Site Data: Reference air density
Reference Air Density is used to convert user-specified air flow parameters conventionally expressed as volume flow rates to mass flow rates for use in load calculations and simulations. It is also used to convert calculated and simulated HVAC system and ventilation mass flow rates to volume flow rates for display in ApacheHVAC and VistaPro. This includes volume flow rates for the MacroFlo external vent and internal vent room variables. Flow rates expressed as volume flows in VE are converted to mass flows in Apache for the purpose of thermal simulation, and subsequently converted back to volume flow in results data. Thus, a larger reference air density will typically yield larger heating and cooling loads since the volumetric heat capacity of a substance at a given temperature is directly proportional to its density.
Because mass flow is used to determine MacroFlo vent gain, thermal gains resulting from MacroFlo ventilation are unaffected. Adjustment of Reference Air Density also does not influence the actual weather file data or the presentation of that data on the psychrometric chart in VistaPro.
The opening-level variables 'Volume flow in' and 'Volume flow out' are actual volume flows calculated by MacroFlo for the air entering or leaving using variable density. However,the room-level results variables 'MacroFlo external vent' and 'MacroFlo internal vent' are MacroFlo mass flows converted to volume flows using the Reference Air Density. This convention ensures consistency with other room-level volume flow rates, so that when summed over all sources, reported net volume flow into the room (being here a proxy for net mass flow) should be zero.
The value is conventionally set to 1.2 kg m-3 via the Standard option. However, the user has the freedom to override this value and specify their own value.
This can be set explicitly by using the Custom control, wherein the user may enter any value between 0.12 kg m-3 and 2.05 kg m-3 (values outside this range are unrealistic from the perspective of building physics).
Alternatively, the value can be calculated via the Derived control. This allows the user to enter an elevation (between 0 m and 9000 m), a reference temperature (between -100 °C and 60 °C) and a relative humidity. The reference air density will then be calculated via a standard atmospheric model to be equal to that of air at the specified elevation above sea level, at the specified temperature and the specified relative humidity.
Note that the elevation, temperature and relative humidity specified here are not used outside of this calculation; the only value that is used elsewhere is the reference air density.
The reference air density specified in this section is, however, not used for NCM simulations; for such cases, Apache will disregard the user-specified value and instead use the standard value of 1.2 kg m-3.
It should also be noted that, when using ApacheHVAC, any auto-sizing calculations should be carried out with respect to the reference air density that will be used for the final Apache simulation.
