Tips to Increase Efficiency and Heat Recovery in a Boiler System
As boilers age, they can become less efficient, causing an
increase in fuel costs. The main cause
of energy inefficiency is system heat loss.
Here are several ways to reclaim the heat loss in an effort to reduce
facility operating costs:
1. Keep up
with annual boiler maintenance – Maintaining a boiler according to
manufacturer recommendations is the simplest way to ensure it will continue to run efficiently.
water treatment – Check that the boiler system water is treated properly. Particulates clog up internal boiler tubes,
and over time the system loses its heat transfer ability. In most applications, a deaerator is a good
solution. It can increase the water
temperature to 227° F and remove a lot of the particulates from the water.
3. Install a
high-turndown burner - Increasing the burner turndown rate will increase
energy savings and reduce maintenance.
Energy savings increases when on-off cycles are reduced. Each on-off cycle is followed by a purge
cycle. During a purge cycle, a large
volume of air passes through the boiler, resulting in heat being blown out of
the stack. Installing a high-turndown
burner enables a boiler to consistently run at its most efficient point.
4. Add VSD
controls -- Installing variable speed drives (VSDs) on boiler feed or
deaerator pumps allows an operator to fine-tune the system to run at optimal conditions, while saving energy.
This is because VSDs enable a motor to operate only at the speed needed at a
parallel positioning - Adding parallel positioning to a boiler enables it to
run at its most capable efficiency point.
Parallel positioning uses dedicated actuators for the fuel and air
valves. Burners that incorporate parallel
positioning are able to maintain excess air levels more precisely.
6. Include O2 trim - Another way
to maintain peak efficiency is to add an oxygen sensor/transmitter in the
exhaust gas. The sensor/transmitter
continuously senses oxygen content and provides a signal to the controller that
trims the air damper and/or fuel valve, maintaining the originally commissioned oxygen concentration under
varying conditions. This minimizes
excess air while optimizing the air-to-fuel ratio.
7. Integrate lead/lag
– Lead/lag sequences the operation of multiple boilers, matching system
load. Lead/lag enables boilers
to operate in sync with fluctuating steam loads, reducing the
possibility of process failure and needless boiler cycling.
Another way to improve energy efficiency is to incorporate
heat recovery into a boiler system. Several
options are listed below. The right type
of equipment for a boiler system depends upon several factors, including:
boiler type, fuel used, stack material, and operating conditions.
Standard economizer -- Economizers transfer energy from the
boiler exhaust gas to the boiler feedwater in the form of “sensible heat.” Sensible heat is created by the transfer of
the heat energy of the exhaust gas to boiler feedwater. An economizer captures and redirects sensible
heat from the hot flue gas that normally goes up the boiler stack. This sensible heat increases the temperature
of boiler feedwater or makeup water. On
average, adding a standard economizer increases boiler system efficiency by 3
percent to 5 percent.
Condensing economizer – A condensing economizer captures both
sensible and latent energy from the flue gases leaving the boiler. Two types of condensing economizers are
available. There is a standard one-stage
condensing economizer, or two-stage condensing economizer. In the two-stage system, the first stage is
non-condensing and the second stage is condensing, so the system can heat two
sources of water. Adding a condensing
economizer increases energy efficiency between 4 percent and 8 percent.
Blowdown heat recovery unit --A blowdown heat recovery unit is the most effective method of
purging destructive solids from any steam boiler system. The unit transfers heat energy normally lost during
continuous blowdown, transferring it to the cold makeup water. The payback in fuel savings for a blowdown
heat recovery unit is typically less than a year.
Flash economizer -- A flash economizer uses the surface blowdown
to heat up the makeup water. It recovers
blowdown energy in the form of flash steam (latent heat) and blowdown (sensible
heat). By capturing both sensible and
latent heat, a flash economizer
is able to recover more heat than the Blowdown Heat Recovery Unit. Payback for a flash economizer often is less
than a year.
U-tube heat exchangers -- Water-to-steam or water-to-water u-tube
heat exchangers can be added to preheat a boiler feedwater system or deaerator. These heat exchangers operate similar to a blowdown heat recovery unit. U-tube
heat exchangers can be used just about anywhere to transfer heat between
a hot fluid and a cold fluid.
To help companies predict the annual cost savings for
making certain energy-efficient upgrades, there is a software program called BOOST™ (Boiler Operation Optimization
Savings Test). The facility-specific
financial analysis is available through Cleaver-Brooks representatives.
For more information about increasing boiler system
efficiency, or heat recovery products, or to locate a Cleaver-Brooks
representative, visit cleaverbrooks.com.
Featured in the September Issue of Process Heating
Download: Tips to Increase Efficiency and Heat Recovery in a Boiler System