of avoiding an acoustical problem is to place any rooftop equipment away from acoustically critical area. If possible, rooftop equipment should not be located directly above areas such as: offices, conference rooms, executive office areas and classrooms. Ideal locations are above corridors, utility rooms, toilet facilities, or other areas where higher sound levels are acceptable.

Several basic guidelines for unit placement should be followed to minimize sound transmission through the building structure:

1

Never cantilever the condensing section 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 a column or main support beam to minimize roof deflection and vibratory noise.

3

If the roof structure is very light, roof joists should 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 transmission.

There are several other sources of unit sound, i.e., supply fan, compressors, exhaust fans, condenser fans and aerodynamic noise generated at the duct fittings. Refer to the ASHRAE Applications Handbook, Chapter 42, 1991 edition for guidelines for minimizing the generation of aerodynamic noise associated with duct fittings.

Trane’s Engineering BulletinRT-EB-80 describes various duct installation considerations specifically addressing indoor sound level concerns. This bulletin includes sound power data on Trane’s IntelliPak Rooftops 20 through 130 tons. Ask your local Trane representative for this informative engineering bulletin.

The VariTrane® Computerized Duct Design Program can be used to analyze the truck duct, run-out duct, VAV control unit and terminal unit noise attenuation. This program quantifies the airborne sound generation that can be expected in each terminal so that the designer can identify potential sound problems and make design alterations before equipment installation.

The Trane Acoustics Program (TAP) allows modeling of rooftop installation parameters. The output of this program shows the resulting indoor NC level for the modeled installation. This program is available from Trane’s Customer Direct Service Network™ (C.D.S.), ask your local Trane representative for additional information on this program.

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 as shown in Figure 17-1 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. Application Considerations