# FAQ - Mass and Energy Balance related questions

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## Where can I get information for Mass and Energy Balances?

• In SysCAD Steady-State (ProBal) mode, a solved model is at steady state (in=out) and therefore no mass accumulation occurs. All Mass and Energy Balance information is provided in the pipe models, i.e. flowrate, temperature, pressure, enthalpy, specific heat, etc. can be found by right clicking on the PIPES.
• The Audit tab does show a summary of the total input and output mass and energy numbers for individual unit operations. Therefore, to check the mass and energy balance around an individual unit, the user can look in the individual model.
• The Links tab shows all of the flows into and out of each unit and also shows the change in selected species across the unit due to reactions.

For information about Energy Balance around reactions, please see Reaction Block Energy Balance.

## What is the difference between Hs and Hz?

The enthalpy of a species, or stream at Temperature, T, and Pressure, P, is calculated using the specific heats with a reference temperature of 0°C and 101.325kPa.

1. SysCAD assigns ALL species an Hs value of 0J/mol at 0°C and 101.325kPa.
2. SysCAD assigns most species an Hz of 0J/mol at 0°C and 101.325kPa.
• However the exception is species that exist in 2 or more phases, where Hz at 0°C and 101.325kPa is calculated as follows:
Hz = Energy of Formation at 0°C and 101.325kPa of the species - Energy of Formation at 0°C and 101.325kPa of the species in the 'lowest' energy phase (g > l > s)

Therefore, if the project contains any species that occur in 2 or more phases, the Hs and Hz values will be different by this amount.

Example

A project has NaCl in both the aqueous and solid forms with the following values:
At 0°C and 101.325kPa:
 NaCl(s): Hf = -7060kJ/kg Hs = 0kJ/kg Hz = 0kJ/kg NaCl(aq): Hf = -7000kJ/kg Hs = 0kJ/kg Hz = 60kJ/kg
Therefore, any streams that contain NaCl(aq) will have a difference between the Hs and Hz values of 60kJ per kg of NaCl(aq).