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Tools of the Trade: Flow Modeling
Tools of the Trade: Flow Modeling
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Tools of the Trade: Flow Modeling

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District energy systems are typically found on college, university, or hospital campuses and central metropolitan areas. These systems produce high-temperature hot water, steam, or chilled water at a …

District energy systems are typically found on college, university, or hospital campuses and central metropolitan areas. These systems produce high-temperature hot water, steam, or chilled water at a central plant, then distribute it through pipes to buildings connected to the system. Customers in those buildings use the steam and hot and chilled water to meet their space and water heating and air-conditioning needs. In this way, individual buildings do not need costly and cumbersome boilers, chillers, or cooling towers.

Given the diversity and complexity of these systems and the campuses they serve, it takes significant effort to troubleshoot existing problems or predict how future changes will affect total demand on the central plant. For consulting engineers and district energy managers, there is a powerful tool to help with this process: flow modeling.

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  • 1. Concepts Fishbeck, Thompson, Carr & Huber, Inc. October 2012Tools of the Trade: Flow ModelingDistrict energy systems are typically found operated, flow modeling software has and engineers had to make educatedon college, university, or hospital campuses evolved to provide a total system view. Flow guesses to solve system problems, orand central metropolitan areas. These models are used by engineers to predict rely on simple trial and error. Either way,systems produce high-temperature hot water, system performance, design pipelines, and one thing is certain: when parts of asteam, or chilled water at a central plant, size and select pumps and other equipment. piping system are inadequately sized, thethen distribute it through pipes to buildings The software gives a clear picture of how entire system may suffer. Flow modelingconnected to the system. Customers in a piping system operates by calculatingthose buildings use the steam and hot and system flow velocities and pressures;chilled water to meet their space and water it shows the interactionheating and air-conditioning needs. In this of the pipelines, pumps,way, individual buildings do not need costly components, and valves inand cumbersome boilers, chillers, or cooling the system.towers. System owners andGiven the diversity and complexity of these managers can use modelssystems and the as part of a largercampuses they flow modeling master planning effort;serve, it takes they can also utilize asignificant effort software has evolved model to troubleshootto troubleshoot their system.existing to provide a totalproblems or Because of the inherentpredict how system view. complexity in a districtfuture changes energy system, it canwill affect total demand on the central plant. be a challenge to diagnoseFor consulting engineers and district energy deficiencies or understandmanagers, there is a powerful tool to help their causes. Flow modelswith this process: flow modeling. simulate system operation and give owners andBasics of Flow Modeling managers an accurate picture to help themFlow modeling is the computerized simulation formulate a plan to correctof fluid flow through a conduit; in this case, the trouble.pipe. Originally created to help engineersand designers size individual pipelines and Before flow modeling,understand how a particular piping system district energy system owners
  • 2. Model Calibration As with any computer simulation, flow modeling results are only as good as the input data. Unfortunately, district energy flow modeling can rarely be done without including some data that must be estimated, and this is most commonly building demands. For this reason, it is crucial to calibrate the model. Calibration is a method that compares empirical or real system performance data against the predicted results of the model. When they differ, the model’s assumptions must be reevaluated. Sometimes additional field investigation may be required to verify physical system characteristics. A more detailed look at individual building demands can sometimes identify unexpected loads. Empirical data used to calibrate the The best system operation information When real performance data and model is obtained in a variety of is obtained from discussions with the model predictions differ, the model’s ways. If system flow and pressure operating and maintenance staff by assumptions must be reevaluated. monitoring is not in place, data discussing their observations on the In such instances, additional field collectors can be placed at strategic operating characteristics of the system investigation is required to verify locations. and its components. physical system characteristics and identify unexpected loads.software makes it possible to simulate the operation of the total piping system. Once the computersimulation reflects the piping system’s actual operation, plant personnel can try various modificationsto the piping system model with less fear of failure. Total System ViewCreating The Model A total system view that includes infrastructureThe model is usually easiest to understand if it is created to resemble the actual system layout. Existing needs when consideringdistrict energy maps are a good place to start by entering pipe lengths, fittings, source points, and use new buildings, buildingpoints. Because actual lengths, sizes, and fittings are more critical to the flow model than they may be use changes, andfor the use the map was intended, a field survey of the system is usually necessary. This is frequently a development is crucialsignificant effort when creating the model. to ensuring your system will operate at itsSystem Demand Loads optimum level. Until theDetermining the system’s demand loads is often more challenging; a load must be determined for advent of flow modelingeach connection point on the system. Original building construction and design documents can help in software, district energythis effort, but must be used cautiously, unless equipment sizing and selection factors are known. An system managers andexperienced engineer can prove to be a valuable resource by estimating building demands based on their the engineers whoexpected maximum load on the distribution system. This process becomes much easier if the system ismetered at each point of use. If meters are present, they must have been in place long enough to have helped them design andrecorded peak demand data. Keeping in mind total system diversity (80% is a good initial estimate), the troubleshoot their systemsrecorded peak output at the central plant is a good check for the total of all system loads. relied on time-consuming hand calculations andModel Uses experience to get the job done. Although thereA good district energy system model will help identify and quantify performance deficiencies. This includes is still no substitute fordeficiencies you have suspected but have been unable to measure or understand their causes. The modelcan then be used to demonstrate the effectiveness of proposed system changes aimed at resolving those experience, flow modelingdeficiencies, and identify the best solution before you make a capital investment. software has given system owners a powerful tool toOnce your current system model is calibrated, it is easy to evaluate different scenarios, such as unplanned understand and maintainfailures in components or pipe sections, to identify the system’s vulnerability. These scenarios should also their systems in the faceinclude planned system shutdowns for maintenance and repairs. Changes to reduce these vulnerabilities, of fluctuating demandssuch as developing looped systems, can also be explored. With the model of your system in place, you arenow in a position to provide definitive input into planning building changes and uses of the areas served and process changes.by the district energy system. www.ftch.com/concepts

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