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Tips to Increase Efficiency and Heat Recovery in a Boiler System

September 2, 2012

Condensing Economizer (1) 162x129

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.

2.   Proper 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 given moment. 

5.  Incorporate 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