46 TRG-TRC001-EN
notes
period six
Psychrometric Analyses
Using the part-load conditions from the previous example, the room’s sensible
heat gain is reduced from 80,000 Btu/hr to 47,000 Btu/hr while the latent heat
gain remains the same.
Sensing the reduction in dry-bulb temperature due to the lower sensible heat
gain, the room thermostat assumes control of the reheat coil. The cooling coil is
controlled to provide a constant supply air temperature
(D = 56.5°F DB), while the reheat coil is controlled to add just enough heat to
the supply air to offset the reduction in room sensible-heat gain.
Since the supply airflow is constant and the desired room dry-bulb temperature
and sensible heat gain are known, we can calculate the required “re-heated”
supply air temperature.
Since the supply air leaving the coil is sensibly heated (i.e., no moisture is
added or removed), it moves horizontally along a constant humidity-ratio line
from D to E. The resulting supply air conditions are 65.4°F DB, 58.9°F WB.
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Figure74
47,000 Btu/hr
1.085 78°F Supply DB–()×
3,430 cfm=
Supply DB 65.4°F=