In most hydraulic models used in district heating networks, in each pipe, the flow rate and pressure head calculations are decoupled from the calculation of water temperature. This is justified because the hydraulic behaviour is minimally impacted by temperature. However, there are important preliminary considerations:
· For the head, the Bernoulli equation is used to describe the forms of energy of water in a pipe (coming from static pressure, kinetic energy, and gravitational potential energy). A pressure difference at two different points in a system causes flow. For changes in head in a water distribution network, the velocity head becomes negligible, and its elevation is included- making each head a piezometric head.
· The pressure differences that cause fluids to flow can be due to changes in: elevation, pressurised flows (pumps, valves, etc). These changes are affected by pipe fittings, pumps, valves, impedances, and pipe friction. Friction is a form of head loss which in iVN, is calculated using Darcy-Weisbach friction formulae.
In a district heating network, the hydraulic model is used for computing the flow rate and pressure head for each pipe. iVN uses the nodal method in combination with demand-driven analysis to assume a steady-state solution. This involves using the continuity equation- but as water is assumed to be incompressible and its properties are temperature invariant, the problem becomes framed in terms of volume flow rate. The Newton-Raphson method is used to iteratively solve this problem.
