Modular Climate Changer - Page 23

• Available space. High filtration efficiencies require more space (lower velocity) and may enlarge the air handler’s footprint.
• Maintenance. A high degree of filtration can lower cleaning costs in the occupied space.

        Application considerations:

• Exceeding the filter’s face velocity limit will increase its resistance (as well as fan energy consumption) and necessitate more frequent maintenance or replacement.
  
• Use disposable prefilters with HEPA filters to enhance filtration efficiency and reduce filter replacement costs, especially at system start-up or when dust loading is heavy.
  
• Exercise special care to avoid moisture carryover whenever final filters are used. Never place the filter module downstream of a cooling coil without providing an intervening source of reheat (e.g., fan motor); see Figure A-5.
  
• Treating filter media with an antimicrobial coating can reduce the likelihood of microbial contamination.
• Gas-Phase Filters. Equipping the filter module with a carbon or potassium- permanganate filter bed will reliably remove a variety of odors and gases (excluding carbon monoxide and carbon dioxide) from the air stream.

          Application considerations:

• Gas-phase filters must be designed ("tuned") for the specific contaminants to be removed.
  
• Gas-phase filters are expensive — i.e., first cost is high, and the additional fan energy required to overcome the extra pressure drop increases the air handler’s operating cost.
  
• Effective filtration requires low-velocity airflow, increasing the air handler’s length/footprint.
  
• Humidity impairs filtration efficiency. As with particulate filters, never place a gas-phase filter module downstream of a cooling coil without providing an intervening source of reheat (e.g., fan motor); see Figure A-5.

Access/Blank Module

Incorporate access or blank modules in the air handler design to provide access to internal components for cleaning, maintenance and service, or to promote proper airflow through the unit.

Coil Module

Coil modules temper all (full-face) or part (modified-size) of the passing air stream by heating, cooling or dehumidifying it with a factory- mounted coil. To select the right coil for an application’s unique requirements, optimize its capacity, face velocity, pressure drop and construction.

Available coils fall into two categories:

• "Unit coils" are designed exclusively for use in Modular Climate Changer air handlers. They have ½-inch OD tubes; designers can choose either 0.016- and 0.025-inch tube walls, and specify 2 to 8 rows. Though they offer fewer options than "shipping coils," "unit coils" have a lower first cost and larger face area; they also require less distance (module length), reducing the air handler’s footprint.
  
• "Shipping coils" can be applied in either Modular Climate Changers or "built-up" air handling systems. Typified by 5 /8 -inch OD tubes, these coils can be configured with 1 to 12 rows, and 0.020-, 0.025-, 0.035- or 0.049-inch tube walls. Other options available include copper fins, 1-inch OD steam coils, and ½-inch Delta-Flo ™ coils that optimize air pressure drop and capacity with an economical fin design. Compared to "unit coils," "shipping coils" offer greater design flexibility; however, they have a higher first cost, a smaller face area, and require a longer module to promote proper air distribution.

Application considerations for chilled water and DX coils:

• Size the coil to prevent moisture carryover due to high airflow velocities. Velocities up to 600 fpm are acceptable depending on air conditions and coil fin arrangement.
  
• Properly size the condensate trap to provide positive drainage; see Figure 7.
  
• Specify two-way-sloped drain pans (Figure 6) to eliminate level seams and promote condensate flow directly to the drain outlet. Consider stainless- steel construction to prolong drain pan life.
  
• Provide adequate freeze protection for chilled water coils; see "AHU Functions: Comply with ASHRAE Standard 62-1989" earlier in this section. Application considerations for hot water coils:
  
• Heating with hot water presents an attractive alternative for buildings without a ready source of steam.
  
• Providing effective freeze protection is more difficult for hot-water preheat coils than it is for steam. To minimize the risk of coil freeze-up, use face-and- bypass dampers and operate the coil at full capacity.

          Application considerations for steam coils: 

• Properly pipe and trap the coil to provide positive drainage.
  
• Steam coils are less susceptible to freeze-up than hot water coils. Trane N and NS steam distributing coils use steam pressure to blow condensate from the coil. For additional freeze protection, use face-and-bypass dampers and operate the coil at full capacity.