Download Load Balancing Energy Conservation Opportunities How to Test for

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 heatrecovery equipment, consider the following example.
Example: Determine the energy savings associated with installing an
economizer or flue-gas air preheater on the boiler from the previous
example. Assume that the excess-air control system from the previous
example has already been implemented.
Available Data
Current energy consumption 1,032,460 therms/yr
Boiler rated capacity 600 boiler horsepower
Operating hours 8,500 hr/yr
Exhaust stack gas analysis 2% Oxygen (by volume, dry)
Minimal CO reading
Load Balancing
Energy Conservation Opportunities
How to Test for Energy Conservation Potential
Information needed to determine energy conservation
opportunities through load-balancing techniques requires
a plant survey to determine (1) total steam demand and
duration at various process throughputs (profile of steam
load versus runtime), and (2) equipment efficiency
characteristics (profile of efficiency versus load).
Steam Demand
The efficiency of each boiler should be documented at a
minimum of four load points between half and maximum
load. A fairly accurate method of obtaining unit
efficiencies is by measuring stack temperature rise and
percent O2 (or excess air) in the flue gas or by the input/
output method defined in the ASME power test codes. Unit
efficiencies can be determined with the aid of Figure 5.3,
5.4, or 5.5 for the particular fuel fired. For pump(s) and
fan(s) efficiencies, the reader should consult
manufacturers’ performance curves. An example of the
technique for optimizing boiler loading follows.
Example: A plant has a total installed steam-generating
capacity of 500,000 lb/hr, and is served by three boilers
having a maximum continuous rating of 200,000, 200,000,
and 100,000 lb/hr, respectively. Each unit can deliver
superheated steam at 620 psig and 700°F with feed water
supplied at 250°F. The fuel fired is natural gas priced into
the operation at $3.50/106 Btu. Total plant steam averages
345,000 lb/hr and is relatively constant.
The boilers are normally operated according to the
following loading (top of following page).
Analysis. Determine the savings obtainable with optimum
steam plant load-balancing conditions.
STEP 1. Begin with approach (a) or (b).
a) Establish the characteristics of the boiler(s) over
the load range suggested through the use of a
consultant and translate the results graphically
as in Figures 5.11 and 5.12.
b) The plant determines boiler efficiencies for each
unit at four load points by measuring unit stack temperature rise and
percent O2 in the flue gas.
With these parameters known, efficiencies are
obtained from previous Figures 5.3, 5.4, or 5.5. Tabulate
the results and graphically plot unit efficiencies and unit
heat inputs as a function of steam load. The results of such
an analysis are shown in the tabulation and graphically
illustrated in Figures 5.11 and 5.12.
(Unit input) = (unit output)/(efficiency)