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Document Type: FAQ
Product(s): Hevacomp Mechanical Designer
Version(s): All
Original Author: Bentley Technical Support Group
An iterative process is used to carry out pipe sizing in Netsys. What follows is an explanation of the methods used.
Initially pipe sizes are estimated by calculating flow rates at emitters based on the following formula:
This gives an initial flow rate based on the flow and return temperatures at the boiler. The program then works these flow rates back through the system to give a total flow rate at the boiler. Pipe sizes are estimated based on these initial flow rates.
Pipe work emissions are then calculated and used to re-compute the flow rates through the system and are worked back to give a new total flow rate at the boiler. This flow rate is then proportioned through the system by connected load. Below is a simple example:
Pipes are then resized on the new flow rates and heat emissions recomputed again. This process continues until no pipes change size.
Customers often look at parts of their system in isolation and use the equation at the top of the page to check flow rates to an emitter using the flow and return temperatures at the boiler (set in System Defaults). However the flow and return temperatures at the emitters will not be the same as at the boiler due to mains losses and so this calculation is inaccurate. The example below shows how the use of the boiler flow and return temperatures differs from what the true flow and return temperature may be.
Using the flow rate equation at the beginning of this document the customer would get the following flow rate for the pipe feeding Emitter 3.
However due to losses from the mains the flow and return temperatures at the emitter will not be 86oC and 76oC but may be more like 85oC and 80oC. The equation will then change thus:
This shows that the flow rate at the emitter could be more than double the value calculated using the boiler flow and return temperatures.
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