Moist-Air Enthalpy: Temperature Alone Does Not Fix the Energy Inventory

Define the Phase or Process Boundary

ASHRAE's psychrometric relationships expresses moist-air specific enthalpy per mass of dry air, combining dry-air and water-vapor contributions. For a common approximate reference, h=1.006T+W(2501+1.86T), with T in°C, W in kg water/kg dry air and h in kJ/kg dry air.

Review a Hypothetical Comparison

At an assigned 35 °C, a fictional stream with W=0.015 has h≈73.70 kJ/kg dry air. Another stream at the same temperature with W=0.005 has h≈48.04 kJ/kg dry air. Their difference is 25.66 kJ/kg dry air despite equal thermometer values. Multiplying this difference by an assigned dry-air flow 0.8 kg/s gives about 20.53 kW. Using 0.8 kg/s of total moist air would be a different mass basis and needs conversion.

Keep the Evidence with the Decision

Record the enthalpy reference and flow basis before comparing air duties or mixing calculations. Distinguish a state property from heat exchanged across a complete process boundary. Include condensed liquid, injected water or other energy paths when present; one dry-air enthalpy difference cannot represent omitted streams.

The states and flow are hypothetical. The approximation supplies no actual air-system capacity or permissible processing condition.

Does the energy comparison include the humidity content on a consistent dry-air basis?

This blank worksheet is for your own project. It contains no H M machine trial result.

Customer Questions

Primary References

These references support the technical principles discussed in this guide. The worked examples and review questions are educational.

• ASHRAE's psychrometric relationships
• Reference from hmpharmamachines.com