Sensible and Latent Loads
This three-point outline (with interspersed comments) describes how
the calculator accounts for latent loads. This is a high-level
algorithmic progression from
operating conditions to runtimes. Portions of this are also discussed on
the "Equipment Response to Loads" (previous) page.
- Evaporator Coil Conditions: Outside air
(ventilation) and indoor air mix before passing over the
coil. Conditions in the mixed stream depend on weather
(outdoor dry-bulb and wet-bulb) and indoor air (dry-bulb,
setpoint, and room humidity).
The analysis of latent and sensible loads starts at the evaporator coil. It is the conditions at the coil that determine how much of the system's total capacity is available for handling sensible loads and how much of that capacity will be used for dehumidification. The coil conditions are determined by mixed-air calculations that consider the contributions from the two air streams (indoor return air and outdoor ventilation air).
- Latent and Sensible System Capacity:
Components of total capacity are based on mixed air
- Rated system capacities and S/T ratio (at AHRI test
conditions) are projected to coil conditions.
- ASHRAE psychrometric routines (thermodynamic properties of moist air)
- DOE-2 total-capacity curves for
- EnergyPlus Apparatus
dew-point and bypass-factor algorithms for
calculating latent and sensible components of
- EnergyPlus Apparatus dew-point and bypass-factor algorithms for calculating latent and sensible components of total capacity
The three sub-bullets list the algorithmic sources for the projection calculations. The result of the projection is sensible and latent capacities at coil conditions. A significant portion of the computational code of the calculator is in support of this step.
- Rated system capacities and S/T ratio (at AHRI test conditions) are projected to coil conditions.
- Runtimes are determined by the
system’s sensible capacity to maintain
the indoor dry-bulb (satisfy sensible loads) as controlled
by the thermostat.
- Building Sensible Loads: Load-line established to match (1) the system sensible capacity at outdoor design conditions and (2) the sensible internal loads when outdoor conditions equal the setpoint.
- Building Latent Loads (condensate): Calculated using S/T ratio at coil conditions and sensible loads (see “LLdE” and “S/T” columns in the bin tables).
This statement on runtimes reflects how a thermostatically controlled air conditioner works. It simply tries to keep the dry-bulb temperature from rising. That is, as sensible loads act to increase the temperature of the room air (and eventually trigger the thermostat), the air conditioner runs (cooling the air) until the thermostat says to stop.
Runtimes are basically the ratio of the sensible load to the sensible capacity. So by establishing the sensible capacity of the system we can determine how long it has to run (to satisfy the thermostat).
The first sub-bullet describes the two outdoor conditions that establish the sensible-load line (design and setpoint).
While the unit runs to counteract sensible loads, it also dehumidifies. To calculate condensate formation (latent coil load), the split between sensible and latent capacities (the S/T ratio) and the sensible load (SL), can be used. S/T is short for the ratio of the sensible to total capacities.
LL = TL * L/T = (SL/(S/T)) * (1 - S/T)
This LL (Latent Load) is reported in the RTUCC as the “LLdE” column in the bin calculations. Another column that gives a good indication of how the coil conditions are affecting the sensible and latent capacities is the “S/T” column.
Systems in humid climates will have a lower portion of their capacity available for sensible cooling (lower S/T ratios).
Assumptions about Indoor Humidity:
The two plots below show the difference between the outside and inside humidity ratio as a function of the outdoor-indoor temperature difference (left is raw hourly; right is daily average). This data is generated from EnergyPlus modeling of a medium-sized office building. This shows a positive difference during the summer cooling season (ΔT greater than -10), and a negative difference in winter. Summer cooling, and resulting dehumidification, suppresses the indoor humidity ratio.
The RTUCC has two ways of modeling indoor humidity. The default is to assume that that indoor humidity ratio tracks with the outdoor humidity ratio. This assumption produces mixed-air (coil conditions) more humid than would be seen in an hourly simulation like EnergyPlus. The corresponding latent coil loads will be higher in the RTUCC than a corresponding hourly model like EnergyPlus. The alternate assumption of fixed indoor humidity assumes that some other conditioning device is acting to control the indoor humidity to a fixed level. The latent coil loads of the RTUCC should be similar to that of an hourly simulation which also uses a fixed humidity assumption.