| Application and Design Information (cont.) Ductwork Alternatives — Static Regain
Round Versus Rectangular
Round Versus
Rectangular Ductwork
Round duct substantially reduces low frequency noise immediately adjacent
to the equipment room. This is due to the geometry of
the duct, However, if the duct is damaged or is significantly out of round, this low frequency attenuation is
significantly reduced. Round duct also dramatically
reduces duct leakage. The main drawbacks of round duct are typically size and cost.
However, round duct can be extremely cost effective
when using static regain high velocity round duct
design advantages.
Static Regain Round Duct Design
Consider static regain design methods using round duct which can reduce
installed ductwork cost by $50 to $100/ton. Refer to
the Trane applications engineering manual "VARIABLE AIR VOLUME DUCT DESIGN"
for more detailed information.
The ductwork system remains balanced at all load conditions when the
static regain Trane duct design program is used and the
equipment is installed correctly. The main concern is the higher risk of flow regenerated noise resulting from improper
installation.
Free Cooling Opportunities And Alternatives
Free cooling is available with either airside or waterside economizers. The
advantages and disadvantages of each type are listed
below. In general the waterside economizer is preferred over the airside economizer.
Waterside Economizer
The waterside economizer substantially
reduces the compressor energy requirements. This
economizer uses cooling tower water.
Costly humidification is not required as the return air usually satisfies the 30 to 50 percent relative humidity winter
requirement.
Typically no external wall penetrations or control systems are required
for exhaust ducts. Typical venting may utilize the
toilet exhaust system. The mechanical rooms can be located in the center of the building.
Control of the waterside economizer is provided as an integral part of the Trane unit control module (UCM). Two control
options, modulating standard and modulating energy
saving, are offered with the waterside economizer.
The first cost of the self-contained equipment only is slightly higher. However the total system installed cost is
typically lower than with an airside economizer.
The disadvantages include higher airside pressure drop and a higher head on
condenser water pumps.
The condenser water piping generally does not have to be insulated
because the humidity is typically lower when the
condenser water is very cold.
The coils may be mechanically cleanable (optional) for ease in maintenance
versus the expensive and difficult chemical cleaning
methods. This allows the equipment to be restored to full efficiency in the event
fouIing occurs.
Airside Economizer
The airside economizer substantially reduces compressor, cooling tower, and condenser water pump energy requirements.
The airside economizer has a lower airside pressure drop than waterside economizers and requires minimum maintenance.
Outside air is used for free cooling.
It also reduces tower make up water needs and related water treatment.
Drawbacks include the potential requirement for humidification in the winter
and large penetrations on the exterior of the building.
Also, the location of the mechanical equipment room is pushed toward an
exterior wall to minimize the ductwork requirements.
Otherwise, huge interior air shafts are required.
The control of the exhaust system can provide a significant problem to be
overcome.
The disadvantages of the airside economizer include an installed cost that is
higher.
Isolation Recommendations
Unit
For most manufacturers external isolation is required. However, the Trane
unit is internally isolated so that external isolation
may not be required. Note that the internal isolation keeps the vibration from reaching and damaging the unit controls. Use
of the isolation pads shipped with the unit is recommended. For fan speeds below 800
rpm, contact your local Trane office for details.
The incremental cost of isolation is small in comparison to the consequences
of installed isolation problems. Therefore, when in
doubt, external isolation should be used.
Ductwork
Design duct connections to the unit using a flexible material such as heavy
coated canvas. If a fire hazard exists, the recommended
material is Flexweave 1000, Type FW-30 or equivalent, Consult local codes. |
