Heat exchanger field checks usually start with a simple question: how much heat appears to move from one stream to the other under the measured conditions? The first-pass arithmetic uses flow, fluid properties, and temperature change to estimate hot-side duty and cold-side duty. If those two numbers disagree, the problem may be measurement, assumptions, heat loss, phase behavior, or the exchanger itself.
The familiar relationship Q = U x A x LMTD is useful, but it is easy to overread. If an app reports UA, that is a product back-calculated from measured duty and temperature driving force. It is not a selected U-value, installed area, fouling factor, or manufacturer thermal rating.
This guide explains the screening math and the source boundary. Final exchanger selection, cleaning, repair, pressure-boundary work, glycol decisions, and code compliance need current source text, manufacturer data, site measurements, and qualified thermal or mechanical review.
Start with Two-Sided Duty
For a single-phase liquid screening calculation, each side can be estimated as Q = flow x density x Cp x temperature change. In U.S. customary water/glycol work, the app uses GPM x lb/gal x 60 x BTU/lb-degF x degF to get BTU/hr.
Hot-side and cold-side duty should be close only when the measurements, units, fluid-property basis, and heat-loss assumptions are close. A mismatch is not automatically an exchanger fault. It can come from probe placement, flow-meter scaling, density or Cp assumptions, unsteady operation, bypassing, heat loss to ambient, phase change, or mislabeled inlet and outlet points.
LMTD Needs the Right Arrangement
LMTD represents the logarithmic mean of the terminal temperature differences. For ideal counterflow, the terminal differences are hot inlet minus cold outlet and hot outlet minus cold inlet. For ideal parallel flow, they are hot inlet minus cold inlet and hot outlet minus cold outlet.
If either terminal difference is zero or negative, the LMTD screen is not valid. Multi-pass shell-and-tube exchangers, crossflow units, air-cooled exchangers, condensers, evaporators, and many real plant arrangements require a correction factor or a different rating method. That correction is not optional for decision use.
UA Product Is Not Area or U-Value
When all four terminal temperatures and the duty estimate are known, UA can be back-calculated as Q divided by LMTD. That number is useful for screening and trending, but it is still a product. It does not say what the surface area is, what the clean or fouled U-value should be, or whether the installed exchanger is correctly sized.
Separating U from A requires exchanger geometry, surface definition, flow regime, fluid properties at operating temperature, wall material, fouling basis, correction factors, and usually manufacturer or thermal-rating software. A source-aware trend can be useful; a single UA value is not purchase or repair proof.
Approach Temperature Is a Clue, Not a Cleaning Order
Approach temperature is the closest terminal temperature difference in the simplified model. A tightening or widening approach can be useful when trended against a clean baseline under comparable flow and load, but it does not identify the cause by itself.
Fouling, reduced flow, changed process load, bypass leakage, air binding, control-valve position, different glycol concentration, steam quality, measurement drift, and ambient losses can all change the apparent performance. Cleaning or repair decisions need pressure-drop data, process history, inspection findings, chemistry, safety controls, downtime planning, and qualified review.
Design, Pressure, and Product Boundaries
Heat exchangers can also be pressure vessels, code-regulated equipment, sanitary equipment, hazardous-process equipment, or manufacturer-warranty equipment. Thermal duty arithmetic does not check metallurgy, corrosion, tube vibration, nozzle loads, relief scenarios, inspection, repair, cleaning chemistry, gasket limits, pressure drop, or product listing.
For shell-and-tube work, TEMA standards and manufacturer datasheets matter. For pressure-boundary work, ASME BPVC or the adopted jurisdictional code may apply. For glycol systems, current product data and field concentration testing matter. The app keeps these as source pointers and warnings, not hidden assumptions.
Heat Exchanger Duty Calculator
Calculate heat exchanger duty (Q), LMTD, and required UA value. Supports counterflow and parallel flow with water and glycol fluids.