Combined Cycle Power Plant HVAC

Combined Cycle HVAC Design Fundamentals

Combined cycle power plants integrate gas turbines (GT), heat recovery steam generators (HRSG), and steam turbines (ST) to achieve thermal efficiencies of 55-60%. Each area presents distinct HVAC challenges driven by heat generation rates, ambient air quality requirements, and operational constraints. The HVAC design must address turbine intake air conditioning, equipment cooling, personnel comfort, and heat rejection while minimizing parasitic power consumption.

Gas Turbine Intake Air Conditioning

The gas turbine performance depends critically on inlet air conditions. Power output and efficiency vary inversely with inlet air temperature according to:

$$P_{GT} = P_{ref} \left(1 - \alpha \frac{T_{inlet} - T_{ref}}{T_{ref}}\right)$$

where $P_{GT}$ is turbine power output, $P_{ref}$ is reference power at $T_{ref}$ (typically 15°C), and $\alpha$ ranges from 0.5 to 0.7 depending on turbine design.

Inlet cooling systems increase power output during hot ambient conditions:

$$\Delta P = \dot{m}{air} c_p (T{ambient} - T_{cooled}) \times \frac{\eta_{thermal}}{\eta_{compression}}$$

Common intake cooling technologies include evaporative cooling (reducing temperature to within 3-5°C of wet bulb), inlet chilling (mechanical refrigeration to 7-13°C), and hybrid systems. The cooling load for a typical 250 MW gas turbine with inlet chilling is:

$$Q_{cooling} = \dot{m}_{air} c_p \Delta T = 600 , \text{kg/s} \times 1.006 , \text{kJ/kg·K} \times (35 - 10) , \text{°C} = 15,090 , \text{kW}$$

Filtration removes particulates (>99% efficiency for particles >10 μm) to prevent compressor fouling and erosion. Pressure drop across filters must remain below 100-150 Pa to avoid excessive performance penalties.

Heat Recovery Steam Generator Zone HVAC

The HRSG operates at exhaust gas temperatures from 550-650°C at the inlet to 90-110°C at the stack. Radiant heat flux from casing surfaces drives ventilation requirements:

$$q_{rad} = \epsilon \sigma A (T_s^4 - T_{amb}^4)$$

where $\epsilon$ is surface emissivity (0.85-0.95 for insulated steel), $\sigma = 5.67 \times 10^{-8} , \text{W/m}^2\text{·K}^4$, and $T_s$ is surface temperature (typically 50-70°C with proper insulation).

Ventilation flow rates for enclosed HRSG buildings follow:

$$\dot{V} = \frac{Q_{total}}{c_p \rho (T_{exit} - T_{inlet})}$$

For a building temperature rise limit of 5-8°C above ambient, typical ventilation rates range from 15-25 air changes per hour. Natural ventilation through louvers and roof vents can provide this flow if $\Delta T$ exceeds 5°C, generating buoyancy-driven flow:

$$\dot{V}{natural} = C_d A \sqrt{2g H \frac{\Delta T}{T{avg}}}$$

where $C_d$ is discharge coefficient (0.6-0.65), $A$ is effective vent area, $H$ is height differential, and $\Delta T$ is temperature difference between indoor and outdoor air.

Steam Turbine Hall Ventilation

The steam turbine hall contains the steam turbine-generator, condenser, feedwater heaters, and auxiliary equipment. Heat rejection from these components totals 0.5-1.5% of plant thermal input. For a 400 MW combined cycle plant with 800 MW thermal input:

$$Q_{hall} = 0.01 \times 800 , \text{MW} = 8 , \text{MW} = 8,000 , \text{kW}$$

This heat load comprises:

• Turbine casing radiation: 40-50%
• Feedwater heater and piping losses: 25-35%
• Generator losses: 15-20%
• Auxiliary equipment: 5-10%

Ventilation systems must maintain indoor temperatures below 40°C during summer operation and provide 50-80 Pa positive pressure to prevent ingress of dust and moisture. Supply air distribution uses high-velocity jets at the roof level with downward flow patterns to create air washing effects over equipment.

Control Room and Electrical Equipment HVAC

Control rooms require precision air conditioning to maintain 22-24°C ±2°C and 45-55% RH for instrumentation reliability and operator comfort. The cooling load calculation includes:

$$Q_{total} = Q_{equipment} + Q_{lights} + Q_{envelope} + Q_{occupants} + Q_{ventilation}$$

Equipment heat loads reach 100-200 W/m² for modern distributed control systems (DCS). Redundant HVAC units with automatic failover provide reliability. Dedicated computer room air conditioning (CRAC) units serve electrical equipment rooms and battery rooms.

Electrical equipment rooms housing switchgear, motor control centers, and variable frequency drives generate 50-150 W/m² heat loads. Ventilation maintains temperatures below 40°C, extending equipment life and preventing nuisance trips.

Combined Cycle HVAC Area Comparison

System Architecture

graph TD
    A[Ambient Air] -->|Filtration & Cooling| B[Gas Turbine Intake]
    B --> C[Gas Turbine]
    C -->|550-650°C Exhaust| D[HRSG]
    D -->|Steam| E[Steam Turbine]

    F[HVAC Plant] -->|Chilled Water| G[Control Room CRAC]
    F -->|Chilled Water| H[Electrical Rooms]
    F -->|Cooled Air| I[GT Inlet Cooling]

    J[Natural Ventilation] -->|Louvers/Vents| D
    K[Forced Ventilation] --> L[Turbine Hall]
    K --> M[GT Enclosure]

    N[Heat Sources] -.->|Radiant Heat| D
    O[Equipment Losses] -.->|Convective Heat| L
    P[GT Waste Heat| -.->|Exhaust Heat| D

    style C fill:#f96,stroke:#333,stroke-width:2px
    style D fill:#fc9,stroke:#333,stroke-width:2px
    style E fill:#9cf,stroke:#333,stroke-width:2px
    style F fill:#9f9,stroke:#333,stroke-width:2px

Energy Balance and Parasitic Load Optimization

The HVAC system parasitic load directly reduces net plant output. For combined cycle plants, HVAC typically consumes 0.3-0.8% of gross output. Optimization strategies include:

Free Cooling: Using ambient air when outdoor temperature permits (spring/fall/winter) eliminates chiller operation:

$$Q_{free} = \dot{m}{air} c_p (T{indoor} - T_{outdoor})$$

Available when $T_{outdoor} < T_{indoor} - 2\text{°C}$.

Variable Speed Drives: Fan power varies with cube of speed, so 80% flow reduces power to 51% of design:

$$P_{fan} = \frac{\dot{V} \Delta P}{\eta_{fan}} \propto \dot{V}^3$$

Waste Heat Recovery: Extracting low-grade heat from jacket water or lube oil cooling systems for winter heating reduces boiler fuel consumption or electric heating loads.

Design Standards and References

Combined cycle HVAC design follows:

• NFPA 850: Recommended Practice for Fire Protection for Electric Generating Plants
• ASHRAE Applications Handbook: Chapter 27, Power Plants
• ISO 2314: Gas Turbines - Acceptance Tests
• ASME PTC 22: Gas Turbines Performance Test Code
• IEEE 666: Design of Auxiliary Systems for Generating Stations

Site-specific considerations include ambient temperature extremes (affecting GT performance), relative humidity (impacting evaporative cooling potential), and dust/salt concentrations (driving filtration requirements). Coastal installations require corrosion-resistant materials and enhanced filtration for salt-laden air. Desert sites need robust filtration (>99.5% efficiency) and evaporative cooling where water is available.

Operational Considerations

HVAC system operation integrates with plant dispatch strategy. During peak demand periods with high ambient temperatures, inlet cooling maximizes GT output despite increased parasitic load. During off-peak periods, HVAC systems may operate in reduced modes to minimize auxiliary power consumption.

Filter replacement schedules balance pressure drop penalties against maintenance costs. Differential pressure monitoring triggers filter changes when $\Delta P$ exceeds 125-150 Pa. Automated filter condition monitoring systems optimize replacement intervals and prevent forced outages due to excessive pressure drop.

Components

• Combined Cycle Gas Turbine (CCGT)
• Heat Recovery Steam Generator (HRSG)
• Multi-Pressure HRSG
• Efficiency 55 To 60 Percent