ConceptsTools of the Trade: Flow ModelingDistrict 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.

Basics of Flow ModelingFlow modeling is the computerized simulation of fluid flow through a conduit; in this case, pipe. Originally created to help engineers and designers size individual pipelines and understand how a particular piping system

operated, flow modeling software has evolved to provide a total system view. Flow models are used by engineers to predict system performance, design pipelines, and size and select pumps and other equipment. The software gives a clear picture of how a piping system operates by calculating system flow velocities and pressures; it shows the interaction of the pipelines, pumps, components, and valves in the system.

System owners and managers can use models

as part of a larger master planning effort; they can also utilize a model to troubleshoot their system.

Because of the inherent complexity in a district energy system, it can

be a challenge to diagnose deficiencies or understand their causes. Flow models simulate system operation and give owners and managers an accurate picture to help them formulate a plan to correct the trouble.

Before flow modeling, district energy system owners

and engineers had to make educated guesses to solve system problems, or rely on simple trial and error. Either way, one thing is certain: when parts of a piping system are inadequately sized, the entire system may suffer. Flow modeling

Fishbeck, Thompson, Carr & Huber, Inc.

October 2012

onceptsC

flow modeling software has evolved

to provide a total system view.

software makes it possible to simulate the operation of the total piping system. Once the computer simulation reflects the piping system’s actual operation, plant personnel can try various modifications to the piping system model with less fear of failure.

Creating The ModelThe model is usually easiest to understand if it is created to resemble the actual system layout. Existing district energy maps are a good place to start by entering pipe lengths, fittings, source points, and use points. Because actual lengths, sizes, and fittings are more critical to the flow model than they may be for the use the map was intended, a field survey of the system is usually necessary. This is frequently a significant effort when creating the model.

System Demand LoadsDetermining the system’s demand loads is often more challenging; a load must be determined for each connection point on the system. Original building construction and design documents can help in this effort, but must be used cautiously, unless equipment sizing and selection factors are known. An experienced engineer can prove to be a valuable resource by estimating building demands based on their expected maximum load on the distribution system. This process becomes much easier if the system is metered at each point of use. If meters are present, they must have been in place long enough to have recorded peak demand data. Keeping in mind total system diversity (80% is a good initial estimate), the recorded peak output at the central plant is a good check for the total of all system loads.

Model UsesA good district energy system model will help identify and quantify performance deficiencies. This includes deficiencies you have suspected but have been unable to measure or understand their causes. The model can then be used to demonstrate the effectiveness of proposed system changes aimed at resolving those deficiencies, and identify the best solution before you make a capital investment.

Once your current system model is calibrated, it is easy to evaluate different scenarios, such as unplanned failures in components or pipe sections, to identify the system’s vulnerability. These scenarios should also include planned system shutdowns for maintenance and repairs. Changes to reduce these vulnerabilities, such as developing looped systems, can also be explored. With the model of your system in place, you are now in a position to provide definitive input into planning building changes and uses of the areas served by the district energy system.

www.ftch.com/concepts

The best system operation information is obtained from discussions with the operating and maintenance staff by discussing their observations on the operating characteristics of the system and its components.

Empirical data used to calibrate the model is obtained in a variety of ways. If system flow and pressure monitoring is not in place, data collectors can be placed at strategic locations.

When real performance data and model predictions differ, the model’s assumptions must be reevaluated. In such instances, additional field investigation is required to verify physical system characteristics and identify unexpected loads.

Total System View A total system view that includes infrastructure needs when considering new buildings, building use changes, and development is crucial to ensuring your system will operate at its optimum level. Until the advent of flow modeling software, district energy system managers and the engineers who helped them design and troubleshoot their systems relied on time-consuming hand calculations and experience to get the job done. Although there is still no substitute for experience, flow modeling software has given system owners a powerful tool to understand and maintain their systems in the face of fluctuating demands and process changes.

Model CalibrationAs 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.