Effectiveness – NTU heat exchanger calculator
Rate or size a heat exchanger from stream capacity rates, inlet temperatures, exchanger geometry, and the ε – NTU relationships.
Calculation setup
Performance predicts heat duty and outlet temperatures; design determines required area.
Choose the flow arrangement whose ε – NTU correlation matches the exchanger.
Positive whole number; tube passes are assumed compatible with the selected shell arrangement.
Units
Changing units converts both stream mass-flow entries.
Changing units converts both specific-heat entries.
Absolute temperatures are converted; temperature differences remain physically equivalent.
Changing units converts the overall heat-transfer coefficient.
Used for entered area in performance mode and reported area in design mode.
Cold fluid properties
Required, positive finite flow rate.
Required, positive finite value treated as constant over the exchanger.
Required cold-stream inlet temperature.
Required only for design calculations; must lie above the cold inlet and below the hot inlet.
Hot fluid properties
Required, positive finite flow rate.
Required, positive finite value treated as constant over the exchanger.
Required; must exceed the cold-stream inlet temperature.
Required only for design calculations; must lie below the hot inlet and above the cold inlet.
Heat-transfer surface
Required, positive finite coefficient on the same area basis as A.
Required in performance mode; positive heat-transfer surface area.
Heat transfer summary
Calculating the startup example...
Stream thermal states
| Stream | Capacity rate | Inlet temperature | Outlet temperature | Heat-rate change |
|---|
How to use the Effectiveness – NTU calculator
What this calculator does
This calculator analyzes a two-stream heat exchanger with the effectiveness – NTU method. In Performance calculation mode, it uses the inlet conditions, stream heat-capacity rates, overall heat-transfer coefficient, area, and selected flow arrangement to estimate effectiveness, heat duty, and both outlet temperatures. In Design problem mode, it uses specified inlet and outlet temperatures to infer effectiveness and NTU, then calculates the heat-transfer area required at the entered overall coefficient. The method is a steady-state engineering model with constant specific heats; it does not predict pressure drop, fouling growth, phase-change details, local temperature fields, or mechanical suitability.
When to use it
Use the calculator to rate an existing exchanger when outlet temperatures are unknown, to estimate the surface area needed for a target thermal duty, to compare parallel, counterflow, shell-and-tube, and common crossflow configurations, or to sanity-check a hand calculation before detailed simulation. The ε – NTU approach is especially useful when inlet temperatures and UA are known but outlet temperatures are not; the NPTEL heat-exchanger lecture on effectiveness and NTU explains that motivation and the governing nondimensional quantities.
How to calculate
- The calculator opens with a complete counterflow demonstration: performance mode, 1.20 kg/s cold flow at 4180 J/(kg·K) and 20 °C, 0.80 kg/s hot flow at 2200 J/(kg·K) and 120 °C, U = 350 W/(m²·K), and A = 7.50 m². Its XLSX workbook is available immediately.
- Choose Type of calculation and Heat exchanger. If you select the multiple-shell-pass option, enter the integer Shell passes.
- Choose Mass-flow unit, Specific-heat unit, Temperature unit, Overall-U unit, and Area unit. Changing a unit converts the live values rather than relabeling them.
- Replace the cold and hot stream values. Performance mode needs each stream's mass flow, specific heat, and inlet temperature plus U and area. Design mode additionally needs both outlet temperatures and solves for area.
- Read the primary result, effectiveness, NTU, actual and maximum heat rates, capacity ratio, capacity rates, area, and outlet temperatures. The stream table reports the same canonical model by fluid.
- Select Download Excel to create a fresh workbook from the current validated inputs. Reset clears the demonstration data rather than restoring it; after reset, Excel export stays disabled until you enter a complete valid state again.
Input guide
Type of calculation is required and accepts Performance calculation or Design problem. Performance is appropriate when A is known; Design is appropriate when target terminal temperatures are known. Heat exchanger is required and selects the ε – NTU correlation. A common mistake is choosing a configuration that does not represent the actual flow mixing. Shell passes is required only for the multiple-shell-pass model; enter a whole number such as 2, not a tube-pass count.
Mass-flow unit applies to both Mass flow rate (mc) and Mass flow rate (mh); positive decimal values such as 1.20 kg/s and 0.80 kg/s are required. Higher mass flow raises that stream's heat-capacity rate and can change which stream limits qmax. Specific-heat unit applies to Specific heat capacity (cpc) and Specific heat capacity (cph); positive values such as 4180 and 2200 J/(kg·K) are required. Do not mix J and kJ without changing the unit selector.
Temperature unit applies to all terminal temperatures. Inlet temperature (Tci) and Inlet temperature (Thi) are required, with the hot inlet above the cold inlet; examples are 20 °C and 120 °C. In design mode, Outlet temperature (Tco) and Outlet temperature (Tho) are also required and must imply heat moving from hot to cold. The cold- and hot-side heat duties must agree closely; inconsistent target temperatures are rejected instead of silently violating energy conservation.
Overall-U unit applies to Overall heat transfer coefficient (U), a required positive coefficient such as 350 W/(m²·K). Larger U means a smaller area is needed for the same NTU. Area unit applies to Area of heat transfer (A), which is required only in performance mode; a value such as 7.50 m² increases NTU in direct proportion. Do not enter projected footprint when the U value is based on a different heat-transfer area convention.
Output guide
Primary result is actual heat rate q in performance mode and required heat-transfer area A in design mode. Effectiveness (ε) is the dimensionless ratio q/qmax and should lie between 0 and 1. Number of transfer units (NTU) equals UA/Cmin and measures exchanger thermal size relative to the smaller stream capacity rate. Actual heat rate (q) and Maximum heat rate (qmax) are reported in kW; qmax is Cmin(Thi – Tci). Capacity ratio (Cr) is Cmin/Cmax. Cold capacity rate (Cc) and Hot capacity rate (Ch) are in kW/K. Required / entered area (A) uses the selected area unit. Cold outlet (Tco) and Hot outlet (Tho) use the selected temperature unit. The Stream thermal states table repeats stream identity, capacity rate, inlet temperature, outlet temperature, and heat-rate change so the two energy-balance sides can be checked directly.
Worked example
With the startup values, Cc = 1.20×4180 = 5016 W/K and Ch = 0.80×2200 = 1760 W/K, so Cmin = 1760 W/K, Cmax = 5016 W/K, and Cr ≈ 0.350877. The entered UA is 350×7.50 = 2625 W/K, giving NTU ≈ 1.49148. For counterflow, the correlation gives ε ≈ 0.715571. The maximum heat rate is 1760×(120 – 20) = 176.00 kW, so q ≈ 125.94 kW. The resulting outlet temperatures are approximately 45.11 °C on the cold side and 48.44 °C on the hot side. Those values are the first-open calculator and workbook checkpoints.
Learn more
For deeper derivations and worked heat-transfer instruction, see the University of Colorado Boulder LearnChemE heat-transfer screencasts. For a research-oriented discussion of balancing and ε – NTU analogies, the LibreTexts heat-exchanger effectiveness notes provides additional context.
How the ε – NTU model works
The first step is to form each stream heat-capacity rate, C = ṁcₚ. The smaller value is Cmin and the larger is Cmax. Their ratio Cr = Cmin/Cmax stays between zero and one. The theoretical maximum heat transfer is qmax = Cmin(Thi – Tci), because the lower-capacity stream can undergo the larger temperature change for a given duty.
NTU = U·A / Cmin and ε = q / qmax
In performance mode, the selected exchanger correlation converts NTU and Cr into effectiveness. The calculator then evaluates q = εqmax and applies the steady-state energy balance to find both outlet temperatures. In design mode, the specified terminal temperatures determine q and ε first. A monotonic numerical solve then finds the NTU that reproduces that effectiveness for the chosen exchanger correlation, and area follows from A = NTU·Cmin/U.
Crossflow and shell-and-tube correlations are geometry models, not universal identities. If a real exchanger has unusual baffle leakage, bypass, maldistribution, phase change, or strongly temperature-dependent properties, use a model calibrated to that equipment.