Multisided Heat Exchanger Capabilities
Heat exchangers with multiple sides are very common in many processes such as LNG or NGL. Because of their smaller footprint, they may provide a more economical alternative. There are many types and configurations of multisided exchangers including brazed aluminum, spiral wound, or plate and fin. The Symmetry process software platform includes a comprehensive offering for simulating these types of unit operations in steady state and dynamics with varying degrees of fidelity. This article describes the main features available along with the tradeoffs and considerations required to specify them.
Figure 1. Process using multisided exchangers
A multisided exchanger can be solved with three main types of specifications: design (energy balance), semi-rating (UA specifications), and rating (detailed configuration and geometry). There is also a link to HTRI, which is leading software for the design and rating of heat exchangers, that can be enabled directly from the heat exchanger window.
1. Design Specifications
This is the fastest way to specify the unit operation. The user must provide enough specifications to satisfy the degrees of freedom to calculate the energy and material balance. This includes temperatures, delta temperatures, duties, pressures and flows. The solver will calculate as soon as information is available. Figure 2 shows an exchanger with 3 sides with temperature specifications. In this example, the temperatures are specified in the streams, but it is also common to set the specifications as delta temperatures and pressures in the Sides Data frame of the Summary tab.
Figure 2. Exchanger solved with temperature specifications
The most common information is included in the main tab but there are other tabs with valuable information such as the Plot tab where the user can inspect the composite curves and get a visual indication in case the exchanger is pinched or undersized.
Figure 3. Plot tab from multisided exchanger
2. Semi-Rating Specifications
This mode of specification uses UA as the specification variable. UA specifications are useful to estimate the performance of the exchanger based on the inlet temperatures and a fixed heat transfer coefficient. They can be combined with other types of specifications such as delta temperatures. There are three types of UA variables but the most common type is the UA per side which can be found in the Side Data frame and is defined as the UA of that side Vs the opposite composite side. For example, the UA for a hot side is evaluated against the cold composite side. Figure 4 shows a heat exchanger with two UA specifications per side. Note how the Overall App T is 16.23 F which indicates that the closest temperature difference between the cold and hot composite is 16.23 F. In this case the approach temperature happens at the outlet of the LNG stream which is getting closer to the feed temperature of the coldest refrigerant (N2).
Figure 4. Multisided exchanger with UA specifications for the Hot sides
The second type of UA is the Overall UA found in the Main Data frame. This value is calculated based on the hot Vs cold composite curves. This specification is often combined with an overall approach temperature or delta temperatures (Figure 5).
Figure 5. Multisided exchanger specified with an Overall UA and Approach T
The final type of UA is more commonly used “behind the scenes” when doing detailed rating calculations. This UA specification is the side to side UA and is found at the bottom of the Sides tab. This UA is based on the interaction of two sides (for example, side 1 Vs side 3). These specifications are relevant if the user has a good sense of the actual configuration of the cold box.
2.1 Simple UA models
One disadvantage of UA specifications is that they do not capture changes in heat transfer coefficient when the flow changes but there is not enough geometry information to create a detailed rating case. A unique feature in Symmetry is the ability to create simple models for the Side UA specifications. To enable them, first solve the exchanger with Side UA specifications (this will be used as reference). Then go to the settings tab and click on the “UA Simple Model” (Figure 6). This will replace Side UA specifications for a simple equation with shape:
Current_UA = Reference_UA * (CurrentFlow / ReferenceFlow) ^ ScaleExponent
This equation creates a relationship between the reference UA and the flow thus providing a more realistic dependency when conditions change. You can regress the ScaleExponent to match plant data if available.
Figure 6. Simple UA model to enhance Side UA specifications
3. Detailed Rating
Detailed rating is enabled from the Detailed Geometry frame by selecting VMG as the engine (Figure 7). Due to the nature of the dynamics engine, this is the only mode available for transient simulations. This feature allows the user to input detailed geometry for the exchanger including the definition of zones and flow paths and passes (see the manual for more details). Heat transfer coefficients and pressure drops are automatically estimated when using the rating mode.
Figure 7. Rating engine selection
Figure 8. Rating geometry configuration
This mode of solution is the most predictive but requires the most effort to configure. The convergence of these unit operations may become challenging. As a result, we have created a very robust convergence algorithm for complex configurations with many sides with nonlinear heating curves and highly pinched. This algorithm is called “Wall T Solver” and is enabled at the bottom of the Settings tab. It is strongly recommended to use the Simple Flash Models when using this algorithm.
Figure 9. Complex detailed multisided exchangers solved with the Wall T Solver
Speed up with Flash Models
The heat exchanger depends on PH flashes and the semi-rating and rating models require several of these calculations. The multisided heat exchanger provides a feature to drastically accelerate convergence time without losing accuracy by internally creating simpler thermo models that are valid within the range of conditions of the heat exchanger. To enable this feature, go to the Setting tab and click on “Use Simple Flash Models”.
The heat exchanger will automatically create the models and you should see an immediate speed up if using rating or semi-rating specifications. There is also another setting called “View Simple Flash Models” used to validate the accuracy of the flash models (Figure 10).
Figure 10. Flash model validation
There are a wide variety of options available in the Symmetry process platform to model multisided heat exchangers in steady state and dynamics depending on the level of accuracy required and information available. The simple UA models offer a flexible alternative to avoid going to a full geometry-based model while still getting the benefit of a heat transfer coefficient that changes with flow. The ability to accelerate calculations with the “flash models” feature significantly reduces the tradeoff between accuracy and speed when going from a balance-based to a rating-type specification.
Please contact us if you have any questions or feedback related to these types of exchangers.
Raul Cota, Ph.D., P. Eng.