Heat exchanger engineering guide

1. Define conditions and heat balance

Identify inlet and outlet states, mass flow rates, allowable pressure drops and operating range. For steady single-phase flow at constant mass flow with negligible pressure effects on enthalpy, use an appropriate average constant-pressure heat capacity to estimate each side’s temperature-related enthalpy-rate change.

Q̇h and Q̇c are the positive magnitudes of the temperature-related enthalpy-rate changes, in W; ṁ is mass flow in kg/s, cp is specific heat at constant pressure in J/(kg·K), and temperature differences may be in K or °C. They approximate each side’s net heat-transfer rate only when shaft work, changes in kinetic and potential energy, and other mass flows across the boundary are negligible. At steady state with negligible external loss, storage and leakage, the two should be close. If not, check flows, temperatures, units, heat loss and measurement error; use a complete energy balance for significant shaft work, such as fans or compressors. See the MIT steady-flow energy equation.

2. Identify flow arrangement and approaches

In parallel flow, both streams enter at the same end; in counterflow, they enter at opposite ends. Lay out the inlets and outlets and pair temperatures at the same end.

• Parallel: ΔT₁ = Th,in − Tc,in; ΔT₂ = Th,out − Tc,out.
• Counterflow: ΔT₁ = Th,in − Tc,out; ΔT₂ = Th,out − Tc,in.

This simple model requires both terminal approaches to be positive. In counterflow, the cold outlet may exceed the hot outlet temperature, while the hot stream must remain hotter at each common end. Near a small approach, check the pinch, measurement uncertainty and required area.

3. Choose LMTD or ε-NTU

Known terminal temperatures: LMTD

ΔTlm is in K; for equal terminal differences its limit is their common value. The equation applies to steady single-pass parallel or counterflow with approximately constant heat-capacity rates and U, and negligible external heat loss. For multipass or crossflow equipment, assess an appropriate correction, such as Q̇ = U A F ΔTlm,counter, where F is a dimensionless factor suited to the arrangement and terminal temperatures. Do not assume F = 1 for arbitrary equipment.

Unknown outlets: ε-NTU

C is heat-capacity rate in W/K, U is overall coefficient in W/(m²·K), and A is area in m² on the same basis as U; Cr, NTU and ε are dimensionless. Given flow arrangement, inlet temperatures, positive capacity rates and UA, use the appropriate effectiveness relation to obtain ε, then duty and outlets. The site’s tool covers ideal single-pass parallel and counterflow only, with constant rates and U, no phase change and negligible heat loss; use a suitable model for complex equipment.

Calculate LMTD · Calculate ε-NTU effectiveness · Estimate heat-transfer area

4. Overall coefficient, fouling and corrections

Total thermal resistance can include hot-side convection, wall conduction, cold-side convection, fouling and contact resistance. When each term is expressed as a whole-device resistance in K/W:

hh and hc are convection coefficients in W/(m²·K), and Ah and Ac are the corresponding areas in m². Here Rf,h and Rf,c are fouling resistances already expressed in K/W. If a source provides area-specific fouling resistance Rf″ in m²·K/W, first divide by its corresponding area. When tube inner and outer areas differ, use a consistent Uref and Aref; the plane-wall calculator does not substitute for a cylindrical-wall model.

Fouling changes with service and operating time; a single value is not a universal allowance. Assess multipass/crossflow corrections, maldistribution, bypass and other geometry effects for the equipment. Evaluate heat transfer alongside pressure drop.

Plane-wall resistance · Smooth-tube single-phase turbulent convection estimate (subject to the tool’s stated conditions)

5. Example: cross-check both methods

Assume ideal single-pass counterflow, steady single-phase flow, constant properties and U, and negligible heat loss. The hot stream has ṁh = 2 kg/s and cph = 1000 J/(kg·K), cooling from 100 °C to 70 °C. The cold stream has ṁc = 1 kg/s and cpc = 1000 J/(kg·K), heating from 20 °C to 80 °C. Take U = 500 W/(m²·K), with area on the same basis.

• Both sides give 60,000 W; counterflow terminal approaches are 20 K and 50 K.
• ΔTlm = 30/ln(50/20) ≈ 32.74 K; A = 60,000/(500 × 32.74) ≈ 3.67 m².
• Cmin = 1000 W/K, Cmax = 2000 W/K and Cr = 0.5. UA ≈ 1833 W/K, NTU ≈ 1.833; ideal counterflow ε ≈ 0.75. Thus Q̇max = 1000 × (100 − 20) = 80,000 W and Q̇ ≈ 60,000 W.

This ideal example illustrates consistency between methods. U is assumed; equipment-specific fouling, pressure drop and multipass corrections are not included.

6. Property and result checks

• Select density, specific heat, viscosity and conductivity for the correct fluid, temperature, pressure and phase; assess segmented or variable-property models for large temperature changes.
• Check mass versus volume flow, absolute versus gauge pressure, dynamic versus kinematic viscosity, and kW/W and kJ/J conversions.
• Check both-side energy balance, approach direction, pinch, physical range of ε, the area basis of U and A, and the order of magnitude of duty and area.
• Separately assess pressure drop, fouling, corrosion, material pressure and temperature limits, margins and applicable requirements. Test uncertain inputs; final design needs reviewed data and methods.

Liquid water and dry air properties · Discrete saturated water and steam states · Back to references