21
I
Determine cooling and heating loads at design
conditions.
Given:
Required Cooling Capacity (TC) . . . . . .67,000 Btuh
Sensible Heat Capacity (SHC). . . . . . . .46,000 Btuh
Required Heating Capacity . . . . . . . . . .85,000 Btuh
Outdoor Entering-Air Temperature db . . . . . . . .95 F
Outdoor Entering-Air Temperature wb. . . . . . . .75 F
Outdoor-Air Entering Airflow Cfm . . . . . . 450 Cfm
Outdoor-Air Winter Design Temperature . . . . . . 0° F
Indoor-Air Winter Design Temperature . . . . . . .70 F
Air to room including outdoor air . . . . . . 2000 Cfm
External Static Pressure . . . . . Supply — 0.60 in. wg
Return — 0.2 in. wg
Indoor-Air Temperature db (room air) . . . . . . . .78 F
Indoor-Air Temperature wb (room air) . . . . . . . .65 F
Indoor-Air Exhaust Cfm . . . . . . . . . . . . . . 450 Cfm
Electrical Characteristics (V-Ph-Hz) . . . . . . 230-1-60
Vertical discharge unit with Energy$Recycler
required.
II
Determine fan speed and power requirements
at design conditions.
Before entering the Fan Performance tables, calculate
the total static pressure required based on unit com-
ponent. From the given and the Economizer Static
Pressure tables on page 48 find:
External static pressure supply
0.6 in. wg
External static pressure return
0.2 in. wg
Accessory static — None
0.0 in. wg
Total Static
0.8 in. wg
Enter the Fan Performance table for vertical
discharge, standard motor, 48HJ006 at 0.80 in. wg
and 2000 cfm. The rpm is 1303 and bhp is 1.41.
NOTE: Convert bhp to Fan Heat and Watts using the
formula found in the note following the Evaporator-
Fan Motor Efficiency table on page 49.
For this example:
Watts = (746 x Bhp)/(motor efficiency)
Watts = (746 x 1.41)/(0.74) = 1421 watts
Indoor Fan Heat = watts x 3.413 Btuh/watt
= 1421 x 3.413 = 4851 Btuh
III
Select Energy$Recycler based on outdoor
entering cfm.
Using Energy$Recycler cooling rating tables found in
the Energy$Recycler Product Data choose 62AQ060
unit.
Using 450 cfm outdoor supply airflow, 95 F OD db,
and 75 F OD wb find performance of the 62AQ060
at these conditions:
Energy$Recycler Gross Cooling
Capacity is
13,000 Btuh
Energy$Recycler Gross Sensible
Capacity is
10,100 Btuh
Compressor power is
1.06 kW
Energy$Recycler Leaving db is
73 F
Energy$Recycler Leaving wb is
66.9 F
IV
Using the simplified* method below, calculate
the approximate mixed air temperature for the
SRT evaporator coil.
Using the outdoor air entering cfm, the room cfm and
the room exhaust airflow with their respective db and
wb temperatures determine the db and wb entering
the rooftop evaporator coil.
a) Estimate the mixed air db to the evaporator coil.
t mix db = ((cfm oa x t oa db) + ((cfm ra – cfm exh)
x t rm db)) / ((cfm ao + (cfm ra – cfm exh)))
= ((450 cfm x 73.0 F) + ((2000 cfm – 450 cfm)
x 78 F)) / ((450 cfm + (2000 cfm – 450 cfm)))
= 76.9 F
mixed air into the rooftop evaporator
coil
b) Estimate the mixed air wb to the evaporator coil.
t mix wb* = ((cfm oa x t oa wb) + ((cfm ra – cfm
exh) x t rm wb)) / ((cfm oa + (cfm ra – cfm exh)))
= ((450 cfm x 66.9 F) + ((2000 cfm – 450 cfm)
x 65 F)) / ((450 cfm + (2000 cfm – 450 cfm)))
= 65.4 F wb
mixed air temperature into the
rooftop evaporator
*Simplified method of determining wet bulb (wb)
temperature of mixture. This approximation is
used because the wb lines in the area of the psy-
chrometric chart in the area used in the calcula-
tion is relatively linear, providing a close approxi-
mation. A more accurate solution can be found
using the E-Cat program.
LEGEND
V
Determine the cooling load requirement for the
rooftop unit.
Required Cooling Capacity is
67,000 Btuh
Less Cooling Capacity supplied
by the Energy$Recycler
–13,000 Btuh
Rooftop cooling load required is
54,000 Btuh
VI
Select the rooftop unit based on mixed air
entering conditions and cooling load.
Enter cooling capacity table at outdoor entering tem-
perature 95 F, mixed air entering evaporator at
2000 cfm, 76.9 F db, and 65.4 F wb.
The 48HJ006 will provide a total gross cooling
capacity of 59,420 Btuh and sensible cooling of
43,420 Btuh.
Because these values were not at 80 F entering db,
they were calculated based on the notes following the
Cooling Capacity tables.
NOTE: Unit ratings are gross capacities and do not
include the effect of evaporator-fan motor heat. To
calculate net capacities see Steps VII and VIII.
cfm —
cubic feet per minute of air
db
—
dry bulb
exh —
Energy$Recycler discharge
mix —
mixture of o return air
oa
—
outside air leaving Energy$Recycler
ra
—
return air
sa
—
supply air at coil (sa = oa + ra – exh)
t
—
temperature
TC
—
total capacity gross
SHC —
sensible capacity
wb
—
wet bulb
Selection procedure (with 48HJ006 example)
