CFD Assessment and HVAC System Design for Buildings

HVAC Building

Every good HVAC (Heating, Ventilation and Air Conditioning) system design must be able to ensure a proper air renovation inside the building it is installed at the lowest possible cost. This objetive is reached through the correct positioning and orientation of the supply and extraction air systems, and an adequate value of air changes per hour, also called air change rate. The use of CFD to assess and optimize the installation’s design is paramount in order to reach the best results.

There are different measurement parameters that allow the evaluation of the ventilation system quality:
First of all, the above-mentioned number of air changes per hour (ACPH) in the building. It is defined as the rate of the air volume added to a finite space in one hour divided by that spaces volume. If a perfectly mixed model is considered, the air renovations per hour parameter is a measure of how many times the air in this space is replaced each hour.

This parameter, despite being useful, may not represent completely how renewed is the air inside the space, since the disposal of the ventilation systems, as well as the elements inside the space, can generate flow patterns that make some areas be under ventilated. For this reason, it is also useful using the concept of age of air.

The local mean age of air is defined as the time that particles contained in a differential volume around a point (as is in the case in a cell of a CFD simulation) have been inside the space. 

If it is assumed that the age at the inlets is 0 (which equals to say that pumped air is completely new), this parameter evaluates the residence time that particles spend in the building from they enter until leaving.

Air Age Contours

This value does help assessing whether areas are renewing the air more frequently, since the residence time in them is lower. From this value, it is possible to obtain the mean age of air (MAA) in the building, which is calculated as the average of the local mean age for each point in the space.
The efficiency (ε) of the ventilation system is defined as the ratio between the minimum time that a particle spends in the space from the input to the output, and twice the mean age of air. This concept allows to seek a balance between a good air quality and a reduced air flow. 

HVAC Efficiency Formula

In addition, the evaluation of the minimum residence time enables the possibility of tracking for short-circuits of new air in the system. Along with this parameter and assuming a complete mixing model it can be stablished that:

  1. The optimal value is obtained at ε=50%
  2. For ε<50%, it can be considered that the space is lack of ventilation, or that the minimum residence time is too low, indicating a possible air short-circuit
  3. For ε>50%, it can be considered that the air Flow ratio is over dimensioned and it is advisable to decrease it

CFD techniques allow not only to determine the age of air in a room. It is also possible to introduce pollutant emission sources, chemical reactions and thermal effects in the models. This way, the pollutant concentration in the area of study can be analyzed. This measure can provide an even more valuable information than the age of air in terms of predicting more accurately which areas of the room are not being properly renovated, and optimize the HVAC installation from the pre-design stage. 

The concentration of a chemical compound in a large volume of air is often measured in milligrams of compound per cubic meter of air (in some cases per kilograms) giving a unit of particles per million (ppm).

To put this in perspective, let’s consider a Waste Water Treatment Plant (WWTP). These facilities are attached to a series of emissions of contaminants, perhaps the more characteristic being the hydrogen sulfide (H2S), produced during the decomposition of some amino acids, as well as the reduction of sulfates to sulphites by certain microorganisms.
Hydrogen sulfide is a colorless, flammable gas with an atomic weight of 34 g/mol and a density of about 1.5 kg/m3 at ambient conditions, being slightly heavier than air, tending to accumulate near the ground. Its odor is very unpleasant and it is associated to “rotten eggs”.

Contaminant Contours

The limit of perceptibility for human beings is 0.02 ppm, with some people being able to detect it at 0.0005 ppm. At higher concentrations can result toxic, produce metabolic changes and even cause death. In addition to this, it is one of the main causes of corrosion in this type of facilities, attacking in moist ambiences to iron and concrete with ease.
The dispersion effect that the air impulsion from the HVAC system can cause over these particles can be properly analyzed with a CFD model, along with the study of alternatives in the flow inputs, system location and effect of the different sources. The SDEA_Engineering team presents a broad experience in the HVAC and Computational Fluid Dynamics field and can help in the design assessment and study of the HVAC system.

Bellows Expansion Joint Analysis using FEA

FSE Drawing

This article explains the use of advanced FEA analysis for evaluating the structural behaviour of Economizer Heat Exchanger equipment and compares the designs obtained through different design codes.

This example is a heat exchanger with a bellow expansion joint and the reason of performing this study is because of a transition zone between two parts with different thickness, in order to assess the failure risk in that zone. On one hand, the heat exchanger design has been performed according to ASME VIII Div. 1 code using design by rules and shell thickness value obtained is 40 mm. On the other hand, the design of expansion joint has been performed according to Standards of the Expansion Joint Manufacturers Association (EJMA) and the minimum required thickness obtained is 30 mm. Is there a discrepancy? Keep reading if you are interested in finding what is going on. 

The SDEA advanced engineering team put their hands on the problem using design by analysis on the bellow design. 

Fixed Tubesheet Heat Exchanger

Flexible Shell Elements (FSE) are often used in fixed tubesheet heat exchanger in order to reduce shell and tube longitudinal stresses or tube-to-tubesheet joint loads. Ligth gauge bellows type expansion joints within the scope of the EJMA Standard are not included within the purview of this section.

The Tubular Exchanger Manufacturers Association (TEMA) Code, according to RCB-8 part, provided guidelines for determining stresses using a 2-dimensional Axisymmetric Finite Element Model (FEA) for the FSE or FSE combinations.

According to the problem described above, and considering the guidelines provided by TEMA for modelling FSE, the procedure used to obtain the answer to this problem is exposed in the following lines:

Boundary Conditions for FSE analysis RCB-8.42

At first, the CAD model (2D Axisymmetric geometry of FSE) is meshed considering eight node quadratic axisymmetric elements and the boundary conditions shown in figure RCB-8.42.  The axial translation of the FSE is restringed at the FSE axial plane, pressure of shell side is applied in the inner face of FSE and an axial displacement is applied for stress determination.

In order to evaluate the stress, it’s necessary to establish the minimum number of stress classification lines (SCL) and to compute linearized both membrane and membrane+bending stress intensities at each SCL in order to compare these stress values with the allowable stress limits defined in the design Code.

Stress Classification Lines for Evaluating Stresses RCB-8.62

The methodology for evaluating stresses is not specified in part RCB-8, so the code chosen for this purpose is ASME VIII Divisions 1 and 2.

In the context of bellows expansion joints design, Division 1 provides the allowable stress in bellows, according to part 6 of Mandatory Appendix 26 (Design of U-shaped unreinforced bellows):

  • S1 ≤ S
  • S2 ≤ S 
  • S3 + S4 ≤ KMS

where S1 is the circumferential membrane stress in bellows tangent; S2, circumferential membrane stress in bellows; S3 and S4 are meridional membrane and bending stresses, respectively, all of them obtained due to pressure. S is the allowable stress of bellows material at design temperature. Stresses in bellows due to deflection are only considered in fatigue analysis.

Stress in Bellows

Km is equal to 1.5·Ysm for as-formed bellows and 1.5 for annealed bellows, where Ysm is the yield strength multiplier which depends on the  material. The value of Km varies between 1.5 and 3.0Similarly, EJMA Standard collects the same stresses criteria in 4.13.1 (Design Equations for Unreinforced Bellows).
It is important to remark the fact that the combination of membrane plus bending stress on the convolution peak is compared against the allowable stress, scaled by a factor of 3 in this case. This is paramount and takes into account the hardening effect from the forming manufacture method, where the material goes well beyond yielding, resulting in hardening after the process.

Design Equations for Unreinforced Bellows [EJMA Standard]

On the other hand, according to Table 5.6 of ASME Section VIII Division 2, the next stresses classification should be used, depending on the location and the origin of the stresses:

  • Pm < S
  • PL < S 
  • PL + PB < 1.5S

where Pm is the maximum primary membrane stress; PL, local membrane stress and PB, primary bending stress.

So, in order to analyse the stresses in the different Stress Classification Lines of FSE, stresses classification provided by Division 2 and allowables stresses in bellows provided by Division 1 (and EJMA) are considered.

Stress Classification Lines for Stress Evaluation

Bellows SCLs Contours

Von Mises Stress Contours of FSE

2D-Axisymmetric FEA is performed for the FSE applying the boundary contidions described above and the stress values are evaluated in one of each SCL chosen for this model. According to the criteria exposed previously, table with SCL values is presented and the stress values obtained are below of the allowable ones:

SCL Results

Stress Classification Lines Results

This work was addressed in order to give an answer to the question if there is some discrepancy between design codes used for the design of the heat exchanger (ASME VII Div 1 Code used for calculating the minimum required thickness of the shell and EJMA Code used for mechanical design of bellow expansion joint) and to predict the structural behaviour in the transition zone between both. In this scope, a FEA method was used in order to analyse this problem. In view of the FEA analysis results , it can be reasuring to see the good behaviour of the system and the transition zone between the parts with different thickness: it is safe under the design conditions for different pressure and temperature parameters considered. It’s a good practice to combine Design by Rules with Design by Analysis, using FEA methods in order to validate the design by rules performed beforehand.

Pressure Vessels Design

Stationary Tubesheet Configurations

Pressure Vessels are used in the industry to store process fluids under certain temperature and pressure conditions. Commonly, pressure vessels have cylindrical or spherical shape, can be disposed in a horizontal or vertical way and are provided of heads with different shapes (Ellipsoidal, Hemispherical, Toroidal, etc.). These equipments are fitted with different elements such as nozzles (used to control the entry and/or exit of the fluids), cooling or heating jackets to maintain the temperature of the stored fluid, or other non-pressurized elements capable of supporting and transporting the equipment such as saddles, skirts, legs and lugs. In the SDEA_Engineering solutions team we accumulate ample experience in pressure vessels among other process engineering equipment design.

Design by Rules approach is a widely used method in several industrial sectors. This approach is based on a set of rules that specifies the design method, loads, allowable stress and materials requirements. The objective of design by rules is to determine the minimum thickness of the part considering design pressure, allowable materials and the specific formulas for the component geometry.

The most used and recognized code was proposed by the American Society of Mechanical Engineers (ASME), and is known as ASME Boiler and Pressure Vessel Code (BPVC). Other commonly used codes are EN-13445 (Europe), BS 5500 (UK), CODAP (France) and AD Merkblatt (Germany).

In particular, ASME Section VIII, Division 1 provides formulas for determining thickness and maximum allowable stress of basic components for Pressure Vessels. Equipment design according to this code does not require a detailed assessment of stresses.

The design method uses design pressure, allowable stress and specific formulas according to geometry of the component subject to design. ASME requires that pressure vessels are designed considering the most severe conditions subjected to the vessel, including the normal operation and other conditions such as start-up and shut-down. It’s necessary to have in mind several considerations for the adequate design. One of them is the design pressure, in order to determine it is necessary to know the Maximum Allowable Working Pressure (MAWP) of the component. That is, the maximum pressure permissible at the top of the vessel in its normal operating position at a specific temperature. Other consideration is the Design Temperature: the maximum and minimum design temperatures will determine the maximum (and minimum) allowable stress for the material to be used in the vessel. The minimum design temperature would be MDMT (Minimum Design Metal Temperature). Other important factor is the maximum allowable stress of the material which, as mentioned, depends on the temperature design and these values of stresses include certain safety margins. Also, it’s necessary to take into account an extra thickness in order to avoid the negative effects that may appear due to corrosion. All design considerations for the complete design of a pressure vessel are collected in paragraphs UG-16 to UG-35 of ASME VIII Division 1.

The designer must be familiar with the different failures that may occur and to take them into account in the design stage. These different failure modes are grouped in four categories:

  • Material: Can be due to improper material selection and / or material corrosion, among others.
  • Design: An incorrect input data or design procedure can lead to design failure
  • Fabrication: Due to gross fabrication errors or poor welding qualities
  • Service: Change in service condition or unexpected operations

The four previous categories describe “why” the vessel failure may occur. The failure types are: Elastic deformation, excessive plastic deformation, brittle fracture, stress rupture, plastic instability, high strain, stress corrosion and corrosion fatigue. More details about fatigue failure can be found here

In order to avoid equipment failure, the designer must take into account which loads will be involved in the process: steady loads applied continuously such as internal or external pressure, thermal loads and wind loads among others, are sometimes combined with Non-Steady loads, variable and short duration, like earthquakes, erection, transportation or start up – shut down. In particular, part UG-22 of the mentioned code collects all loadings to be considered in the design of a vessel.

On the other hand, several processes involve cyclic loads and thermal stresses, and other approaches more adequate for these cases are needed. Usually, instead of design by rules, design by analysis is used. This approach is collected by ASME in Section VIII Division 2. The combination of both approaches (Design by rules and Design by Analysis) results in a safer, more accurate and economically efficient design.

Condenser retrofitting relying on CFD

Condenser CFD

Sometimes, pressure equipment life needs to be extended above the initial design requirements and that represents an engineering challenge. Even less frequently, the design point also changes, making a priority to understand if the original design can withsthand the new operational settings without eroding the equipment performance.  

This article provides a rough view of a project developed by SDEA where CFD modelling was used including a multi-phase solver and in-house developed functions to replicate the real behaviour of the condenser operational conditions. 

This condenser was part of a combined cycle power plant using a Rankine cycle. The aim of the low pressure equipment is to reduce the turbine back pressure and condensate the steam before it gets back into the boiler. A large number of tubes are placed inside the condenser, with a cooling fluid [brine in this case] running inside them. They are below the incoming steam dew point that takes energy away, condensing and separating by gravity the created droplets.

Condenser Geometry

Condenser geometry simplified for CFD

Most condensers have been designed using analytic methods, which usually are lumped models that can reproduce more or less the overall performance but can not provide detailed data on the insights of the process. The main drawback for this approach is that small details like the tube bundle geometry or undetected stagnant high-aged fluid are critical if an optimized condenser is aimed at. The main solution is to build laboratory prototypes and do physical test with the associated high costs.

SDEA engineering team developed a comprehensive CFD model to emulate the condensation process with a high level of detail. Nevertheless, the vessel has two tube bundles with 14726 tubes each, resulting in very expensive computational model if  every single tube was to be included in the final CFD model. 

Bearing in mind the previous information, the main objectives of this study were to predict the velocities, temperature, density and pressure drop distribution inside the condenser for different operating conditions: those for the original design and for the upgraded normal conditions.

Due to the time scales involved in this project a simplified strategy was carefully selected after an exhaustive literature review, in order to achieve a good balance between accuracy and model complexity. 

Condenser CFD Mesh

Condenser geometry CFD mesh

In this section the main assumptions will be explained and justified in order to provide a glimpse of not just SDEA’s CFD capabilities, also the physics understanding and the skills needed to carry on with projects like this and to complete all the goals stated in the project. 

The cooling system is composed by two different bundles with more than 14K tubes. Modelling each single tube is unaffordable, so a porous media approach was assumed for this CFD model. This method does not require solving the detailed flow and temperature fields around each tube.

The tube bundle is regarded as a distributed resistance region. The resistance is based in the theory developed by Patakar and Spalding. Many different models have adopted this approach and it proves to match experimental data with reasonable accuracy.

Porous Media CFD

Porous media submodelling

Porous media for the tube bundle region is modelled by the addition of a momentum source term to the standard fluid flow equations. The source added is a two terms expression including a viscous loss term and inertial loss contribution. Next equation details thematrix form included in the momentum transport balance for the tube bundle and the impingement rods region. 

Due to the obvious anisotropic resistance of the tube bundle an orthotropic [different behaviour on each of the three axes of a Cartesian reference frame aligned with the tube bundle] scheme was adopted. 

A local model was built for each direction and exposed to a wide range of velocities to obtain the equivalent pressure drop for each velocity field condition. The thermal resistance for the tubes has been calculated according to values and correlations obtained by several research papers. These include various terms in order to get the total value of the thermal resistance.

Thermal Resistance Condenser CFD

Thermal resistance vs cooling water velocity plot

The water side thermal resistance, Rw, is a function of the coolant water conductivity, flow characteristics and internal diameter of the tube.

The fouling thermal resistance, Rd, has been taken as an average value typically found in this type of condensers in previous research, with the usual value for the tubes heat transfer coefficient varying between 10000 and 35000 S.I. units depending on the position in the bundle.

Thermal resistances detailed above is an important issue. The heat transfer between the cooling tubes and the steam is important, in this matter different phenomena contributes on the overall energy transfer and the condensation flow rate depends due the mass transfer model used  strongly on that heat balance. 

The fluid is modelled as a single phase multi-component gas composed of steam; non-condensate was not included in the model for simplicity.  The condensation mass sink term for the steam fraction is then computed based on the local heat transfer in each control volume according to the frequently used thermal resistance mass transfer model. 

The thermal resistance mass transfer equation included T as the local gas phase temperature, L is the latent heat of condensation of the steam, A is the heat exchange area of the tubes in each control volume, V is the volume of the control volume and R the thermal resistance previously described.

A linear mean coolant temperature profile, Tw, is assumed along the tube bundle and is used to compute the heat transfer between the coolant and gas phase.

Final CFD results including plots and contours

After running different operational conditions cases, a comparison of velocities and temperatures was done to ensure the propper behavior of the condenser under the new set of conditions. Image above shows a glimpse on the type of results and insights you can get from an advanced CFD model like the one devoloped in this case.

Keep your curiosity in good shape.

Developing an exciting project in Athens

Proud to announce another exciting international project. Now the project has been executed in Athens, Greece.

We have collaborated with “Armos SA” in the new “Agia Sofia Stadium” project for the AEK sports club in Athens, performing the GRC (Glass fiber Reinforced Concrete) design verification and the fixings design and calculations.

To validate and improve the panel’s design, the SDEA advanced engineering team created a FEA (Finite Element Analysis) model starting from the base drawings. Several load cases are then performed over that model based on the relevant building normative, such as wind or earthquake forces, with its corresponding safety factors. On the final step, the FEA results are assessed and compared with the limits allowed by the standards to ensure that the design is safe and therefore, validated to move to the manufacturing process.

GRC is a composite material that is used extensively worldwide, both as a functional and decorative construction material in building and civil engineering. The high-strength embedded glass fiber properties combined with ones from the concrete matrix result in a much more resistant and lighter material. This allows the GRC to stand above other solutions or materials when it comes to manufacture complex facades with intricate designs, as it is able to be easily precast in any shape or texture we want by using a mold, offering almost limitless personalization possibilities. In addition to its structural performance, the precast method offers low execution times, which translates into lower manufacturing costs. On top of that, it can also be designed to add an extra insulation layer (both acoustic and thermal) to any type of building, having an impact in energy savings.

More new international projects are coming. Keep an eye on our news in LinkedIn

Thermal Stress Analysis Of Dissimilar Welding Joints using FEA

Weld Residual Stress Cut





Finite Element to foreseen residual stress from the welding process

Most welding process involve local heating, therefore temperature distribution is not uniform and structural thermal stresses beyond the material yield [strongly dependent on the temperature] show up, which can create issues on the final design related to fatigue or even geometry distortions.

Material Yield Temperature Graphics

This local temperature field is enough to create metallurgical and phase changes. As the pool solidifies and shrinks, it begins to exert stresses on the surrounding weld metal and heat affected zones. When it first solidifies and the weld metal is hot, yield is very low and exerts little stresses. As it cools to ambient temperature, this stresses in the weld area increase and eventually reach the yield point leading to a tensile stress state prone to giving problems during the equipment operational life.

As it cools to ambient temperature, this stresses in the weld area increase and eventually reach the yield point leading to a tensile stress state prone to giving problems during the equipment operational life.

Often pressure vessel suppliers are asked by their clients on how the residual stresses would influence the expected structural performance. Those residual stresses, that in tension may lead to high local stress in weld regions of low notch toughness and, as a result, may initiate brittle cracks that can be propagated by low overall stresses that are normally present. Also, notice that residual stresses can be reduced using thermal or mechanical relief methods, but not always post weld treatments are performed for different reasons.

Residual Stress and Temperature contours

SDEA engineering team used a Finite Element Analysis [FEA] approach to foresee the residual stresses from a dissimilar weld between the tubular plate and the shell [see figure 1]. SA-516 Gr 70 against SA-965 F304 will be welded with ER 309L Mo.

The welding procedure includes the geometry detail of each pass and a two-staged coupled simulation involving a thermal stage and a posterior mechanical step that use the temperature field as an input from the previous model. This can be done because changes in the mechanical state do not cause a change in the thermal state, but the opposite doesn’t apply; so that in this study, computation of the temperature history during welding and subsequent cooling is completed first and then this temperature field is applied to the mechanical model as a body force to perform the residual stress analysis.

Heat input during welding is modelled by a distributed heat flux applying on individual element based data provided by the client for each pass. The amount of heat input is obtained from the next expression, that depends on electrode speed, energy applied and a welding efficiency factor.


Principal stress and contours after weld is finished

Is important to highlight the fact that the reduction in yield for temperatures above 700-800 °C will govern the melting transition. Image above shows the expansion coefficient used for the both materials to be welded, notice the different thermal properties are the ones responsible for different heat diffusion and different thermal strain response.

As expected, the high temperature in each pass relaxes the stresses as yield drops with temperature. Weld metal melts and gets to flow state where stresses are relieved; this effect is clear and can be seen in the images around.

As expected tensile stresses appear on the weld surface. Next plot represents the Von Mises stress on the weld surface for the top and bottom side:

The model was also assessed for protection against ratcheting. Ratcheting is a behavior in which plastic deformation accumulates due to, in this case, thermal stress. To evaluate the model against ratcheting, as well as in the previous point from this document, an elastic-plastic analysis was used, according to the guidelines provided by “ASME BPVC.VIII.2-2017” Part 5, point 5.5.7.

For this analysis, same model from plastic collapse was used. In addition to its boundary conditions, three thermal cycles were calculated, as required in the normative, by setting the following temperature profiles on the interior faces:

Summary:

  • Weld residual stresses were calculated using a non-linear FEA coupled thermo-mechanical analysis. As expected, tensile stresses appear at ambient temperature on the weld surface close to the material yield [in the range of 400 MPa].
  • Stresses on the model were calculated using a non-linear FEA coupled thermo-mechanical analysis under operating loads, achieving convergence according to “ASME BPVC.VIII.2-2017 Part5” and thus satisfying the elastic plastic protection criteria
  • Elastic-plastic ratcheting assessment was performed after three thermal cycles, according to “ASME BPVC.VIII.2-2017 Part5”. Two main dimensions on the shell were measured during the cycles, showing no permanent deformation due to the effect of the thermal loads. This condition satisfies the elastic plastic ratcheting assessment criteria as stipulated by the standard.

If you are interested in stress analysis please contact us.

Keep your curiosity in good shape.  

CFD in HVAC for railway applications

CFD simulation for HVAC studies

When travelling, arriving to your destination on time is not the only concern. Comfort also plays a very important role on the travelling experience and may have an impact on your evaluation about the trip.

Railway transportation is one of the most used systems in the world, from high-speed trains allowing people to travel long distances quickly to underground railway facilitates mass transit for people movement inside a big city.

What all these types of trains have in common is their need for a Heating, Ventilation and Air Conditioning [HVAC] system to ensure environmental comfort inside the vehicle. In order to regulate these conditions, there is a strict normative related to comfort for both the passenger cars and driving cabins.

These regulations, specified in Standards such as EN-13129 or EN-14813, determine the comfort parameters inside the vehicles related to air temperature, humidity, air renovation and surface temperature in a wide range of locations, with the final objective of guaranteeing the maximum passenger comfort during the travel.

Computational Fluid Dynamics [CFD] is a powerful tool that can help assessment in the design phase of HVAC systems, since it can simulate the flow and temperature distribution inside the carriages and any environmental and input conditions even before the system is built. With these previous evaluations, it can be easier to determine optimal flow distribution, impulsion parameters, flow rates, poorly optimized areas and so on. CFD also counts with the advantage of not being restrained by any physical components, so data can be analysed in any spot of the geometry, giving even more additional information that can be useful to the designer than the parameters asked by the Standards.

Both EN-13129 and EN-14813 stablish the probing coordinates and sections during test conditions; so in the CFD model these locations can be fully monitored during the calculation to ensure the convergence and stability, and later on, these data can be compared to the requirements asked in the Standards.

Seating Measuring Points

HVAC systems have to meet an exhaustive process of evaluation in terms of thermal homogeneity in a large amount of different locations: seats, aisles, bathrooms, windows, walls, etc. in order to provide a comfortable feeling to the passengers along the ride. Air velocity is also regulated in order to prevent that any area is exposed to any jet stream or, on the contrary, be improperly conditioned. All these requirements involve a deep understanding of fluid dynamics and air behaviour.

SDEA Solutions provides a fully customized service using its knowledge on CFD HVAC modelling, simulation and expertise on Standards to fulfil any client’s request in HVAC systems assessment. Our services include the whole process of designing the geometry from drawings or modification of a given one to the creation of a high quality model to be simulated and a complete analysis of the results, giving the client the precise data they need to know in order to better understand and improve their system, with complete and exhaustive reports made to ensure that every condition and result is well interpreted.

TLD Tuned Liquid Dampers

FEA for TLD Tuned Liquid Dampers

One of the greatest challenges when it comes to design steel stack structures such as chimneys, is to deal with oscillating loads as dynamic wind forces and seismic spectrum, as commented in this article. These loads threaten the design with coupling the excitation with its natural frequency, leading the structure to failure through resonance due to the high stresses produced by the oscillation amplitude.

Two main strategies can be followed to address this issue: either increasing the natural mode of the structure or damping its response to the excitation.

The objective of the first way (natural mode improvement) is to run away from the dangerous resonance frequencies. 

Figure 1 - Vortex induced oscillating loads on a cylindrical profile due to the wind action

This can be achieved by geometrical changes on the design such as plate thickness enhancement or a gain of chimney’s diameter, or by the addition of new structural elements as stiffeners or supports. Although this method is conceptually simple and does not require an extra designing effort other than adding some more material, is has other limitations. Usually, geometrical changes as wider diameter or the addition of new elements are restricted by other features of the whole project. 
 

Figure 2 - TLD pools coupled oscillation

Neighbour structures limiting the available volume or chimney’s exhaust properties are examples of these limitations. Other inconvenience for this solution, and the one who uses to have the last word, is the cost-efficiency. An increment on plate’s thickness involve a not negligible increment on the project’s expense moving it away further than the structure’s natural mode.

The other path to follow in order to guard the structure against oscillations is dramatically increasing the structure’s damping by the addition of a damping system. 

Even though there are different methods to achieve this, this article will be focused on the tuned liquid dampers (TLD).

The mission of the TLD is to dissipate the energy absorbed by the structure from oscillating loads, through a mass-spring-damper equivalent system that moves desynchronized with the chimney at its natural frequency. 

It is, in essence, a series of pools connected to the structure that moves against its pendulum movement. The phenomenon responsible of the energy mitigation effect inside the pools is called sloshing, and is defined as the movement of liquid inside another object. An example of sloshing familiar to almost everyone can be seen every time you walk with a soup plate. 

The human gait frequency (acting as the excitation) is close to the plate mode, so each step amplifies the soup’s wave motion, finally causing it to ruin your menu by splitting out of the rim. 

Figure 3 - Amplitude vs Frequency graph comparing a chimney with and without damper

On the other hand, if you are carrying a cup of coffee at a standard walking speed, despite it creates waves on the surface, the excitation frequency remains lower that the coffee’s natural mode, and as long as you don’t speed up, you breakfast will be safe.

Figure 4 - Plate & cup first oscillating mode shapes

The parameters that define geometrically a TLD, controlling its behaviour, are its interior [Rint] and exterior radius [Rext], the number of pools in which the TLD is divided, and the liquid height [H] that controls the liquid volume and thus the liquid mass. This liquid mass is divided in two distinguishable masses: the impulsive mass, the one that moves creating the sloshing phenomenon, and the convective mass, that has no relative movement with the TLD. Note that despite it has its influence on the sloshing frequency, the amount of convective mass also acts as dead load on the chimney, moving its natural frequency, so it has to be taken into consideration when tuning the assembly.

Figure 5 - TLD main dimensions and water mass distribution

The interior radius is used as the start point to calculate the previously mentioned variables [as it is more or less set by the chimneys geometry itself] along with the chimneys natural frequency and modal mass, acting as the focus result for the tune and as a reference for the total liquid mass respectively. Then, using the linear wave theory and the Graham & Rodríguez equations, the first group of parameters is obtained. Because of the influence of the TLD’s convective mass on the base chimney’s natural mode, an interpolation process has to be performed in order to obtain a fine tuning for the liquid damper.

Figure 6 - TLD sloshing mode shape at tuned frequency

To optimize this method’s accuracy and asses its results, SDEA’s advanced engineering team assembled a coupled FEA modal analysis plus harmonic response computational model. A fluid-structure interaction model along with a fluid free surface boundary condition is needed to capture the sloshing effect on the TLD’s pools. The next image shows the pools from a TLD where it can be appreciated that the liquid from all pools move in the same direction when they get coupled with the chimney’s response to external excitation.

Although this solution for the problem introduced in this article has a bigger technical load than enhancing the structure plate’s thickness, the cost-efficiency and adaptability of the damper installation compensate these cons by relieving the projects budget and providing certain design flexibility to overcome hypothetical installation issues.

STADT latest generation solution, STADT Lean Propulsion ®

The Norwegian company STADT has for many years been a frontrunner in development of electric propulsion for ships. The company’s latest generation solution, STADT Lean Propulsion ®   have already been awarded for its revolutionary design with a set  uniqueness’s : very high efficiency, STEALTH-silence, low signature,  reduced weight, high reliability and low maintenance cost  for shipowners.

To meet new requirements STADT now release a new feature in

Lean Propulsion ,  The  VariAC .      

This option give customers the possibility to take increased effect /
advantage of variable speed from Diesel or LNG-gensets  fitted with
variable speed-control.   

STADT VariAC  operates
on an AC grid with variable frequency in a range of 45-65 Hz,  and will
give ship operators more efficient and precise control of power-load and ship-speed
via a  seamless regulation of  propeller speed (RPM) and Pitch in
three different power-modes – Low, Medium  and High-power  – very scalable for propulsion systems in the range 1 to more than
 50 MW pr propeller.

The first version of STADT   VariAC
  is designed
and developed  in cooperation with PON –

Et bilde som inneholder overvåke, skjerm, innendørs, skjermbilde

Automatisk generert beskrivelse

Caterpillar, with the utilization of
Caterpillars and MaK products.

By the new VariAC  feature,  STADT have increased the Lean Propulsion® system benefits in a set areas, and will secure

  • Overall
    lower fuel consumption and emission, especially on low and medium power
  • Lower
    noise level during operation
  • Less
    Lube oil consumption
  • Less
    wear and tear on genset with lower maintenance cost
  • Higher
    Propulsion efficiency

“In cooperation with leading LNG and
Diesel-generator vendors, we see that we with the STADT VariAC  can utilize  latest generation
motor-technology  to even lower operational cost for our customers” says
Hallvard Slettevoll, ceo at  STADT AS.

STADT AS

Moljevegen 50

N 6083 GJERDSVIKA

Norway

                                                                                              
www.stadt.no