Skip to main content
Shops & Outbuildings Free Pro Features Available

Heat Exchanger Duty Calculator - Q, LMTD, UA & Approach Temperature

Calculate heat transfer duty, log mean temperature difference, and required surface area for shell-and-tube and plate exchangers

Calculate heat exchanger thermal duty (Q), log mean temperature difference (LMTD), overall heat transfer coefficient (U), and required heat transfer area (A) for counterflow, parallel flow, and crossflow configurations. Enter inlet and outlet temperatures for hot and cold streams to get duty in BTU/hr, LMTD with correction factor, approach temperature, and effectiveness. Supports shell-and-tube, plate-and-frame, air-cooled, and double-pipe heat exchangers with built-in U-value ranges for common fluid pairs.

Pro Tip: Approach temperature is the single most important number for monitoring heat exchanger health. Track it monthly. When approach temperature increases by 5-10 degrees F from the clean baseline, it is time to clean or inspect the exchanger. Fouling that costs $500 to clean today will cost $5,000 in wasted energy and emergency downtime if you wait until performance collapses. Trend the data, do not just react to failures.

Calculate pipe pressure drop through exchanger piping

Pipe Pressure Drop Calculator →

Check pump head for exchanger circulation

Pump Affinity Laws Calculator →

Look up pipe schedule for exchanger connections

Pipe Schedule Reference →

Convert material hardness for tube material selection

Hardness Converter →
Start Using This Tool See How It Works

PREVIEW All Pro features are currently free for a limited time. No license key required.

Heat Exchanger Duty Calculator
Pop Out

How It Works

  1. Enter Stream Temperatures

    Input inlet and outlet temperatures for both the hot side and cold side in degrees F or C. If you only know three of the four temperatures, enter the known values plus the flow rate and fluid type, and the calculator will determine the missing temperature from the energy balance.

  2. Select Flow Configuration

    Choose counterflow (most efficient, highest LMTD), parallel flow (simpler construction), or crossflow (air-cooled exchangers). For shell-and-tube with multiple passes, select the pass configuration and the calculator applies the appropriate LMTD correction factor (F).

  3. Enter Flow Rates and Fluids

    Input flow rates in GPM, lb/hr, or kg/hr for each stream. Select fluid type to load specific heat and density. Supports water, glycol solutions (20-60%), steam, thermal oil, air, and custom fluids with user-entered properties.

  4. Review Duty and LMTD

    See heat transfer duty Q (BTU/hr, kW), LMTD (degrees F or C), approach temperature, and thermal effectiveness. The energy balance is verified to ensure hot side duty matches cold side duty within rounding tolerance.

  5. Calculate Required Surface Area

    Select an overall U-value from the built-in reference table or enter a known value. The calculator determines required heat transfer area A from Q = U x A x LMTD. Compare to your existing exchanger surface area to evaluate whether it is adequately sized.

Built For

  • Mechanical engineers sizing new heat exchangers for process heating and cooling applications
  • Plant operators monitoring exchanger performance and detecting fouling from rising approach temperatures
  • HVAC engineers calculating heating and cooling coil capacity for air handler design
  • Process engineers evaluating heat recovery opportunities between hot and cold process streams
  • Maintenance planners scheduling heat exchanger cleaning based on performance degradation trends
  • Energy auditors quantifying energy waste from fouled or undersized heat exchangers
  • Reliability engineers troubleshooting exchangers that are not meeting duty requirements

Features & Capabilities

LMTD with Correction Factor

Calculates log mean temperature difference for counterflow, parallel flow, and multi-pass configurations. Applies the LMTD correction factor (F) for shell-and-tube exchangers with unequal passes, using the P-R method per TEMA standards.

Approach Temperature Monitor

Computes approach temperature (the minimum temperature difference between the two streams) and compares it to the clean design value. Flags exchangers where approach has increased, indicating fouling or reduced flow that needs attention.

U-Value Reference Library

Built-in overall heat transfer coefficient ranges for common service pairs: water-to-water (200-500 BTU/hr-ft2-F), steam-to-water (250-750), water-to-air (5-30), oil-to-water (20-60), and condensing refrigerant (100-300). Select a pair to get typical clean and fouled U-values.

Surface Area Calculator

Determines required heat transfer area from the fundamental equation Q = U x A x LMTD. Compare calculated area to installed area to identify exchangers that are undersized or have excessive fouling reducing effective area.

Effectiveness-NTU Method

Alternative rating method using heat exchanger effectiveness (actual duty / maximum possible duty) and NTU (number of transfer units). Useful when outlet temperatures are unknown and you need to predict what an existing exchanger will do at off-design conditions.

Comparison

Exchanger Type U-Value Range (BTU/hr-ft2-F) Pressure Rating Cleaning Access Best Application
Shell & Tube 50-750 High (300+ PSI) Tube side: mechanical; Shell side: chemical Process industry, high pressure, fouling fluids
Plate & Frame 200-1200 Moderate (150 PSI) Excellent (plates separate) HVAC, food/beverage, frequent cleaning
Air-Cooled (Fin-Fan) 5-30 Moderate Fin cleaning, tube rodding Where cooling water is unavailable
Double Pipe 50-500 High Easy (disassemble) Small duties, high pressure, pilot plants
Brazed Plate 200-1000 Moderate (400 PSI) Chemical only (no disassembly) Refrigeration, compact HVAC, clean fluids

Frequently Asked Questions

LMTD (Log Mean Temperature Difference) is the effective average temperature driving force across a heat exchanger. For counterflow: LMTD = (dT1 - dT2) / ln(dT1/dT2), where dT1 and dT2 are the temperature differences at each end. LMTD is always between dT1 and dT2 but weighted toward the smaller difference. It accounts for the logarithmic nature of heat transfer as temperatures change along the exchanger length.
Approach temperature is the closest temperature difference between the hot and cold streams, typically at the cold-side outlet end in a counterflow exchanger. A smaller approach means more heat recovery but requires more surface area (and cost). Design approach temperatures typically range from 5-20 degrees F. Monitoring approach temperature over time is the best way to detect fouling. If approach increases by 5-10 degrees F from clean baseline, the exchanger needs cleaning.
U-value represents the total resistance to heat transfer through the exchanger, including hot-side film coefficient, tube wall conduction, fouling resistances on both sides, and cold-side film coefficient. It is expressed in BTU/hr-ft2-F (or W/m2-K). Higher U means better heat transfer. U-values depend on fluid types, velocities, and fouling condition. Clean water-to-water exchangers achieve U = 200-500; fouled exchangers may drop to 50-150.
Counterflow (fluids flow in opposite directions) is almost always preferred because it achieves higher LMTD and can theoretically heat the cold fluid above the hot fluid outlet temperature. Parallel flow (same direction) has a lower LMTD and cannot heat the cold fluid above the hot outlet temperature. Parallel flow is used only in special cases: to limit cold-side outlet temperature, for thermal shock protection, or when equipment layout requires it.
Fouling adds thermal resistance on the tube surfaces, effectively reducing the U-value. Typical fouling resistances: clean cooling tower water 0.001 hr-ft2-F/BTU, river water 0.002-0.003, city water 0.001, light hydrocarbon 0.001, heavy hydrocarbon 0.003-0.005. A fouling resistance of 0.002 on both sides of a water-to-water exchanger can reduce U by 30-40%, requiring that much more surface area to maintain duty. TEMA standards specify recommended fouling factors by fluid service.
The correction factor F accounts for the reduced thermal effectiveness of multi-pass shell-and-tube exchangers compared to pure counterflow. F ranges from 0 to 1.0, with 1.0 being pure counterflow. For a 1-shell-2-tube-pass exchanger, F is typically 0.8-0.95. If F drops below 0.75, the exchanger configuration is thermodynamically inefficient and you should add shells in series. F depends on the P and R parameters calculated from the four stream temperatures.
Calculate the required duty Q from your process requirements, determine the LMTD from stream temperatures, select the appropriate U-value (use the fouled value for realistic sizing), and calculate required area A = Q / (U x LMTD x F). If the calculated area exceeds your installed exchanger's surface area, it is undersized. Common symptoms of an undersized exchanger include high approach temperature, inability to reach design outlet temperatures, and frequent fouling from operating at high heat flux.
Disclaimer: This calculator provides engineering estimates for heat exchanger sizing and performance evaluation. Actual heat transfer performance depends on fluid properties, flow distribution, fouling condition, and equipment design details. U-value ranges are typical values for preliminary design. Final heat exchanger selection requires detailed thermal design per TEMA, ASME, or API standards. ToolGrit is not responsible for heat exchanger design or procurement decisions.

Learn More

Shops & Outbuildings

Heat Exchanger Basics: LMTD, UA, Fouling & Approach Temperature

Understanding heat exchanger duty calculations: LMTD method, UA rating, fouling factors, approach temperature, and when to clean or replace exchangers.

Read Guide →
Industrial

Mechanical Insulation Thickness: ASTM C680 Guide

How to calculate insulation thickness using ASTM C680 method. Heat loss, surface temperature, condensation prevention, and material selection for pipes and equipment.

Read Guide →
Industrial & Plant

Steam Properties and Boiler Operations Guide

How to use steam tables for steam trap sizing, boiler efficiency calculations, flash steam recovery, deaerator verification, and superheated steam applications. Covers hf, hfg, hg, and specific volume.

Read Guide →

Related Tools

Shops & Outbuildings Live

Shop Heater BTU Sizing Calculator

Calculate the exact BTU output your shop or garage heater needs. Factors in wall R-values, ceiling insulation, slab edge loss, overhead door infiltration, and air changes per hour to size propane, natural gas, and electric heaters correctly.

Launch Tool →
Shops & Outbuildings Live

Overhead Door Infiltration Loss Calculator

Calculate heat loss through overhead doors in shops, garages, and warehouses. Compares open-door vs closed-door losses, seal condition impact, and annual cost of infiltration with payback on door seals and high-speed doors.

Launch Tool →
Shops & Outbuildings Live

Long-Run Voltage Drop Calculator

Calculate voltage drop for long wire runs to detached shops, barns, garages, and outbuildings. Compares copper vs aluminum, shows motor starting voltage impact, and recommends the right wire size for your distance and load.

Launch Tool →