12-01-2011, 08:48 PM
ABSTRACT
A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. Inside the container is a liquid under its own pressure that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Heat pipes have an effective thermal conductivity many thousands of times that of copper. Heat pipes can be built in almost any size and shape
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CONTENTS
CHAPTERS PAGE NO.
1. INTRODUCTION 1
2. HEAT PIPE BASICS 3
3. COMPONENTS OF A HEAT PIPE 6
4. SHAPES AND DESIGNS OF HEAT PIPES 8
5. HEAT PIPES FOR INLET AIR COOLING 10
6. HPHE FOR WASTE HEAT RECOVERY 12
7. CONCEPTUAL SCHEME 16
8. CONCLUSIONS AND FUTURE WORK 19
9. REFERENCES 21
1. INTRODUCTION
Engine Air Pre-cooling
The inlet air temperature of a compressor has a considerable effect
on the engine performance, especially during hot summer climates when high
atmospheric temperatures cause reduction in net power output. After-coolers are
used to cool the air delivered by the compressor for improving engine power
output, but the mass flow rate of air to the turbocharger compressor cannot be
increased by means of after-cooling.
Several methods have been suggested to cool the compressor
inlet air. This is known as engine air pre-cooling. Heat pipes can be a possible
solution to the air inlet temperature cooling. It is proposed that the evaporator of
the heat pipe heat exchanger will remove the heat from the inlet air, while the
heat rejection from the condenser section to the surroundings can be achieved by
a number of ways. One of the ways is by the utilization of waste heat.
A proposed scheme is to use a turbocharger with an engine air
pre-cooling system and a waste heat recovery system making use of the efficient
heat transfer characteristics of the heat pipes.
1.2 Effect of Ambient Air Temperature on performance of engines
. The turbocharger performance and operation, like gas turbines, strongly depends on the ambient air temperature. As the pressure and temperature of the ambient air changes, so does the air density. As the air density increases, as the air mass flow rate increases, which allows the compressor to move a higher mass of air for the same volumetric flow rate. Turbochargers are designed to work at temperatures that optimize the operation of the coupled turbocharger and engine system. During warmer months, a turbocharger may work at off-design conditions because of lower air density, which then moves the operating point away from the point of peak performance.
fig 1.1 Schematic of turbocharger assembly
fig 1.2 A typical performance map for a compressor
2. HEAT PIPE BASICS
2.1 What are heat pipes?
A heat pipe is a device that efficiently transports heat from its one
end to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors.
Heat input at the evaporator vaporizes the working fluid and this vapor travels to the condenser section. Here the latent heat is rejected via condensation. The vapor of the working fluid condenses and the condensate returns to the evaporator by means of capillary action. A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid.
The length of the heat pipe can be divided into 3 parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container. Fins may be attached to the evaporator and the condenser portion to increase heat transfer rate depending upon the application.
fig 2.1 Cut out section of a heat pipe
2.2 Operating principle
A heat pipe operates on a closed two phase cycle. As previously mentioned, there is liquid-vapor equilibrium inside the heat pipe. When the heat is supplied to the evaporator, this equilibrium breaks down as the working fluid evaporates.The generated vapor is at a higher pressure than the liquid and it travels to the condenser section through the vapor space provided. Vapor condenses giving away its latent heat of vaporization to the heat sink. The capillary pressure created in the menisci of the wick pumps the condensed fluid back to the evaporator section.
The cycle repeats and the heat is continuously transported from evaporator to condenser in the form of latent heat of vaporization. When heat is applied to the evaporator, the liquid recedes into the pores of the wick and thus the menisci at the liquid-vapor interface are highly curved. At thecondenser end, the menisci at the liquid-vapor interface are nearly flatduring the condensation. The difference in the curvature of menisci driving force that circulates the fluid against the liquid and vapor pressure losses and body forces such as gravity (if there is an adverse tilt with respect to the ground).
fig 2.2 Heat transfer mechanism in a heat pipe
fig 2.3 Driving forces in the wick structure
3. COMPONENTS OF A HEAT PIPE
3.1 Working fluid
Every heat pipe application has a particular temperature range in which the heat pipe needs to operate. As a rule of thumb, the useful range extends from the point where the saturation pressure is greater than 0.1 atm and less than 20 atm. Below 0.1 atm, the vapor pressure limit may be approached while above 20atm, the container thickness must be increased to a point where the heatpipe operation is limited by the increased thermal resistance.
Water is the most economical and efficient working fluid. In some applications, however, a mixture called glycol water is used as the working fluid.
3.2 Wick structure
The important requirements of a wick structure are listed below:
•Should be compatible with the wick and container material
•High latent heat
•High thermal conductivity
•High surface tension
•Low liquid and vapor viscosities
•Wettability of wick and wall materials
The wick structure in a heat pipe facilitates liquid return to the evaporator from the condenser. The main purposes of the wick are to generate the capillary pressure, and to distribute the liquid around the evaporator section of heat pipe. Figure 6 shows various wick structures. The most commonly used wick structure is a wrapped screen wick. At this point, we will define a parameter called ‘Mesh Number’ which is used to specify a particular wrapped screen wick. It is defined as the number of wires per linear inch, counted from the centerof any wire to a point exactly one inch distant, including the fractional distance between wires thereof. For example a mesh number of 60 per inch can beinterpreted as 60 × 60 mesh wires per inch. A mesh could be square or rectangular mesh.. Obviously, higher mesh number represents a finer grid.
.
fig 3.1 Various wick structures
4. SHAPES AND DESIGNS
The most frequently used shapes on heat pipes are listed below. Keep in mind that many other shapes can be easily tooled.
4.1 Rounds
Simple Bend
d 2 3 4
R 1.5 2 2.6
Return Loop
d 2 3 4
D 8 10 12
Offset
d 2 3 4
l 8 12 14
"h" can be anything
Loop (flat)
d 2 3 4
D 4 5 6
Motion Loops
d ± 0.76 mm
D ± 1.0 mm
l ± 1.0 mm
d3 4 * *
D 3.6 5 6
4.2 Mini Flat
Easy Bend
± 0.5 mm
t 1.3 1.6 2.4
R 0.8 1.0 1.2
Hard Bend t = 2.3 ± 0.5 mm
W 6.2 8..7
R* 12 15
t = 2.0 mm thick ± 0.5 mm
W 3.6 5.1 8.8
R* 4 11 15
t = 1.6 mm thick ± 0.5 mm
W 2.8 4.5 7.0
R* 3 4 15
Change Plane Center
Change Plane Flush
: any angle 0° to 90°
t: any mini flat thickness
5. HEAT PIPES FOR INLET AIR COOLING.
5.1. HEAT EXCHANGER
The solid model of a finned-tube heat pipe heat exchanger is shown in Fig 4.1 that gives a general idea of a heat pipe heat exchanger concept.Staggered arrangement of tubes is shown in this figure. The upper portion of theheat exchanger is the condenser section while the lower portion is the evaporator,placed in the inlet air duct. The inlet air passes through this staggered bank oftubes. The condenser section is shown above the evaporator because we are using gravity assisted heat pipes.
fig 5.1 Heat Pipe Heat Exchanger(gravity assisted)
fig 5.2. Schematic for inlet air cooling and
condenser section heat removal
The ‘Part 1’ illustrated in Fig 4.2 essentially removes the heat from the air. This is achieved by the evaporator section of the HPHE. This heat lost by the ambient air is the heat gained by the condenser section of the HPHE and we will address this heat removal problem as the ‘Part 2’ shown in Figure 13. Figure 14 shows different ways of achieving heat rejection to the surrounding (Part 2). The heat removal by implementing a waste heat recovery system will be discussed in detail in the next chapter.
6. HPHE FOR WASTE HEAT RECOVERY
6.1 Research work on waste heat recovery using heat pipes
Interesting research work has been done to utilize heat pipes in a waste heat recovery application. Akyurt et al. (1994), in their paper on performance of a heat pipe based on waste heat recovery and utilization for a gas-turbine engine, present results of a modeling and simulation study on combined performance of a power system. In this model, water in a steel type two-phase thermo siphon loop extracts waste energy from the stack of the gas turbine and delivers it to the generator of an aqua-ammonia absorption chiller.
Fig6.1.Schematic arrangement for waste heat recovery and utilization system
6.2. Working of Absorption Chillers
Absorption chillers use heat instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.
In the chiller, refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant revaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device. The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water.
Compared with mechanical chillers, absorption chillers have a low coefficient of performance (COP = chiller load/heat input). However, absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat. Vapor compression chillers, by contrast, must be motor- or engine-driven.
Low-pressure, steam-driven absorption chillers are available in capacities ranging from 100 to 1,500 tons. Absorption chillers come in two commercially available designs: single-effect and double-effect. Single-effect machines provide a thermal COP of 0.7 and require about 18 pounds of 15-pound-per-square-inch-gauge (psig) steam per ton-hour of cooling. Double-effect machines are about 40% more efficient, but require a higher grade of thermal input, using about 10 pounds of 100- to 150-psig steam per ton-hour.
A single-effect absorption machine means all condensing heat cools and condenses in the condenser. From there it is released to the cooling water. A double-effect machine adopts a higher heat efficiency of condensation and divides the generator into a high-temperature and a low-temperature generator.
6.3 Condenser Section Cooling with PCM
When any material is heated two things happen. First the material stores energy, which is called sensible heat. Second, the material gets warmer. When the material melts, it changes from a solid to a liquid phase. A large amount of heat is stored at the melting point. The temperature does not increase during the melting process but the heat addition continues during this process. The energy stored in the material is called latent heat.
The amount of energy stored at melting point is called the heat of fusion. When the material freezes, then the latent heat isreleased. This latent heat can be effectively utilized in cooling-heating applications by using phase change materials (PCM).
In the conceptual model discussed so far, the heat pipe heat exchanger is to be used for inlet air cooling of turbocharger compressor. Its condenser section can be cooled by using an absorption chiller.
In desert areas like Arizona and Nevada, the difference between day and night time temperatures is large. During summer, the outside day time temperature can be as high as 105°F while the night time temperature may drop to 50-70°F. These natural conditions can be used to cool the condenser without installing an absorption chiller. Phase change materials may play an important role in achieving this purpose.
6.3.1. Conceptual Scheme for Condenser Section Cooling with PCM
Fig 6.2 shows a conceptual scheme for condenser side cooling with a phase change material. The PCM, in solid state initially, is contained in a box around the heat pipes and is direct contact with the heat pipe surface.
During day operation, the heat pipe will reject its heat to the PCM. The phase change material is adiabatically insulated from the surrounding atmosphere or else it would gain heat from hot outside air. Thus, it can accept heat for at least 8 hours.
During the night, when the outside air temperature drops to 50-60°F, the adiabatic insulation is removed and the PCM is exposed to the outside air. This exposure to cool air for approximately 10-12 hours helps solidify the PCM and prepares it for the next cycle.
fig 6.2. Condenser Section Cooling with PCM
7. CONCEPTUAL SCHEME
7.1 Scheme for Inlet Air Cooling and
Waste Heat Recovery using HPHE
Figure 6.1 shows a conceptual layout of the inlet air cooling waste heat recovery system. For this layout, the HPHE #1 is placed in the inlet air duct to the turbocharger compressor while the HPHE #2 is placed in the turbine exhaust duct for the recovery of waste heat. This system will work as follows.
fig 7.1. Conceptual Scheme for Inlet Air Cooling And Waste Heat Recovery using HPHE
The turbine exhaust will pass through the evaporator section of the HPHE # 2.Condenser section of this HPHE will be placed in the hot water circuit of the indirect type of absorption chiller. The water will gain the heat from the HPHE and will be fed to the generator of the absorption chiller. The chilled water
generated by the absorption chiller will be circulated through the condenser section of the HPHE #1. This is basically a shell and tube heat exchanger arrangement. Depending upon the type of absorption chiller, the cooling liquid will be either glycol water (if it is an aqua-ammonia absorption chiller) or just chilled water (if it is a Li-Br absorption chiller). The heat gained by the cooling liquid is essentially the heat removed by the evaporator section of the HPHE #1,from the inlet air to the compressor.
In the proposed system, the air to be cooled will pass through a staggered bank of finned heat pipes, placed in the air duct, somewhere between the air filter housing and the turbocharger compressor inlet. This cooled air will then enter the compressor. Before the compressed air is supplied to the engine, it will be cooled by the after-cooler. The hot exhaust gas will then expand in the turbine. The turbine exhaust in turn will again pass through the HPHE # 2.
7.2. Cost of a HPHE
Generally, heat pipe heat exchanger units available in the market are produced for air to air heat recovery applications. A prototype of the conceptual model described in this thesis needs to be developed upon availability of funds. An approximate cost estimate for a heat pipe heat exchanger was obtained from Advanced Cooling Technologies Inc. (Lancaster, Pennsylvania) a heat pipe manufacturing company.
According to the previous models for the approximate cost of a HPHE unit will be $3500. Another cost estimate was obtained from Innergytech Inc (Canada) which was based on the air to air recovery units that this company manufactures. According to these specifications, the cost of ThermogainTM( A registered trade name of Innergytech Inc) is $1950.00.
7.3. Advantages
• Very high thermal conductivity of heat pipes results in efficient heat transfer.
• Apart from the kinetic energy, heat contained in the exhaust gases is also made use of, resulting in high fuel efficiency.
• Very efficient in arid climatic conditions where there is a great difference between the day and night temperatures.
• Precludes the engine from climatic variations
7.4. Disadvantages
• High cost ( as high as $ 1950 - $ 3500 )
• The technology is still under research.
• Heat rejection to the atmosphere may not
be practically obtained in all situations
8. CONCLUSIONS AND FUTURE WORK
8.1 Considerations
The considerations in using heat pipes are;
•Installation cost of the indirect fired absorption chiller is higher and it is not readily available in the low capacity units (less than 7 tons). The smallest unit commercially available is 10 tons. However, if this method of cooling is applied to an entire compression station, then the extra capacity is not an issue
•The concept to insert the waste heat recovery HPHE condenser directly into the generator instead of hot water circuit is dismissed since it would require significant design changes.
•While direct-fired absorption chillers are commercially available with an installation cost of about $5,000, these chillers require a continuous supply of natural gas. If these units are installed, then the second HPHE for waste heat recovery is of no use, but in turn the waste heat is not utilized.
•Cooling of the condenser section of heat pipe with PCM is not practical at this
time because of the long hours of operation, the cost associated with the purchase of PCM, and the insulating properties of paraffins may provide resistance to the heat flux.
8.2. Overall Conclusion
The overall conclusion is that a heat pipe air cooling system could be installed that would increase the density of the air entering the compressor. In warmer climates, this system would preclude the engine from operating under high ambient conditions, since the ambient air temperature would be maintained constant throughout the warmer seasons. If a heat pipe system is to be installed, at present the best technology is with a heat recovery absorption unit.
Of course, if extra cooling capacity is available via evaporative cooling systems in dry climates, this extra capacity could be used to cool the heat pipes, significantly lowering the equipment cost.
8.3. Future Work
Future work could be developing a prototype of an inlet-air cooling HPHE based on the cost estimation given in the previous chapter. If the PCM slurry cooling is to be done, the research work must be done on micro-encapsulated PCM slurry cooling.
9. REFERENCES
1. Ait-Ali, Mohand A, 1996, “Optimum Power Boosting of Gas Turbine Cycles With Refrigerated Inlet Air and Compressor Intercooling” Proceedings of the 1996 International Gas Turbine and Aeroengine Congress & Exhibition, Jun 10-13,Burmingham, UK, 96-GT-393
2. Anderson, William G., Hoff, Sandra, Winstanley, Dave ,1993, “Heat pipe cooling of turbo shaft engines”, American Society of Mechanical Engineers, GT-220, 12p
3. Faghri Amir, 1995, Heat Pipe Science and Technology, Taylor & Francis.
4. Peterson G.P., 1994, An Introduction to Heat Pipes, John Wiley and Sons, Inc.
5. http://lanl.gov/orgs/pa/science21
A heat pipe consists of a sealed aluminum or copper container whose inner surfaces have a capillary wicking material. Inside the container is a liquid under its own pressure that enters the pores of the capillary material, wetting all internal surfaces. Applying heat at any point along the surface of the heat pipe causes the liquid at that point to boil and enter a vapor state. When that happens, the liquid picks up the latent heat of vaporization. The gas, which then has a higher pressure, moves inside the sealed container to a colder location where it condenses. Thus, the gas gives up the latent heat of vaporization and moves heat from the input to the output end of the heat pipe. Heat pipes have an effective thermal conductivity many thousands of times that of copper. Heat pipes can be built in almost any size and shape
[attachment=8182]
CONTENTS
CHAPTERS PAGE NO.
1. INTRODUCTION 1
2. HEAT PIPE BASICS 3
3. COMPONENTS OF A HEAT PIPE 6
4. SHAPES AND DESIGNS OF HEAT PIPES 8
5. HEAT PIPES FOR INLET AIR COOLING 10
6. HPHE FOR WASTE HEAT RECOVERY 12
7. CONCEPTUAL SCHEME 16
8. CONCLUSIONS AND FUTURE WORK 19
9. REFERENCES 21
1. INTRODUCTION
Engine Air Pre-cooling
The inlet air temperature of a compressor has a considerable effect
on the engine performance, especially during hot summer climates when high
atmospheric temperatures cause reduction in net power output. After-coolers are
used to cool the air delivered by the compressor for improving engine power
output, but the mass flow rate of air to the turbocharger compressor cannot be
increased by means of after-cooling.
Several methods have been suggested to cool the compressor
inlet air. This is known as engine air pre-cooling. Heat pipes can be a possible
solution to the air inlet temperature cooling. It is proposed that the evaporator of
the heat pipe heat exchanger will remove the heat from the inlet air, while the
heat rejection from the condenser section to the surroundings can be achieved by
a number of ways. One of the ways is by the utilization of waste heat.
A proposed scheme is to use a turbocharger with an engine air
pre-cooling system and a waste heat recovery system making use of the efficient
heat transfer characteristics of the heat pipes.
1.2 Effect of Ambient Air Temperature on performance of engines
. The turbocharger performance and operation, like gas turbines, strongly depends on the ambient air temperature. As the pressure and temperature of the ambient air changes, so does the air density. As the air density increases, as the air mass flow rate increases, which allows the compressor to move a higher mass of air for the same volumetric flow rate. Turbochargers are designed to work at temperatures that optimize the operation of the coupled turbocharger and engine system. During warmer months, a turbocharger may work at off-design conditions because of lower air density, which then moves the operating point away from the point of peak performance.
fig 1.1 Schematic of turbocharger assembly
fig 1.2 A typical performance map for a compressor
2. HEAT PIPE BASICS
2.1 What are heat pipes?
A heat pipe is a device that efficiently transports heat from its one
end to the other. It utilizes the latent heat of the vaporized working fluid instead of the sensible heat. As a result, the effective thermal conductivity may be several orders of magnitudes higher than that of the good solid conductors.
Heat input at the evaporator vaporizes the working fluid and this vapor travels to the condenser section. Here the latent heat is rejected via condensation. The vapor of the working fluid condenses and the condensate returns to the evaporator by means of capillary action. A heat pipe consists of a sealed container, a wick structure, a small amount of working fluid that is just sufficient to saturate the wick and it is in equilibrium with its own vapor. The operating pressure inside the heat pipe is the vapor pressure of its working fluid.
The length of the heat pipe can be divided into 3 parts viz. evaporator section, adiabatic section and condenser section. In a standard heat pipe, the inside of the container is lined with a wicking material. Space for the vapor travel is provided inside the container. Fins may be attached to the evaporator and the condenser portion to increase heat transfer rate depending upon the application.
fig 2.1 Cut out section of a heat pipe
2.2 Operating principle
A heat pipe operates on a closed two phase cycle. As previously mentioned, there is liquid-vapor equilibrium inside the heat pipe. When the heat is supplied to the evaporator, this equilibrium breaks down as the working fluid evaporates.The generated vapor is at a higher pressure than the liquid and it travels to the condenser section through the vapor space provided. Vapor condenses giving away its latent heat of vaporization to the heat sink. The capillary pressure created in the menisci of the wick pumps the condensed fluid back to the evaporator section.
The cycle repeats and the heat is continuously transported from evaporator to condenser in the form of latent heat of vaporization. When heat is applied to the evaporator, the liquid recedes into the pores of the wick and thus the menisci at the liquid-vapor interface are highly curved. At thecondenser end, the menisci at the liquid-vapor interface are nearly flatduring the condensation. The difference in the curvature of menisci driving force that circulates the fluid against the liquid and vapor pressure losses and body forces such as gravity (if there is an adverse tilt with respect to the ground).
fig 2.2 Heat transfer mechanism in a heat pipe
fig 2.3 Driving forces in the wick structure
3. COMPONENTS OF A HEAT PIPE
3.1 Working fluid
Every heat pipe application has a particular temperature range in which the heat pipe needs to operate. As a rule of thumb, the useful range extends from the point where the saturation pressure is greater than 0.1 atm and less than 20 atm. Below 0.1 atm, the vapor pressure limit may be approached while above 20atm, the container thickness must be increased to a point where the heatpipe operation is limited by the increased thermal resistance.
Water is the most economical and efficient working fluid. In some applications, however, a mixture called glycol water is used as the working fluid.
3.2 Wick structure
The important requirements of a wick structure are listed below:
•Should be compatible with the wick and container material
•High latent heat
•High thermal conductivity
•High surface tension
•Low liquid and vapor viscosities
•Wettability of wick and wall materials
The wick structure in a heat pipe facilitates liquid return to the evaporator from the condenser. The main purposes of the wick are to generate the capillary pressure, and to distribute the liquid around the evaporator section of heat pipe. Figure 6 shows various wick structures. The most commonly used wick structure is a wrapped screen wick. At this point, we will define a parameter called ‘Mesh Number’ which is used to specify a particular wrapped screen wick. It is defined as the number of wires per linear inch, counted from the centerof any wire to a point exactly one inch distant, including the fractional distance between wires thereof. For example a mesh number of 60 per inch can beinterpreted as 60 × 60 mesh wires per inch. A mesh could be square or rectangular mesh.. Obviously, higher mesh number represents a finer grid.
.
fig 3.1 Various wick structures
4. SHAPES AND DESIGNS
The most frequently used shapes on heat pipes are listed below. Keep in mind that many other shapes can be easily tooled.
4.1 Rounds
Simple Bend
d 2 3 4
R 1.5 2 2.6
Return Loop
d 2 3 4
D 8 10 12
Offset
d 2 3 4
l 8 12 14
"h" can be anything
Loop (flat)
d 2 3 4
D 4 5 6
Motion Loops
d ± 0.76 mm
D ± 1.0 mm
l ± 1.0 mm
d3 4 * *
D 3.6 5 6
4.2 Mini Flat
Easy Bend
± 0.5 mm
t 1.3 1.6 2.4
R 0.8 1.0 1.2
Hard Bend t = 2.3 ± 0.5 mm
W 6.2 8..7
R* 12 15
t = 2.0 mm thick ± 0.5 mm
W 3.6 5.1 8.8
R* 4 11 15
t = 1.6 mm thick ± 0.5 mm
W 2.8 4.5 7.0
R* 3 4 15
Change Plane Center
Change Plane Flush
: any angle 0° to 90°
t: any mini flat thickness
5. HEAT PIPES FOR INLET AIR COOLING.
5.1. HEAT EXCHANGER
The solid model of a finned-tube heat pipe heat exchanger is shown in Fig 4.1 that gives a general idea of a heat pipe heat exchanger concept.Staggered arrangement of tubes is shown in this figure. The upper portion of theheat exchanger is the condenser section while the lower portion is the evaporator,placed in the inlet air duct. The inlet air passes through this staggered bank oftubes. The condenser section is shown above the evaporator because we are using gravity assisted heat pipes.
fig 5.1 Heat Pipe Heat Exchanger(gravity assisted)
fig 5.2. Schematic for inlet air cooling and
condenser section heat removal
The ‘Part 1’ illustrated in Fig 4.2 essentially removes the heat from the air. This is achieved by the evaporator section of the HPHE. This heat lost by the ambient air is the heat gained by the condenser section of the HPHE and we will address this heat removal problem as the ‘Part 2’ shown in Figure 13. Figure 14 shows different ways of achieving heat rejection to the surrounding (Part 2). The heat removal by implementing a waste heat recovery system will be discussed in detail in the next chapter.
6. HPHE FOR WASTE HEAT RECOVERY
6.1 Research work on waste heat recovery using heat pipes
Interesting research work has been done to utilize heat pipes in a waste heat recovery application. Akyurt et al. (1994), in their paper on performance of a heat pipe based on waste heat recovery and utilization for a gas-turbine engine, present results of a modeling and simulation study on combined performance of a power system. In this model, water in a steel type two-phase thermo siphon loop extracts waste energy from the stack of the gas turbine and delivers it to the generator of an aqua-ammonia absorption chiller.
Fig6.1.Schematic arrangement for waste heat recovery and utilization system
6.2. Working of Absorption Chillers
Absorption chillers use heat instead of mechanical energy to provide cooling. A thermal compressor consists of an absorber, a generator, a pump, and a throttling device, and replaces the mechanical vapor compressor.
In the chiller, refrigerant vapor from the evaporator is absorbed by a solution mixture in the absorber. This solution is then pumped to the generator. There the refrigerant revaporizes using a waste steam heat source. The refrigerant-depleted solution then returns to the absorber via a throttling device. The two most common refrigerant/ absorbent mixtures used in absorption chillers are water/lithium bromide and ammonia/water.
Compared with mechanical chillers, absorption chillers have a low coefficient of performance (COP = chiller load/heat input). However, absorption chillers can substantially reduce operating costs because they are powered by low-grade waste heat. Vapor compression chillers, by contrast, must be motor- or engine-driven.
Low-pressure, steam-driven absorption chillers are available in capacities ranging from 100 to 1,500 tons. Absorption chillers come in two commercially available designs: single-effect and double-effect. Single-effect machines provide a thermal COP of 0.7 and require about 18 pounds of 15-pound-per-square-inch-gauge (psig) steam per ton-hour of cooling. Double-effect machines are about 40% more efficient, but require a higher grade of thermal input, using about 10 pounds of 100- to 150-psig steam per ton-hour.
A single-effect absorption machine means all condensing heat cools and condenses in the condenser. From there it is released to the cooling water. A double-effect machine adopts a higher heat efficiency of condensation and divides the generator into a high-temperature and a low-temperature generator.
6.3 Condenser Section Cooling with PCM
When any material is heated two things happen. First the material stores energy, which is called sensible heat. Second, the material gets warmer. When the material melts, it changes from a solid to a liquid phase. A large amount of heat is stored at the melting point. The temperature does not increase during the melting process but the heat addition continues during this process. The energy stored in the material is called latent heat.
The amount of energy stored at melting point is called the heat of fusion. When the material freezes, then the latent heat isreleased. This latent heat can be effectively utilized in cooling-heating applications by using phase change materials (PCM).
In the conceptual model discussed so far, the heat pipe heat exchanger is to be used for inlet air cooling of turbocharger compressor. Its condenser section can be cooled by using an absorption chiller.
In desert areas like Arizona and Nevada, the difference between day and night time temperatures is large. During summer, the outside day time temperature can be as high as 105°F while the night time temperature may drop to 50-70°F. These natural conditions can be used to cool the condenser without installing an absorption chiller. Phase change materials may play an important role in achieving this purpose.
6.3.1. Conceptual Scheme for Condenser Section Cooling with PCM
Fig 6.2 shows a conceptual scheme for condenser side cooling with a phase change material. The PCM, in solid state initially, is contained in a box around the heat pipes and is direct contact with the heat pipe surface.
During day operation, the heat pipe will reject its heat to the PCM. The phase change material is adiabatically insulated from the surrounding atmosphere or else it would gain heat from hot outside air. Thus, it can accept heat for at least 8 hours.
During the night, when the outside air temperature drops to 50-60°F, the adiabatic insulation is removed and the PCM is exposed to the outside air. This exposure to cool air for approximately 10-12 hours helps solidify the PCM and prepares it for the next cycle.
fig 6.2. Condenser Section Cooling with PCM
7. CONCEPTUAL SCHEME
7.1 Scheme for Inlet Air Cooling and
Waste Heat Recovery using HPHE
Figure 6.1 shows a conceptual layout of the inlet air cooling waste heat recovery system. For this layout, the HPHE #1 is placed in the inlet air duct to the turbocharger compressor while the HPHE #2 is placed in the turbine exhaust duct for the recovery of waste heat. This system will work as follows.
fig 7.1. Conceptual Scheme for Inlet Air Cooling And Waste Heat Recovery using HPHE
The turbine exhaust will pass through the evaporator section of the HPHE # 2.Condenser section of this HPHE will be placed in the hot water circuit of the indirect type of absorption chiller. The water will gain the heat from the HPHE and will be fed to the generator of the absorption chiller. The chilled water
generated by the absorption chiller will be circulated through the condenser section of the HPHE #1. This is basically a shell and tube heat exchanger arrangement. Depending upon the type of absorption chiller, the cooling liquid will be either glycol water (if it is an aqua-ammonia absorption chiller) or just chilled water (if it is a Li-Br absorption chiller). The heat gained by the cooling liquid is essentially the heat removed by the evaporator section of the HPHE #1,from the inlet air to the compressor.
In the proposed system, the air to be cooled will pass through a staggered bank of finned heat pipes, placed in the air duct, somewhere between the air filter housing and the turbocharger compressor inlet. This cooled air will then enter the compressor. Before the compressed air is supplied to the engine, it will be cooled by the after-cooler. The hot exhaust gas will then expand in the turbine. The turbine exhaust in turn will again pass through the HPHE # 2.
7.2. Cost of a HPHE
Generally, heat pipe heat exchanger units available in the market are produced for air to air heat recovery applications. A prototype of the conceptual model described in this thesis needs to be developed upon availability of funds. An approximate cost estimate for a heat pipe heat exchanger was obtained from Advanced Cooling Technologies Inc. (Lancaster, Pennsylvania) a heat pipe manufacturing company.
According to the previous models for the approximate cost of a HPHE unit will be $3500. Another cost estimate was obtained from Innergytech Inc (Canada) which was based on the air to air recovery units that this company manufactures. According to these specifications, the cost of ThermogainTM( A registered trade name of Innergytech Inc) is $1950.00.
7.3. Advantages
• Very high thermal conductivity of heat pipes results in efficient heat transfer.
• Apart from the kinetic energy, heat contained in the exhaust gases is also made use of, resulting in high fuel efficiency.
• Very efficient in arid climatic conditions where there is a great difference between the day and night temperatures.
• Precludes the engine from climatic variations
7.4. Disadvantages
• High cost ( as high as $ 1950 - $ 3500 )
• The technology is still under research.
• Heat rejection to the atmosphere may not
be practically obtained in all situations
8. CONCLUSIONS AND FUTURE WORK
8.1 Considerations
The considerations in using heat pipes are;
•Installation cost of the indirect fired absorption chiller is higher and it is not readily available in the low capacity units (less than 7 tons). The smallest unit commercially available is 10 tons. However, if this method of cooling is applied to an entire compression station, then the extra capacity is not an issue
•The concept to insert the waste heat recovery HPHE condenser directly into the generator instead of hot water circuit is dismissed since it would require significant design changes.
•While direct-fired absorption chillers are commercially available with an installation cost of about $5,000, these chillers require a continuous supply of natural gas. If these units are installed, then the second HPHE for waste heat recovery is of no use, but in turn the waste heat is not utilized.
•Cooling of the condenser section of heat pipe with PCM is not practical at this
time because of the long hours of operation, the cost associated with the purchase of PCM, and the insulating properties of paraffins may provide resistance to the heat flux.
8.2. Overall Conclusion
The overall conclusion is that a heat pipe air cooling system could be installed that would increase the density of the air entering the compressor. In warmer climates, this system would preclude the engine from operating under high ambient conditions, since the ambient air temperature would be maintained constant throughout the warmer seasons. If a heat pipe system is to be installed, at present the best technology is with a heat recovery absorption unit.
Of course, if extra cooling capacity is available via evaporative cooling systems in dry climates, this extra capacity could be used to cool the heat pipes, significantly lowering the equipment cost.
8.3. Future Work
Future work could be developing a prototype of an inlet-air cooling HPHE based on the cost estimation given in the previous chapter. If the PCM slurry cooling is to be done, the research work must be done on micro-encapsulated PCM slurry cooling.
9. REFERENCES
1. Ait-Ali, Mohand A, 1996, “Optimum Power Boosting of Gas Turbine Cycles With Refrigerated Inlet Air and Compressor Intercooling” Proceedings of the 1996 International Gas Turbine and Aeroengine Congress & Exhibition, Jun 10-13,Burmingham, UK, 96-GT-393
2. Anderson, William G., Hoff, Sandra, Winstanley, Dave ,1993, “Heat pipe cooling of turbo shaft engines”, American Society of Mechanical Engineers, GT-220, 12p
3. Faghri Amir, 1995, Heat Pipe Science and Technology, Taylor & Francis.
4. Peterson G.P., 1994, An Introduction to Heat Pipes, John Wiley and Sons, Inc.
5. http://lanl.gov/orgs/pa/science21
