Air Conditioning System Selection for HVAC Engineers

Air Conditioning System Selection for HVAC Engineers

AC system selection impacts comfort delivery, energy consumption, maintenance requirements, and life cycle costs. No single system suits all applications; selection depends on building type, load characteristics, and owner priorities.

System Types Overview

Direct Expansion (DX) Systems

Characteristics:

• Refrigerant directly cools air
• Packaged or split configurations
• Capacities: 2-150 tons per unit

Advantages:

• Lower first cost
• Simple installation
• Fast response
• No water piping/treatment

Disadvantages:

• Less energy efficient
• Limited capacity modulation (most systems)
• Refrigerant line length limits (typically < 150 ft)

Applications: Small commercial, retail, light industrial

Chilled Water Systems

Characteristics:

• Central chiller produces 40-45°F water
• Distributed to air handlers/fan coils
• Capacities: 50-10,000+ tons central plant

Advantages:

• Highest efficiency (especially part-load)
• Unlimited piping runs
• Central equipment (easier maintenance)
• Thermal storage potential
• Free cooling capability

Disadvantages:

• High first cost
• Complex controls
• Water treatment required
• Freeze protection needed

Applications: Large commercial, hospitals, universities, high-rise

Variable Refrigerant Flow (VRF)

Characteristics:

• Multiple indoor units on common refrigerant circuit
• Capacities: 6-60 tons per outdoor unit
• Simultaneous heating and cooling (heat recovery models)

Advantages:

• High efficiency (especially part-load)
• Zone-level control
• Simultaneous heating/cooling
• Compact indoor units

Disadvantages:

• High first cost
• Refrigerant line length limits (500-1,000 ft)
• Complex controls/troubleshooting
• Limited outdoor air integration

Applications: Medium commercial, hospitality, multi-tenant residential

Selection Criteria Matrix

Energy Analysis Example

100-ton office building annual cooling energy:

• DX rooftop units (10 EER): 1,200,000 kWh/year
• VRF system (15 EER avg): 800,000 kWh/year
• Chilled water (17 EER avg): 700,000 kWh/year

At $0.12/kWh:

• DX: $144,000/year
• VRF: $96,000/year
• Chilled water: $84,000/year

Life Cycle Cost Considerations

First Cost (typical, $/ton):

• DX: $3,000-$5,000
• VRF: $5,000-$7,000
• Chilled water: $6,000-$10,000

Maintenance ($/ton-year):

• DX: $200-$300
• VRF: $250-$350
• Chilled water: $150-$250

Equipment Life:

• DX: 15-20 years
• VRF: 20-25 years
• Chilled water: 20-30 years (chillers), 15-25 years (AHUs)

Practical Applications

1. < 50 tons: DX or VRF
2. 50-200 tons: VRF or chilled water (depends on zoning needs)
3. > 200 tons: Chilled water
4. Renovation: VRF (minimal shaft/pipe modifications)
5. Mission critical: Chilled water (redundancy, reliability)

Related Technical Guides:

• Chiller Performance Analysis
• Cooling Load Calculations
• Refrigeration Cycle Design

References:

• ASHRAE Handbook of HVAC Systems and Equipment
• ASHRAE GreenGuide, 5th Edition