeling

A simulation model of the AHE was established based on the mass balance, solution balance, energy balance, and heat and

mass transfer equations of each section of the AHE. Some assumptions are introduced to simplify the model:

(1) There is no pressure drop for the water vapor going from the generator to the condenser or from the evaporator to the absorber. Therefore, each section of generator and condenser share one vapor pressure; and each

Hot Primary Water Generator

Secondary Water supply

Primary 90 Water supply

Primary water

50

AHE

Secondary water

40 15.6m3 h-1

Primary Water return

Condenser

Absorber HEX Secondary Water

25 Secondary 2.4m3 h-1 Heating Station Water return

(a)

Evaporator

(b)

Fig. 4 – Principle of the multi-section vertical AHE: (a) external parameters; and (b) internal heat exchange T-Q chart.

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Table 1 – External designed condition of multi-section vertical AHE. Primary water flow rate (kg s−1)

Secondary water flow rate (kg s−1)

Primary water supply temperature (°C)

Primary water return temperature (°C)

Secondary water supply temperature (°C)

Secondary water return temperature (°C)

3.8

90

30

51

40

0.7

section of absorber and evaporator share one vapor pressure. (2) The coil pipes in the condenser and evaporator are covered completely with saturated water film. Therefore, we used lumped parameter method to describe the heat and mass transfer processes from the hot/cooling water flows inside the coil pipes to the water vapor. The heat and mass transfer processes in each section of the four main parts are listed in Eqs. (1)–(20). Generator:

Evaporator:

 rw,in = m  rw,o + m  v,e m

(16)

 rw,in ⋅ hrw,in + Q e = m  rw,o ⋅ hrw,o + m  v,e ⋅ h v,e m

(17)

 w,e ⋅ Cp ⋅ (t w,e,in − t w,e,o ) Qe = m

(18)

ΔTe =

(t w,e,in − t sa,e ) − (t w,e,o − t sa,e ) ln ((t w,e,in − t sa,e ) (t w,e,o − t sa,e ))

(19)

 s,g,in = m  s, g,o + m  v,g m

(1)

Q e = UAe ⋅ ΔTe

 s,g,in ⋅ Xs,g,in = m  s, g, o ⋅ X s, g , o m

(2)

 s,g,in ⋅ hs,g,in + Q g = m  s , g , o ⋅ hs , g , o + m  v,g ⋅ h v,g m

(3)

 w,g ⋅ Cp ⋅ (t w,g,in − t w,g,o ) Qg = m

(4)

This model is developed for a three-section vertical AHE. Therefore the inlet and outlet refrigerant water enthalpies for each section of evaporator are not equal. This is considered in Eq. (17). The steady state model is solved by Engineering Equation Solver (EES) developed by Klein (2010).

ΔTg =

(t w,g,in − ts,g,in ) − (t w,g,o − ts,g,o ) ln ((t w,g,in − t s,g,in ) (t w,g,o − t s,g,o ))

Q g = UAg ⋅ ΔTg

(5)

(6)

Condenser:

 v,c ⋅ (hv,c − hsa,c ) = m  w,c ⋅ Cp ⋅ (t w,c,o − t w,c,in ) m

(7)

(t sa,c − t w,c,in ) − (t sa,c − t w,c,o ) ln ((t sa,c − t w,c,in ) (t sa,c − t w,c,o ))

(8)

ΔTc =

 w,c ⋅ Cp ⋅ (t w,c,o − t w,c,in ) = UAc ⋅ ΔTc m

3.2. Design parameters of the multi-section vertical absorption heat exchanger The multi-section vertical absorption heat exchanger was first designed and simulated based on the structure and process shown in Fig. 3. The design parameters, including water temperature, flow rate, and UA (product of the heat transfer coefficient and the heat transfer area, representing the equivalent heat transfer area) of each part are listed in Tables 1 and 2.

3.3. (9)

Absorber:

 s,a,in = m  s,a,o − m  v,a m

(10)

 s,a,in ⋅ Xs,a,in = m  s,a,o ⋅ Xs,a,o m

(11)

 s,a,in ⋅ hs,a,in − Q a = m  s,a,o ⋅ hs,a,o − m  v,a ⋅ hv,a m

(12) (13)

The working condition of the multi-section vertical AHE is determined by the outside, primary, and secondary water temperatures and flow rates, which then determine the heat supply of the AHE. The changes in outside air temperature

Table 2 – Internal equivalent heat transfer area (kW K−1).a

1.4

UAa,2

(t s,a,in − t w,a,o ) − (t s,a,o − t w,a,in ) ΔTa = ln ((t s,a,in − t w,a,o ) (t s,a,o − t w,a,in ))

(14)

Q a = UAa ⋅ ΔTa

(15)

Performance in different working conditions

3.3.1. Outlet water temperature change in different working conditions

UAg,1  w,a ⋅ Cp ⋅ (t w,a,o − t w,a,in ) Qa = m

(20)

1.8 a

UAg,2

UAg,3

UAc,1

UAc,2

UAc,3

UAa,1

1.4

1.4

2.1

2.1

2.1

1.8

UAa,3

UAe,1

UAe,2

UAe,3

UASHX

UAHEX

1.8

2.9

2

1.8

1.1

13.8

The subscript numbers represent different sections; SHX represents the solution heat exchanger, and HEX represents the water plate heat exchanger.

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Table 3 – Designed outside parameters in different working conditions. Heat supply (kW)

Primary water supply temperature (°C)

Primary water return temperature (°C)

Secondary water supply temperature (°C)

Secondary water return temperature (°C)

90.0 83.1 76.3 69.5 62.6 55.8

30.2 29.3 28.5 27.6 26.7 25.9

51.0 48.1 45.2 42.3 39.4 36.5

40.0 38.2 36.4 34.6 32.8 31.0

175.1 157.6 140.1 122.6 105.1 87.6

during the whole heating season result in different heat loads of the building (and therefore, different heat demands of the AHE). Table 3 lists the designed different heat output conditions by changing only the supply primary water temperature. The indoor temperature is assumed to be constant at 22 °C during the whole heating season, and the flow rates of the primary water and secondary water remain unchanged. In district heating systems, the heat supplier usually controls the primary water supply temperature to change the heating load according to the outside temperature, as shown in Table 3. The heat supplier decreases the primary water supply temperature from 90 °C to 55.8 °C to change the heat supply from 100% to 50%. The indoor temperature is unchanged, the secondary water temperature decreases, and the temperature difference in the secondary water side is lower. As a result, the primary water return temperature decreases from 30.2 °C to 25.9 °C, which is comparatively stable and low enough to recover low-grade waste heat from industrial activities.

3.3.2.

Pressure distribution for different heating loads

Fig. 5 shows strictly decreasing pressure from the top section of the generator–condenser chamber to the bottom section of the absorber–evaporator chamber. According to Fig. 5a, the condensing pressure in each section of the generator–condenser chamber decreases as the heat load ratio gets lower and lower. At the same time, the pressure difference between neighboring sections decreases. The largest condensing pressure

difference between two neighboring sections occurs at a full load; this 3 kPa pressure difference is the design basis of the U-pipe length. The evaporating pressure demonstrated in Fig. 5b presents similar changes as condensing pressure. However, the largest pressure difference between two neighboring sections is only 0.7 kPa, which is much smaller than the condensing pressure difference.

3.3.3.

Concentration distribution for different heating loads

Fig. 6 illustrates the inlet solution concentration change in different working conditions. From the top section to the bottom section of the generator, the solution is generated, so the concentration gets higher and higher; from the top section to the bottom section of the absorber, the solution absorbs water, so the concentration gets lower and lower. As the heat load goes down, the concentration in each part decreases; thus, the total volume of the solution increases, while the total volume of the refrigerant water decreases. From a 100% load to a 50% load, the concentration of the solution changes from 39% to 50%, which is an important result for the design of working pair quantity. Meanwhile, the multi-section vertical AHE has a comparatively low risk of crystallization since the highest concentration in all the working conditions is just 50.2%.

3.3.4. AHE

Performance evaluation of the multi-section vertical

According to the process described in Fig. 4a, the multisection vertical AHE actually completes a heat exchange process

18

5.5 First Section

First Section

Second Section

5

Third Section

Evaporating pressure (kPa)

Condensing pressure (kPa)

16 14 12

10 8

Third Section 4.5

4

3.5

3

6 4 50%

Second Section

60%

70% 80% Heat Load Ratio

(a)

90%

100%

2.5 50%

60%

70% 80% Heat Load Ratio

90%

100%

(b)

Fig. 5 – Simulated pressure in each section of the multi-section vertical AHE: (a) condensing pressure; and (b) evaporating pressure.

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54

54

First Section

First Section 52

Second Section Inlet solution concentration (%)

Inlet solution concentration (%)

52

Third Section

50 48 46 44 42

Second Section Third Section

50 48 46 44 42 40

40 38 50%

60%

70% 80% Heat Load Ratio

90%

100%

38 50%

60%

70% 80% Heat Load Ratio

(a)

90%

100%

(b)

Fig. 6 – Simulated concentration of solution into each section of the multi-section vertical AHE: (a) generator; and (b) absorber.

from the primary water to the secondary water. To evaluate the performance of the multi-section vertical AHE, heat exchange effectiveness ε, which refers to the effectiveness of an ordinary plate heat exchanger, is used to indicate the heat exchange performance on the primary water side (Xie and Jiang, 2015). The expression of ε is written as Eq. (21):

ε=

t 1,in − t 1,o t 1,in − t 2,in

(21)

where t1,in is the inlet primary water temperature, t1,o is the outlet primary water temperature, and t2,in is the inlet secondary water temperature.

1.35

For an ordinary plate heat exchanger, ε ≤ 1, but for the AHE, ε > 1. Similar to plate heat exchangers, the ε of the AHE is mainly influenced by the heat exchange area and flow rate ratio (the flow rate of the secondary water/primary water). A greater heat exchange area or a larger flow rate ratio will increase effectiveness ε. Meanwhile, the effectiveness is also slightly influenced by the inlet temperature of the primary and secondary water, as shown in Fig. 7. However, the influence of the inlet temperature is smaller than that of the heat exchange area or flow rate ratio. Another factor that significantly influences effectiveness is the internal structure of the AHE. The effectiveness difference between the ordinary one-section AHE and three-section AHE for a given total heat exchange area and

1.35

t1,in = 80

t2,in = 50 t2,in = 45 t2,in = 40

t1,in = 90 t1,in = 110

1.3

1.3

t1,in = 30

t1,in = 100 1.25

1.25

1.2

1.2

1.15

t1,in = 35

1.15

t2,in = 40

1.1

t1,in = 90

1.1 2

4

6

8 10 Flow rate ratio

(a)

12

14

16

2

4

6

8 10 Flow rate ratio

12

14

(b)

Fig. 7 – Effectiveness in different inlet conditions: (a) a given inlet secondary water temperature; and (b) a given inlet primary water temperature.

16

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1.35

Table 4 – Measurement sensors and instrument accuracy.

Three sections

Sensor and instrument

1.3

1.25

Thermocouple Absolute pressure transducer Electromagnetic flowmeter Data acquisition/switch unit

One section

Accuracy ±0.1 °C ±0.5% FS ±1.0% FS ±1.0 °C (T-type thermocouple) ±(0.0035% RDG + 0.0005% FS)

1.2

1.15

1.1 2

4

6

8 10 Flow rate ratio

12

14

16

Fig. 8 – Effectiveness for different structures.

a given external condition is shown in Fig. 8. The effectiveness of the one-section AHE is markedly lower than that of the three-section AHE. Moreover, the difference is larger for a greater flow rate ratio, indicating that the multi-section vertical AHE is more suitable for the extremely unmatched flow rate ratio heat exchange process represented in Fig. 4.

4. Development and application of the multisection vertical absorption heat exchanger 4.1. The developed and applied multi-section vertical absorption heat exchanger The first multi-section vertical AHE prototype was developed and applied in the city of Chifeng in Inner Mongolia in January of 2014 (Jiang and Xie, 2015), as shown in Fig. 9. The heat user is an office building with a heating area of 5000 m2; the terminal device is heating radiator. The AHE prototype has the following dimensions: 1.1 m × 1.1 m × 5.1 m (L × W × H). The rated heat output is 180 kW. The secondary water temperature

requirement in harsh winter is 50 °C/40 °C. The AHE has been in constant use for two years without problems. Because of the vertical design and pressure gradient established from top to bottom, there is no need for engineers to control the AHE during the whole heating season. The multi-section vertical AHE can work stably and be controlled as easily as plate heat exchangers. This AHE has an independent primary water pump (for buildings located far away from power plants or other heat suppliers), a secondary water pump, and a secondary water supply and pressure control system, making it an independent smallscale building-level substation (Jiang et al., 2015).

4.2. Testing performance of the multi-section vertical absorption heat exchanger The inlet and outlet water temperatures of the primary water and secondary water were measured using thermocouples. The pressure of different sections was measured by silicon capacitance absolute pressure transducers. The flow rates of the primary water and secondary water were measured by electromagnetic flowmeters. The temperature, pressure, and flow rate data were collected and stored using a data acquisition/ switch unit. The accuracies of the measurement sensors and instruments are shown in Table 4. Fig. 10 illustrates the heat balance by calculating the heat supply on the primary and secondary water sides. The unbalance can be controlled below 5%.

4.2.1.

Water temperature change during the past two years

The test data of the first multi-section vertical AHE from January to April of 2014 are shown in Fig. 11a. The first-year data include the data from the harsh winter to the end of winter. The designed heating rate is 180 kW, and the real heating demand during the harsh winter was 150 kW. With the change of the

Fig. 9 – First prototype multi-section vertical AHE in the city of Chifeng: (a) upward view; (b) side view; and (c) outside view.

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pump was powered off, the pressure difference disappeared; when the pump was powered on again, the pressure gradient was rebuilt quickly. The first section evaporating pressure was 2–3 kPa lower than the third section condensing pressure, so the solution could flow from generator to absorber under the driven force of the pressure gradient and gravity through the U-type pipe. The evaporating pressure difference between two neighboring sections was 0.5–1.5 kPa in harsh winter, and decreased when the heat load was lower. The decreasing pressure from top to bottom could ensure stable solution flow from the first section of the generator finally to the third section of the absorber section by section without a solution pump.

200 Heat supply on secondary water side (kW)

20%

5%

150 20%

100

50

4.2.3. 0 0

50 100 150 Heat supply on primary water side (kW)

200

Fig. 10 – Heat balance between the primary and secondary water.

outside temperature, the heat output changed from 80% to 30% of the rated heat rate, the supply primary water temperature changed from 90 °C to 57 °C, and the return primary water temperature fluctuated between 25 °C and 30 °C. The return primary water temperature was 15 K–20 K lower than it could be for a traditional plate heat exchanger, and was comparatively stable during winter. Moreover, the supply/return temperature of the secondary water was 50 °C/40 °C in harsh winter, which could adequately satisfy the user demand. The test data from October of 2014 to April of 2015 are shown in Fig. 11b. According to the data from the second year, when the supply primary water temperature decreased from 90 °C to 85 °C, the return primary water temperature could still be below 30 °C during most of the heating season. Furthermore, some problems from the heat supplier power plant led to huge fluctuations in the supply primary water temperature during the second heating season. However, the AHE could still work well in these conditions, and the return primary water temperature was comparatively stable. Fig. 12 presents tested external primary water and secondary water inlet/outlet temperature through generator, condenser, absorber, and evaporator. In high load condition, the temperature difference reached 20 K in generator, 8 K in condenser, 10 K in absorber, and 18 K in evaporator. The large temperature difference made it possible to reach lower outlet return primary water temperature from evaporator.

4.2.2.

Pressure distribution performance

Fig. 13 shows the condensing pressure and evaporating pressure in each section (measured by an absolute pressure transducer). The pressure in each section was higher in the harsh winter and lower toward the end of winter. The pressure gradient was successfully established, and the pressure difference between two neighboring sections was stable during a long period of time. The condensing pressure difference between two neighboring sections was 2–3 kPa in harsh winter, and decreased when the heat load was lower. When the solution

Regulation performance

Fig. 14 shows the regulation performance of this multisection vertical AHE. The heat output can be controlled by changing the temperature of the supply primary water or the flow rate of the primary water to meet the heat demand. The supply primary water temperature is usually controlled by a power plant, so for each substation, only the flow rate of the primary water can be controlled to change the heat supply to terminal users. Fig. 14a shows the change of heat output with the primary water flow rate when the temperature of the supply primary water was steady. There is a good linear relationship between the heat output and the primary water flow rate. A comparison of the regulation performance between the multisection vertical AHE and ordinary plate heat exchanger is illustrated in Fig. 14b, in which the vertical axis is the heat output ratio (heat output/rated heat output), and the horizontal axis is the primary water flow rate ratio (flow rate/rated flow rate). There is an obvious nonlinear relationship between the heat output and the primary water flow rate for conventional plate heat exchangers. Therefore, the regulation performance of the multi-section vertical AHE is superior to that of a traditional plate heat exchanger.

4.2.4.

Heat exchange effectiveness

Fig. 15 illustrates the tested effectiveness during the past two years. The flow rate ratio changed from 7 to 9 during most of the heating season, and the effectiveness was in the range of 1.2–1.3. The effectiveness can be higher if the flow rate ratio between the secondary side and the primary side is larger, meaning that the performance of the multi-section vertical AHE can be higher by increasing the flow rate ratio (i.e., increasing the secondary water flow rate). Therefore, if the terminal device is a radiant floor heating system, the flow rate ratio will be higher and the effectiveness will increase. In Section 4.2 the test performance of the first multisection vertical AHE prototype is presented. First, we present the external primary water and secondary water temperature change during the past two heating seasons. The return primary water temperature was below 30 °C and stable. Second, we present the internal pressure distribution in three sections of generator–condenser chamber and three sections of absorber–evaporator chamber. The pressure separation was successful and could ensure stable solution flow from top to bottom. Third, we evaluate the regulation performance of the novel AHE. The new AHE has better regulation performance than

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international journal of refrigeration 71 (2016) 69–84

100

Primary Water Supply

90

Temperature ( )

80 70 60

Secondary Water Supply

50

Secondary Water Return

40 30

Primary Water Return

20 10 0

4/26

4/21

4/16

4/11

4/6

4/1

3/27

3/22

3/17

3/12

3/7

3/2

2/25

2/20

2/15

2/10

2/5

1/31

1/26

1/21

1/16

1/11

1/6

Date

(a) 100

Primary Water Supply

90

Temperature ( )

80 70 60

Secondary Water Supply

50 40 30

Secondary Water Return

20

Primary Water Return

10 0

4/26 4/21 4/16 4/11 4/6 4/1 3/27 3/22 3/17 3/12 3/7 3/2 2/25 2/20 2/15 2/10 2/5 1/31 1/26 1/21 1/16 1/11 1/6 1/1 12/27 12/22 12/17 12/12 12/7 12/2 11/27 11/22 11/17 11/12 11/7 11/2 10/28 10/23 10/18 10/13 Date

(b) Fig. 11 – Water temperatures in two seasons: (a) 2013–2014 heating season; and (b) 2014–2015 heating season.

traditional plate heat exchanger. Finally, we calculated the effectiveness of the new AHE.

4.3.

Simulation model validation using test data

The validation of the simulation model mentioned in Section 3 is carried out by comparing the test data. The input parameters are the flow rates of the primary and secondary water, as well as the inlet primary water temperature and the inlet secondary water temperature. The output parameters are the outlet water temperature and the pressure of each section. The uncertainties of the calculated parameters are listed in Table 5. Fig. 16 presents a comparison of the test data and simulation results during both high and low load conditions. The solid line is the test data, and the dotted line represents the simulation results. The differences between the test data and the simulation results in both conditions were below ±0.3 °C. The

simulation model can present the temperature change of the inlet and outlet water, and the results are close to real working conditions. Fig. 17 presents a comparison of the test data and simulation results for condensing and evaporating pressure during high-load conditions and low-load condition. The blue line represents the test data, and the green line represents the simulation results. (For interpretation of the references to color

Table 5 – Uncertainty analysis. Calculated parameter Outlet primary water temperature Outlet secondary water temperature Condensing pressure Evaporating pressure

Uncertainty ±0.55 °C ±0.55 °C ±2% ±3%

international journal of refrigeration 71 (2016) 69–84

70

100 90 80 70 60 50 40 30 20 10 0

60 Temperature ( )

Temperature ( )

80

50 40 30 20 10 0 4/21 4/16 4/11 4/6 4/1 3/27 3/22 3/17 3/12 3/7 3/2 2/25 2/20 2/15 2/10 2/5 1/31 1/26 1/21 1/16 1/11

4/21 4/16 4/11 4/6 4/1 3/27 3/22 3/17 3/12 3/7 3/2 2/25 2/20 2/15 2/10 2/5 1/31 1/26 1/21 1/16 1/11 Date

Date

(a)

(b) 80

60

70 Temperature ( )

Temperature ( )

50 40 30 20 10

60 50 40 30 20 10 0

0

4/21 4/16 4/11 4/6 4/1 3/27 3/22 3/17 3/12 3/7 3/2 2/25 2/20 2/15 2/10 2/5 1/31 1/26 1/21 1/16 1/11

4/21 4/16 4/11 4/6 4/1 3/27 3/22 3/17 3/12 3/7 3/2 2/25 2/20 2/15 2/10 2/5 1/31 1/26 1/21 1/16 1/11 Date

Date

(c)

(d)

Fig. 12 – Inlet/outlet water temperature in each component: (a) generator; (b) condenser; (c) absorber; and (d) evaporator.

in this text, the reader is referred to the web version of this article.) The simulation results and test data have differences of no more than 0.2 kPa most of the time, which is acceptable considering the uncertainty of the calculations. The outlet temperatures and pressures of each section are the most important parameters for the design of the multisection vertical AHE. The simulation model in Section 3 can provide a close situation to real working conditions. Therefore, the accuracy of the model is acceptable.

4.4.

Application prospects

In the past two years, this small-scale multi-section vertical AHE has already been applied in several projects in Inner Mongolia and in Hebei province of northern China, including government buildings and residential communities. The multisection vertical AHE has many advantages (e.g., a small site area, a lower return primary water temperature, and nearly no control requirements). This makes it possible to replace the plate heat exchangers and apply the device in a novel absorption district heating system to recover some of the industrial low-grade waste heat, which can significantly reduce coal consumption and decrease air pollution in northern China during the heating season. This multi-section vertical AHE will be used as a smallscale substation throughout new residential communities, supplying heat for single buildings to replace traditional heating

stations; the total heating demand will be more than 500,000 m2 by the end of 2016. This will be a new direction for district heating system development in the future.

5.

Conclusions

An innovative multi-section vertical AHE is introduced in this paper. The multi-section vertical AHE was used in a novel district heating system based on the absorption process to replace a traditional plate heat exchanger. This AHE has six sections in the vertical direction to realize a large external fluid inlet/ outlet temperature difference in each component of generator, condenser, absorber and evaporator. Thus, the return primary water temperature can be lower than 30 °C, which can be used to recover industrial low-grade waste heat as heat source. Heat exchange effectiveness is introduced to evaluate the performance of the multi-section vertical AHE. The effectiveness of a plate heat exchanger is lower than 1, while the effectiveness of the AHE is higher than 1. The effectiveness of the multi-section AHE is higher than that of the traditional onesection AHE. A prototype machine was designed according to simulations in different working conditions, and then developed and applied in a real project. The performance of the prototype multi-section AHE was tested. The results are as follows:

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16

First section

Absolute pressure (kPa)

14 12

Second section

10 8

Third section

6 4 2 0

4/21

4/11

4/1

3/22

3/12

3/2

2/20

2/10

1/31

1/21

1/11

Date

(a) 8

Absolute pressure (kPa)

7

First section

6 5

Second section

4 3

Third section

2 1 0

4/21

4/11

4/1

3/22

3/12

3/2

2/20

2/10

1/31

1/21

1/11

Date

(b) Fig. 13 – Pressure distribution: (a) condensing pressure; and (b) evaporating pressure. 180 1.00

140

0.90

120

0.80

Heat output ratio

Heat output (kW)

160

100 80 60 40

0.70 0.60 AHE Plate HEX

0.50 0.40

20

0.30

0 0

0.5

1 1.5 2 Primary flow rate (m3h-1)

(a)

2.5

3

0.30

0.40

0.50

0.60

0.70

0.80

0.90

Flow rate ratio

(b)

Fig. 14 – Regulation performance of the multi-section vertical AHE: (a) field test AHE regulation performance; and (b) comparison of the AHE and a plate heat exchanger.

1.00

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(1) The return primary water temperature remained below 30 °C throughout the whole heating season; and it was comparatively stable in spite of some fluctuations of working conditions. (2) High load condition requires large primary water and secondary water inlet/outlet temperature difference in each component. The multi-section AHE realized 20 K, 18 K, 8 K and 10 K inlet/outlet temperature difference through generator, evaporator, condenser and absorber, respectively. (3) The pressure gradient was successfully established and stabilized in the six sections from top to bottom that ensured the fluid flow stability of the AHE. (4) The tested effectiveness of the prototype machine is 1.2– 1.3, which is better than common plate heat exchanger, and could be higher if the terminal device is a radiant floor heating system. (5) The regulation performance of this AHE is better than plate heat exchanger. There is a good linear relationship between the heat output and the primary water flow rate.

1.4 1.35 1.3 1.25 1.2 1.15 1.1

2013-2014 heating season 2014-2015 heating season

1.05 1 2

4

6 8 Flow rate ratio

10

12

Fig. 15 – Effectiveness of the multi-section vertical AHE in a real project.

100

Primary Water Supply 90

Temperature ( )

80 Test data

70 60

Simulation result

Secondary Water Supply

50 40 30

Secondary Water Return Primary Water Return

20 1/25

1/27

1/29

1/31

2/2

2/4

2/6

2/8

2/10

2/12

Date

(a) 65

Primary Water Supply 60

Temperature ( )

55 50

Test data

45

Simulation result

40

Secondary Water Supply

35 30 25

Secondary Water Return Primary Water Return

20 3/26

3/28

3/30

4/1

4/3

4/5

4/7

4/9

Date

(b) Fig. 16 – Temperature simulation and test results: (a) high load; and (b) low load.

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international journal of refrigeration 71 (2016) 69–84

16

First-condensing

Pressure (kPa)

14 12

Second-condensing

10

Third-condensing

8

First-evaporating 6 4 2

Second-evaporating Third-evaporating

0 1/25

1/27

1/29

Test data 1/31

2/2

2/4

Simulation result

2/6

2/8

2/10

2/12

Date

(a) 12

Pressure (kPa)

10 8 6

First-condensing Second-condensing Third-condensing First-evaporating

4 2

Second-evaporating Third-evaporating Test data

0 3/26

3/28

3/30

4/1

4/3

Simulation result 4/5

4/7

4/9

Date

(b) Fig. 17 – Pressure simulation and test results: (a) high load; and (b) low load.

(6) The simulation model is verified using test result. The tested data fit well with simulation result. The multi-section vertical AHE has already been used in several projects in the past two years. This small-scale multisection vertical AHE will be applied in many more projects in Inner Mongolia and Hebei province of northern China. Using the multi-section vertical AHE in a novel district heating system will be a new direction for district heating system development in the future.

Acknowledgments The authors gratefully acknowledge the support from the National Natural Science Foundation of China (No. 51306098, No.

51138005), the Innovative Research Groups of National Natural Science Foundation of China (No. 51521005), and the Independent Research Program from Ministry of Education of China (No. 20151080470).

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