The intent of the design process is to achieve the most efficient air handling system to meet the customer’s requirements for air quality, accessibility, reliability, noise criteria, initial cost and operating efficiency.
Factors that govern the unit selection include local building ventilation codes, supply versus ambient air temperature differentials, heating and cooling loads, acoustical performances, and physical limitations. The selection of the unit can then be completed through the following steps :
- Casing type selection ￼
- Unit size selection ￼
- Coil selection ￼
- Accessory selection ￼
- Determination of total static pressure ￼
- Fan type selection and motor horsepower
The following paragraphs outline a suggested procedure for selecting a Racan Carrier air handler.
Casing type selection
The designer must identify the type of application and its special requirements. Theaters, schools, hospitals, laboratories, etc., all have different and specific requirements. Casing panels must be compatible with conditions such as noise, indoor air pollutants, bacterial control, presence of chemicals or a combination of these.
When in doubt regarding the panel selection and its application, contact your local Racan Carrier representative.
Unit size selection
Selection of the optimum unit size can be made based on the required air volume. The heating load, cooling load, and ventilation requirements, any of which may be a maximum, will establish an airflow requirement. The unit air volume for cooling/heating depends on the sensible space cooling/heating load and the design dry bulb temperature differential. The minimum air volume is obtained using the following formula:
CFM = sensible space load (BTUH) 1.08 x temp. differential
The required air volume for ventilation is generally less than that of cooling or heating but must not be overlooked particularly in the case of industrial processes. The unit size can be selected based on the maximum air volume required. If the mechanical room has dimensional limitations that do not allow the use of pre-dimensioned units, the unit can be customized to meet the customer’s needs. Contact your local Racan Carrier representative for further information.
Moisture carryover is of major concern. Therefore, the cooling coil is normally the primary factor governing unit size selection. Air volumes shown in this catalogue are cooling CFM calculated at 500 ft./min. nominal capacity. In certain cases, air volume can be increased over maximum tabulated CFM if the cooling coil has proper fin spacing and if a special fin treatment has been applied. Contact your local Racan Carrier representative for more information.
When only heating coils are involved, reasonable static pressure drops become the governing factor in unit size selection.
Once the coil face area has been selected, the coil performance can be obtained from Racan Carrier or vendor software.
When two or more coils are installed in series in a cooling coil section, all coils must have identical dimensions. Caution should be exercised when locating the coils in certain applications such as freeze protection preheat process or reheat of air following a dehumidification process.
A wide choice of accessories such as filters, dampers, air mixers, and access sections allows for numerous combinations to ensure proper efficiency, reliability, and serviceability.
For air mixing, a mixing box or an economizer section allows a simple way of introducing and/or exhausting air to and from the system. When air stratification is a concern, an air mixer section can be added downstream the fresh air intake or after a face and bypass section. To facilitate maintenance, access sections can be located anywhere in the system. Plenum fans have a bearing mounted in the inlet airstream. It is mandatory that an access section be provided upstream of the plenum fan section for maintenance.
Face and bypass sections allow a different way of controlling air temperature. Internal face and bypass must be used with the small face area coil only. External face and bypass sections can be used with large and extralarge face area coils and require the addition of an external bypass duct.
Determination of total static pressure
The total static pressure resistance consists of the sum of the external static pressure of the distribution system and the internal unit resistance.
The internal unit resistance is calculated by adding the resistance of the coils and of the various unit components and accessories. The face damper pressure drop must be added to the adjacent coil(s) pressure drop(s) and the result compared to the bypass damper pressure drop. The designer must use the higher value.
On blow-thru units, the pressure drop value of only one coil is used, the higher of either the cooling or heating coil. The higher coil pressure drop must be added to the blow-thru section pressure drop. If zone dampers are used, the pressure drop must also be added.
When plenum fans are used, system pressure drop must be evaluated and added accordingly.
Fan type selection and motor horsepower
Once the CFM and total resistance pressure drop are known, the fan selection process can begin.
Airfoil wheels have the following advantages : high efficiency, low noise, and non-overloading characteristics. Backward inclined wheels can also be considered when initial cost is a concern.
When air handlers are floor mounted, the supporting structure must be leveled and rigid enough for satisfactory unit operation.
If a unit has to be ceiling hung, structural beams must be field supplied and installed underneath the unit for proper support. Structural beams must be sized according to industry standards. Maximum deflection of beams under the unit shall be 1/360 of unit length without exceeding ½”.
Basic Racan Carrier air handlers are designed with all accesses and coil connections on one side, permitting unit installation adjacent to a wall. On the access side, it is recommended to maintain 40″ clearance for routine servicing. Additional room should be allowed for coil removal.
Note that access doors are available on one or both sides, as necessary.
Good ductwork layout will minimize system resistance and regenerated sound. Ducts to and from units should allow straight and smooth airflow (see figure 36). Avoid sharp turns at the fan discharge, particularly turns opposed to wheel rotation. Long radius turning vanes are recommended. Avoid abrupt changes in duct sizes. See figure 37 for good fan outlet practices.
Racan Carrier air handlers’ fan discharge, double duct, mixing box, and economizer sections are provided with raw edge duct connections. The multizone damper assembly is provided with 1” perimeter flange and J-clips at each zone damper.
When noise is a concern, it is recommended to install silencers through mechanical room walls on both supply and return paths. When mechanical room walls cannot allow enough noise reduction to adjacent rooms, silencer sections can be incorporated in the air handler.
Piping and drain pan traps
Piping should comply with accepted industry standards. Undue stress should not be applied at the coil header connection. Support the pipe network independently of the coils with adequate piping flexibility for thermal expansion.
All drain pans should have traps to permit the condensate to drain freely. The traps should be sized for draw-thru applications (negative pressure within the casing) or for blow-thru applications (positive pressure within the casing) according to figure 38.
Optional drain connections are sometimes required in other sections, such as humidifier sections, mixing box sections or economizer sections.
To ensure that noise and vibration levels are within the accepted design criteria of the project, good acoustical and engineering practices should be applied during the early stages of the design.
Racan Carrier air handlers include factory installed 2″ deflection internal isolators. The standard spring isolators are specifically selected for each fan wheel and motor combination. Because internal isolation minimizes vibration at the source (fan & motor), there is seldom a need for flexible connections on ductwork or coil piping.
Internal isolation provides an opportunity to significantly reduce installation costs.
Racan Carrier air handlers are built with acoustically rated cabinet panels. Fan sections are designed in accordance with AMCA® recommendations, hence optimizing the acoustical performance.
Racan Carrier sound attenuators can be provided as an integral part of air handlers to reduce the inlet or discharge sound levels. Special configurations for low sound level applications are available from the factory.
For sound critical applications, special “Mock-up” tests can be conducted at the factory.
Contact the factory to obtain the unit inlet, discharge, and radiated sound levels for your application.