In industrial environments requiring sub-freezing temperatures, conventional single-stage compression often proves inadequate. This article explores key concepts in thermal engineering, from the core technology of two-stage compression to thermal property parameters affecting heat transfer, along with essential electrical protection measures that ensure safe equipment operation.

When extremely low evaporation temperatures are required, single-compressor systems face significant challenges due to excessive compression ratios, leading to dramatically reduced efficiency. Two-stage compression technology solves this problem by employing two compressors working in series. The first compressor raises refrigerant pressure from evaporation pressure to intermediate pressure, while the second further compresses it to condensation pressure.

This staged approach effectively reduces the compression ratio at each stage, significantly improving overall refrigeration efficiency. An alternative configuration integrates both compression stages within a single compressor unit. Two-stage compression has become essential for deep-freezing applications and other industrial processes demanding ultra-low temperatures.

Precise understanding of thermal properties is crucial in thermal engineering, as they directly impact heat transfer efficiency. Key parameters include:

• Overall Heat Transfer Coefficient (K-value): This critical metric evaluates a surface's thermal performance, representing heat transfer per unit area and temperature difference. Lower K-values indicate better insulation. Calculated as K = 1/(F × W), where F is surface area and W is thermal resistance.
• Thermal Resistance (W): The inverse of K-value, representing a material's resistance to heat flow. Higher values indicate better insulation.
• Heat Transfer Coefficient (α): Measures heat exchange efficiency between fluids and solid surfaces, with different coefficients for convection, condensation, and boiling heat transfer.
• Thermal Conductivity (λ): Quantifies a material's internal heat transfer capability, with higher values indicating better conductivity. Typically measured in W/m·K.

Thermal load represents the heat energy that must be removed (for cooling) or added (for heating) to maintain desired temperatures in a space or object. This parameter is fundamental for designing and selecting air conditioning, refrigeration, and freezing systems.

Calculation methods range from detailed analysis to simplified estimates. For office spaces, typical cooling loads range 0.128–0.174 kW/m² (110–150 kcal/m²h), while heating requirements approximate 0.058 kW/m³ (50 kcal/m³h) of room volume.

Heat quantifies the kinetic energy of molecular motion. The joule (J) serves as the SI unit, though the calorie (cal) remains historically significant, defined as the energy needed to raise 1 gram of water by 1°C (1 cal = 4.18605 J).

Reliable electrical protection is vital for thermal systems. Two key components include:

• Noise Filters: In variable-frequency drive systems, these components suppress electromagnetic interference from inverters, preventing equipment disruption and improving signal reliability.
• Circuit Breakers (NFB): Serving as primary power switches, these devices interrupt circuits during overcurrent (exceeding 125–200% of rated value) or short-circuit conditions, preventing equipment damage.