Feedwater Temperature Steam Boiler Rating

Overview

Engineering reference data for Feedwater Temperature Steam Boiler Rating in steam condensate.

Key Formulas

Steam Quality

$x = \frac{m_v}{m_{total}}$

Mass fraction of vapor in two-phase mixture.

Enthalpy of Wet Steam

$h = h_f + x \cdot h_{fg}$

Specific enthalpy of wet steam.

Flash Steam

$m_{flash} = m_{liquid} \frac{h_f - h_{f2}}{h_{fg2}}$

Steam generated when condensate flashes to lower pressure.

Condensate Load

$m_c = \frac{Q}{h_{fg}}$

Condensate generated by heat transfer.

Variables

Practical Example

To illustrate the impact of feedwater temperature, consider a boiler designed to produce 10,000 kg/h of steam at 10 bar gauge. The design assumes feedwater at 100°C (sensible heat already provided). If the feedwater temperature drops to 80°C, the boiler must supply additional energy to heat that extra 20°C before the water can vaporize, effectively reducing its capacity to produce steam at the rated pressure.

Calculation of Capacity Reduction

The reduction in boiler capacity can be estimated using the following relationship, which compares the enthalpy (heat content) of the steam to the enthalpy of the feedwater at different temperatures:

$\text{Capacity Reduction (\%)} = \left(1 - \frac{h_{steam} - h_{fw,design}}{h_{steam} - h_{fw,actual}}\right) \times 100$

Where:

• $h_{steam}$ = Specific enthalpy of the steam at operating pressure (kJ/kg).
• $h_{fw,design}$ = Specific enthalpy of feedwater at the design temperature (kJ/kg).
• $h_{fw,actual}$ = Specific enthalpy of feedwater at the actual (lower) temperature (kJ/kg).

Sample Impact Data

The following table shows the approximate capacity reduction for a boiler operating at 10 bar gauge, with a design feedwater temperature of 100°C, as the actual inlet temperature decreases.

Interactive Charts

References

• Original Source