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Refrigeration Cycle Design for HVAC Engineers

Refrigeration Cycle Design for HVAC Engineers

Refrigeration system design balances capacity, efficiency, and reliability. Proper component sizing and control ensures optimal performance across varying load and ambient conditions.

Vapor Compression Cycle Components

  1. Evaporator: Absorbs heat, evaporates liquid refrigerant
  2. Compressor: Increases pressure and temperature
  3. Condenser: Rejects heat, condenses vapor to liquid
  4. Expansion device: Reduces pressure, controls flow

Compressor Selection

Types:

  • Reciprocating: 1-100 tons, high efficiency, good part-load
  • Scroll: 2-60 tons, quiet, reliable
  • Screw: 20-500 tons, good part-load, industrial
  • Centrifugal: 100-10,000 tons, highest efficiency at full load

Displacement:

$$V_{displacement} = \frac{Capacity \times v_1}{\eta_v}$$

Where:

  • $v_1$ = specific volume at compressor suction (ft³/lb)
  • $\eta_v$ = volumetric efficiency (0.65-0.85)

Superheat Control

Superheat: Temperature above saturation at evaporator outlet

$$SH = T_{suction} - T_{sat,evap}$$

Target: 10-15°F

Too low: Liquid slugging risk, compressor damage Too high: Reduced capacity, higher discharge temperature

Control: Thermostatic expansion valve (TXV) or electronic expansion valve (EEV)

Subcooling

Subcooling: Temperature below saturation at condenser outlet

$$SC = T_{sat,cond} - T_{liquid}$$

Target: 10-20°F

Benefits:

  • Prevents flash gas in liquid line
  • Increases capacity (~0.5% per °F)
  • Improves efficiency

Evaporator Sizing

Heat transfer:

$$Q = UA_{evap} \times LMTD$$

Log mean temperature difference:

$$LMTD = \frac{(T_{in} - T_{evap}) - (T_{out} - T_{evap})}{\ln\frac{T_{in} - T_{evap}}{T_{out} - T_{evap}}}$$

Typical approach: 8-15°F (difference between evaporating temperature and leaving chilled water)

Condenser Sizing

Similar to evaporator:

$$Q_{cond} = Q_{evap} + W_{comp}$$

Approach: 10-15°F (condensing temperature above entering condenser water)

Capacity Modulation

Methods:

  1. On-off: Simple, cycling losses
  2. Cylinder unloading: 25/50/75/100% steps
  3. Variable speed: Continuous modulation, highest efficiency
  4. Hot gas bypass: Inefficient, use only for critical control

Practical Applications

  1. R-410A residential AC: 13-21 SEER
  2. R-134a chiller: 0.50-0.65 kW/ton
  3. Ammonia industrial: Highest efficiency, toxic
  4. CO₂ transcritical: Low GWP, high pressure

Related Technical Guides:

References:

  • ASHRAE Handbook of HVAC Systems and Equipment, Chapter 38: Compressors
  • ASHRAE Handbook of Fundamentals, Chapter 30: Thermophysical Properties of Refrigerants