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.

1. Concepts Fishbeck, Thompson, Carr & Huber, Inc.
October 2012
Tools of the Trade: Flow Modeling
District energy systems are typically found operated, flow modeling software has and engineers had to make educated
on college, university, or hospital campuses evolved to provide a total system view. Flow guesses to solve system problems, or
and 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 a
steam, or chilled water at a central plant, size and select pumps and other equipment. piping system are inadequately sized, the
then distribute it through pipes to buildings The software gives a clear picture of how entire system may suffer. Flow modeling
connected to the system. Customers in a piping system operates by calculating
those buildings use the steam and hot and system flow velocities and pressures;
chilled water to meet their space and water it shows the interaction
heating and air-conditioning needs. In this of the pipelines, pumps,
way, individual buildings do not need costly components, and valves in
and cumbersome boilers, chillers, or cooling the system.
towers.
System owners and
Given the diversity and complexity of these managers can use models
systems and the as part of a larger
campuses they flow modeling master planning effort;
serve, it takes they can also utilize a
significant effort software has evolved model to troubleshoot
to troubleshoot their system.
existing to provide a total
problems or Because of the inherent
predict how system view. complexity in a district
future changes energy system, it can
will affect total demand on the central plant. be a challenge to diagnose
For consulting engineers and district energy deficiencies or understand
managers, there is a powerful tool to help their causes. Flow models
with this process: flow modeling. simulate system operation
and give owners and
Basics of Flow Modeling managers an accurate
picture to help them
Flow modeling is the computerized simulation formulate a plan to correct
of fluid flow through a conduit; in this case, the trouble.
pipe. Originally created to help engineers
and 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 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. Total System View
Creating The Model A total system view that
includes infrastructure
The model is usually easiest to understand if it is created to resemble the actual system layout. Existing needs when considering
district energy maps are a good place to start by entering pipe lengths, fittings, source points, and use new buildings, building
points. Because actual lengths, sizes, and fittings are more critical to the flow model than they may be use changes, and
for the use the map was intended, a field survey of the system is usually necessary. This is frequently a
development is crucial
significant effort when creating the model.
to ensuring your system
will operate at its
System Demand Loads optimum level. Until the
Determining the system’s demand loads is often more challenging; a load must be determined for advent of flow modeling
each connection point on the system. Original building construction and design documents can help in software, district energy
this effort, but must be used cautiously, unless equipment sizing and selection factors are known. An system managers and
experienced engineer can prove to be a valuable resource by estimating building demands based on their the engineers who
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
helped them design and
recorded peak demand data. Keeping in mind total system diversity (80% is a good initial estimate), the troubleshoot their systems
recorded peak output at the central plant is a good check for the total of all system loads. relied on time-consuming
hand calculations and
Model Uses experience to get the job
done. Although there
A good district energy system model will help identify and quantify performance deficiencies. This includes is still no substitute for
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
experience, flow modeling
deficiencies, and identify the best solution before you make a capital investment. software has given system
owners a powerful tool to
Once your current system model is calibrated, it is easy to evaluate different scenarios, such as unplanned understand and maintain
failures in components or pipe sections, to identify the system’s vulnerability. These scenarios should also their systems in the face
include planned system shutdowns for maintenance and repairs. Changes to reduce these vulnerabilities,
of fluctuating demands
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
and process changes.
by the district energy system.
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