The First Law of Thermodynamics, also called the law of energy conservation, states that energy cannot be created or destroyed, only converted between forms such as heat, work, and internal energy. In HVAC systems, the first law requires that energy input through electrical or mechanical work equals the sum of heat removed from indoor spaces plus heat rejected to outdoor air, accounting for all energy transformations within the system.
Energy Balance Equations
The first law is expressed as Q equals W plus delta U, where Q is heat transfer, W is work input, and delta U is change in internal energy. For air conditioning cycles, energy balance requires that heat absorbed indoors plus compressor work equals heat rejected outdoors. This relationship holds regardless of system complexity or efficiency levels.
System Analysis Applications
HVAC engineers use first law analysis to determine refrigerant mass flow rates, compressor power requirements, and heat exchanger capacities. If a system requires removing 24,000 BTU per hour from indoor air with a 2.0 coefficient of performance, it consumes 12,000 watts of electrical input. Energy balances across evaporators, compressors, and condensers ensure proper system design and troubleshooting accuracy.