Table of Contents
Advertisement
The curb should be located according to the location
recommendations above, and properly sealed to pre-
vent moisture and air leakage into and out of the duct
system. Flexible collars should be used when connect-
ing the duct work to prevent unit noise transmission
and vibration into the building.
Duct work should be supported independently of the
unit.
ACOUSTICAL CONSIDERATIONS
2
The eco
unit is designed for lower sound levels than
competitive units by using flexible fan connections, fan
spring isolators, double-wall construction, and lower
speed and horsepower fans. For VAV applications,
VFDs are used instead of inlet guide vanes. Additional
sound attenuation can be obtained using compressor
sound blankets and field-supplied sound attenuators
when necessary.
Even with these equipment design features, the acous-
tical characteristics of the entire installation must never
be overlooked. Additional steps for the acoustical char-
acteristics of a rooftop installation should be addressed
during the design phase of a project to avoid costly al-
terations after the installation of the equipment. During
the design phase of a project, the designing engineer
should consider, at a minimum, the impact of the equip-
ment location, rooftop installation, building structure,
and duct work.
SELECTION PROCEDURE
Given:
Required total cooling capacity of 600 mbh and sen-
sible cooling capacity of 450 mbh with evaporator en-
tering air conditions of 83°F dry bulb and 67°F wet bulb.
Design ambient temperature is 95°F dry bulb. Supply
air requirements are 17500 cfm of air at 2.25 IWG ex-
ternal static pressure. Power supply is 460V/3ph/60Hz
YORK INTERNATIONAL
and the unit requires a modulating economizer, 2-inch
pleated filters, bottom supply and bottom return air open-
ings and is constant volume.
Select Unit:
1. Determine the internal static pressure drop of the
cabinet by referencing Table 8.
Wet evaporator coil
Bottom supply opening
Bottom return opening
2-inch pleated filters
Economizer openings
Modulating economizer dampers
2. Determine the total static pressure by adding the
internal to the external static pressure.
TSP = 1.46 IWG + 2.25 IWG
= 3.71 IWG total static pressure
3. Determine the BHP of the supply fan from Table 6
using the supply air flow and total static pressure.
From the table, we interpolate to get 15.1 BHP. As-
suming a drive loss of 3% and a motor efficiency of
90%, we can calculate the heat rejection of the sup-
ply fan motor as:
(2545 x 15.1)/(0.90 x (1-0.03)) = 44.0 mbh
Required Cooling Capacities:
Total = 600 + 44.0 = 644 mbh
Sensible = 450 + 44.0 = 494 mbh
4. Required total and sensible capacities are 644 mbh
and 494 mbh, respectively. Using the Cooling Per-
formance Data starting with Tables 2, locate the
table with the correct ambient air temperature. Next,
trace the 83°F entering air dry bulb temperature to
match the 17,500 cfm and 67°F entering wet bulb
temperature condition. The resulting conditions are,
from the table, 645 mbh total cooling capacity and
497 mbh sensible cooling capacity. Thus, a 50-ton
unit is selected.
FORM 100.50-EG1
0.54
0.14
0.13
0.10
0.24
0.31
Total
1.46 IWG
11
Advertisement
Table of Contents
