14
I Determine the actual cooling and heating
loads at the desired dry bulb and wet bulb
conditions.
Assume cooling load at desired dry bulb 80 F and
wet bulb 66 F conditions are as follows:
Given:
Total Cooling (TC). . . . . . . . . . . . . . .11,750 Btuh
Sensible Cooling (SC) . . . . . . . . . . . . .8,650 Btuh
Entering-Air Temperature db . . . . . . . . . . . . 80 F
Entering-Air Temperature wb . . . . . . . . . . . . .66 F
II Determine the following design parameters.
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.
For applications using multiple units, the water pres-
sure drop should be kept as close as possible across
units to make water balancing easier. Enter the
50KQL12 Performance Data tables and find the
proper indicated water flow and water temperature.
For example:
Entering Water Temp . . . . . . . . . . . . . . . . . 90 F
Water Flow (Based upon
10 F rise in temp) . . . . . . . . . . . . . . . . . . 3.1 gpm
Airflow Cfm . . . . . . . . . . . . . . . . . . . . . . 308 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 tables at the design water flow and water
temperature. Read the total and sensible cooling
capacities.
NOTE: Interpolation is permissible, extrapolation is
not.
For example:
Enter the 50KQL12 Performance Table at design
water flow and water temperature. Read Total
Cooling, Sensible Cooling and Heat of Rejection
capacities:
Total Cooling . . . . . . . . . . . . . . . . . .12,200 Btuh
Sensible Cooling . . . . . . . . . . . . . . . . 8,900 Btuh
Heat of Rejection . . . . . . . . . . . . . . .15,900 Btuh
NOTE: It is quite normal for water source heat
pumps to be selected on cooling capacity only since
the heating output is usually greater than the cooling
capacity. Heating capacity is selected based on
different entering water conditions than cooling
capacity.
IV Determine the correction factors associated
with the variable factors of dry bulb and wet
bulb using the Corrections Factor tables
found in this book.
Using the following formulas to determine the cor-
rection factors of dry bulb and wet bulb:
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
V Determine entering air and airflow correction
using the Corrections Factor tables found in
this book.
The nominal airflow for 50KQL12 is 350 cfm. The
design parameter is 325 cfm.
308/350 = 88% of nominal airflow
Use the 88% row in the Nominal Cfm Correction
table.
The Entering Air Temperature wb is 66 F. Use the
66.2 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
requirements established in Step I. If the capacities
are within 10% of the load requirements, the equip-
ment 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 Water temperature rise calculation and
assessment.
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.
Table
Ent Air
Airflow
Corrected
Corrected
Total Cooling
= 12,200 x 0.983 x 0.979 = 11,741
Corrected
Sensible Cooling =
89,900 x 1.036 x 0.936 =
8,630
Corrected
Heat of Rejection = 15,900 x 0.985 x 0.979 = 15,333
Actual Temperature
Rise
=
Correction of Heat Rejection
gpm x 500
Actual Temperature
Rise
=
15,333
=
9.9 F
3.1 x 500
Selection procedure (with 50KQL12 example)
