Calculator
Example data table
| Case | Suction Temp | Suction Pressure | Discharge Pressure | k | η | Ideal T₂ (°C) | Actual T₂ (°C) |
|---|---|---|---|---|---|---|---|
| Portable air unit | 25 °C | 100 kPa | 700 kPa | 1.40 | 0.75 | ~195.6 | ~252.9 |
| Plant instrument air | 30 °C | 110 kPa | 800 kPa | 1.40 | 0.80 | ~207.0 | ~250.5 |
| Higher efficiency case | 20 °C | 101 kPa | 600 kPa | 1.40 | 0.85 | ~165.1 | ~194.3 |
Formula used
This tool assumes adiabatic compression with a selected specific heat ratio k. It first estimates the ideal isentropic outlet temperature, then applies isentropic efficiency.
- Pressure ratio: r = P₂ / P₁
- Isentropic outlet temperature: T₂s = T₁ · r^((k−1)/k)
- Actual outlet temperature: T₂ = T₁ + (T₂s − T₁) / η
Use absolute pressures whenever possible, and keep units consistent.
How to use this calculator
- Enter suction temperature and select the correct unit.
- Enter suction and discharge pressures, preferably absolute values.
- Set k for the working gas, or keep air at 1.40.
- Enter isentropic efficiency to reflect real compressor performance.
- Enable pressure drops if inlet filters or coolers add losses.
- Press Calculate to view results above the form.
- Use export buttons to save the report as CSV or PDF.
Compressor Discharge Temperature in Construction Air Systems
Why discharge temperature matters
On construction sites, portable compressors and plant air headers often operate at high pressure ratios, which can raise discharge temperature quickly. Elevated discharge temperature accelerates lubricant breakdown, reduces hose and coupling life, and can overwhelm aftercoolers and moisture separators. In pneumatic tool lines, hotter air carries more water vapor and can increase downstream condensation once it cools, affecting tool reliability and filter loading.
Key inputs and realistic assumptions
This calculator uses suction temperature, suction pressure, discharge pressure, the specific heat ratio (k), and isentropic efficiency (η). For air, k is commonly about 1.40. Efficiency typically falls between 0.60 and 0.85 depending on compressor type, maintenance, and operating point. Including suction and discharge pressure drops helps reflect real installations where inlet filters, coolers, and long temporary lines change the effective pressure ratio.
Example field scenario
Suppose the inlet temperature is 25 °C, suction pressure is 100 kPa, discharge pressure is 700 kPa, k is 1.40, and η is 0.75. The pressure ratio is 7.0 and the model estimates an ideal discharge temperature near 196 °C, with an actual discharge temperature near 253 °C after accounting for efficiency. If you enable pressure drops, the effective ratio increases and the predicted discharge temperature rises further, which is often what crews observe when long hoses restrict flow.
Collecting dependable site data: Measure suction temperature where air enters the machine, not near a hot engine bay. Use the most reliable pressure readings available, and prefer absolute pressure; if only gauge pressure is available, add local atmospheric pressure before calculating. When a system includes filters, dryers, or long pipe runs, estimate pressure drops so the output reflects the compressor’s true operating load.
Interpreting results and next actions
Use the ideal value as a benchmark and the actual value for decisions. If the predicted discharge temperature is high, reduce pressure ratio, improve cooling, verify separator performance, and check compressor condition. Exporting the CSV or PDF supports shift reports, maintenance records, and commissioning checks for temporary air networks.
FAQs
1) What does “isentropic efficiency” represent?
It compares ideal compression work to real compression work. Lower efficiency means more heat is generated for the same pressure ratio, raising discharge temperature. Use manufacturer data when available, otherwise a practical range is 0.60–0.85.
2) Should I use gauge or absolute pressure?
Absolute pressure gives the correct pressure ratio. If you only have gauge pressure, add local atmospheric pressure (about 101 kPa at sea level) to both suction and discharge readings before calculating.
3) What k-value should I use for air?
For dry air near ambient conditions, k ≈ 1.40 is commonly used. If the gas composition differs or humidity is very high, k can shift slightly, which changes the predicted temperature.
4) Why do pressure drops increase discharge temperature?
Pressure drops reduce effective suction pressure or increase required discharge pressure at the compressor. That raises the effective pressure ratio, and the temperature rise scales strongly with pressure ratio in adiabatic compression.
5) Does this include intercooling or multi-stage compression?
No. It models a single adiabatic compression step. Multi-stage systems with intercoolers usually have lower final discharge temperatures than a single stage at the same overall pressure ratio.
6) What if my calculated temperature seems too high?
Re-check units, confirm you used absolute pressures, and verify the efficiency value. Also confirm suction temperature location and consider pressure drops. If values remain high, inspect cooling, separators, and compressor condition.
7) How can I use the exported report onsite?
The CSV supports quick log sheets and trend checks across shifts. The PDF is useful for commissioning packs, maintenance documentation, and communicating expected discharge temperatures to crews handling hoses, dryers, and filters.
Practical notes for construction use
Discharge temperature affects hose ratings, lubrication life, aftercooler sizing, and water carryover risk. Higher pressure ratios and lower efficiency quickly raise temperature, especially in hot ambient conditions. When compressors feed tools, shotcrete rigs, or instrument air skids, stable discharge temperature helps protect seals and downstream filters. For field checks, measure inlet temperature near the compressor suction, and use the most accurate pressure readings available. If you only have gauge pressure, convert to absolute pressure by adding local atmospheric pressure before calculating. For long temporary lines, consider pressure drops because the compressor may run at higher discharge pressure to achieve the required delivery pressure. When discharge temperature is high, improve cooling, reduce pressure ratio, or verify the compressor’s mechanical condition.
Use verified inputs to prevent overheating and failures onsite.