(most efficient)
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How to Use the Heat Exchanger Calculator
Pick the exchanger type and the application, which sets a starting U-value, then choose counterflow, parallel flow or cross flow. Use Size mode to find the area for known temperatures, or Rate mode to find the duty of an exchanger whose area you know.
For water, glycol, oil or air, enter the inlet and outlet temperatures, plus mass flow and specific heat if you want the duty from the stream. For a condensing or evaporating refrigerant, pick the refrigerant stream type and enter its saturation temperature instead.
Enter the clean U-value and pick a fouling allowance. In Size mode, give the duty directly or let it come from a mass flow, using a latent heat for a refrigerant stream. In Rate mode, enter the exchanger area and the mass flows.
The results show the duty, LMTD and effective LMTD, the effectiveness and NTU, the required area with your design margin, and a plate count if you enter an area per plate. Rate mode returns the duty and both outlet temperatures.
Heat exchanger design for refrigeration and HVAC
Every condenser, evaporator, economizer, cascade exchanger and liquid subcooler is a heat exchanger, and the design question is the same each time: how much surface moves the duty at the temperature difference that is available. The calculator answers it two ways. The LMTD method gives the average driving temperature difference and the area from Q = U × A × LMTD. The effectiveness-NTU method gives the same area from the fraction of the maximum possible heat transfer that the exchanger achieves, and it is the method that works when the area is fixed and you want the duty.
Duty, LMTD and the area equation
In the condenser example, water enters at 29°C and leaves at 35°C at 2.4 kg/s, so the duty is 2.4 × 4.18 × 6 K, or 60.19 kW. The refrigerant condenses at 40°C, which leaves end temperature differences of 11 K and 5 K and an LMTD of 7.61 K. A clean U of 3,500 W/m²K with 0.0001 m²K/W of fouling gives an effective U of 2,593 W/m²K, and the required area is 60,192 W ÷ (2,593 × 7.61), or 3.05 m². With the default 10% design margin the specified area is 3.36 m².
Counterflow, parallel flow and cross flow
When both streams change temperature, the arrangement changes the area. For water cooled from 60°C to 40°C against water warmed from 10°C to 30°C, at the same effective U of 2,593 W/m²K, counterflow needs 0.537 m², cross flow needs 0.596 m² and parallel flow needs 0.649 m², which is 21% more than counterflow. Cross flow is solved with the effectiveness relation for both streams unmixed, and the calculator reports the correction factor F, 0.90 in this case, which is the effective LMTD divided by the counterflow LMTD. A refrigerant coil with a fixed saturation temperature is the exception, because one isothermal stream makes all three arrangements give the same area.
Condensing and evaporating refrigerant streams
A condensing or evaporating refrigerant holds its saturation temperature, so the calculator treats that side as isothermal and sets the heat capacity ratio to zero. The duty of such a stream is its mass flow times the latent heat, and you can also enter a known duty directly. The economizer example shows both streams checking each other. Liquid at 40°C is subcooled to 25°C at 0.30 kg/s with a specific heat of 1.47 kJ/kg·K, giving 6.62 kW, while refrigerant evaporates at 10°C at 0.035 kg/s with a latent heat of 190 kJ/kg, giving 6.65 kW. The two duties agree within 0.5%, the calculator uses their average of 6.63 kW, and with a clean U of 2,000 W/m²K and an LMTD of 21.64 K the area is 0.153 m².
Economizer and subcooler exchangers
An economizer or liquid subcooler uses a portion of the refrigerant to cool the main liquid line before the expansion device. That removes flash gas from the liquid line and raises the refrigerating effect per kilogram. Brazed plate exchangers suit the duty because they reach a high U in a compact body. Work out the flash fraction and the exchanger duty first with the economizer cycle calculator, then enter that duty here to get the area and plate count.
Stream properties and glycol
Specific heat sets the duty from a mass flow, so enter a value that matches the fluid. Water is 4.18 kJ/kg·K and air is 1.006. Liquid halocarbon refrigerants run roughly 1.3 to 2.0 depending on the fluid and temperature, and glycol mixes sit below water, around 3.3 to 3.8 depending on the type and concentration. Use the glycol system calculator to settle the mix for the freeze protection you need, then enter the specific heat the supplier lists for that concentration and temperature.
Catalogue ratings, plate count and design margin
Plate exchanger makers publish performance at stated conditions, and AHRI Standard 400 (I-P) and 401 (SI) covers how liquid-to-liquid exchangers are rated, so compare the calculated area with the catalogue at your own temperatures and flows. The optional area-per-plate field converts the design area into a plate count. In the condenser example, 3.36 m² at 0.075 m² per plate is 45 plates. The design margin is for uncertainty in U and flow, and it is separate from fouling, which is already in the effective U.
Fouling resistance
Fouling is the deposit of scale, biofilm, corrosion products or oil on a heat transfer surface, and it adds resistance in series: 1 ÷ U effective = 1 ÷ U clean + Rf. The calculator takes Rf as the total for both sides. A light value of 0.0001 m²K/W suits treated water in a plate exchanger, 0.0002 is close to the figure the TEMA standards use for city or well water on one side, and 0.0005 covers heavy fouling from hard or untreated water. In the condenser example, fouling at 0.0001 raises the area by 35% over the clean case. Oil carried over from a compressor also coats refrigerant-side surfaces and lowers U over the life of the unit.
Approach temperature and water-side pressure drop
The minimum approach is the smaller of the two end temperature differences, and area climbs quickly as it closes, which is why the calculator flags anything under 3 K and warns below 1 K. In the condenser example the effectiveness is 54.5% and the approach is 5 K. Water flow trades against it, because more flow lowers the temperature rise and lifts the LMTD, but it also raises the exchanger pressure drop and the pump energy. The exchanger pressure drop comes from the manufacturer's data, and the pipe pressure drop calculator sizes the connecting pipework for the flow you choose.
Frequently Asked Questions
The heat exchanger equation is Q = U × A × LMTD, so for a given duty and U the area is Q divided by U times LMTD. A higher LMTD means less surface. For the same four temperatures, counterflow gives the highest LMTD, because the coldest fluid meets the coldest hot fluid at one end and the warmest at the other. Take water cooled from 60°C to 40°C against water warmed from 10°C to 30°C: counterflow has an LMTD of 30.0 K, parallel flow only 24.9 K. At an effective U of 2,593 W/m²K that is 0.537 m² for counterflow and 0.649 m² for parallel flow, 21% more. Counterflow is also the only arrangement that lets the cold stream leave warmer than the hot stream leaves, which matters when the two heat capacity rates are close.
Brazed plate exchangers in refrigeration usually have a clean U of roughly 2,000 to 5,000 W/m²K for refrigerant and water, and 3,000 to 6,000 W/m²K for water and water. The calculator starts at 3,500 for refrigerant and water, 4,000 for water and water, and 2,000 for a refrigerant subcooler or economizer, and you can overwrite it. Shell-and-tube exchangers with refrigerant on the shell side and water in the tubes are lower, about 800 to 1,500 W/m²K. These are starting values only. Actual U depends on the refrigerant, the water velocity, the plate pattern and the plate count, so size the final selection with the manufacturer's rating at your own conditions.
A condensing or evaporating refrigerant stays at its saturation temperature, so you pick that stream type and enter one temperature. The calculator treats the stream as isothermal, with a heat capacity ratio of zero, and the effectiveness becomes 1 − exp(−NTU) for every flow arrangement. That is why a condenser cooled by water at 29°C in and 35°C out, condensing at 40°C, needs the same 3.05 m² whether the exchanger is counterflow, parallel or cross-flow. The duty comes from the water side or from the refrigerant mass flow times latent heat. Desuperheating and subcooling zones are not split out, so a condenser with large superheat or subcooling needs a zone-by-zone check.
Fouling adds a thermal resistance in series with the clean exchanger, so the effective U is 1 ÷ (1/U clean + Rf). The fouling choice in the calculator is the total resistance for both sides combined: 0.0001 m²K/W for light fouling, 0.0002 for moderate and 0.0005 for heavy. In the condenser example, 0.0001 cuts U from 3,500 to 2,593 W/m²K and raises the required area by 35%. The design margin is a separate percentage added on top for uncertainty in U and flow, and it defaults to 10%. Do not add a second allowance for fouling on top of the resistance you already entered.