Summary

When condensate is discharged from a high pressure steam system to a system of lower pressure, a proportion of the condensate may vaporise to form low pressure steam. This is known as flash steam and may be utilised in low pressure heating systems to improve plant efficiency. This article discusses the generation of flash steam and presents a method by which it can be quantified.

Definitions

\( F \):Proportion of flash steam generated (kg steam/kg condensate)
\( h_1 \):Enthalpy of condensate at the steam system pressure (kJ/kg).
\(h_2 \):Enthalpy of condensate at release pressure and corresponding saturation temperature (kJ/kg).
\(h_{vap} \):Specific enthalpy of vaporisation at the condensate release pressure (2257 kJ/kg at 101.325 kPa)
\( m \):Mass flow rate of condensate (kg/s)
\( q \):Waste heat in condensate stream (kW)


Introduction

When hot condensate is passed from a high pressure system to a low pressure system (commonly the atmosphere) flash steam is generated. This is commonly diagnosed as a steam trap fault, however it is simply the surplus enthalpy of the hot condensate boiling off a proportion of the water as the condensate adjusts to its new conditions.

Flash steam is generated when the temperature of the condensate is higher than the steam saturation temperature at the pressure it is being discharged to. The quantity of flash steam generated for condensate being discharged from pressure \( P_1 \) to pressure \( P_2 \) may be calculated as follows:

\[ \displaystyle F = \frac{h_1 - h_2}{h_{vap,2}} \]

The quantity of flash steam calculated above represents the excess heat present in the condensate stream. This waste heat may be quantified using the mass flow rate of the condensate stream as shown below:

\[ \displaystyle q = m \times F \times h_{vap, 2} \]

The energy flow rate in flash steam can be significant, particularly in applications such as boiler blowdown, often accounting for 40% of the total energy lost through the condensate stream. Therefore in order to achieve maximum efficiency, where economical, steam systems should reclaim flash steam for use in systems requiring low pressure heating such as boiler feed water pre-heating.



Example

Consider a low pressure steam system at 7 barg, here the steam will have a saturation temperature \( T_{s,1} = 170.5°C \) and an enthalpy \(h_1 = 721.6 kJ/kg \). The condensate for this system is discharged to an atmospheric vented vessel where the steam saturation temperature is \( T_{s,2} = 100°C \), corresponding enthalpy \( h_2 = 419.1 kJ/kg \) and heat of vaporisation \( h_{vap,2} = 2257 kJ/kg \).

Flash steam system

The fraction of flash steam generated on condensate discharge to the atmospheric vessel may be calculated as follows:

\[ \displaystyle \begin{align} F &= \frac{\left( 721.6 kJ/kg - 419.1 kJ/kg \right)}{2257 kJ/kg} \\ &= 0.134 \text{kg steam/kg condensate} \end{align} \]

Therefore 0.134 kg of flash steam may be reclaimed per kilogram of condensate discharged, this represents approximately 42% of the energy in the condensate stream.

Further Reading

  1. Spirax-sarco, The Steam and Condensate Loop
  2. Woodruff, E., Lammers, H., (Author), Lammers, T. ,Steam Plant Operation, 2011, McGraw-Hill Professional
  3. Stultz, S. C., Steam: Its Generation and Use, 2005, Babcock & Wilcox Co