Trane Voyager TC Series, User Manual

Page: 13 / 108

RT-PRC024-EN
13
Selection Procedure
Dehumidification Selection
Note: Dehumidification selection
procedure is the same for
both cooling (TCD) and gas/
electric (YCD) models.
Determine normal unit
cooling and heating
capacities as previously
described in the selection
procedures on prior page.
Typical 20 ton TFD241C
6400 cfm Total Supply airflow
2560 cfm Outside Air (40%)
3840 cfm Return Air
0.41" External Static Pressure
OA Conditions
Part load day and raining
68°F db
67°F wb
66.5 dp
95% RH
RA' conditions
75°F db
63°F wb
52% RH
55.9 dp
Step 1:
Determine the mixed/entering air
condition (MA)
MA = (% outside air*outside air
drybulb temperature) + (% return
air*return air dry-bulb temperature)
MA = (0.40*68°F) + (0.60*75°F)
MA = 72.20°F db
Note: Repeat for wet-bulb
temperature (wb).
Plot on psychrometric chart.
MA
72.2°F db
64.7°F wb
Step 2:
Determine the additional static
pressure drop for a reheat unit
Table 54 shows a static pressure drop
of 0.35" for the reheat coil and an
additional .04 for the mandatory 2"
pleated filters required when
ordering the dehumidification option.
Total static pressure=
1.0 + 0.035 + 0.04 = 1.075
(
≅
1.1 for manual calculations)
Do not forget to also add any
additional static from other
accessories.
Table 45 (airflow table for 20 ton
dehumidification units) indicates that
a standard motor and drive is needed
for this airflow and static pressure
range.
Step 3a:
Determine leaving evaporator
temperature (SA')
Leaving Evaporator Temperature =
SA'
Utilizing the manual Cooling
Capacity selection method as
previously described, find the leaving
evaporator temperatures with the
formula:
Subtract your sensible
Δ
temp from
the entering db and latent
Δ
temp
from the entering wb to determine
the leaving evaporator db & wb
(temperatures without the addition of
fan heat).
51.7 db
51.03 wb
Connect MA and SA' on
psychrometric chart with cooling
curve.
Step 3b:
Determine leaving unit temperature
in standard cooling mode
Repeat Step 3a substituting net
sensible or latent capacity for gross
sensible or latent capacity to find the
leaving unit temperature including
fan heat.
48.9 db 53.6 db
47.0 wb 51 wb
84% RH
49% dp
Figure 2. Psychrometric Chart
Δ
Temp = gross sensible or
gross latent cooling
capacity in Btuh
________________
(cfm) (1.085)
MA
SA'
RA
OA
OA
RA
68°F DB,
67°F WB
75°F DB,
52% RH
MA
72.2°F DB
65°F WB
SA'
46.6°F DB
SA SA
REHEAT
SA
72.2°F DB
57°F WB
46.0°F WB
38% RH
Step 4:
Determine reheat temperature rise
Using the leaving evaporator temp,
go to
Table 59 and find the reheat
temperature rise for that particular
cfm: 17.55°F db
Note: Reheat temperature rise is
based on supply airflow and
leaving evaporator coil
temperature.
Step 5:
Determine leaving unit sensible
temperature with reheat active (SA)
Reheat temperature (obtained in step
3) + (SA' + fan heat) = SA
(SA’ + fan heat) = leaving unit temperature
in standard cooling mode from step 3b.
17.55°F db + 53.6°F = 71.2°F db
SA=71.2°F
Since reheat adds only sensible heat,
the dewpoint temperature will
remain constant so follow the
dewpoint temperature line across the
psychrometric chart to find the new
wb temperature.
51 wb
29.4 dp
20.8 RH
Consider the Psychrometric Chart. If
the space relative humidity is equal
to or above the space relative
humidity setpoint, the
Dehumidification option will:
• Energize compressor or both
compressors (2 stage
compressor units).
• Hot gas reheat valve is energized
and hot gas is diverted to the
reheat coil.
• Dehumidification/reheat is
terminated when space humidity
is reduced to 5% below relative
humidity setpoint.
At MA, air enters the RTU. The RTU
filters, cools, and dehumidifies the
air as it moves through the
evaporator coil. Air leaves the
evaporator coil saturated at the
preset dew point condition (SA') and
is reheated by the hot gas reheat coil
to deliver 71°F (SA) supply air to the
space.