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50 TRG-TRC001-EN
notes
period six
Psychrometric Analyses
At part load, when the SHR of the room is reduced from 0.80 to 0.70, the VAV
system responds by reducing the quantity of 56.5°F air supplied to the room to
match the reduced sensible heat gain. The part-load sensible heat gain of
47,000 Btu/hr and the constant supply air temperature, 56.5°F DB, are used to
determine the required part-load air quantity.
When the conditioned supply air D enters the room, it mixes with room air
along the part load SHR line from D to A’. This quantity and temperature of
supply air are suitable to absorb the room's sensible heat gain, but are unable
to completely absorb the latent heat gain. The result is a shift in the room
conditions from the design point A to 78°F DB and 59% RH (A’). While the
simple VAV system does a better job of controlling room humidity than the
simple constant-volume system, it is still unable to maintain the desired
condition of 50% RH.
To more accurately determine the final room conditions, you would cycle
through the psychrometric chart again. First, use the new room condition A’ to
calculate the mixed-air entering conditions. Then use the coil curves to find the
condition of the 56.5°F dry-bulb supply air. Finally, draw the SHR line through
this new supply air condition to find the resulting room conditions. Repeating
this process a few times allows the room condition to converge and be equal to
the condition used in the previous iteration.
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Figure 80
Supply Airflow (cfm)
47,000 Btu/hr
1.085 78°F 56.5°F–()×
2,015 cfm==