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 inTable
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, ideal 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 which is close to, or over, a 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 localTrane 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.
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