6.6 Energy Recovery Devices, HVAC Acoustics & Vibration Isolation

Energy Recovery Devices, HVAC Acoustics & Vibration Isolation

Code-required outdoor ventilation air (ASHRAE Standard 62.1) imposes a major load on the central cooling and heating coils — heating 100% outdoor air in a Minneapolis winter or dehumidifying Miami summer air. The HVAC&R specification lists Energy Recovery (enthalpy wheels, heat pipes, run-around systems) as testable equipment, and its Supportive Knowledge area adds acoustics and vibration control. This section closes both topics.

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1. Air-to-Air Energy Recovery Effectiveness

Energy-recovery devices transfer heat (and possibly moisture) from the exhaust airstream to the outdoor supply airstream without mixing the streams, shrinking the central coil load. Every device is rated by effectiveness, the fraction of the available stream-to-stream driving difference actually transferred:

$\varepsilon = \frac{X_{s,o} - X_{s,i}}{X_{e,i} - X_{s,i}}$

where $X$ is dry-bulb temperature (sensible effectiveness), humidity ratio (latent effectiveness), or enthalpy (total effectiveness); $s$ denotes the supply/outdoor stream, $e$ the exhaust stream, $i$ entering, and $o$ leaving. Sign convention exam trap: in winter the equation raises supply temperature toward the exhaust value; in summer it lowers enthalpy toward the exhaust value — always subtract in the direction of approach to exhaust conditions for a cooling application.

2. Device Types Compared

Engineering Checks Examiners Target

• Frost control: when exhaust air cools toward freezing inside the device, wheels modulate speed and plate devices need frost-prevention preheat or face-and-bypass dampers.
• Cross-leakage: wheel EATR is typically held below roughly 5% using a purge sector between the streams; wheel sections are avoided downstream of hazardous exhaust.
• Fan-energy penalty: the device adds $0.4$–$1.0$ in. w.g. of pressure drop per airstream, which the fans pay every operating hour — recovered energy must exceed the parasitic fan power.
• Run-around economics: the coil loop trades the pump and two coil pressure drops against the ability to serve widely separated ducts.

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3. HVAC Acoustics: Levels, Logarithmic Addition & Noise Criteria

Sound is a small pressure fluctuation; because hearing spans $20,\mu\text{Pa}$ to $20\text{ Pa}$, engineering uses logarithmic decibel scales.

Sound Pressure Level vs. Sound Power Level

$L_p = 20 \log_{10}\!\left(\frac{p_{rms}}{p_{ref}}\right)\text{ dB} \quad (p_{ref} = 20\,\mu\text{Pa}) \qquad L_w = 10 \log_{10}\!\left(\frac{W}{W_{ref}}\right)\text{ dB} \quad (W_{ref} = 10^{-12}\text{ W})$

Manufacturers publish sound power ($L_w$, a property of the source, by octave band); an occupant experiences sound pressure ($L_p$), which depends on distance and the room. In a free field a point source drops 6 dB per doubling of distance; indoors, reverberant buildup reduces the drop-off.

Decibel Addition of Multiple Sources

$L_{p,\text{total}} = 10 \log_{10}\left(\sum_i 10^{L_{p,i}/10}\right)$

Two equal sources combine to just +3 dB above one; a source 10 dB quieter than the dominant one adds only ≈ 0.4 dB and is acoustically irrelevant. Sound-mitigation design therefore attacks the loudest path first.

Noise Criteria (NC) Curves

NC curves rate a measured room octave-band spectrum with a single number tied to speech interference and comfort. Frequently tested design targets:

Sound Control Along the Source–Path–Receiver Chain

• Source: select fans at their peak-efficiency operating point and check published octave-band sound power, not just a single dBA rating.
• Path (duct-borne): duct silencers (lined parallel-baffle attenuators), acoustical liner, and flexible duct; elbows, branches, and end reflections give useful natural attenuation before paying for a silencer.
• Breakout noise: sound radiates through duct walls themselves — heavy-gauge or double-wall duct is specified where ducts cross quiet rooms.
• Receiver: ceiling tile, wall panels, and carpet add room absorption and trim the reverberant level; the Sabine reverberation time is $RT_{60} = 0.049,V/A$ seconds with volume $V$ in ft³ and total absorption $A$ in sabins (ft²).

4. Vibration Isolation of Rotating HVAC Equipment

Fans, pumps, and compressors inject vibration into the structure, which reradiates as audible noise. Isolators are sized on the frequency ratio $r = f / f_n$ between the forcing frequency $f$ and the mounted natural frequency $f_n$:

$T = \frac{\sqrt{1 + (2\zeta r)^2}}{\sqrt{(1 - r^2)^2 + (2\zeta r)^2}} \approx \frac{1}{|1 - r^2|}\ \text{(undamped)} \qquad \text{Isolation efficiency} = (1 - T) \times 100\%$

• Isolation ($T < 1$) requires $r > \sqrt{2} \approx 1.41$; near $r = 1$ the mounting amplifies motion, so isolators carry damping to survive startup/shutdown coast-through of resonance.
• For a steel spring, $f_n \approx 3.13 / \sqrt{\delta_{in}}$ Hz where $\delta$ is static deflection in inches: a 1-inch deflection spring gives $f_n \approx 3.1$ Hz.
• Practical pairings: 3600-rpm (60 Hz) direct-drive equipment isolates well on ≈ 0.5-in deflection springs, while 600-rpm (10 Hz) machines need 2–3 in of static deflection plus inertia bases; flexible pipe and duct connectors prevent flanking paths around the isolators.

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5. Common Exam Traps & PE Pro-Tips

• Trap 1 — Confusing Sensible and Total Effectiveness: An enthalpy wheel's 75% total effectiveness does not mean 75% of the dry-bulb difference and 75% of the humidity-ratio difference are both handled identically in every application — apply the stream property the question names ($T$, $W$, or $h$).
• Trap 2 — Arithmetic dB Addition: Never add decibels arithmetically. $80\text{ dB} + 80\text{ dB} = 83\text{ dB}$, not 160 dB.
• Trap 3 — Isolation Below $\sqrt{2}$: An isolator with $r$ between 1 and $\sqrt{2}$ is not "slightly helpful" — it is not isolating at all, and $r \approx 1$ actively amplifies.
• Trap 4 — Assuming Enthalpy Wheels Eliminate Cross-Leakage: Only the run-around loop truly decouples the streams; wheels manage but do not eliminate EATR.