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Sensible and Latent Loads

Some of the content on this page is also discussed on the "Equipment Response to Loads" (previous) page. This content is focused on how the RTU Comparison Calculator accounts for latent loads (see discussion below for an explanation of the outline).

  1. 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, set point, and room humidity).
  1. Latent and Sensible System Capacity: Components of total capacity are based on mixed air conditions.
    • 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 evaporator coils
        • EnergyPlus Apparatus dew-point and bypass-factor algorithms for calculating latent and sensible components of total capacity
  1. Run times are determined by the system’s sensible capacity to maintain the indoor dry-bulb (satisfy sensible loads) as controlled by the thermostat.
    • Building Sensible LoadsLoad-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).


Discussion:

Comments on #1:

The analysis of latent and sensible loads starts at the evaporator coil. It’s 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).

Comment on #2:

Once coil conditions are known, then the rated capacities can be projected to them. This projection takes the characteristics of the system at AHRI test conditions (Outdoor dry-bulb = 95F, Inside dry-bulb=80F, Inside wet-bulb=67F) and projects to the mixed-air coil conditions in the simulation (as affected by indoor setpoint and weather-bin conditions).

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 RTUCC is in support of this step.

Comment on #3:

This bullet starts off with a statement that 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 (cools the air) until the thermostat says to stop.

Run times 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 sub-bullets here give some detail on the sensible load line (which will probably have been discussed to some degree in previous slides). This bullet describes the two outdoor conditions that establish the 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 above 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 (delta T 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.