Feed-effluent preheater — duty from the flowsheet, geometry from EDR

A feed/effluent preheater, built so the shell-and-tube rating in the Analysis panel has a real duty to rate. The flowsheet answers how much heat; the rating answers whether this exchanger can move it — the split Aspen sells as EDR, and it had no curated example.

From the flowsheet. Hot reactor effluent (130 mol/s at 420 K) preheats 120 mol/s of cold feed to 360 K, transferring 1.53 MW and leaving the hot side at 373 K, against an LMTD of 66.4 K. Those are the numbers you carry into the rating.

The rating (Analysis > HX design). Put a candidate bundle against that duty — a 0.6 m shell, 320 tubes of 19 mm OD on a 4.88 m length, two tube passes, 0.25 m baffle spacing — and the Bell-Delaware method returns U = 681 W/m2K, 93.2 m2 available against 53.6 m2 required: 74% over-surface, adequate. It also exposes the correction factors that actually decide the shell-side coefficient (segmental-cut j_c = 0.91, leakage j_l = 0.67, bypass j_b = 0.54, combined 0.33), which is where a real rating lives — most of the difference between an ideal bank and the bundle you can buy is leakage and bypass.

Reading the margin. 74% over-surface is generous, not automatically right: it buys fouling allowance and turndown, and costs capital and residence time. The rating is the tool for trading those off — shrink the bundle and watch the margin close.

Honesty notes. Shell-side characterization (Reynolds, Prandtl, conductivity, tube-side coefficient) is supplied as screening input rather than derived from the stream, so this sizes and checks a bundle, it does not replace a vendor's thermal design. The exchanger in the flowsheet is specified by outlet temperature; its own u_a rating mode is the alternative when you want the duty to fall out of the geometry instead.