Engineering task and calculation objective
The DSTR module calculates the radiation and conduction loss of steam generators — the heat that boiler walls, casing and fittings release to the surroundings by radiation and convection. This loss component enters the boiler efficiency and must be quantified in every efficiency determination by the indirect method.
The basis is the Energietechnische Arbeitsmappe (14th edition, 1995), a German power-engineering data compilation. The full-load radiation loss is determined from the maximum steam generator output, a surface factor and the mean heat flux density of the enclosing surfaces. For part-load operation, a load factor accounts for the fact that the absolute loss remains nearly constant while its relative share of the firing thermal capacity rises as the load drops — the module therefore reports the full-load and part-load radiation losses separately.
In practice, the radiation loss of a steam generator is needed for acceptance tests, efficiency guarantees, fuel balances and for evaluating part-load operating regimes in power stations and industrial boiler plants.
Standard and calculation basis: Energietechnische Arbeitsmappe 14. Auflage: 1995
Calculation workflow
- Enter boiler data: The maximum steam generator output of the boiler is specified; it is the reference quantity for the relative radiation loss. For part-load studies, the steam generator output at part load is entered in addition.
- Set surface factor and heat flux density: The surface factor links the boiler output to the heat-releasing enclosing surface; the mean heat flux density describes the area-specific loss of the casing. Both quantities depend on boiler type, fuel and insulation quality and are chosen from the guide values of the Arbeitsmappe.
- Calculate the full-load radiation loss: From the surface factor, the heat flux density and the maximum steam generator output, the radiation loss at full load is obtained — in absolute terms and as a fraction of the boiler output.
- Evaluate part-load behaviour: The part-load radiation loss is determined via the load factor, the ratio of part-load to full-load output. Since the surface temperatures of the boiler hardly drop at part load, the absolute loss is approximately preserved and its relative share grows in inverse proportion to the load.
Input quantities
| Quantity | Symbol | Unit |
|---|---|---|
| Surface factor | C | m²/W^.7 |
| Mean heat flux | q | W/m² |
| Maximum boiler duty | QN | W |
| Boiler duty at partial load | QNφ | W |
Calculated results
| Quantity | Symbol | Unit |
|---|---|---|
| Load factor | φ | – |
| Radiation loss at maximum load | lV | – |
| Radiation loss at partial load | lT | – |
Frequently asked questions
Why does the relative radiation loss increase at part load?
The radiation and conduction loss depends on the surface area and surface temperature of the boiler — both change very little when the load is reduced, as long as the boiler stays at operating temperature. The absolute loss in kW therefore remains nearly constant while the reference output falls: at half load, the relative loss fraction approximately doubles. This is a major reason why boiler efficiency drops in low-load operation.
How does the calculation relate to EN 12952-15 and the former DIN 1942?
The acceptance-test rules for water-tube boilers (today EN 12952-15, formerly DIN 1942) also contain approaches for the radiation and conduction loss as a function of the rated boiler output. The Arbeitsmappe method yields comparable guide values for balancing and preliminary design purposes; for contractually binding acceptance tests, however, the agreed acceptance code governs.
What uncertainties lie in the surface factor and the heat flux density?
Both are empirical values that represent boiler type (water-tube or shell boiler), age and condition of the insulation in a lumped way. With damaged or moisture-soaked casing, uninsulated fittings or unusual geometry, the actual loss can be considerably higher than the calculated value; in case of doubt, surface temperature measurements (thermography) provide clarity.