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Application Considerations |
Heat selections other than gas heat will not be affected by
altitude. Nominal gas capacity (output) should be multiplied by the factors given in Table
17-3 before calculating the heating supply air temperature.
Acoustical Considerations
Proper placement of rooftops is critical to reducing
transmitted sound levels to the building. The ideal time to make provisions to reduce
sound transmissions is during the design phase. And the most economical means of avoiding
an acoustical problem is to place the rooftop(s) away from acoustically critical areas. If
possible, rooftops should not be located directly above areas such as: offices, conference
rooms, executive office areas and classrooms. Instead, idea locations might be over
corridors, utility rooms, toilets or other areas where higher sound levels directly below
the unit(s) are acceptable.
Several basic guidelines for unit placement should be
followed to minimize sound transmission through the building structure:
1
Never cantilever the compressor end of the unit. A
structural cross member must support this end of the unit.
2
Locate the unit’s center of gravity close to or over
column or main support beam.
3
If the roof structure is very light, roof joists must
be replaced by a structural shape in the critical areas described above.
4
If several units are to be placed on one span, they should
be staggered to reduce deflection over that span.
It is impossible to totally quantify the effect of building
structure on sound transmission, since this depends on the response of the roof and
building members to the sound and vibration of the unit components. However, the
guidelines listed above are experience-proven guidelines which will help reduce sound
transmissions.
Clearance Requirements
The recommended clearances identified with unit dimensions
should be maintained to assure adequate serviceability, maximum capacity and peak
operating efficiency. A reduction in unit clearance could result in condenser coil
starvation or warm condenser air recirculation. If the clearances shown are not possible
on a particular job, consider the following:
• Do the clearances available allow for major service
work such as changing compressors or coils?
• Do the clearances available allow for proper outside
air intake, exhaust air removal and condenser airflow?
• If screening around the unit is being used, is there
a possibility of air recirculation from the exhaust to the outside air intake or from
condenser exhaust to condenser intake?
Actual clearances which appear inadequate should be
reviewed with a local Trane sales engineer.
When two or more units are to be placed side by side, the
distance between the units should be increased to 150 percent of the recommended single
unit clearance. The units should also be staggered for two reasons:
1
To reduce span deflection if more than one unit is placed
on a single span. Reducing deflection discourages sound transmission.
2
To assure proper diffusion of exhaust air before contact
with the outside air intake of adjacent unit.
Duct Design
It is important to note that the rated capacities of the
rooftop can be met only if the rooftop is properly installed in the field. A well
designed duct system is essential in meeting these capacities.
The satisfactory distribution of air throughout the system
requires that there be an unrestricted and uniform airflow from the rooftop discharge
duct. This discharge section should be straight for at least several duct diameters to
allow the conversion of fan energy from velocity pressure to static pressure.
However, when job conditions dictate elbows be installed
near the rooftop outlet, the loss of capacity and static pressure may be reduced through
the use of guide vanes and proper direction of the bend in the elbow. The high velocity
side of the rooftop outlet should be directed at the outside radius of the elbow rather
than the inside. |
