Enter Radiator Conditions
Use consistent SI units. The coolant inlet must be warmer than the air inlet.
Example Data Table
| Input | Example value | Unit | Purpose |
|---|---|---|---|
| Coolant flow | 0.25 | kg/s | Sets coolant heat capacity rate. |
| Coolant inlet | 95 | °C | Defines the hot-side starting temperature. |
| Air flow | 1.20 | kg/s | Sets available cooling-air capacity. |
| Air inlet | 25 | °C | Defines ambient air entering the core. |
| Clean UA | 850 | W/K | Represents radiator thermal conductance. |
| Fouling resistance | 0.00010 | K/W | Reduces conductance for deposits or dirt. |
Formula Used
The calculation uses the effectiveness–NTU method. It first determines how much heat each stream can carry per degree of temperature change.
C = ṁ × cpHere, C is heat capacity rate in W/K, ṁ is mass flow rate in kg/s, and cp is specific heat capacity in J/kg·K.
UAeffective = 1 ÷ (1 ÷ UAclean + Rfouling)This converts a clean conductance into an effective conductance after the added fouling resistance.
NTU = UAeffective ÷ Cmin Qmax = Cmin × (Thot,in − Tcold,in) Q = ε × QmaxEffectiveness ε is selected from standard equations for crossflow, counterflow, or parallel flow. Outlet temperatures follow from energy balance.
Thot,out = Thot,in − Q ÷ Chot | Tcold,out = Tcold,in + Q ÷ CcoldHow to Use This Calculator
- Enter coolant and air flow rates in kilograms per second.
- Enter specific heat capacities that match your actual fluids.
- Enter inlet temperatures. Coolant must enter hotter than air.
- Choose crossflow for most finned radiators unless another arrangement applies.
- Select direct UA when known, or calculate UA using U and area.
- Add fouling resistance to compare clean and aged radiator performance.
- Select Calculate Heat Exchange. Read duty, outlet temperatures, effectiveness, and LMTD.
- Download a CSV or PDF after a valid result appears.
Radiator Heat Exchanger Performance
Why These Calculations Matter
A radiator transfers heat from a hot liquid to moving air. Its output affects reliability, electronics cooling, process safety, and comfort. A useful calculation begins with measured flow rates and inlet temperatures. These values show how much energy each stream can carry. The calculator converts them into heat capacity rates. It then estimates how strongly the streams can exchange energy. This approach is often more informative than comparing temperatures alone. It identifies whether flow, surface area, or resistance limits cooling.
Heat Capacity Rate Controls Temperature Change
Heat capacity rate equals mass flow rate multiplied by specific heat capacity. A stream with a small capacity rate changes temperature quickly. A stream with a large capacity rate changes temperature slowly. The smaller rate controls the theoretical maximum heat duty. In vehicle radiators, coolant has a high specific heat capacity. Air often has a lower capacity rate because its density is low. Increasing airflow can improve heat removal. Increasing coolant flow can help, but its benefit may become small.
NTU and Effectiveness Explain Performance
The number of transfer units compares available conductance with the smaller heat capacity rate. Higher NTU normally increases heat exchange. Effectiveness expresses actual heat duty as a fraction of the maximum possible duty. An effectiveness of 0.70 means the exchanger achieves seventy percent of that limit. The value is not a simple efficiency rating. It depends on flow arrangement, capacity-rate ratio, and thermal conductance. These factors must be considered together. High effectiveness can still produce limited duty when inlet temperature difference is small.
Choosing a Flow Arrangement
Counterflow usually provides the strongest temperature driving force. It often gives the highest effectiveness for the same UA value. Parallel flow is simpler to model but normally has lower effectiveness. Crossflow is common in radiators because air moves across coolant tubes. The calculator uses a both-streams-unmixed crossflow approximation. This helps preliminary sizing. Detailed design may require manufacturer curves, fin efficiency, airflow distribution, and fan performance data. Use measured conditions whenever final selection depends on close margins.
Fouling Changes Real Output
Deposits inside tubes reduce heat transfer. Dirt between fins also restricts airflow. Both effects lower the effective UA value. The fouling resistance input represents the added thermal barrier. Even a small resistance can reduce duty when the clean radiator has high conductance. Regular cleaning preserves airflow and keeps calculated performance closer to reality. Coolant quality also matters. Corrosion products and scale can cause persistent losses. Compare clean and fouled outputs when planning maintenance or checking an older unit.
Design Checks Before Installation
Review predicted outlet temperatures, heat duty, effectiveness, and LMTD together. A low air outlet rise can signal insufficient airflow measurement. A large coolant drop may indicate low coolant flow or excessive load assumptions. Check that calculated heat duty matches the system heat source. Add safety margin for ambient changes, fan cycling, and manufacturing variation. Do not treat this calculator as a replacement for testing. It is a clear first-pass tool for comparing alternatives. Good measurements create dependable radiator heat exchanger performance estimates.
Frequently Asked Questions
1. What does heat duty mean?
Heat duty is the calculated rate of heat removed from coolant and absorbed by air. This calculator reports it in kilowatts.
2. Which units should I enter?
Use kg/s for flow, J/kg·K for specific heat, °C for temperatures, W/K for UA, W/m²·K for U, and m² for area.
3. Why must coolant enter hotter than air?
The model assumes heat flows from coolant to air. Reversing those temperatures requires a different physical interpretation and setup.
4. Which flow arrangement suits a normal radiator?
Crossflow is normally appropriate because air crosses coolant tubes. Use counterflow or parallel flow only when the physical path clearly matches them.
5. What is a UA value?
UA combines overall heat-transfer coefficient and active heat-transfer area. A larger value generally supports more heat exchange for the same conditions.
6. What does fouling resistance represent?
It represents extra thermal resistance from scale, corrosion deposits, blocked passages, or dirty fins. Higher values reduce effective UA and predicted duty.
7. Is the crossflow equation exact?
It is a standard approximation for both streams unmixed. Detailed core geometry, bypass air, and fin efficiency can change actual performance.
8. How is LMTD used here?
LMTD summarizes the terminal temperature driving force. It is reported as a useful check, especially when comparing predicted operating points.
9. Can I use glycol coolant?
Yes. Enter the mass flow rate and specific heat capacity for the actual glycol-water concentration and operating temperature.
10. Can this model a room heating radiator?
Yes, when water is the hot stream and air is the cool stream. Choose inputs that reflect the radiator and airflow conditions.
11. What accuracy should I expect?
Accuracy depends on reliable flow, property, UA, and fouling inputs. Good measurements create dependable radiator heat exchanger performance estimates.