Product data

13
I Determine the actual cooling and heating
loads at the desired dry bulb and wet bulb
conditions.
Given:
Total Cooling (TC) . . . . . . . . . . . . . . .24,500 Btuh
Sensible Cooling (SC) . . . . . . . . . . . . .21,800 Btuh
Entering-Air Temperature db . . . . . . . . . . . . 80.0 F
Entering-Air Temperature wb. . . . . . . . . . . . . .65 F
II Determine the following design parameters
from Performance Data tables.
Determine entering water temperature, water flow
rate (gpm), airflow (cfm), water flow pressure drop
and design wet and dry bulb temperatures. Airflow
cfm should be between 300 and 450 cfm per ton.
Unit water pressure drop should be kept as close as
possible to each other to make water balancing eas-
ier. For the 50P1H024 unit example, the given
design parameters are as follows:
Given:
Entering Water Temperature . . . . . . . . . . . . . .90 F
Water Flow (Based upon
12 F rise in temperature) . . . . . . . . . . . . 6.0 gpm
Airflow . . . . . . . . . . . . . . . . . . . . . . . . . . 750 cfm
III Select a unit based on total cooling and total
sensible cooling conditions. Unit selected
should be closest to but not larger than the
actual cooling load.
Enter Performance Data tables at the design water
flow and water temperature. Read the total and sensi-
ble cooling capacities.
NOTE: Interpolation is permissible, extrapolation is
not.
For example, enter the 50P1H024 (PSC Blower)
Performance Table at design water flow and water
temperature. Read Total Cooling, Sensible Cooling
and Heat of Rejection capacities:
Total Cooling . . . . . . . . . . . . . . . . . . .23,400 Btuh
Sensible Cooling. . . . . . . . . . . . . . . . .17,500 Btuh
Heat of Rejection . . . . . . . . . . . . . . . .30,200 Btuh
Read the Heat Capacity. If the Heat Capacity
exceeds the design criteria specified in the scope, it is
acceptable.
NOTE: It is normal for water source heat pumps to
be selected on cooling capacity only since the heating
output is usually greater than the cooling capacity.
IV Determine the correction factors associated
with the variable factors of dry bulb and wet
bulb using the correction factor tables found
in this book.
Use the following formulas to determine the correc-
tion factors of total cooling, sensible cooling, and
heat of rejection:
a. Corrected Total Cooling = tabulated total cooling
x wet bulb correction x airflow correction.
b. Corrected Sensible Cooling = tabulated sensible
cooling x wet/dry bulb correction x airflow
correction.
c. Corrected Heat of Rejection = tabulated heat
of rejection x wet bulb correction x airflow
correction.
V Determine entering air and airflow correction
using the correction factor tables found in this
book.
The nominal airflow for the 50P1H024 is 800 cfm.
The design parameter is 750 cfm. Actual airflow
(from table) is 850 cfm.
750/850 x 100 = 88% of actual airflow:
Use the 87.5% row in the Airflow Correction Table.
The Entering-Air Temperature is 65 F wb. Use the
65 F row in the Entering Air Correction Table.
Using the following formulas to determine the cor-
rection factors of entering air and airflow correction:
Compare the corrected capacities to the load require-
ments established in Step I. If the capacities are
within 10% of the load requirements, the equipment
is acceptable. It is better to undersize than oversize as
undersizing improves humidity control, reduces
sound levels and extends the life of the equipment.
VI Calculate and assess the water temperature
rise.
Calculate the water temperature rise and assess the
selection using the following calculation:
For example, using the Corrected Heat of Rejection
from the last step:
If the units selected are not within 10% of the load
calculations, review what effect changing the GPM,
water temperature and/or airflow will have on the
corrected capacities. If the desired capacity cannot be
achieved, select the next larger or smaller unit and
repeat Steps I through VI.
ADDITIONAL REFERENCE CALCULATIONS
HEATING
LWT = EWT – (HE / GPM x 500)
LAT = EAT + (HC / CFM x 1.08)
COOLING
LWT = EWT + (HR / GPM x 500)
LAT (db) = EAT (db) – (SC / CFM x 1.08)
Latent Cooling (LC) = Total Cooling (TC) – Sensible
Cooling (SC)
Table Ent Air Airflow Corrected
Corrected
Total Cooling
= 23,400 x 0.9681 x 0.9889 = 22,402
Corrected
Sensible Cooling
= 17,500 x 1.1213 x 1.0484 = 20,572
Corrected
Heat of Rejection
= 30,200 x 0.9747 x 0.9393 = 27,649
Actual Temperature
Rise
=
Corrected Heat of Rejection
GPM x 500
Actual Temperature
Rise
=
27,649
= 9.2 F
6.0 x 500
Selection procedure (50P1H024 unit example)