How to Test for Heat-Recovery Potential

How to Test for Heat-Recovery Potential
In assessing overall efficiency and potential for heat recovery, the parameters of significant importance are temperature and fuel type/sulfur content. To obtain a meaningful operating flue-gas temperature measurement and a basis for heat-recovery selection, the unit under consideration should be operating at, or very close to, design and optimum excess-air values as defined on Table 5.2. Temperature measurements may be made by mercury or bimetallic element thermometers, optical pyrometers, or an appropriate thermocouple probe. The most adaptable device is the thermocouple probe in which an iron or chromel constantan thermocouple is used. Temperature readout is accomplished by connecting the thermocouple leads to a potentiometer. The output of the potentiometer is a voltage reading which may be correlated with the measured temperature for the particular thermocouple element employed. To obtain a proper and accurate temperature measurement, the following guidelines should be followed: 1. Locate the probe in an unobstructed flow path and sufficient distance, approximately five diameters downstream or upstream, of any major change of direction in the flow path.
2. Ensure that the probe entrance connection is relatively leak free.
3. Take multiple readings by traversing the cross-sectional area of the flue to obtain an average and representative flue-gas temperature.
Modifications or Additions for Maximum Economy
The installation of economizers and/or flue-gas air preheaters on units not presently equipped with heat-recovery devices and those with minimum heat-recovery equipment are practical ways of reducing stack temperature
while recouping flue-gas sensible heat normally rejected to the stack. There are no “firm” exit-temperature guidelines that cover all fuel types and process designs. However, certain guiding principles will provide direction to the lowest practical temperature level of heat rejection. The
elements that must be considered to make this judgment include (1) fuel type, (2) flue-gas dew-point considerations, (3) heat-transfer criteria, (4) type of heat-recovery surface, and (5) relative economics of heat-recovery
equipment. Tables 5.5 and 5.6 may be used for selecting the lowest practical exit-gas temperature achievable with installation of economizers and/or flue-gas air preheaters. As an illustration of the potential and methodology for recouping flue-gas sensible heat by the addition of heat-recovery equipment, consider the following example.