Model of a pipe with flow resistance and optional heat exchange
with environment.

#### Heat loss calculation

There are two possible configurations:

- If
`useMultipleHeatPorts=false`

(default option),
the pipe uses a single heat port for the heat exchange with the
environment. Note that if the heat port is unconnected, then all
volumes are still connected through the heat conduction elements
`conPipWal`

. Therefore, they exchange a small amount of
heat, which is not physical. To avoid this, set
`useMultipleHeatPorts=true`

.
- If
`useMultipleHeatPorts=true`

, then one heat port
for each segment of the pipe is used for the heat exchange with the
environment. If the heat port is unconnected, then the pipe has no
heat loss.

#### Pressure drop calculation

The default value for the parameter `diameter`

is
computed such that the flow velocity is equal to
`v_nominal=0.15`

for a mass flow rate of
`m_flow_nominal`

. Both parameters, `diameter`

and `v_nominal`

, can be overwritten by the user. The
default value for `dp_nominal`

is two times the pressure
drop that the pipe would have if it were straight with no fittings.
The factor of two that takes into account the pressure loss of
fittings can be overwritten. These fittings could also be
explicitly modeled outside of this component using models from the
package Modelica.Fluid.Fittings.
For mass flow rates other than `m_flow_nominal`

, the
model Buildings.Fluid.FixedResistances.PressureDrop
is used to compute the pressure drop.

For a steady-state model of a flow resistance, use Buildings.Fluid.FixedResistances.PressureDrop
instead of this model.

- January 22, 2016, by Michael Wetter:

Corrected type declaration of pressure difference. This is for
#404.
- October 30, 2015, by Michael Wetter:

Improved documentation for #455.
- February 5, 2015, by Michael Wetter:

Renamed `res`

to `preDro`

for #292.
- September 13, 2013 by Michael Wetter:

Replaced `nominal`

with `default`

values as
they are computed using the default Medium values.
- February 22, 2012 by Michael Wetter:

Renamed `useMultipleHeatPort`

to
`useMultipleHeatPorts`

and used heat port connector from
`Modelica.Fluid`

package for vector of heat ports.
- February 15, 2012 by Michael Wetter:

Revised implementation and added default values.
- February 12, 2012 by Wangda Zuo:

First implementation.

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