Calculator Inputs
Example Data Table
| Case | Air Flow kg/s | Air In °C | Air Out °C | Water Flow kg/s | Water In °C | U W/m²K | Expected Use |
|---|---|---|---|---|---|---|---|
| Small recovery coil | 0.80 | 55 | 35 | 0.45 | 18 | 38 | Quick duty estimate |
| Process air cooler | 1.20 | 60 | 35 | 0.70 | 20 | 45 | Area check |
| Large ventilation coil | 2.50 | 48 | 28 | 1.35 | 15 | 52 | Fan and pump review |
Formula Used
Air side heat duty: Qair = mair × Cpair × (Tair,in − Tair,out)
Water side heat duty: Qwater = mwater × Cpwater × (Twater,out − Twater,in)
Predicted water outlet: Twater,out = Twater,in + Qair ÷ (mwater × Cpwater)
Counterflow LMTD: LMTD = (ΔT1 − ΔT2) ÷ ln(ΔT1 ÷ ΔT2)
Effective coefficient: Ueffective = 1 ÷ ((1 ÷ Uclean) + Rf,air + Rf,water)
Required area: Area = Q ÷ (Ueffective × LMTD × correction factor) × safety allowance
Fan power: Pfan = air volume flow × air pressure drop ÷ fan efficiency
Pump power: Ppump = water volume flow × water pressure drop ÷ pump efficiency
How to Use This Calculator
- Enter air and water mass flow rates. Select matching flow units.
- Choose the common temperature unit for all temperature fields.
- Enter air inlet, air outlet, and water inlet temperatures.
- Leave water outlet blank when you want the tool to predict it.
- Enter specific heat values, overall coefficient, fouling factors, and correction factor.
- Add installed area when you want to compare available capacity.
- Enter pressure drops and efficiencies for fan and pump power estimates.
- Press calculate. Review the result table above the form.
- Download the CSV or PDF report for records.
Air to Water Heat Exchanger Calculation Guide
Practical Heat Transfer Review
Air to water heat exchangers move thermal energy between moving air and circulating water. They appear in coils, condensers, heat recovery units, process coolers, dryers, and comfort systems. A clear calculation helps designers check duty before choosing coil size, pumps, fans, and controls. This calculator keeps the main steps together. It compares the air side with the water side. It also estimates the area needed from overall heat transfer data.
Duty and Balance
The first task is heat duty. Air duty uses mass flow, specific heat, and the air temperature drop. Water duty uses mass flow, specific heat, and the water temperature rise. When the outlet water temperature is blank, the tool predicts it from the air side duty. When both sides are entered, the difference shows how well the two sides balance. A small difference is expected. A large difference suggests a wrong unit, measured flow error, or unrealistic outlet temperature.
Driving Force and Area
The second task is temperature driving force. The log mean temperature difference turns two terminal temperature gaps into one useful average. Counterflow normally gives a stronger driving force than parallel flow. Crossflow often needs a correction factor. Enter a factor below one when baffles, mixed flow, or coil geometry reduce performance. The required area then follows from duty divided by overall coefficient and corrected driving force.
Advanced Inputs
Useful inputs improve the estimate. Fouling resistances reduce the clean overall coefficient. Safety margin increases the required area for practical selection. Installed area allows a quick capacity comparison. Face area and density estimate air velocity. Pressure drops and efficiencies estimate fan and pump power. These values do not replace manufacturer coil software, but they give a fast engineering screen.
Design Notes
Use consistent design assumptions. Keep dry bulb, water temperature, and flow rates from the same condition. For moist air coils, latent heat can matter. Add a separate moisture load when condensation occurs. For freezing risk, check the lowest water temperature and control strategy. For hot water coils, confirm that materials and seals suit the temperature. Review final selections against codes, pressure ratings, and vendor data. Document every assumption. That habit makes later troubleshooting easier and supports confident conversion between thermal demand, flow, area, and power. Always compare final results with tested coil performance curves too carefully.
FAQs
What does this calculator estimate?
It estimates heat duty, water outlet temperature, LMTD, corrected area, capacity rates, effectiveness, air velocity, fan power, and pump power for air to water heat exchange checks.
Can I leave water outlet temperature blank?
Yes. Leave it blank when the air side duty is your main known load. The calculator then predicts water outlet temperature from water flow and specific heat.
What is LMTD?
LMTD means log mean temperature difference. It converts two end temperature differences into one average driving force for exchanger area calculations.
When should I use a correction factor?
Use a correction factor when flow is not ideal counterflow. Crossflow coils, mixed streams, bypassing, and special geometry often need a factor below one.
Why are fouling factors included?
Fouling adds thermal resistance on heat transfer surfaces. It lowers the effective overall coefficient and increases required exchanger area for practical design.
Can this calculator handle imperial inputs?
It accepts mass flow in pounds per hour and temperature in Fahrenheit. Results are converted internally and reported mainly in SI engineering units.
Why does effectiveness exceed one sometimes?
Effectiveness above one means the selected temperatures or flow rates are not physically consistent. Check outlet temperatures, units, and stream direction.
Does this replace manufacturer sizing?
No. It is best for early screening, conversion checks, and educational review. Final coil selection should use tested vendor data and project requirements.