The Heat Is On-Part I
Boiler flue gas heat recovery in the laundry process

By Lee R. Kemberling

Editor's Note: Click here for the second part of the article.

The use of heat remaining in exhaust gasses from the boiler has been practiced for many years. The main reason has been to reduce the primary fuel requirement of the boiler or of another separate process by preheating fluids used in the process. This technique has been carefully applied and studied, and the application is well documented with respect to fuel savings and hardware. Recent increases in fuel costs have resulted in a renewed emphasis on flue gas heat recovery. Many linen supply boiler operators are unaware of new direct contact stack economizers and avoid stack gas heat reclaimers based on outdated information. Previously, it was not uncommon for old technology with limited heat recovery capacity to develop mechanical and corrosion problems. Based on today's costs, stack economizers' return on investment runs as high as 100% per year and returns over 50% per year are common. 

What's available
Flue gas recovery equipment fall into three general types. One or all three may be utilized, depending on the process:

• Flue gas to water is the type most applicable to laundry boiler applications. 
  Flue gas to combustion air heat exchangers may have an application in very high pressure steam boilers, but are usually not cost effective in the ranges encountered in the laundry industry.
  
• Recirculating a portion of the flue gas is usually reserved for dryer, oven recuperators or other applications utilizing extremely high levels of "excess air," and are not utilized on boilers.

This study limits investigation to boiler flue gas exhaust as a waste heat source with process water as the cooling medium. Present practice includes the use of tubular or direct contact condensing-type stack economizers. Consideration is limited to boilers or heaters using either gas or oil as fuels, since these are the fuels used in the laundry industry. Alternate fuels present special problems requiring additional investigation. 

The best way
The process of converting hydrocarbons to heat energy is a precise science. Today's increasing fuel costs demand an understanding of the process of adding oxygen (obtained from air) to fuel, which produces heat. This allows the operation of a boiler or heater at optimum efficiency and minimizes the natural loss of purchased energy to the stack. Stack economizers represent the best way of improving boiler plant efficiency. Stack gas heat recovery should be considered only after optimum efficiency has been obtained in the existing boiler. 

The amount of heat given off by a fuel varies with its composition. Some of the heat produced is used to vaporize water formed in the combustion reaction. This heat is only available to condensing heat recovery systems. This amount is about 10% in gas-fired systems and about 5% in #6 oil-fired systems.

A typical installation of each type of heat exchanger is shown in the following diagrams. Diagram #1 shows the tubular type unit as it would be used when hot water for process is used for cooling the flue gas. Diagram #2 shows the recently developed "condensing type direct contact" stack economizer as it would be used when hot water for process is used to extract heat from exhaust gas.

In the past, and presently in some applications, boiler flue gas heat reclaimers were constructed as the tubular type, consisting of either tubes, finned tubes, plate type, or coils. These are inserted in the breaching, or stack, of the boiler. Water or some other cooling medium flows within the tubes and the hot exhaust gas passes over and around the tubes. With tubular units, the recommended cooling of the exhaust gasses has been limited to 270°F - 280°F because products of combustion begin to condense at this temperature and "cold end" corrosion occurs. These products of condensation are corrosive (weak acids) and will destroy the tubular stack economizer-and may even damage the boiler, breaching and stacks. Caution should be exercised on any application of this type unit to prevent accidental cooling of the flue gasses below the critical condensing temperatures.

The newly developed direct contact stack economizer has found wide acceptance due to its higher efficiency and non-corrosive features. The development of these highly efficient direct contact stack gas coolers has eliminated the condensation barrier temperature (270°F), and flue gas is now being safely cooled to temperatures equal to or below the ambient combustion air supply temperature, and within 5°F of the incoming temperature of the water to be heated in the Economizer. On a 350 BHP firetube boiler installation at a uniform laundering plant in Wisconsin, boiler stack gasses entered the direct contact stack economizer at 550°F and were discharged at 76°F, the flue gas discharge temperature was 6°F lower than the boiler combustion air supply temperature of 82°F.

When the flue gas is cooled below the critical condensing temperature, a bonus is obtained. Not only does the stack economizer recover the sensible heat (due to change in flue gas temperature), but the heat of evaporation lost in the combustion process, vaporizing the water formed, is recovered during the phase change. In #2 oil-fired systems, this represents approximately 1,206 Btu per lb. of fuel. In natural gas-fired systems the heat of condensation given up to heat the water is about 2,358 Btu per lb. of fuel (Diagram #3).

Because Direct Contact Condensing Heat Exchangers operate at about atmospheric pressure, they have an upper limit on the cooling medium effluent temperature. Where water is the medium, the upper limit varies from 110°F to 145°F, depending on the flue gas temperature and plant altitude. While this precludes the use of these units on the boiler feed water, they can be used effectively on the makeup water for the boiler. TR