13.4 Energy Recovery Systems: Enthalpy Wheels, Fixed-Plate Exchangers, Heat Pipes & Runaround Loops

13.4 Energy Recovery Systems: Enthalpy Wheels, Fixed-Plate Exchangers, Heat Pipes & Runaround Loops

Outdoor air ventilation represents one of the largest thermal energy loads in modern commercial HVAC systems. In a $100%$ Dedicated Outdoor Air System (DOAS), conditioning raw ambient air requires substantial sensible and latent energy. Energy Recovery Ventilation (ERV) and Heat Recovery Ventilation (HRV) systems capture thermal energy from departing building exhaust air to pre-condition incoming outdoor ventilation air. ASHRAE Standard 90.1 establishes prescriptive mandates for energy recovery based on supply airflow ($\text{CFM}$), percent outdoor air, and local climate zone.

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1. Governing Heat Exchanger Thermodynamics & AHRI Standard 1060

AHRI Standard 1060 (Performance Rating of Air-to-Air Heat Exchangers for Energy Recovery Ventilation Equipment) defines thermal effectiveness ($\epsilon$) across sensible, latent, and total enthalpy domains:

+---------------------------------------------------------------------------------------------------------+
| ENERGY RECOVERY AIRSTREAM CONFIGURATION & NOMENCLATURE                                                  |
+---------------------------------------------------------------------------------------------------------+
|                                                                                                         |
|   Outdoor Air (Supply Inlet: State 1) -------> [ ENERGY RECOVERY ] -------> Leaving Supply (State 2)    |
|   (T_s1, W_s1, h_s1, CFM_s)                    [    EXCHANGER    ]         (T_s2, W_s2, h_s2)           |
|                                                [                 ]                                      |
|   Leaving Exhaust (State 4) <----------------- [ (Sensible/Total)] <------- Exhaust Air (Inlet: State 3)|
|   (T_e2, W_e2, h_e2)                                                       (T_e1, W_e1, h_e1, CFM_e)    |
|                                                                                                         |
+---------------------------------------------------------------------------------------------------------+

AHRI 1060 Thermal Effectiveness Formulations

1. Sensible Effectiveness ($\epsilon_s$): $\epsilon_s = \frac{\dot{m}_s \cdot c_p \cdot (T_{s2} - T_{s1})}{\dot{C}_{\min} \cdot (T_{e1} - T_{s1})}$ When supply and exhaust mass flow rates are balanced ($\dot{m}_s = \dot{m}_e$): $\epsilon_s = \frac{T_{s2} - T_{s1}}{T_{e1} - T_{s1}} \implies T_{s2} = T_{s1} + \epsilon_s (T_{e1} - T_{s1})$

2. Latent Effectiveness ($\epsilon_L$): $\epsilon_L = \frac{\dot{m}_s \cdot (W_{s2} - W_{s1})}{\dot{m}_{\min} \cdot (W_{e1} - W_{s1})}$ When mass flows are balanced: $\epsilon_L = \frac{W_{s2} - W_{s1}}{W_{e1} - W_{s1}} \implies W_{s2} = W_{s1} + \epsilon_L (W_{e1} - W_{s1})$

3. Total (Enthalpy) Effectiveness ($\epsilon_t$): $\epsilon_t = \frac{\dot{m}_s \cdot (h_{s2} - h_{s1})}{\dot{m}_{\min} \cdot (h_{e1} - h_{s1})}$ When mass flows are balanced: $\epsilon_t = \frac{h_{s2} - h_{s1}}{h_{e1} - h_{s1}} \implies h_{s2} = h_{s1} + \epsilon_t (h_{e1} - h_{s1})$

Where:

• $T = \text{Dry-bulb temperature } (^\circ\text{F})$
• $W = \text{Humidity ratio } (\text{lbm}{\text{water}}/\text{lbm}{\text{dry air}} \text{ or } \text{grains/lbm})$
• $h = \text{Enthalpy } (\text{Btu/lbm}_{\text{dry air}})$
• $\dot{C}_{\min} = \min(\dot{m}_s c_p, \dot{m}_e c_p) = \text{Minimum heat capacity rate}$

Psychrometric Energy Transfer Rates

$\dot{Q}_{\text{total}} = 4.5 \cdot \text{CFM} \cdot |h_{s1} - h_{s2}| \quad (\text{Btu/h})$

$\dot{Q}_{\text{sensible}} = 1.08 \cdot \text{CFM} \cdot |T_{s1} - T_{s2}| \quad (\text{Btu/h})$

$\dot{Q}_{\text{latent}} = 4,840 \cdot \text{CFM} \cdot |W_{s1} - W_{s2}| \quad (\text{Btu/h})$

2. Comparison of Energy Recovery Device Technologies

Four primary heat exchanger architectures are utilized in commercial HVAC engineering:

+---------------------------------------------------------------------------------------------------------+
| COMPARATIVE MATRIX OF ENERGY RECOVERY TECHNOLOGIES                                                      |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| FEATURE             | ROTARY WHEEL      | FIXED-PLATE       | HEAT PIPES        | RUNAROUND COIL LOOP   |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Energy Transferred  | Total (Sensible + | Sensible or Total | Sensible Only     | Sensible Only         |
|                     | Latent Moisture)  | (Membrane Plate)  | (Phase change DX) | (Water/Glycol loop)   |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Typical Sensible Eff| 65% – 80%         | 60% – 78%         | 45% – 65%         | 45% – 60%             |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Typical Latent Eff  | 60% – 75%         | 0% (Metal) to     | 0%                | 0%                    |
|                     |                   | 55% (Membrane)    |                   |                       |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Cross-Contamination | Low (EATR 1-3%    | Zero (EATR = 0%)  | Zero (EATR = 0%)  | Absolutely Zero       |
| (EATR Risk)         | with purge seal)  |                   |                   | (EATR = 0.0%)         |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Duct Proximity      | Adjacent Airstreams| Adjacent Airstreams| Adjacent Airstreams| Remote Airstreams     |
| Requirement         | Required          | Required          | Required          | Permitted (Any dist)  |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Moving Parts        | Yes (Motor/Belt)  | No                | No (Passive)      | Yes (Hydronic Pump)   |
+---------------------+-------------------+-------------------+-------------------+-----------------------+
| Ideal Application   | DOAS, Schools,    | Commercial Office,| Laboratories, ORs,| Hazardous Lab Exhaust,|
|                     | High-density Occ. | Cleanrooms        | Wrap-around Dehum.| Industrial Retrofit   |
+---------------------+-------------------+-------------------+-------------------+-----------------------+

1. Rotary Enthalpy Wheels

A porous aluminum or polymer matrix coated with a desiccant (such as silica gel or $3\text{Å}$ molecular sieve) slowly rotates between counter-flowing supply and exhaust airstreams at $15$ to $30\text{ RPM}$. As it passes through the warm/humid airstream, the matrix absorbs heat and moisture, which it transfers to the cooler/drier airstream. A purge sector with positive supply-to-exhaust pressure differential ($\Delta P_{\text{sup}} > \Delta P_{\text{exh}}$) flushes entrained exhaust air back into the exhaust duct, keeping carryover below $1%$.

2. Fixed-Plate Exchangers

Consists of alternating thin plates forming isolated cross-flow or counter-flow air channels. When constructed of aluminum or polypropylene, it transfers sensible heat only. When constructed of polymeric semi-permeable membranes (water-vapor permeable), it functions as an energy recovery plate, transferring both sensible heat and latent moisture with zero liquid or gas bypass.

3. Heat Pipe Heat Exchangers

Sealed tubes charged with a phase-change working fluid (e.g., R-134a or methanol) with an internal capillary wick. Heat from the warm airstream evaporates liquid refrigerant at the bottom, vapor travels to the cold airstream where it condenses and releases latent heat, and capillary action/gravity returns the liquid. Used in wrap-around coil configurations around a cooling coil to provide passive free reheat and enhanced dehumidification.

4. Runaround Coil Loops

Comprises two standard hydronic finned-tube coils—one in the exhaust duct and one in the supply duct—connected by a closed hydronic piping loop circulating a water-glycol mixture driven by a small pump. Its defining advantage is that the supply and exhaust ducts do not need to be adjacent (can be hundreds of feet apart, or on different floors), making it the mandatory choice for hazardous Class 4 laboratory exhaust systems where cross-contamination must be zero.

3. Leakage Metrics: EATR & OACF

To ensure air quality, AHRI Standard 1060 defines two critical leakage parameters:

+---------------------------------------------------------------------------------------------------------+
| AIR LEAKAGE METRICS IN ROTARY ENERGY RECOVERY                                                          |
+---------------------------------------------------------------------------------------------------------+
|                                                                                                         |
|   1. Exhaust Air Transfer Ratio (EATR):                                                                 |
|      Measures the fraction of exhaust air recirculated into the supply air:                            |
|                                                                                                         |
|        EATR = (C_s2 - C_s1) / (C_e1 - C_s1) * 100%                                                      |
|                                                                                                         |
|      Where C is the concentration of an inert tracer gas (e.g., SF6). Target: EATR < 1% to 3%.          |
|                                                                                                         |
|   2. Outdoor Air Correction Factor (OACF):                                                              |
|      Ratio of outdoor entering airflow to supply leaving airflow:                                       |
|                                                                                                         |
|        OACF = CFM_s1 / CFM_s2                                                                           |
|                                                                                                         |
|      - OACF > 1.0 : Outdoor supply air leaks into the exhaust airstream (proper fan arrangement).        |
|      - OACF < 1.0 : Exhaust air leaks into the supply airstream (dangerous condition).                   |
|                                                                                                         |
+---------------------------------------------------------------------------------------------------------+

Fan Arrangement for Minimum Cross-Contamination

To prevent contaminant transfer in rotary wheels, the supply blower must be located downstream (draw-through) of the wheel and the exhaust blower must be located downstream (draw-through), ensuring the supply plenum is at a higher static pressure than the exhaust plenum ($P_{\text{supply}} > P_{\text{exhaust}}$).

4. Frost Formation & Low-Temperature Freeze Protection

In severe winter conditions, moisture in the warm exhaust airstream condenses on the cold exchanger surface. If the heat exchanger surface temperature drops below $32^\circ\text{F}$ ($0^\circ\text{C}$), condensation freezes into frost, blocking airflow and causing fan overload.

Theoretical Frost Threshold Temperature

The outdoor air temperature at which frost begins to form ($T_{\text{frost,oa}}$) is derived by setting the leaving exhaust air surface temperature to $32^\circ\text{F}$:

$T_{\text{frost,oa}} = \frac{32^\circ\text{F} - (1 - \epsilon_s) \cdot T_{e1}}{\epsilon_s}$

Where $T_{e1}$ is indoor return/exhaust air dry-bulb temperature (typically $70^\circ\text{F}$).

Example Frost Threshold Calculation

For an ERV with $\epsilon_s = 0.75$ and indoor exhaust at $T_{e1} = 70^\circ\text{F}$:

$T_{\text{frost,oa}} = \frac{32 - (1 - 0.75)(70)}{0.75} = \frac{32 - (0.25 \times 70)}{0.75} = \frac{32 - 17.5}{0.75} = \frac{14.5}{0.75} = \mathbf{19.33^\circ\text{F}}$

When outdoor air drops below $19.3^\circ\text{F}$, frost protection controls must engage.

Frost Mitigation Control Strategies

1. Preheat Coil: A hydronic or electric preheat coil warms entering outdoor air to just above $T_{\text{frost,oa}}$ (e.g., to $25^\circ\text{F}$), preventing frost while maintaining full energy recovery.
2. Variable Speed Wheel Modulation: Slowing the wheel rotation speed (from $20\text{ RPM}$ down to $1-3\text{ RPM}$) reduces sensible effectiveness, elevating the leaving exhaust temperature above $32^\circ\text{F}$.
3. Face and Bypass Dampers: Bypasses a portion of outdoor air around the core, reducing thermal loading and raising exhaust surface temperature.
4. Timed Exhaust Defrost: Shuts off the outdoor air damper periodically, allowing warm exhaust air to thaw the heat exchanger matrix.

5. Worked Engineering Calculation: Dedicated Outdoor Air Enthalpy Wheel Sizing

Problem Statement

A $100%$ Dedicated Outdoor Air System (DOAS) delivers $8,000\text{ CFM}$ of conditioned outdoor air to an office building. An enthalpy wheel provides total energy recovery with rated sensible effectiveness $\epsilon_s = 0.72$ and latent effectiveness $\epsilon_L = 0.68$. Exhaust air mass flow equals supply air mass flow.

• Summer Design Outdoor Air (State 1): $T_{s1} = 95.0^\circ\text{F DB}$, $W_{s1} = 0.0160\text{ lbm}{w}/\text{lbm}{da}$ ($112.0\text{ gr/lb}$), $h_{s1} = 40.5\text{ Btu/lbm}$.
• Indoor Exhaust Air (State 3): $T_{e1} = 75.0^\circ\text{F DB}$, $W_{e1} = 0.0090\text{ lbm}{w}/\text{lbm}{da}$ ($63.0\text{ gr/lb}$), $h_{e1} = 28.2\text{ Btu/lbm}$.

Calculate:

1. Leaving supply air dry-bulb temperature ($T_{s2}$) and humidity ratio ($W_{s2}$).
2. Leaving supply air enthalpy ($h_{s2}$).
3. Sensible cooling load reduction ($\text{Btu/h}$).
4. Latent cooling load reduction ($\text{Btu/h}$).
5. Total cooling capacity avoided (in $\text{Tons}$ of refrigeration).

Step-by-Step Solution

1. Leaving Supply Air Properties ($T_{s2}, W_{s2}$):

• Leaving Dry-Bulb Temperature: $T_{s2} = T_{s1} - \epsilon_s (T_{s1} - T_{e1}) = 95.0 - 0.72 \times (95.0 - 75.0) = 95.0 - 0.72(20.0) = 95.0 - 14.4 = \mathbf{80.6^\circ\text{F DB}}$
• Leaving Humidity Ratio: $W_{s2} = W_{s1} - \epsilon_L (W_{s1} - W_{e1}) = 0.0160 - 0.68 \times (0.0160 - 0.0090) = 0.0160 - 0.68(0.0070) = 0.0160 - 0.00476 = \mathbf{0.01124\text{ lbm}_w/\text{lbm}_{da}}$
  • In grains: $W_{s2} = 0.01124 \times 7,000 = 78.68\text{ gr/lb}$.

2. Leaving Supply Enthalpy ($h_{s2}$):

• Psychrometric approximation: $h = 0.240 T + W(1061 + 0.444 T)$ $h_{s2} = (0.240 \times 80.6) + 0.01124 \times [1061 + (0.444 \times 80.6)] = 19.344 + 0.01124 \times [1096.79] = 19.344 + 12.328 = \mathbf{31.67\text{ Btu/lbm}}$

3. Sensible Heat Reduction:

$\dot{Q}_{\text{sensible}} = 1.08 \times \text{CFM} \times (T_{s1} - T_{s2}) = 1.08 \times 8,000 \times (95.0 - 80.6) = 8,640 \times 14.4 = \mathbf{124,416\text{ Btu/h}}$

4. Latent Heat Reduction:

$\dot{Q}_{\text{latent}} = 4,840 \times \text{CFM} \times (W_{s1} - W_{s2}) = 4,840 \times 8,000 \times (0.0160 - 0.01124) = 38,720,000 \times 0.00476 = \mathbf{184,307\text{ Btu/h}}$

5. Total Avoided Cooling Capacity:

$\dot{Q}_{\text{total}} = \dot{Q}_{\text{sensible}} + \dot{Q}_{\text{latent}} = 124,416 + 184,307 = 308,723\text{ Btu/h}$ $\text{Tons of Avoided Cooling} = \frac{308,723\text{ Btu/h}}{12,000\text{ Btu/(h}\cdot\text{Ton)}} = \mathbf{25.73\text{ Tons}}$

Engineering Summary: The enthalpy wheel reduces the peak chiller sizing required for outdoor air treatment from $32.8\text{ Tons}$ down to $7.1\text{ Tons}$, saving $25.73\text{ Tons}$ ($78.4%$) of peak refrigeration capacity.

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6. NCEES Reference Handbook Navigation Strategies

• AHRI 1060 Effectiveness Equations: Search "Energy Recovery" or "Effectiveness" under HVAC to find the sensible, latent, and total effectiveness equations.
• Psychrometric Equations: Search "Sensible Heat Rate" ($q_s = 1.08 \cdot \text{CFM} \cdot \Delta T$), "Total Heat Rate" ($q_t = 4.5 \cdot \text{CFM} \cdot \Delta h$), and "Latent Heat Rate" ($q_L = 4840 \cdot \text{CFM} \cdot \Delta W$).
• Psychrometric Charts: Locate the ASHRAE Psychrometric Chart No. 1 ($32^\circ\text{F}$ to $120^\circ\text{F}$) to determine enthalpy and humidity ratios from dry-bulb and wet-bulb temperatures.