DOE steam tip sheet examples reproduce to their printed rounding, and each assumes that saved energy displaces boiler production
Three DOE Steam Tip Sheets, on condensate return, blowdown heat recovery and flash steam, implemented as verified calculations and compared with their printed worked examples.
Keywords: steam systems; condensate return; blowdown heat recovery; flash steam; energy efficiency; verification
Abstract
The U.S. Department of Energy's Steam Tip Sheets are widely used worked examples for estimating the savings from steam-system measures. We implemented three of them, on condensate return, blowdown heat recovery and flash steam, as verified calculations and compared nine printed monetary and physical results with the computed values; intermediate printed percentages were checked separately. All 9 printed monetary and physical results are reproduced; the largest relative difference among them is 2.5 × 10-3, and every nonzero difference, including a 2.5 × 10-2 difference in an intermediate percentage, is attributable to rounding of the printed value. The calculations reject all 28 invalid-input probes and agree exactly between the native library and a browser implementation. The condensate and blowdown examples convert heat saved into fuel saved through a boiler efficiency; the flash example values regenerated steam directly at a steam price. All three therefore assume that the energy saved displaces boiler production. This is valid only where boiler firing follows steam demand; at sites where turbines or minimum firing rates fix boiler output, the same measure saves electricity or nothing.
1. Introduction
Steam-system savings are commonly estimated with short worked methods such as the DOE Steam Tip Sheets [1]. Tip Sheet 8 values returning condensate instead of discarding it: the water, sewage and chemical treatment saved, and the fuel no longer needed to heat cold makeup water. Tip Sheet 10 values recovering heat from boiler blowdown with a heat exchanger, using a table of recoverable heat by blowdown rate and pressure. Tip Sheet 12 values flashing high-pressure condensate to low-pressure steam.
Because these methods are reused across many sites, two questions matter: whether their printed arithmetic is reproducible, and which assumptions travel with it when it is applied elsewhere. This study addresses both.
2. Methods
2.1 Implementation
Each worked example was implemented in a C++ library and checked against an independent Python reference using exact decimal arithmetic. Inputs are those printed in the tip sheets, including the energy price of 8.00 USD per MMBtu, which the condensate and blowdown examples apply to fuel and the flash example applies to steam. The tip sheets do not state a heating-value basis, and the fuel figures are labelled accordingly. The source calculation paths retain the tip sheets' approximations, such as a unit heat capacity for water; thermodynamic paths using IAPWS-IF97 properties [2] are computed separately and reported alongside.
2.2 Verification
Each computed result was compared with its printed value as a relative difference. Energy balances internal to each calculation were evaluated. Robustness was tested with 28 invalid-input probes: negative flows, zero efficiencies, equal pressures, pressure pairs outside the printed table and overflowing values. A browser build of the calculators was compared with the native library across 30 values, including refusals.
3. Results
3.1 Reproduction of the printed results
All 9 printed monetary and physical results are reproduced (Table 1, Figure 1). The condensate water saving is printed as a total rounded to the nearest ten dollars. An intermediate quantity, the share of steam energy above the makeup-water enthalpy that remains in the returned condensate, computes to 10.72 percent and is printed as 11, a relative difference of 2.5 × 10-2 that is also due to rounding. The blowdown ratio is exactly 3,200 over 53,200 lb/h, 6.015 percent, printed as 6.0. The flash fraction taken from the printed table is reproduced exactly; computed from IAPWS-IF97 properties, it is 0.1028, a 0.2 percent difference absorbed by the table's rounding.
Table 1. Printed and computed results of the three worked examples.
| Printed result | Printed | Computed | Relative difference |
|---|---|---|---|
| Condensate: water, sewage and chemicals, USD/yr | 33,760 | 33,764.99 | 1.5 × 10-4 |
| Condensate: fuel, USD/yr | 88,000 | 88,000 | 0 |
| Condensate: total, USD/yr | 121,760 | 121,764.99 | 4.1 × 10-5 |
| Blowdown: blowdown ratio, percent | 6.0 | 6.015 | 2.5 × 10-3 |
| Blowdown: energy saved, MMBtu/yr | 8,500 | 8,500 | 0 |
| Blowdown: cost saved, USD/yr | 68,000 | 68,000 | 0 |
| Flash: flash fraction, lb/lb | 0.103 | 0.103 | 9.7 × 10-17 |
| Flash: low-pressure steam, lb/h | 515 | 515 | 0 |
| Flash: savings, USD/yr | 40,924 | 40,923.81 | 4.6 × 10-6 |
3.2 Calculation checks
Internal energy balances close to 1.0 × 10-16. Of 28 invalid-input probes, 0 were accepted; each was refused rather than returning a number. The browser implementation matches the native library to a relative difference of 0 across 30 values.
3.3 The embedded assumption
The condensate example credits fuel saved at a boiler efficiency of 0.80 and the blowdown example at 0.80. The flash example applies no boiler efficiency: it values the regenerated low-pressure steam directly at 8.00 USD per MMBtu of steam energy. In all three, the saving exists because the arithmetic assumes the boiler produces less when the site requires less steam or less makeup heating.
4. Discussion
The arithmetic of the three tip sheets is reliable. Their savings, however, are conditional on how the site's boilers respond. Where turbines draw a fixed steam flow, or boilers operate at a minimum firing rate, steam saved downstream does not reduce firing and is routed elsewhere in the network [3]. A measure valued by a tip sheet may then save electricity rather than fuel, at a different price and carbon intensity, or nothing. The tip sheets correctly quantify the heat; whether that heat displaces boiler production is a property of the site.
5. Limitations
- The tip sheets do not state an HHV or LHV basis; fuel figures carry the source's unspecified basis.
- Fuel, steam and water prices are the tip sheets' example values, not current prices.
- Nine monetary and physical results are compared; intermediate printed percentages are discussed but not tabulated.
- Printed approximations are retained in the source paths; thermodynamic paths are reported separately and not judged against printed values.
- No insulation example had a verifiable worked result, so none was evaluated.
6. Conclusions
All 9 printed monetary and physical results of DOE Steam Tip Sheets 8, 10 and 12 are reproduced within their printed rounding. Applying them to a specific site requires a step they do not include: confirming that boiler firing at that site follows the steam saved.
References
- U.S. Department of Energy, Advanced Manufacturing Office. Steam Tip Sheets 8, 10 and 12: Return Condensate to the Boiler (DOE/GO-102012-3410); Recover Heat from Boiler Blowdown (DOE/GO-102012-3408); Flash High-Pressure Condensate to Regenerate Low-Pressure Steam (DOE/GO-102012-3400). Washington, DC, 2012.
- International Association for the Properties of Water and Steam. Revised Release on the IAPWS Industrial Formulation 1997 for the Thermodynamic Properties of Water and Steam, IAPWS R7-97(2012). 2012.
- Smith R. Chemical Process Design and Integration, 2nd edition. Wiley, Chichester, 2016.