Heat Transfer by Air Movement
ApacheSim models the following types of air movement:
· Pre-specified air exchanges, classified as infiltration, natural ventilation or mechanical ventilation. These air exchanges may be sourced from outside air, outside air modified by a temperature offset, air at a (possibly varying) temperature defined by an absolute profile or air from another room. The rate of air flow is specified before the simulation, but may be made to vary with time by means of a profile. If the profile is a formula profile, the air flow rate may also vary with simulation variables such as room air temperature.
· Air flows calculated by MacroFlo. MacroFlo calculates natural ventilation air flows arising from wind and stack pressure (buoyancy). It also takes account of flow imbalances generated by HVAC systems. MacroFlo runs in tandem with ApacheSim and the calculations of the two programs are interdependent.
· Air flows specified or calculated by ApacheHVAC. Like MacroFlo, ApacheHVAC is fully integrated with ApacheSim and its ducted mechanical ventilation rates are superimposed on other air flows dealt with by ApacheSim.
The rate of heat transfer associated with a stream of air entering a space is
(22)
where
is the air mass flow rate (kg/s),
is the specific heat capacity of air at constant pressure (J/kg/K),
is the supply temperature of the air (ºC ), and
is the room mean air temperature of the air (ºC ).
Equation 22 embodies an assumption that the air displaced by the supply air is at the room mean air temperature, which is consistent with ApacheSim’s stirred tank model of the room air.
Air supply rates may be specified in various ways. A rate specified in terms of a volume flow in l/s is converted to a mass flow rate using a reference air density of 1.2 kg/m 3 . A rate specified in terms of l/s/m 2 is multiplied by room floor area and then converted as above. A rate specified in terms of air changes per hour is converted to a mass flow rate m using
(23)
where
= 1.2 kg/m
3 is the reference air density,
is the room volume (m
3 ) and
is the air change rate (ach).
The water vapour gain associated with the air supply, which plays a part in the room’s latent balance, is
(24)
where
is the water vapour gain (kg/s)
is the humidity ratio of the supply air (kg/kg)
is the humidity ratio of the room air (kg/kg)
Carbon dioxide gain is treated in a similar way:
(25)
where
is the carbon dioxide gain (kg/s)
is the carbon dioxide concentration of the supply air (kg/kg)
is the carbon dioxide concentration of the room air (kg/kg)
In the case of an air supply for which the supply temperature is defined by an absolute profile, the supply humidity ratio is assumed to be that of the outside air, unless the supply air approaches saturation under this assumption. The supply air percentage saturation is limited by the greater of 90% and the outside air percentage saturation.
The carbon dioxide concentration of outside air is assumed to take the fixed value of 360 ppm (volumetric).
The calculation of air flow rates by MacroFlo and ApacheHVAC is dealt with in the sections devoted to these programs.
