Heat Recovery Steam Generator Design

Heat recovery steam generators (HRSGs) are used in power generation to recover heat from hot flue gases (500-600 °C), usually originating from a gas turbine or diesel engine. The HRSG consists of the same heat transfer surfaces as other boilers, except for the furnace. Since no fuel is combusted in a HRSG, the HRSG have convention based evaporator surfaces, where water evaporates into steam. A HRSG can have a horizontal or vertical layout, depending on the available space. When designing a HRSG, the following issues should be considered:

hrsg-boiler
Figure 1: Schematic of a HRSG boiler
  • The pinch-point of the evaporator and the approach temperature of the economizer
  • The pressure drop of the flue gas side of the boiler
  • Optimization of the heating surfaces

The pinch-point (the smallest temperature difference between the two streams in a system of heat exchangers) is found in the evaporator, and is usually 6-10 °C, which can be seen in Figure 2. To maximize the steam power of the boiler, the pinch-point must be chosen as small as possible. The approach temperature is the temperature difference of the input temperature in the evaporator and the output of the economizer. This is often 0-5 °C. The pressure

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Figure 2: Example of a heat load graph for HRSG boiler

drop (usually 25-40 mbar) of the flue gas side also has an effect on the efficiency of power plant. The heat transfer of the HRSG is primarily convective. The flow velocity of the flue gas has an influence on the heat transfer coefficient. The evaporator of heat recovery boiler can be of natural or forced circulation type. The heat exchanger type of the evaporator can be any of parallel-flow, counter-flow or cross-flow. In parallel-flow arrangement the hot and cold fluids move in the same direction and in counter-flow heat exchanger fluids move in opposite direction.

 

Can 1D Tools be Used to Design an HVAC System?

The heating, ventilation, and air-conditioning (HVAC) system is arguably the most complex system that is installed in a house and it is responsible for a substantial amount of the total house energy used. A right-sized HVAC system will provide the desired comfort and will run efficiently. Right-sizing of a HVAC system is the selection of equipment and the designing of the air distribution system to meet the accurate predicted heating and cooling loads of the house. Rightsizing the HVAC system begins with an accurate understanding of the heating and cooling loads on a space, however, a full HVAC design involves more than just the load estimate calculation as this is only the first step of the iterative HVAC design procedure. Heating and cooling loads are dependent on the building location, sighting, and the construction of the house, whereas the equipment selection and the air distribution design are dependent upon the loads and each other.

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Figure 1: A 3D model representing the HVAC          ducting in a building.
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Figure 2: Ducts layout for HVAC

The initial design iteration starts with tools such as AxCYCLE™, the thermodynamic design, analysis and optimization tool offered by SoftInWay Inc., based on which the initial specification for the equipment selection is obtained. Furthermore, it is more important to study the distribution of the air into the building to further refine the design iterations. Figure 1 shows the ducting layout in a typical building. The ducting design is a major activity not only from the point of pressure loss but also to ensure the air flows into the room effectively.

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Figure 3: Design and Analysis of the                            HVAC ducts in AxSTREAM® NET.

Figure 2 shows the ducting layout with the air discharge ports. The branching of the duct and the location of the discharge ports affect the cooling in the room which needs to be analyzed in detail. However, to begin the initial design process, it is more appropriate to use a 1D tool such as AxSTREAM® NET which can be used to model the flow through the duct and estimate the pressure loss and cooling effectiveness based on whether the thermodynamic cycle can be further fine-tuned iteratively before any detailed 2D/3D analysis is performed.

To learn more about how AxSTREAM® NET can be used for HVAC design and analysis, please write to info@softinway.com.

Importance and Modelling of Internal Combustion Engine Cooling Systems

In an internal combustion engine, combustion of air and fuel takes place inside the engine cylinder and hot gases are generated with temperature of gases around 2300-2500°C which may result in not only burning of oil film between the moving parts, but also in seizing or welding of the stationery and moving components. This temperature must be reduced such that the engine works at top efficienc,  promoting high volumetric efficiency and ensuring better combustion without compromising the thermal efficiency due to overcooling. Most importantly, the engine needs to function both in the sense of mechanical operation and reliability. In short, cooling is a matter of equalization of internal temperature to prevent local overheating as well as to remove sufficient heat energy to maintain a practical overall working temperature.

It is also important to note that about 20-25% of the total heat generated is used for producing brake power (useful work). The cooling system should be designed to remove 30-35% of total heat and the remaining heat is lost in friction and carried away by exhaust gases.

The design of cooling systems depends on whether the engine is air cooled or liquid cooled. Air cooling is generally used in small engines wherein fins or extended surfaces are provided on the cylinder walls, cylinder head, and so on. Heat generated due to combustion in the engine cylinder will be conducted to the fins and when the air flows over the fins, heat will be dissipated to air. The amount of heat dissipated to air depends upon: Amount of air flowing through the fins, fin surface area and the thermal conductivity of metal used for fins.

In water cooling methods, cooling water jackets are provided around the cylinder, cylinder head, valve seats and so on. When the water is circulated through the jackets, it absorbs heat of combustion. This hot water will then be cooling in the radiator partially by a fan and partially by the flow developed by the forward motion of the vehicle. The cooled water is again recirculated through the water jackets either through a pump or thermos-siphon which is based on the principle of density difference in working fluid.

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Figure 1: Cooling water ports in an IC engine cylinder block

Figure 1. shows the cooling water jacket for an IC engine cylinder block. The engine cooling jacket is of complex shape and is influenced by many factors including the shape of the engine block and optimal temperature at which the engine runs. A large cooling jacket would be effective in transporting heat away from the cylinders, but makes the engine bulky and heavier. The cooling water jacket needs to be optimized considering both the cooling effectiveness and engine weight. Hence the flow through the cooling jacket needs to be optimized from the inlet to the outlet covering the lengthwise along the geometry as well as traversing from cylinder block to the head. The optimization is done with the objective of minimizing the fluid pressure loss between inlet and outlet and obtains even distribution of the flow to each cylinder in the engine block and uniform velocities along its flow.

The engine cooling jacket is of complex geometry and performing 3D simulation over this is quite a complex task involving generating the 3D geometry with all the intricate details and preparing the model for performing conjugate heat transfer analysis. As an initial step it is advisable to perform a simple 1D heat and flow network analysis to obtain the heat transfer distribution and data for creating the 3D model using commercial tools such as AxSTREAM NET™.

To know more about how AxSTREAM NET™ can simplify engine cooling system design and analysis, please write to info@softinway.com.

Using 1D Models to Predict the Thermal Growth and Stresses During The Start up and Shutdown Phase of a Steam Turbine

Steam turbines are not just restricted to conventional or nuclear power plants, they are widely used in combined cycle power plants, concentrated solar thermal plants and also geothermal power plants. The operational requirements of a steam turbine in the combined cycle and CSP’s means that they operate under transient conditions. Even in conventional steam turbines, the market requirements are changing with requirements for faster and more frequent start-up which can result into faster deterioration of the equipment and reduced lifespan. During the startup phase, significant heat exchange takes place between the steam and the structural components that include the valves, rotor and casing. The accuracy of the life prediction is strongly affected and dependent on the accuracy of the transient thermal state prediction [1].

Though the expansion of steam takes place in the nozzles and blades, the influence of the leakage steam during the startup phase is significant with steam expanding through the labyrinths resulting in expansions, condensation, and increased velocities which may even reach supersonic levels. During cold start, the flow is minimal, the temperature of the metal is at room temperature and heat exchange happens between the steam and metal parts resulting in thermal stress.

Every designer is interested in making a prediction that is as accurate as possible. This requires modelling the entire flow path with all the intricate details which means generating a complex 3D model,use of extensive computational resources and so on, which ultimately results in more time and cost. Even if one has the luxury of using a complex 3D model with all the intricate details, the question is how to get the appropriate boundary conditions to be applied for the 3D simulation and how to reduce the number of iterations required between the flow analysis and structural analysis. The flow parameters and the area between sealing fins in the labyrinth (refer Fig.1) is varied, not only in each stage, but also within each component which means the heat transfer coefficient being applied to each of these locations is also different.

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   Figure 1: Sealing fins in a turbine stage

 

As seen in Fig. 2, the entire flow through the seals and cavities can be modelled as a 1D model considering both the flow and heat transfer between the fluid and metal. During design phase of the flow path, a streamline analysis tool such as AxSTREAM® can give the leakage flow parameters which can be further detailed using AxSTREAM® NET, the 1D flow and heat transfer module. The results from the 1D module, which gives the thermodynamic parameters and heat transfer coefficients at different zones in the seal, can be used to apply the boundary conditions for performing thermos-structural analysis of the steam turbine.

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                    Figure 2: 1D model of the flow and heat transfer in sealing fins of a turbine stage

 

To learn more about how the AxSTREAM® platform can be used for designing steam turbines, from concept to optimizing operating curves, please contact info@SoftInWay.com

 

 

Optimizing the Cooling Holes in Gas Turbine Blades

To increase the overall performance of the engine and reduce the specific fuel consumption, modern gas turbines operate at very high temperatures. However, the high temperature level of the cycle is limited by the melting point of the materials. Therefore, turbine blade cooling is necessary to reduce the blade metal temperature to increasing the thermal capability of the engine. Due to the contribution and development of turbine cooling systems, the turbine inlet temperature has doubled over the last 60 years.

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Figure 1: Variations of Thermal Efficiency with TIT [1]
The cooling flow has a significant effect on the efficiency of the gas turbine. It has been found that the thermal efficiency of the cooled gas turbine is less than the uncooled gas turbine for the same input conditions (see figure 1). The reason for this is that the temperature at the inlet of turbine is decreased due to cooling and therefore, work produced by the turbine is slightly decreased. It is also known that the power consumption of the cool inlet air is of considerable concern since it decreases the net power output of  the gas turbine.

With this in mind, during  the design phase of gas turbine it is very important to optimize the cooling flow if you are considering both the performance and reliability. Cooled Gas turbine design is quite complicated and requires not only the right methodology, but also the most appropriate design tools, powerful enough to predict the results accurately from thermodynamics cycle to aerothermal design, ultimately generating the 3D blade.

Different cooling methods that are employed depend on the extent of the cooling required. The cooling flow passes through several loops internally and is then ejected over the blade surface to mix with the main flow. The mixing of the cooling flow with the main flow alters the aerodynamics of the flow within the turbine cascade. The cooling flow that is injected into the main flow needs to be optimized, not only in terms of thermodynamic parameters, but also  in terms of the locations to ensure the turbine vanes and blade surfaces are maintained well below the melting surface. The spacing between the holes, both in horizontal and vertical direction, affects not only the surface temperature of the blade, but also the strength of the blade and its overall life.

Performing a 3D analysis for optimizing the flow, spacing, and location of cooling flow is computationally expensive. A 1D flow and heat network simplifies the task of not only arriving at the optimal configuration of cooling holes and location, but also in aerothermal design of the gas turbine flow path and generation

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AxSTREAM NET

of the optimized 3D blades with reduced overall design cycle time. Designers are faced with the challenge of simplifying the complex 3D cooled blade and accurately modelling it. AxNET, the module for 1-dimensional flow and heat transfer provides designers options to not only use the different components and models inbuilt in the module but also customize to represent the 3D blade as accurately as possible in a simplified approach

 

To learn more about how AxSTREAM Net can help you optimize the cooling holes in gas turbine blades, please contact us at sales@softinway.com or info@softinway.com.

Source:

[1] Amjed Ahmed Jasim Al-Luhaibi and Mohammad Tariq, “Thermal Analysis of cooling effect on gas turbine blade” International Journal of Research in Engineering and Technology, eISSN: 2319-1163, pISSN:2321-7308

Axial and Mixed Pump Theory

Axial Pump
Axial Pump

Unlike the centrifugal pump, the performance in axial machines is a function of the action of the blade profiles. Because of this, the main approach in design of axial pumps is focused on blade performance.

Impeller blades of axial flow pumps have a double curvature form at the inlet and at the outlet due to the change in diameter from hub to periphery. Absolute flow before and after the impeller and relative flow along the impeller passage are axisymmetric and potential. There is no radial mixing. Under this condition, each streamline is parallel to the axis of the pump. Fluid passes parallel to the pump axis i.e., along the streamline. Continue reading “Axial and Mixed Pump Theory”

What Are Some Factors Affecting Gas Turbine Operation?

Let’s face it, we know the operations of our gas turbines can’t all be perfect, and we’ll run through calculations, feasibility studies and more to pinpoint the exact cause. But before all of that is accomplished, you should keep a list in the back of your mind of what might be causing your loss in performance, based on common factors that affect gas turbine efficiency and more.

Here’s the list. Continue reading “What Are Some Factors Affecting Gas Turbine Operation?”