HVAC Psychrometric Chart Practice Questions Refrigeration Cycle Essentials

Refrigeration Cycle Key Points

1. Compression

Input: Low-pressure vapor

Output: High-pressure superheated vapor

Work Input (W) = h₂ - h₁

Isentropic efficiency: 65-85%
Discharge temp: 65-100°C (149-212°F)
Key component: Compressor

2. Condensation

Input: High-pressure superheated vapor

Output: High-pressure liquid

Heat Rejected (Q_out) = h₂ - h₃

Subcooling: 5-15°C (9-27°F)
Condensing temp: 10-20°C above ambient
Key component: Condenser

3. Expansion

Input: High-pressure liquid

Output: Low-pressure liquid/vapor mix

Flash Gas % = (h₃ - h₄) / h_fg

Pressure drop: 80-90% of total
Typical ΔT: 30-50°C (54-90°F)
Key component: Expansion device

4. Evaporation

Input: Low-pressure liquid/vapor mix

Output: Low-pressure vapor

Cooling Effect (Q_in) = h₁ - h₄

Evaporating temp: 5-10°C below target temp
Superheat: 5-10°C (9-18°F)
Key component: Evaporator

Performance Metrics

Parameter	Formula	Typical Range
COP (Cooling)	Q_in/W	2.5-4.5
EER	COP × 3.412	8.5-15.4 BTU/W·h
Capacity	ṁ × (h₁ - h₄)	Varies by system size

Common Refrigerants

Refrigerant	GWP	Application
R-134a	1430	Automotive, chillers
R-410A	2088	Residential AC
R-32	675	New residential systems
R-1234yf	4	Automotive replacement

Troubleshooting Guide

Symptom	Possible Cause	Check
Low cooling	Low refrigerant charge	Superheat/subcooling
High head pressure	Dirty condenser	Condenser ΔT
Short cycling	Overcharge/undercharge	Compressor amp draw