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User Guide

Steam Heat Exchanger Stall Analysis Tool

Table of Contents

1. Purpose of the Tool 2. What the Tool Does 3. Governing Physics 4. How to Use the Tool 5. Input Parameters 6. Input Bounds and Validation 7. Assumptions 8. Limitations 9. Appropriate Use Cases

1. Purpose of the Tool

This tool evaluates stall risk in steam-heated heat exchangers operating with condensing steam. It replicates the logic of the Excel workbook Stall Chart.xlsm and follows the Spirax-Sarco stall chart methodology, using validated IF-97 steam properties.

The Core Question

At what operating loads will the heat exchanger lose sufficient steam pressure to drain condensate against the return backpressure (i.e., stall)?

Important: What This Tool Is NOT

This is not a full heat-exchanger design or rating tool (HTRI / Aspen EDR). It is a stall and condensate drainage screening tool intended for concept design, troubleshooting, and justification of pump-assisted condensate removal.

2. What the Tool Does (At a High Level)

For a steam-heated exchanger, the tool:

  1. Uses heat-transfer fundamentals to determine the required steam saturation temperature at each load
  2. Converts that temperature to required steam pressure using IF-97 properties
  3. Compares required steam pressure to condensate return backpressure
  4. Identifies operating regions where:
    • Condensate will drain normally, or
    • Stall and flooding are expected
  5. Repeats this analysis across turndown (load %)
  6. Performs the analysis for both:
    • Clean exchanger condition, and
    • Service / fouled exchanger condition

3. Governing Physics

The tool assumes condensing steam at constant temperature heating a single-phase process fluid.

Heat Transfer

Q = U × A × ΔT_lm

Stall Criterion

Stall is predicted when:

P_steam,HX ≤ P_backpressure

or equivalently:

T_s ≤ T_sat(P_backpressure)

4. How to Use the Tool (Step-by-Step)

Step 1 — Enter Design / Operating Inputs

Provide the required inputs on the left sidebar (described in Section 5 below).

Step 2 — Click "Analyze"

The tool will generate temperature and pressure curves across load (% capacity).

Step 3 — Review the Chart

Look at the curves showing:

  • Required steam temperature (clean & service)
  • Required steam pressure (clean & service)
  • Condensate backpressure limit

Step 4 — Identify Stall Regions

At any load where the required steam pressure drops to or below the backpressure limit, stall is expected. Check the "Min Safe Load" summary cards.

Step 5 — Interpret Results

Use the results to:

  • Identify minimum safe operating load
  • Compare clean vs fouled behavior
  • Justify pump-trap or pressure-powered pump solutions
  • Understand why stall may appear during turndown or fouling

5. Input Parameters (What Each Input Represents)

5.1 Duty at 100% Load

Description: Total heat duty required at full load.

Units: BTU/hr

Role: Governs required steam temperature and pressure. Duty is scaled linearly with % load.

Example: 1,257,060 BTU/hr

5.2 Heat Transfer Surface Area

Description: Effective heat transfer area of the exchanger.

Units: ft²

Role: Larger area reduces required steam temperature. Small area increases stall risk.

Example: 58.71 ft²

5.3 Overall Heat Transfer Coefficient — Clean

Description: Overall heat transfer coefficient for a clean exchanger.

Units: BTU/(hr·ft²·°F)

Role: Defines best-case (new) performance.

Typical Range: 100-200 for shell-and-tube with condensing steam

Example: 140 BTU/(hr·ft²·°F)

5.4 Overall Heat Transfer Coefficient — Service / Fouled

Description: Overall heat transfer coefficient under fouled or aged conditions.

Units: BTU/(hr·ft²·°F)

Role: Shows how fouling shifts stall behavior. Typically less than clean U.

Typical Range: 70-90% of clean U

Example: 110 BTU/(hr·ft²·°F)

5.5 Process Inlet Temperature (Ti)

Description: Temperature of the process fluid entering the exchanger.

Units: °F

Role: Determines cold-end temperature difference. Higher Ti reduces driving temperature difference and increases stall risk.

Example: 0°F

5.6 Process Outlet Temperature (To)

Description: Target temperature of the process fluid leaving the exchanger.

Units: °F

Role: Defines process thermal requirement. Must be greater than Ti.

Example: 300°F

5.7 Condensate Backpressure (Maximum)

Description: Maximum pressure in the condensate return system.

Units: psig

Role: Sets the stall threshold. Higher backpressure increases stall likelihood.

Typical Range: 0-100 psig

Example: 70 psig

What Inputs Do NOT Affect the Stall Outcome

The following quantities do not independently change the stall result:

  • Process flow rate
  • Max feed rate
  • Steam flow rate
  • Condensate flow rate
  • Control valve position

These quantities are derived or interpretive, not governing. The model is duty-driven, not flow-driven.

6. Input Bounds and Validation

To ensure numerical stability and physical validity, the tool enforces the following:

Required Conditions (Errors)

Numerical Guardrails

If violated, the tool explains:

7. Assumptions of the Tool

  1. Condensing steam at saturation temperature
  2. Single-phase process fluid (liquid)
  3. Fixed duty basis across load
  4. Constant overall heat transfer coefficient (clean & service)
  5. Negligible pressure drop within the exchanger
  6. Steady-state operation

These assumptions are consistent with standard stall-chart methodology.

8. Limitations

For exchanger design or guaranteed performance, use HTRI or Aspen EDR.

9. Appropriate Use Cases

This tool is well-suited for:

Key Takeaway

Stall is governed by steam pressure versus condensate backpressure.

This tool quantifies how duty, heat-transfer capability, and process temperatures determine whether condensate can physically drain. Used correctly, it provides clear, defensible insight into when and why stall will occur.

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