Selection procedure
Example 1: Selection for specified total
cooling
I Determine job requirements:
Given:
Total Cooling Load ..................67.0 MBtuh
Sensible Cooling Load ...............41.0 MBtuh
Air Delivery ..........................1800 Cfm
Entering Air Temperature ....75.0 F db, 64.5 F wb
Entering Water Temperature.................43F
External Static Pressure...............0.30 in. wg
Maximum Chilled Water Flow Rate......15.0 Gpm
I Select unit based on required air delivery.
From Fan Performance table on page 22, select a unit
that delivers required airflow. Model 42BH size 20 de-
livers 1800 cfm.
I Determine water temperature rise.
This step is used to determine the actual water tem-
perature rise required to provide the specified cooling
capacity.
(This can be done to meet either specified sensible or
specified total capacity. For this example, we will de-
termine water temperature rise required to meet the
specified total capacity after it has been corrected to
the rating curve capacity. The resulting rating curve
sensiblecapacity willbe determinedin StepIV andthen
corrected to actual capacity. If the sensible capacity
were the critical requirement, the reverse procedure
would be followed.)
A. Determine the air delivery correction factors using
actual unit air delivery (1800 cfm).
Enter the 42BH Cooling Capacity Correction Fac-
tors table, page 7, at 1800 cfm and find that a
42BH size 20 unit has a total heat (TH) factor of
0.94 and a sensible heat (SH) factor of 0.93.
B. Correct the specified total capacity to the required
rating curve capacity.
Specified Total Cap.
Rating Curve Capacity =
TH Factor
67.0 MBtuh
=
0.94
= 71.3 MBtuh
C. Enter the cooling capacity curve for a size 20 unit
with standard 4-row coil at 71.3 MBtuh and project
vertically to the specified entering wet bulb (65 F).
Project from this point horizontally to the right to
the specified entering water temperature (43 F) and
then vertically to read water temperature rise of 9.2
degrees F.
I Determinethe rating curvesensible capacity and
the actual sensible capacity..
A. Enter the cooling curve at the intersection of total
capacity (71.3 MBtuh) and entering wet
bulb temperature (65 F). Read horizontally to the
left to the specified dry bulb (75 F). Read down to
the rating curve sensible capacity of 46.0 MBtuh.
B. Correct the rating curve capacity to actual
capacity:
Actual Sensible Cooling
= SH x Rating Curve Sensible Cooling
= 0.93 (From Step III A) x 46.0 MBtuh
= 42.8 MBtuh
I Determine water flow and pressure drop.
A. Actual Total Cooling
Actual Gpm =
.5 x Temp Rise
67.0
=
.5 x 9.2 (From Step III C)
= 14.6
B. Pressure drop is obtained from the curve on page
18 for the 4-row cooling coil and is 10 ft of
water.
I Find Leaving Air Dry Bulb (LDB) and Leaving
Wet Bulb (LWB) Temperatures.
A. Actual Sensible MBtuh
LDB = EDB −
Actual Cfm x .00108
42.8 (Step IV B)
=75 −
1800 x .00108
= 53.0 F
B. Leaving wet bulb is computed by referring to the
Enthalpy at Saturation table on page 21. First de-
termine that the enthalpy at saturation for the en-
tering (h ) wet bulb of 64.5 is 29.69 Btu per lb
ent
of dry air.
Actual Total Capacity
h=h−
lvg ent
.00445 x Actual Cfm
67.0
= 29.69 −
.00445 x 1800
= 21.33
Again referring to the table, the temperature cor-
responding to 21.33, by interpolation, is 51.8 F,
which is the leaving wet bulb.
I Verify that the selected unit will provide the re-
quired external static pressure of 0.30 in. wg
with the selected coil, options, and ductwork.
A. Determine total static pressure. For an application
with ductwork, the total static pressure is equal to
the sum of the external static pressure, duct grilles,
and component static pressures.
Assume for this example that the selected 42BH
size 20 unit with 4-row cooling coil has a double
deflection grille and throwaway filter. Enter the
42BHComponent Static Resistancetable (page21)
at unit size 20, 1800 cfm, and find the static resis-
tance (in. wg) for the following components:
Cabinet 0.12
4-Row Coil 0.37
Throwaway Filter 0.12
Double Deflection Grille + 0.07
0.68
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