Steam Systems - Economizers
September 17, 2001

Introduction Steaming Gas Temperature Control Parallel vs. Series Boiler Feed Water Preheating Gas Dew Point Temperature Tube Wall Temperature

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Introduction

Economizers serve to improve the efficiency of plant operation by recovering waste heat from the gas stream an putting the energy into generating steam.

Steaming

If steaming occurs in an economizer not designed to generate steam, water hammer and vibration may result.  These problems can be avoided by not designing for too close an approach to the saturation temperature in the economizer.

If steaming cannot be avoided, the economizer can be designed to ensure the steam/water mixture flows upward through the bundle and into the steam drum.  This avoids high points where the steam can separate from the mixture and give rise to hydraulic disturbances.

Gas Temperature Control

To prevent condensation of acid on the shellside it is preferable to bypass on the water side to control the gas exit temperature.  Any reduction in gas flow due to gas bypassing results in a closer temperature approach at the exit of the economizer and increased chance of condensation.  Depending on the degree of instrumentation and automation, the bypass can either be a manual valve or a control valve.

Parallel vs. Series

In a double absorption plant, economizers can be placed prior to the Intermediate and Final Absorber Towers to cool the gas before it enters the towers.  When there are more than two economizers a decision must be made as to whether to place the units in parallel or series with respect to the water side.

In the example given (ie. double absorption plant) it is preferable to place the economizers in series on the water side with the boiler feed water entering the 'Final' economizer first and then the 'Inter' economizer.  This arrangement reduces the chance of condensation in the economizers.

Boiler feed water typically leaves the deaerator at 105°C (221°F).  At this temperature there is a strong possibility that the tube wall temperature will be low enough that condensation will occur on the shell side.  The dewpoint of the gas entering the Intermediate Absorber is expected to be higher than the gas entering the Final Absorber.  In this arrangement the coldest boiler feed water is first fed through the unit ('Final' economizer) where condensation is least likely to occur.  The boiler feed water leaves the 'Final' economizers preheated before it enters the 'Inter' economizers.  The result is a higher tube wall temperature in the 'Inter' economizer and condensation of acid is less likely to occur.

Boiler Feed Water Preheating

Preheating the boiler feedwater can help reduce the possibility of condensation on the shell side of economizers by raising the tube water temperature above the dewpoint of the gas.  This can be done in a number of ways such as increasing the deaerator operating temperature, heat recovery from blowdown, attemporators, steam desuperheating, etc.  These and other techniques are discussed in more detail in BFW Heating Systems.  The disadvantage of using hotter boiler feed water is a closer temperature approach at the gas outlet/water inlet of the economizer which increases its size.  However, the size increase can probably be tolerated by the fact that condensation has been avoided.

There are several techniques that can be used within the economizer itself to raise the tube wall temperature.  The simplest is to design the economizer as a co-current instead of a countercurrent exchanger.  The disadvantage is the unit is much bigger when designed as a co-current unit rather than a countercurrent unit.

An alternative is to place a small bank of tubes at the gas inlet in which the boiler feed water is preheated before entering the primary tube bundle in countercurrent flow to the gas. 

Gas Dew Point Temperature

If process gas is cooled down far enough, acid will condense from the gas onto the cold surface.  The temperature at which condensation begins is called the dew point temperature.   The dew point of the gas is a function of the partial pressure of water and sulphur trioxide in the gas.  The SO3 in the gas comes as a result of the conversion of SO2 to SO3.  Water enters the plant from the small amount that is not removed in the dry tower or from the combustion of hydrocarbons in sulphur.

As the partial pressure of either component is raised, the dew point temperature increases.  Several methods are available to estimate the dew point of the gas.  Unfortunately, the dew points obtained from the different methods may vary by more than 20°C.  

Tube Wall Temperature

Condensation will occur on a cold surface if its temperature is low enough.  In an economizer, the section of tubes which sees the coldest gas will be most prone to condensation on the surface of the tube.  Because of thermal resistances in the surface films and tube wall, the temperature of the tube surface will be inbetween the temperature of the water and gas.  The tube wall temperature should be checked to ensure condensation will not occur.