Vibration Control in HVAC Systems

Vibration Control in HVAC Systems

Vibration control prevents transmission of mechanical oscillations from HVAC equipment to building structures, eliminating structure-borne noise, occupant discomfort, and equipment degradation. Effective isolation requires understanding vibration sources, applying fundamental isolation principles, and selecting appropriate isolation systems.

Vibration Sources in HVAC Equipment

HVAC systems generate vibrations from multiple mechanisms:

Rotating Equipment:

• Unbalanced fans, motors, and compressors produce forces at rotational frequency
• Residual unbalance creates centrifugal forces: F = meω² where m is unbalanced mass, e is eccentricity, ω is angular velocity
• Belt drives introduce frequency components at belt pass frequency

Reciprocating Equipment:

• Compressor pistons generate forces at twice running speed
• Primary forces follow F₁ = mrω²cos(ωt)
• Secondary forces appear at F₂ = mrω²(r/l)cos(2ωt) where r is crank radius, l is connecting rod length

Flow-Induced Vibration:

• Turbulent flow creates broadband excitation
• Vane pass frequency generates tonal components at n×RPM where n is blade count
• Pressure pulsations from fans and compressors couple into piping and ductwork

Structural Resonance:

• Equipment cabinets and mounting structures amplify vibration at natural frequencies
• Panel resonances typically occur between 100-500 Hz

Isolation Principles and Transmissibility

Vibration isolation exploits the mass-spring system where equipment mass sits on compliant isolators. The system exhibits a natural frequency:

$$f_n = \frac{1}{2\pi}\sqrt{\frac{k}{m}}$$

where k is isolator stiffness (N/m) and m is supported mass (kg).

For simplified spring isolators, natural frequency relates to static deflection:

$$f_n = \frac{1}{2\pi}\sqrt{\frac{g}{\delta_{st}}} = \frac{3.13}{\sqrt{\delta_{st}}}$$

where δₛₜ is static deflection in inches, or using SI units:

$$f_n = \frac{15.76}{\sqrt{\delta_{st}}}$$

where δₛₜ is in millimeters.

Transmissibility Analysis

Transmissibility (TR) defines the ratio of transmitted force to applied force. For an undamped system:

$$TR = \frac{F_t}{F_0} = \frac{1}{\left|1 - \left(\frac{f}{f_n}\right)^2\right|}$$

Adding damping (ζ) provides:

$$TR = \frac{\sqrt{1 + (2\zeta f/f_n)^2}}{\sqrt{\left[1 - (f/f_n)^2\right]^2 + (2\zeta f/f_n)^2}}$$

Critical Frequency Regions:

• f/fₙ < √2: Amplification region where TR > 1
• f/fₙ = 1: Resonance where TR approaches infinity without damping
• f/fₙ > √2: Isolation region where TR < 1

Isolation Efficiency:

$$\eta = \left(1 - TR\right) \times 100%$$

ASHRAE Handbook recommends minimum frequency ratio f/fₙ = 3.5 for effective isolation, providing approximately 90% efficiency.

Isolator Types and Selection

┌─────────────────────────────────────────────────────────────┐
│              HVAC ISOLATOR CONFIGURATION                     │
└─────────────────────────────────────────────────────────────┘

STEEL SPRING ISOLATOR          NEOPRENE PAD              AIR SPRING
     ┌─────┐                   ┌─────────┐               ┌─────────┐
     │  M  │ Equipment         │    M    │               │    M    │
     └──┬──┘                   └────┬────┘               └────┬────┘
    ╱╲╱╲╱╲╱╲                  ▓▓▓▓▓▓▓▓▓▓               ╭────────╮
   ╱        ╲ Spring          ▓ Rubber ▓               │  Air   │
  ╱          ╲                ▓▓▓▓▓▓▓▓▓▓               │Bladder │
 └────────────┘ Housing       └─────────┘              ╰────────╯
 ════════════════              ═══════════              ═══════════
    Foundation                  Foundation                Foundation

 δst = 25-75 mm               δst = 3-6 mm              δst = 50-150 mm
 fn = 2-8 Hz                  fn = 10-25 Hz             fn = 1-4 Hz
 Efficiency: 85-95%           Efficiency: 50-70%        Efficiency: >95%


VIBRATION TRANSMISSION PATH CONTROL

Equipment Without Isolation        Equipment With Isolation
        ┌─────┐                           ┌─────┐
        │ Fan │ 1000 N force              │ Fan │ 1000 N force
        └──┬──┘                           └──┬──┘
           │                              ╱╲╱╲╱╲╱ Spring (f/fn=4)
           │                             ╱      ╲ TR = 0.07
           ▼                             └───┬───┘
    ═══════════════                          ▼
    Transmitted: 1000 N               ═══════════════
    (100% transmission)               Transmitted: 70 N
                                      (93% isolation efficiency)

Selection Methodology

Steel Spring Isolators:

• Applications: Chillers, cooling towers, large fans, air handlers
• Static deflection: 25-75 mm (1-3 inches)
• Natural frequency: 2-8 Hz
• Advantages: High efficiency, stable over temperature, long life
• Considerations: Requires lateral restraints, potential corrosion

Elastomeric Isolators (Neoprene, Natural Rubber):

• Applications: Small fans, pumps, unit ventilators
• Static deflection: 3-6 mm (0.125-0.25 inches)
• Natural frequency: 10-25 Hz
• Advantages: Inherent damping (ζ = 0.05-0.15), lateral stability
• Limitations: Limited low-frequency isolation, environmental degradation

Air Springs:

• Applications: Critical environments, variable-speed equipment
• Static deflection: 50-150 mm (2-6 inches)
• Natural frequency: 1-4 Hz
• Advantages: Constant natural frequency regardless of load, excellent low-frequency isolation
• Considerations: Requires compressed air supply, higher maintenance

Isolation Pad Compounds:

• Cork-rubber: General purpose, ζ ≈ 0.10
• Fiberglass: Watertight applications, ζ ≈ 0.08
• Neoprene-cotton duck: High loads, ζ ≈ 0.12

Inertia Bases

Inertia bases provide a rigid mounting platform that increases effective mass and lowers system natural frequency. Mass ratios (base mass/equipment mass) typically range from 1.5:1 to 3:1.

Design Considerations:

The combined system natural frequency becomes:

$$f_{n,combined} = \frac{1}{2\pi}\sqrt{\frac{k}{m_{eq} + m_{base}}}$$

Steel bases typically use 150-300 mm (6-12 inch) concrete thickness or fabricated structural steel channels. The base must exhibit rigid body motion with first flexural mode above 2×operating frequency.

Applications Requiring Inertia Bases:

• Multiple-piece equipment requiring common mounting plane
• Equipment with significant mass eccentricity
• High-frequency rotating equipment (>1800 RPM)
• Equipment subjected to impact loads

Flexible Connections

Flexible connections prevent vibration transmission through piping and ductwork while accommodating thermal expansion and equipment movement.

Flexible Pipe Connectors:

• Rubber expansion joints: Accommodate 13-25 mm (0.5-1 inch) movement
• Braided stainless hoses: High-pressure applications, 6-13 mm (0.25-0.5 inch) movement
• Installation: Mount within 0.5 m of equipment to minimize transmitted vibration

Flexible Duct Connections:

• Canvas or neoprene-coated fabric: Standard HVAC applications
• Length: Minimum 75 mm (3 inches), typical 150-225 mm (6-9 inches)
• Avoid stretched or compressed installation that transmits forces

Electrical Conduit:

• Provide slack loops or flexible conduit within 1 m of isolated equipment
• Rigid conduit effectively short-circuits vibration isolation

Implementation Guidelines

Isolator Sizing Procedure:

1. Determine operating frequency: f_op = RPM/60 Hz
2. Select target frequency ratio: f_op/f_n ≥ 3.5 (ASHRAE minimum)
3. Calculate maximum natural frequency: f_n = f_op/3.5
4. Determine required static deflection: δ_st = (3.13/f_n)² inches
5. Calculate total supported weight including piping and accessories
6. Select isolator type and quantity to achieve required deflection
7. Verify isolator load capacity with safety factor ≥ 1.25

Installation Considerations:

• Level equipment to within 3 mm (1/8 inch) to prevent load redistribution
• Provide seismic restraints per local code requirements
• Install flexible connections at all piping, duct, and electrical penetrations
• Verify clearance for full static deflection plus dynamic movement
• Use housekeeping pads to maintain access for maintenance

References

ASHRAE Handbook—HVAC Applications, Chapter 49: Sound and Vibration Control ASHRAE Handbook—HVAC Systems and Equipment, Chapter 48: Noise and Vibration Control

Technical content addressing vibration isolation fundamentals, transmissibility analysis, and practical implementation for HVAC systems. Analysis based on single-degree-of-freedom theory with considerations for damping and frequency-dependent behavior.

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