This paper aims to design a special exchanger to recover the exhaust gas heat of marine diesel engines used in small and medium-sized fishing vessels, which can then be used to heat water up to 55°C–85°C for membrane desalination devices to produce fresh water. A new exhaust-gas heat exchanger of fins and tube, with a reinforced heat transfer tube section, unequal spacing fins, a mixing zone between the fin groups and four routes tube bundle, was designed. Numerical simulations were also used to provide reference information for structural design. Experiments were carried out for exhaust gas waste heat recovery from a marine diesel engine in an engine test bench utilizing the heat exchanger. The experimental results show that the difference between heat absorption by water and heat reduction of exhaust gas is less than 6.5%. After the water flow rate was adjusted, the exhaust gas waste heat recovery efficiency was higher than 70%, and the exhaust-gas heat exchanger’s outlet water temperature was 55°C–85°C at different engine loads. This means that the heat recovery from the exhaust gas of a marine diesel engine meets the requirement to drive a membrane desalination device to produce fresh water for fishers working in small and medium-sized fishing vessels.
The number of fishing vessels in China ranks first globally. As the main power source of the ships, marine diesel engines have a thermal efficiency of 40% to 45% [
Many types of research have previously been carried out on the use of waste heat from diesel engines. Lu et al. [
Much of the prior research focused on waste heat recovery for diesel engine exhaust gas for large vessels; the heat recovery efficiency was relatively low. However, there were few studies on producing fresh water by using the waste heat of small and medium-sized fishing vessels. Large vessels usually use a Waste Heat Recovery (WHR) steam plant to recover heat from the exhaust gas, characterized mainly by a Heat Recovery Steam Generator (HRSG), and generally including a steam turbine, in some cases a power gas turbine, pumps, heat exchangers, as well as electric machinery [
Parameters | Numerical values |
---|---|
Engine type | Inline, water-cooled, four-stroke, supercharged intercooled, 6-cylinder |
Bore × Stroke (mm) | 138 × 168 |
Total stroke volume (L) | 15 |
Compression ratio | 17 |
Rated speed (rpm) | 1500 |
Rated power (kW) | 295 |
Maximum power (kW) | 324.5 |
Fuel consumption rate (g/kW·h) | ≤232 |
The marine diesel engine rotation speed and load are controlled by an NCK2000 cabinet. One PT-1000 sensor is used to measure the exhaust gas temperature at point E, and four PT-100 sensors are used to measure the water temperatures at points a, b, c to d. A CKLWGYC-D40 turbine flowmeter and DN50-200 insertion exhaust gas flowmeter are used to measure the external circulating water flow rate and exhaust gas mass flow rate, respectively. The specific parameters of the experimental equipment are shown in
Parameters | Range | Accuracy |
---|---|---|
NCK2000 cabinet | 500–7500 rpm | ±10 rpm |
PT-100 sensor | −50°C–200°C | ±0.1°C |
PT-1000 sensor | 0°C–1300°C | ±0.5°C |
CKLWGYC-D40 flowmeter | 0–20 m³/h | ±0.5% |
DN50-200 flowmeter | 11–11310 m³/h | ±1.5% |
There is a strong non-linearity and time lag between the cooling water system and exhaust gas emission system [
As the marine diesel engine is equipped with a double-pipe heat exchanger, the external circulating cooling water will take away a part of the exhaust gas heat and reduce exhaust gas outlet temperature through the double-pipe heat exchanger. Therefore, it is necessary to find the appropriate external circulating cooling water flow rate, which can reduce heat loss of exhaust gas and make the diesel engine operate stably simultaneously. And then, the exhaust-gas heat exchanger can recover more heat from the exhaust gas to drive the membrane desalination device.
With the changes in external circulating water flow rate, the exhaust gas temperature at point E and water temperature at point c, the inlet of internal plate heat exchanger in internal circulating water, are shown in
Load (%) | External circulation water (m³/h) | |||||
---|---|---|---|---|---|---|
10 | 13 | 18 | 10 | 13 | 18 | |
Turbine outlet temperature at E point (°C) | Water temperature at c point (°C) | |||||
100 | 374 | 337 | 330 | 83 | 75 | 72 |
90 | 354 | 322 | 320 | 82 | 75 | 71 |
80 | 336 | 309 | 303 | 82 | 74 | 70 |
70 | 318 | 294 | 284 | 81 | 74 | 70 |
60 | 297 | 275 | 268 | 81 | 73 | 69 |
50 | 276 | 257 | 247 | 80 | 72 | 68 |
25 | 211 | 201 | 184 | 76 | 70 | 65 |
Note: External circulation water flow rate is 10, 13, 18 m³/h; turbine outlet temperature at point E; water temperature at point c, which is the inlet of internal plate heat exchanger in internal water circuit.
In
In order to protect the water quality of the membrane desalination device, an additional seawater heat exchanger is added, which can transfer heat to the membrane desalination device from the hot water of the exhaust-gas heat exchanger. The membrane desalination device to be driven requires 30 kW of heat. The seawater heat exchanger’s heat exchange efficiency is about 80% [
The method of the exhaust gas heat recovery begins with the transferring of exhaust gas heat to the circulating water in the heat exchanger. The fin and tube structure heat exchanger is suitable for gas-liquid heat transfer [
The flow medium in heat exchanger consists of hot fluid and cold fluid. The hot fluid is high-temperature exhaust gas, and the cold fluid is liquid water. The maximum recoverable heat
Logarithmic temperature difference of counter-flow heat exchanger:
The overall heat transfer coefficient of the heat exchanger can be calculated as follows [
Heat transfer area:
In order to verify the rationality of the 2D model structure and provide reference information for the structural design of the exhaust-gas heat exchanger, this paper uses CFD (fluent) software to analyze the internal flow field of the heat exchanger model. Due to the symmetry inside the heat exchanger, half of the heat exchanger is taken for flow field simulation analysis. The numerical calculations for this study will use
Continuity equation:
Momentum equation:
Turbulent kinetic energy
Turbulent kinetic energy dissipation rate
The inlet condition of the heat exchanger model adopts velocity inlet condition, that is, given velocity, turbulent kinetic energy, and dissipation rate. The heat exchanger inlet exhaust gas velocity is set to 50 m/s, and the temperature is set to 350°C. In addition, due to the range of exhaust gas Reynolds number between the simulated fin and tube is 2.8 × 104–5 × 104, which belongs to high Reynolds number turbulence, and the
Mesh divisions are taken to the exhaust-gas heat exchanger with and without a reinforced heat transfer tube section. The 2D model of the heat exchanger is meshed four times to ensure the accuracy of the simulation. The mesh number of the exhaust-gas heat exchanger with and the without reinforced heat transfer tube section are 679375, 541659, 389925, 277723, and 651096, 534670, 374975, 252128, respectively. The exhaust gas velocity is monitored at six equally spaced points in the model X = 0.53 m and Y in the range of 0.05–0.30 m. The exhaust gas velocity at different mesh numbers is shown in
The gas flow simulation is carried out to provide reference information to the structural design of the exhaust-gas heat exchanger. The mesh between the exhaust-gas heat exchanger with reinforced heat transfer tube section and the without is shown in
Based on the above results of the flow field in fins, an exhaust-gas heat exchanger combining unequally spaced fins and a reinforced heat transfer tube section. The 3D structure of the heat exchanger is shown in (1) A reinforced tube unit with a 225 mm peripheral dimension is added at 115 mm in front of the fin and tube heat exchanger unit, which can disturb the flow of exhaust gas and heat the water from four distributors uniformly. (2) The fin part adopts an unequal spacing arrangement to increase the resistance of the middle fins to the exhaust gas and increase the exhaust gas flow rate on the outer fins of the heat exchanger. (3) A mixing zone is set up between the fin groups to enhance the heat transfer effect of the rear half of the fins. (4) The cold fluid in the tube bundle is divided into four routes, which can enhance the uniformity of heat transfer.
Heat exchanger type | Fin and tube counter flow, |
---|---|
Height of fins (m) | 0.6 |
Tube pitch (m) | 0.05 |
Length of the heat exchanger (m) | 1.06 |
Mass of the heat exchanger (kg) | 80 |
The exhaust gas temperature and circulating water temperature of the exhaust-gas heat exchanger were monitored at 25%–100% marine diesel engine loads with the circulating water flow rate of 1–3 m3/h. The temperature difference between exhaust gas and that of circulating water through the exhaust-gas heat exchanger are shown in
According to the average specific heat capacity of the exhaust gas, the recovered heat of the exhaust gas,
As a load of the marine diesel engine increases, the heat of the circulating water and the exhaust gas increases, and the heat of the circulating water side is a little bit less than the exhaust gas side. This means there is a heat loss in the exhaust-gas heat exchanger.
When the circulating water flow rate is 1 m³/h, the proportion of heat difference between the circulating water side and exhaust gas side is large. This means that the heat loss of the exhaust gas side is higher when the circulating water flow rate is small. When the circulating water flow rate is 3 m³/h, the proportion of heat difference between the circulating water side and exhaust gas side becomes small. This means that the heat loss rate of the exhaust gas side is low when the circulating water flow rate is high. Based on the study of these contents, it can provide a reference for improving the heat recovery rate of the exhaust-gas heat exchanger.
The relationship between the heat difference rate
The heat recovery efficiency, η, of the exhaust-gas heat exchanger can be obtained by considering the heat recovered from the water to the recoverable heat from the exhaust gas, and can be written as:
The heat recovery efficiency of the exhaust-gas heat exchanger is shown in
To meet the requirements of seawater desalination, the outlet water temperature of the exhaust-gas heat exchanger needs to be higher than 55°C and lower than 85°C. In all experimental cases, the inlet water temperature of the exhaust-gas heat exchanger is 20°C. In order to recover more heat from the exhaust gas and match the requirements of seawater desalination, a coupling analysis is carried out for the water flow rate and the outlet water temperature of the exhaust-gas heat exchanger with different engine loads, as is shown in
In order to improve the heat recovery efficiency of the exhaust-gas heat exchanger, it can be achieved by changing the water flow rate.
In this paper, a new heat exchanger was designed for the exhaust gas heat recovery of small and medium-sized marine diesel engines. The exhaust gas heat recovery experiments for the marine diesel engine at 25%–100% load and rated speed were conducted to verify the heat transfer effect. The following conclusions are drawn:
According to the characteristics of the small windward surface and high exhaust gas flow rate in the heat exchanger, a compact exhaust-gas heat exchanger with the fin-tube and a reinforced heat transfer tube section was designed. The internal flow fields of the exhaust-gas heat exchanger with reinforced heat transfer tube section and the without were analyzed by CFD (fluent) software, and the rationality of the heat exchanger structure was verified. From the gas flow simulation, the exhaust gas is disturbed by the reinforced heat transfer tube section, and exhaust gas velocity is slowed down. Due to the resistance of the unequal spaced fins, some of the exhaust gas is diffused to flow into the outer fins and enters the fin and tube section more evenly. The exhaust gas is fully mixed in the mixing zone between the two sets of fins, and the dead zone vortex area between fins is small. The difference between the heat absorption by the water in the exhaust-gas heat exchanger and the heat reduction of exhaust gas is within 6.5%, which ensures the accuracy of the experiment. The outlet water temperature of the exhaust-gas heat exchanger can meet the requirements of the membrane desalination device, and the heat recovered from the exhaust gas ranges from 38.6 to 122.9 kW. With the cooling water flow rate adjusted, the heat recovery efficiency of the exhaust-gas heat exchanger is greater than 70%.
The author sincerely thanked the National Key Research and Development Program of China and the Postgraduate Research & Practice Innovation Program of Jiangsu Province, China, for funding, mainly for research on the exhaust-gas heat exchangers for small and medium-sized marine diesel engines.