10.1 Air Handling Units (AHUs), Dedicated Outdoor Air Systems (DOAS) & Fan-Coil Units

10.1 Air Handling Units (AHUs), Dedicated Outdoor Air Systems (DOAS) & Fan-Coil Units

Air-side distribution systems are the primary thermal and ventilation delivery mechanisms in commercial, institutional, and industrial HVAC applications. The primary function of an Air Handling Unit (AHU) or Dedicated Outdoor Air System (DOAS) is to process, filter, condition (heat, cool, humidify, dehumidify), and circulate moist air throughout building zones to satisfy indoor environmental quality, ventilation, and thermal comfort criteria. On the PE Mechanical: HVAC and Refrigeration exam, air-handling equipment questions test psychrometric coil processes, fan heat temperature rise, freeze protection, sensible-latent decoupling, energy recovery effectiveness, and hydronic terminal selection.

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1. Central Air Handling Unit (AHU) Architecture & Component Sequence

A custom or packaged commercial AHU consists of an insulated sheet metal casing enclosing a sequential train of aerodynamic, thermal, and filtration components. The physical arrangement of these components determines thermal performance, acoustic generation, filtration efficiency, and maintenance longevity.

+---------------------------------------------------------------------------------------------------------+
|                                STANDARD DRAW-THROUGH AHU COMPONENT TRAIN                                |
+---------------------------------------------------------------------------------------------------------+
|                                                                                                         |
| Outdoor Air (OA) ---\                                                                                   |
|                      [Mixing Box / Dampers] ==> [Prefilter (MERV 8)] ==> [Final Filter (MERV 13-14)]    |
| Return Air (RA) ----/                                                                                   |
|                                                                                                         |
|   ==> [Preheat / Glycol Coil] ==> [Cooling / Dehum Coil] ==> [Supply Fan] ==> [Sound Attenuator]       |
|                                    (with Drain Pan)         (Draw-Thru)       ===> Supply Air (SA)     |
+---------------------------------------------------------------------------------------------------------+

Standard Component Sequence (Airflow Direction)

1. Air Mixing Plenum (Economizer Section): Interlinked motorized outdoor air (OA), return air (RA), and relief/exhaust air (EA) dampers modulate to control the mixed air ratio and execute airside economizer free cooling.
2. Filtration Bank:
   • Pre-filters: Typically 2-inch or 4-inch pleated filters rated MERV 8 (ASHRAE 52.2) to capture large particulate matter and protect downstream coils.
   • Final Filters: High-efficiency bag, cartridge, or mini-pleat filters rated MERV 13 to 14 (standard LEED/ASHRAE 55 baseline) or MERV 16/HEPA (healthcare/cleanroom applications).
3. Preheat Coil: Located upstream of the chilled water cooling coil in cold climates to temper incoming mixed air above $40^\circ\text{F}$ ($4.4^\circ\text{C}$), preventing coil freeze-up. Utilizes hot water with a dedicated circulating pump, ethylene/propylene glycol, steam distributing tubes (non-freeze type), or face-and-bypass dampers.
4. Cooling and Dehumidification Coil: Finned-tube heat exchanger circulating chilled water ($42^\circ\text{F}$ to $45^\circ\text{F}$) or direct expansion (DX) evaporating refrigerant. Operates with face velocities restricted to $400$ to $500\text{ FPM}$ ($2.0$ to $2.5\text{ m/s}$). Face velocities exceeding $500-550\text{ FPM}$ cause condensed moisture droplet blow-off from fins into the downstream casing.
5. Sloped Condensate Drain Pan: Stainless steel dual-sloped drain pan conforming to ASHRAE Standard 62.1 to prevent standing water, biological slime, and mold amplification. P-traps must be engineered with sufficient water seal depth to overcome negative casing static pressure.
6. Reheat / Heating Coil: Hot water, electric resistance, or indirect gas-fired heat exchanger positioned downstream of the cooling coil for zone temperature control or humidity trim.
7. Supply Air Fan: Centrifugal airfoil, plenum/plug fan, or fan array (FANWALL) driven by electronically commutated (EC) motors or premium-efficiency motors with variable frequency drives (VFDs).
8. Sound Attenuator (Silencer): Factory-engineered dissipative silencer baffles to attenuate blade pass frequencies and broadband fan noise before discharge into the duct system.

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2. Draw-Through vs. Blow-Through Configurations

The relative placement of the supply fan with respect to the cooling coil creates two distinct thermodynamic and aerodynamic architectures:

DRAW-THROUGH CONFIGURATION:                 BLOW-THROUGH CONFIGURATION:
                                            
Coil Under Negative Static Pressure          Coil Under Positive Static Pressure
Fan Heat Added AFTER Cooling Coil           Fan Heat Added BEFORE Cooling Coil
                                            
       Negative (-)       Positive (+)             Positive (+)       Positive (+)
 [Coil] ===> [Fan] ===> Supply Duct           [Fan] ===> [Coil] ===> Supply Duct
               |                                  |
           Fan Heat                           Fan Heat
          Added Here                         Added Here

Comparative Engineering Matrix

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3. Supply Fan Heat Temperature Rise Equations

All mechanical energy losses and thermodynamic work imparted by the fan impeller into the airstream convert into thermal internal energy (heat). For a fan with total pressure rise $\Delta P_t$ (in inches of water gauge, $\text{in. wg}$) and total efficiency $\eta_t$:

$\text{Fan Brake Horsepower: } \text{BHP} = \frac{\text{CFM} \times \Delta P_t\text{ (in. wg)}}{6,356 \times \eta_t}$

$\dot{Q}_{\text{fan heat}}\text{ (Btu/hr)} = \text{BHP} \times 2,544.43\text{ Btu/(hp}\cdot\text{hr)}$

Substituting $\dot{Q} = 1.08 \times \text{CFM} \times \Delta T_{\text{fan}}$ (for standard air density $\rho = 0.075\text{ lbm/ft}^3$ and specific heat $c_p = 0.240\text{ Btu/(lbm}\cdot^\circ\text{F)}$):

$1.08 \times \text{CFM} \times \Delta T_{\text{fan}} = \frac{\text{CFM} \times \Delta P_t}{6,356 \times \eta_t} \times 2,544.43$

$\Delta T_{\text{fan}}\text{ (}{}^\circ\text{F)} = \left(\frac{2,544.43}{6,356 \times 1.08}\right) \times \frac{\Delta P_t}{\eta_t} = 0.3707 \times \frac{\Delta P_t\text{ (in. wg)}}{\eta_t}$

Where:

• $\Delta P_t$ = Total pressure rise across fan ($ ext{in. wg}$)
• $\eta_t$ = Fan total efficiency (decimal, typically $0.60$ to $0.80$)
• If the motor is located inside the airstream (common in packaged units and plug fans), motor electrical inefficiency heat ($1 - \eta_m$) is also transferred to the air:

$\Delta T_{\text{fan+motor}}\text{ (}{}^\circ\text{F)} = 0.3707 \times \frac{\Delta P_t\text{ (in. wg)}}{\eta_t \times \eta_m}$

Exam Rule of Thumb: For a typical commercial AHU with $\Delta P_t = 3.5\text{ in. wg}$ and total fan-motor efficiency $\eta = 0.65$, fan heat produces a temperature rise of $\Delta T = 0.3707 \times \frac{3.5}{0.65} \approx 2.0^\circ\text{F}$. If the leaving cooling coil temperature is $53.0^\circ\text{F}$, the air leaves the draw-through AHU at $55.0^\circ\text{F}$.

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4. Dedicated Outdoor Air Systems (DOAS)

A Dedicated Outdoor Air System (DOAS) is an HVAC architecture that completely decouples building ventilation and latent humidity control from space sensible cooling and heating.

+-----------------------------------------------------------------------------------------+
|                       DECOUPLED DOAS + PARALLEL TERMINAL ARCHITECTURE                   |
+-----------------------------------------------------------------------------------------+
|                                                                                         |
|  100% Outdoor Air ===> [ DOAS AHU ] ======> Dry Neutral Air (70°F DB, 48°F DP)          |
|   (Ventilation &       (Cool to 50°F DP,     - Handles 100% Latent Load                 |
|    Latent Control)      Reheat to 70°F)      - Handles 100% Ventilation Air             |
|                                                      |                                  |
|                                                      v                                  |
|                                            +-------------------+                        |
|                                            |   Occupied Zone   |                        |
|                                            +-------------------+                        |
|                                                      ^                                  |
|                                                      |                                  |
|  Recirculating Zone Air =================> [ Parallel Sensible ]                        |
|                                            [ Terminal Device   ]                        |
|                                            • Fan-Coil Units (FCUs)                      |
|                                            • Active Chilled Beams                       |
|                                            • Radiant Cooling Panels                     |
|                                            • VRF Indoor Units                           |
+-----------------------------------------------------------------------------------------+

Core Engineering Principles of DOAS

1. Latent Load Decoupling: Traditional variable air volume (VAV) systems dehumidify by overcooling mixed air down to $55^\circ\text{F}$ at the central AHU. In spaces with high latent loads (auditoriums, classrooms, gyms), central VAV requires wasteful terminal reheat. DOAS processes 100% outdoor air down to a low dew point ($45^\circ\text{F}$ to $50^\circ\text{F}$), removing all outdoor air moisture plus internal occupant latent generation.
2. Delivery Strategies:
   • Cold Air Delivery ($55^\circ\text{F}$ DB / $48^\circ\text{F}$ DP): DOAS air provides sensible cooling capacity directly to the zone, reducing the sizing of parallel sensible cooling terminals.
   • Neutral Air Delivery ($70^\circ\text{F}$ DB / $48^\circ\text{F}$ DP): DOAS air enters at room temperature, handling zero space sensible load. Parallel terminal systems handle 100% of space sensible loads.
3. ASHRAE Standard 62.1 Multiple Spaces Compliance: In a multi-zone recirculating VAV system, system ventilation efficiency ($E_v$) is governed by the critical zone requiring the highest outdoor air fraction, forcing significant over-ventilation of uncritical zones. Because DOAS delivers dedicated, measured outdoor air directly to each zone, $E_v = 1.0$, reducing total building outdoor air intake by $20%$ to $40%$.

Total Energy Recovery in DOAS (Enthalpy Wheels)

Modern DOAS units incorporate an air-to-air energy recovery device between the exhaust/relief airstream and incoming outdoor airstream:

• Total Enthalpy Wheel: A rotating honeycomb matrix impregnated with a desiccant (silica gel or molecular sieve) rotating at $10$ to $30\text{ RPM}$. It transfers both sensible heat (via matrix thermal mass) and latent moisture (via desiccant adsorption/desorption).
• Effectiveness Equation (Sensible, Latent, or Total):

$\varepsilon = \frac{\dot{m}_{oa} (X_{oa,in} - X_{oa,out})}{\dot{m}_{\min} (X_{oa,in} - X_{ea,in})}$

Where $X$ represents dry-bulb temperature ($T$), humidity ratio ($W$), or enthalpy ($h$), and $\dot{m}{\min} = \min(\dot{m}{oa}, \dot{m}_{ea})$.

$\text{For equal mass flows (}\dot{m}_{oa} = \dot{m}_{ea}\text{): } h_{oa,out} = h_{oa,in} - \varepsilon_t (h_{oa,in} - h_{ea,in})$

$\dot{Q}_{\text{recovered}} = 4.5 \times \text{CFM}_{oa} \times \varepsilon_t \times (h_{oa,in} - h_{ea,in})$

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5. Fan-Coil Units (FCUs) & Parallel Hydronic Terminals

Fan-Coil Units (FCUs) are decentralized zone conditioning units consisting of a small centrifugal fan, filter, and one or two water coils.

2-PIPE FCU (Seasonal Changeover):           4-PIPE FCU (Simultaneous Heating/Cooling):

      +----------------------+                    +----------------------+
      |   Single Coil for    |                    |  Heating  | Cooling  |
      |   Heating OR Cooling |                    |   Coil    |   Coil   |
      +----------------------+                    +----------------------+
          |              |                            |     |     |     |
       Supply          Return                       HW-S  HW-R  CW-S  CW-R
    (Seasonally switched at plant)                (Independent simultaneous loops)

2-Pipe vs. 4-Pipe System Comparison

Chilled Beams (Active vs. Passive)

• Passive Chilled Beams: Natural convection coils mounted at ceiling level. Room air warms, rises, passes through the chilled coil, cools, and sinks back into the occupied zone. Zero fan power, quiet. Handles sensible cooling only.
• Active Chilled Beams: Pressurized primary air from a DOAS unit discharges through aerodynamic nozzles at high velocity, inducing room air through an integrated chilled water coil at an induction ratio of $3:1$ to $5:1$.
• Condensation Prevention Rule: Chilled water supply temperature entering chilled beams must be maintained $2^\circ\text{F}$ to $3^\circ\text{F}$ above the zone dew point temperature (typically $T_{\text{CHWS}} \ge 58^\circ\text{F}$ to $60^\circ\text{F}$). If the zone dew point rises near $T_{\text{CHWS}}$, condensation sensors automatically close the hydronic control valve.

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6. Worked Example: DOAS Enthalpy Wheel & Coil Load Analysis

Problem: A commercial DOAS unit supplies $8,000\text{ CFM}$ of outdoor air to an office building at sea level ($P = 14.696\text{ psia}$, $\rho = 0.075\text{ lbm/ft}^3$). Summer outdoor design conditions are $95.0^\circ\text{F}$ Dry-Bulb (DB) and $78.0^\circ\text{F}$ Wet-Bulb (WB) ($h_{oa} = 41.38\text{ Btu/lbm}$, $W_{oa} = 0.0168\text{ lbm}w/\text{lbm}{da}$). Building exhaust air enters the energy recovery wheel at $75.0^\circ\text{F}$ DB and $62.0^\circ\text{F}$ WB ($h_{ea} = 27.80\text{ Btu/lbm}$, $W_{ea} = 0.0092\text{ lbm}w/\text{lbm}{da}$) with an exhaust flow rate of $8,000\text{ CFM}$.

The DOAS unit features a total enthalpy wheel with a total effectiveness of $\varepsilon_t = 0.72$ and sensible effectiveness of $\varepsilon_s = 0.75$. Downstream of the wheel, a chilled water cooling coil dehumidifies the air to a leaving coil condition of $52.0^\circ\text{F}$ DB and $51.0^\circ\text{F}$ WB ($h_{cc} = 20.80\text{ Btu/lbm}$, $W_{cc} = 0.0078\text{ lbm}w/\text{lbm}{da}$). The draw-through supply fan has a total pressure rise of $\Delta P_t = 2.80\text{ in. wg}$ and total efficiency of $\eta_t = 0.68$.

Find:

1. The condition of the air leaving the enthalpy wheel (enthalpy $h$, dry-bulb temperature $T$, and humidity ratio $W$).
2. The cooling capacity reduction provided by the energy recovery wheel in Tons of Refrigeration.
3. The net cooling coil load in $\text{Btu/hr}$ and Tons.
4. The final supply air dry-bulb temperature ($T_{\text{SA}}$) delivered to the ductwork after fan heat addition.

Step-by-Step Solution:

Step 1: Calculate air properties leaving the enthalpy wheel. $\text{Mass flows are equal (}8,000\text{ CFM OA} = 8,000\text{ CFM EA)} \implies \dot{m}_{oa} = \dot{m}_{\min}$

$h_{\text{wheel,out}} = h_{oa} - \varepsilon_t (h_{oa} - h_{ea}) = 41.38 - 0.72 \times (41.38 - 27.80) = 41.38 - 0.72(13.58)$ $h_{\text{wheel,out}} = 41.38 - 9.778 = 31.60\text{ Btu/lbm}$

$T_{\text{wheel,out}} = T_{oa} - \varepsilon_s (T_{oa} - T_{ea}) = 95.0 - 0.75 \times (95.0 - 75.0) = 95.0 - 15.0 = 80.0^\circ\text{F}$

$W_{\text{wheel,out}} = W_{oa} - \varepsilon_t (W_{oa} - W_{ea}) = 0.0168 - 0.72 \times (0.0168 - 0.0092) = 0.0168 - 0.72(0.0076) = 0.0113\text{ lbm}_w/\text{lbm}_{da}$

Step 2: Calculate thermal energy recovered by the wheel. $\dot{Q}_{\text{recovered}} = 4.5 \times \text{CFM} \times (h_{oa} - h_{\text{wheel,out}}) = 4.5 \times 8,000 \times (41.38 - 31.60)$ $\dot{Q}_{\text{recovered}} = 36,000 \times 9.78 = 352,080\text{ Btu/hr}$

$\text{Cooling Capacity Saved} = \frac{352,080\text{ Btu/hr}}{12,000\text{ Btu/(hr}\cdot\text{ton)}} = 29.34\text{ Tons}$

Step 3: Calculate cooling coil thermal load. $\dot{Q}_{\text{coil, total}} = 4.5 \times \text{CFM} \times (h_{\text{wheel,out}} - h_{cc}) = 4.5 \times 8,000 \times (31.60 - 20.80)$ $\dot{Q}_{\text{coil, total}} = 36,000 \times 10.80 = 388,800\text{ Btu/hr}$

$\text{Coil Capacity (Tons)} = \frac{388,800\text{ Btu/hr}}{12,000\text{ Btu/(hr}\cdot\text{ton)}} = 32.40\text{ Tons}$

(Note: Without the energy recovery wheel, the cooling coil would require $4.5 \times 8,000 \times (41.38 - 20.80) = 740,880\text{ Btu/hr} = 61.74\text{ Tons}$. The wheel reduces cooling coil size by $47.5%$).

Step 4: Calculate fan heat temperature rise and final supply air temperature. $\Delta T_{\text{fan}} = 0.3707 \times \frac{\Delta P_t}{\eta_t} = 0.3707 \times \frac{2.80\text{ in. wg}}{0.68} = 0.3707 \times 4.1176 = 1.526^\circ\text{F} \approx 1.53^\circ\text{F}$

$T_{\text{SA}} = T_{\text{leaving coil}} + \Delta T_{\text{fan}} = 52.0^\circ\text{F} + 1.53^\circ\text{F} = 53.53^\circ\text{F}$

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7. NCEES Reference Handbook Navigation & Exam Tips

• Psychrometric Energy Equations: Look in the HVAC section for $\dot{Q}_s = 1.08 \cdot \text{CFM} \cdot \Delta T$, $\dot{Q}_l = 4840 \cdot \text{CFM} \cdot \Delta W$, and $\dot{Q}_t = 4.5 \cdot \text{CFM} \cdot \Delta h$.
• Fan Power & Heat Relations: Verify whether fan motor is inside or outside the airstream. If the motor is inside, divide total pressure by $\eta_{\text{fan}} \times \eta_{\text{motor}}$.
• Enthalpy Wheel Conservation: When outdoor airflow ($\text{CFM}{oa}$) equals exhaust airflow ($\text{CFM}{ea}$), the effectiveness formula simplifies directly to linear interpolation of enthalpy, temperature, and humidity ratio.