Modular Climate Changer - Page 16

Table A-1 — Multipliers to Adjust Vertical Unit Performance

Dual-Path Layout

Dual-path air handler layouts provide two paths for outdoor air: one for economizer cooling (if used) and the other for ventilation to specifically address ASHRAE Standard 62-1989’s increased minimum outdoor air requirement. These paths effectively work in parallel, and each is provided with its own air treatment components (e.g., filters, heating and cooling coils). Like single-path layouts, dual-path designs can incorporate basic outdoor air, recirculation, exhaust air and energy recovery functions. See Figure A-8.

Designing an air handler with two outdoor paths rather than one (i.e., handling ventilation air separately):

• Reduces or eliminates reheat requirements while providing an effective means of dehumidification for loads with low sensible heat ratios (high latent cooling requirements).

• Avoids increasing supply fan static pressure due to high-pressure-drop components in the ventilation air stream (i.e., increases latent cooling and filtration capacity without increasing fan size).

• Permits downsizing of the ventilation path components.

• Enables compliance with ASHRAE Standard 62’s requirement for measuring outdoor (ventilation) airflow without significantly increasing the first cost of the air handler.

• Provides a cost-effective means to increase ventilation airflow in an existing system.

AHU Arrangements

After schematically organizing the air handler components in a single- or dual-path layout, the components or modules must be physically arranged in a way that fits the space available. Conventional descriptions of air handler arrangements reflect the means of establishing airflow through the coil based on its position relative to the fan. Typically stated, the fan either draws air through a coil located upstream or blows air through a downstream coil.

The Modular Climate Changer adds another dimension to air handler arrangements, allowing designers to combine modules by stacking them on top of each other in space-saving configurations or connecting them together in a side-by-side arrangement with transition panels.

Careful evaluation of each arrangement’s merits is a critical part of the design process.

Draw-Thru or Blow-Thru . . .

Here are four common air handler arrangements, along with general application considerations for each.

Horizontal Draw-Thru

(Figure A-9) This traditional air handler arrangement usually results in the longest unit footprint. Accepted system design practices are generally the only "restrictions" on its application.

However:

• When using a Model Q ™ or plug fan module, provide extra distance (e.g., a medium blank module) between the coil/filter and the fan for proper air distribution.

Vertical Draw-Thru

(Figure A-10) Modular Climate Changer air handlers arranged in this manner typically result in a shorter footprint than horizontal draw-thru units. Vertical coil modules with either 1 /2 -inch "unit" coils or 5 /8 -inch "shipping" coils are available; see "AHU Components: Coil Module." When designing an air handler in this configuration:

• The bottom deck must be longer than the fan module to avoid creating a "cantilever" effect.

• Derate fan performance (speed and brake horsepower) if the coil is in the air-turning section of the unit; see Table A-1. (The Modular Climate Changer selection program does this automatically.)