Exhibition Hall HVAC Systems

Overview of Exhibition Hall Climate Control Challenges

Exhibition halls present exceptional HVAC design challenges due to their enormous conditioned volumes (typically 200,000 to 2,000,000 ft³), highly variable occupancy patterns, and unique operational requirements. These spaces must accommodate extreme load swings from empty hall conditions (setup/teardown) to peak occupancy events with thousands of attendees, exhibit lighting, and equipment heat loads.

The fundamental design challenge stems from the $Q = \dot{m} c_p \Delta T$ relationship applied across vastly different operational states. During setup periods, cooling loads may be as low as 0.3-0.5 W/ft² (building envelope only), while peak event loads can reach 8-12 W/ft² when combining occupant sensible heat (255 BTU/hr per person), exhibit lighting (2-4 W/ft²), audio-visual equipment, and booth displays.

Physical Principles and Load Characteristics

Thermal Stratification Management

Exhibition halls with ceiling heights of 25-60 ft experience significant thermal stratification due to natural convection. The buoyancy-driven flow velocity can be estimated:

$$v = \sqrt{\frac{2g\beta\Delta T H}{C_d}}$$

where:

• $g$ = gravitational acceleration (32.2 ft/s²)
• $\beta$ = thermal expansion coefficient (≈0.0055 K⁻¹ for air)
• $\Delta T$ = temperature difference between ceiling and floor
• $H$ = ceiling height
• $C_d$ = drag coefficient (typically 0.6-0.8)

Without proper air distribution, temperature gradients of 10-15°F floor-to-ceiling are common, wasting energy on conditioning unused upper volumes while occupants experience discomfort at floor level.

Variable Occupancy Load Analysis

Exhibition hall loads vary by operational phase:

The sensible heat ratio (SHR) for exhibition halls typically ranges from 0.75-0.85, reflecting moderate latent loads from occupants and minimal moisture-generating processes.

Forklift and Equipment Exhaust Management

During setup and teardown operations, propane and diesel-powered forklifts introduce carbon monoxide (CO), nitrogen oxides (NOₓ), and particulate matter into the space. A typical 5,000 lb capacity propane forklift produces approximately 200-400 cfm of exhaust containing 1-3% CO by volume.

Dilution Ventilation Requirements

The required dilution ventilation rate to maintain CO below OSHA’s 35 ppm 8-hour TWA:

$$Q_{req} = \frac{Q_{exhaust} \times C_{exhaust}}{C_{allowable} - C_{ambient}}$$

For multiple forklifts operating simultaneously, total ventilation rates of 15,000-50,000 cfm are often necessary during move-in/move-out periods. This represents a separate ventilation mode that must be designed into the system controls.

graph TD
    A[Exhibition Hall Operation] --> B{Operational Mode}
    B -->|Empty/Setup| C[Forklift Exhaust Mode]
    B -->|Event Active| D[High Occupancy Mode]
    B -->|Off-Hours| E[Setback Mode]

    C --> F[Enable 100% Outside Air]
    C --> G[Maximize Air Changes 4-8 ACH]
    C --> H[CO Monitoring Active]

    D --> I[Demand Control Ventilation]
    D --> J[CO2 Monitoring]
    D --> K[Temperature/Humidity Control]

    E --> L[Minimum Ventilation]
    E --> M[Space Temperature 55-60°F]

    H --> N{CO > 25 ppm?}
    N -->|Yes| O[Increase Ventilation Rate]
    N -->|No| P[Maintain Current Rate]

    J --> Q{CO2 > 1000 ppm?}
    Q -->|Yes| R[Increase OA Damper]
    Q -->|No| S[Maintain Ventilation]

Air Distribution Strategies

Underfloor vs. Overhead Supply

Exhibition halls employ two primary air distribution approaches:

Overhead Supply with Long Throw Diffusers:

• Throw distance: $T = V_x / V_t$ where $V_t$ is terminal velocity (typically 50 fpm)
• Requires throw distances of 60-120 ft to reach floor level
• Supply velocities: 1,500-2,500 fpm at diffuser
• Effective for spaces without raised floors

Underfloor Air Distribution (UFAD):

• Supply air temperature: 60-65°F (vs. 55°F overhead)
• Displacement ventilation principles reduce mixing energy
• Utilizes thermal buoyancy for natural stratification control
• Requires raised floor systems (12-18 inches typical)

The effectiveness factor $\varepsilon$ for UFAD systems ranges from 1.0-1.2 compared to 0.8-0.9 for overhead mixing systems, providing 10-25% energy savings through reduced air distribution fan power.

Control Strategies for Variable Loads

Exhibition halls require sophisticated control sequences addressing extreme load variability:

1. Demand-Based Ventilation: CO₂ sensors modulate outside air from 10% minimum to 100% during high occupancy
2. Zone-Based Temperature Control: VAV systems with 1 zone per 2,500-5,000 ft²
3. Operational Mode Selection: Manual or scheduled switching between setup, event, and setback modes
4. Economizer Integration: Free cooling when $T_{OA} < T_{Return} - 2°F$

The control system must accommodate rapid load changes. A typical 100,000 ft² exhibition hall transitioning from empty to occupied can experience load increases of 200-400 tons within 2-3 hours, requiring anticipatory control algorithms that pre-cool the space based on event schedules.

Energy Recovery and Efficiency Considerations

Given the substantial ventilation requirements, energy recovery systems provide significant operational savings. Total enthalpy wheels achieve 70-80% effectiveness, recovering both sensible and latent energy. For a system handling 50,000 cfm with a 40°F temperature difference and 0.020 humidity ratio difference:

$$Q_{recovered} = \dot{m}(h_{OA} - h_{EA}) \times \varepsilon$$

Annual energy savings of 30-40% on conditioning outdoor air are achievable in most climates, with payback periods of 3-5 years for wheel-based recovery systems.

Design Recommendations Per ASHRAE Standards

ASHRAE 90.1 Section 6.5.3 requires energy recovery for systems exceeding 70% outside air at flows above 5,000 cfm. Exhibition halls routinely exceed these thresholds. ASHRAE 62.1 specifies 7.5 cfm/ft² for assembly spaces, though actual design values depend on occupant density calculations per the ventilation rate procedure.

Key design parameters for exhibition halls:

• Design air change rate: 3-6 ACH during events
• Outside air fraction: 15-30% typical, 100% during setup operations
• Supply air temperature: 55-58°F for overhead, 60-65°F for UFAD
• Space design conditions: 72-76°F, 40-55% RH per ASHRAE 55
• Acoustic design: NC 40-45 during events, NC 35 during setup

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