Modeling a closed loop system

  Applies To 
  Product(s): WaterGEMS, WaterCAD
  Version(s): All
  Area:  Modeling
  Original Author: Thomas Walski and Jesse Dringoli, Bentley Systems

Problem

How can I model a closed loop system where the water or other liquid is circulated with no net inflow or outflow? (no tank or reservoir) For example a heat exchanger.

Solution

Typical water distribution systems have a tank, reservoir, groundwater aquifer or some other locations where the hydraulic grade line is known. These are usually set as a reservoir element in the model.

Occasionally, a modeler will need to deal with a closed loop system where the water or other liquid is circulated with no net inflow or outflow. Heat exchangers are an example of such a system. If a closed system with no reservoir or tank is run in a model, an error message is returned.

The key to modeling such a system is to realize that such a system needs a source of water at some time, because even the best systems leak. The system can be recharged:

1. Using a pump to add water that kicks on when the pressure drops

2. Bleeding in water from a municipal system (through a backflow preventer and control valve)

3. Shutting down the system and adding water at a high point.

In any case there is some source of water. This can be modeled by a reservoir connected to the closed loop through a very small pipe (for example a diameter of 1 mm) with a very long length at the point where the system is charged. This will allow you to set the hydraulic grade while not introducing any flow (the headloss through the connection pipe will be so large that you should see no flow.) The elevation of the reservoir can be set to the HGL at which the recharge pump turns on, the opening setting of a valve that feeds the loop or the elevation of the high point where water is added.

If this reservoir HGL is not known, the elevation can be set arbitrarily. In this case, the flows, velocities, pump operating point and head losses will be correct but the HGL and pressure will be arbitrary values depending on the arbitrary reservoir elevation set.

Example:

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