# Drum Model#

## Introduction#

The drum model consists of three main sub-unit operations:

a flash model to separate the saturated steam from the saturated liquid water in the water/steam mixture,

a mixer model to mix saturated liquid water with feed water, and

a water tank model to calculate drum level and pressure drop.

First the water/steam mixture from boiler waterwall tubes (risers) enters the flash model and leaves in two separate streams (liquid water and steam). Then, the saturated water from the flash model is mixed with the feed water stream (typically from the economizer or a water pipe linking the economizer and the drum) and leave the mixer model in a single mixed stream. Finally, the mixed stream enters the water tank of the drum and leaves the vessel through the multiple downcomers (see Figure 1).

Inlet Ports:

water_steam_inlet: water/steam mixture from waterwall

feedwater_inlet: feedwater from economizer/pipe

Outlet Ports:

liquid_outlet: liquid to downcomer

steam_outlet: saturated steam leaving the drum

## Variables#

Model inputs (variable name):

water/steam inlet (water_steam_inlet: flow_mol, enth_mol, pressure)

feedwater inlet (feedwater_inlet: flow_mol, enth_mol, pressure)

drum diameter (drum_diameter)

drum length (drum_length)

number of downcomer tubes (number_downcomers)

downcomer diameter (downcomer_diameter)

drum level (drum_level)

heat duty (heat_duty)

Model Outputs:

vapor outlet (vap_outlet: flow_mol, enth_mol, pressure)

liquid outlet (liq_outlet: flow_mol, enth_mol, pressure)

## Constraints#

As mentioned above, the drum model imports a HelmPhaseSeparator and mixer models, specific documentation for these models can be obtained in: Once the water enters the tank model the main equations calculate water velocity and pressure drop calculation due to gravity based on water level and contraction to downcomer. Water level (drum_level) is either fixed for steady state simulation or calculated for dynamic model (Dynamic = True)

Main assumptions:

Heat loss is a variable given by the user (zero heat loss can be specified if adiabatic)

Pressure change due to gravity based on water level and contraction to downcomer is calculated

Water level is either fixed for steady-state model or calculated for dynamic model

Assume enthalpy_in == enthalpy_out + heat loss + energy accumulation

Subcooled water from economizer and saturated water from waterwall are well mixed before entering the drum

Pressure equality constraint:

Pressure drop in unit:

where: * V: fluid velocity (m/s, liquid only)

Note that the model builds an Pyomo Arc to connect the Liquid_outlet from the self.aFlash unit to the SaturatedWater inlet port of the mixer, and the mixed_state (Mixer outlet) is directly constructed as the Drum control_volume.properties_in. Once the Drum model is constructed, the mixer and flash blocks can be found as self.aDrum.aMixer and self.aDrum.aFlash

## Degrees of Freedom#

Once the unit dimensions have been fixed, the model generally has 5 degrees of freedom. The water/steam mixture inlet state (flow_mol, enth_mol, and pressure) and feewater inlet state (flow_mol and enth_mol). The feedwater inlet pressure is usually free due to the pressure equality mentioned above.

## Dynamic Model#

The dynamic model version of the drum model can be constructed by selecting dynamic=True. If dynamic = True, material accumulation, energy accumulation, and drum level must be calculated. Therefore, a dynamic initialization method has been developed set_initial_conditions to initialize the holdup terms.