Johnston products have been designed for maximum efficiency. Firetube boiler and burner packages with the exhaust stack and breeching attached to them operate as a system. Each item affects how well the other items perform their task.
Refinements in combustion technology and burner designs have made it more critical that everyone reviews the total system to ensure proper operation. This will avoid many problems that are too often considered combustion related but are really system generated, resulting from improper flue gas breeching and stack design.
Packaged firetube boilers are designed to operate at their peak performance with short, straight flue stacks. Other Stack Configurations can be used if they are properly engineering. Improper exhaust system designs can cause problems that come and go, or can vary with stack temperature, ambient temperature, wind conditions, operating status of other boilers connected to the exhaust system, and many other varying conditions.
A few of the problems to check in the event of an erratic operations and/or problems that appear to have no solution are:
1. Stack Height – The stack or chimney should be of sufficient height to extend above the roof of the building or adjoining buildings to avoid down drafts in the stack or the possibility of carrying down combustion gases to undesirable locations such as air inlet ducts or open windows in adjoining or nearby buildings.
(Source: Packaged Firetube Boiler –Engineering Manual–, American Boiler Manufacturers Association, First Edition, page 28)
A very high stack can create a high back pressure (a high positive pressure at the boiler stack connection) that the burner must overcome which may turn into a high draft (a high negative pressure at the boiler stack connection). This can cause burner stability problems after the system has warmed to operating temperatures.
A very high stack that does not have some type of insulation, in addition to the problem listed above, can have widely ranging draft conditions caused by changes in the ambient air temperature. Cool ambient temperatures can chill flue gas products and reduce or eliminate draft conditions that existed with warmer ambient air. These conditions can change from winter to summer, day to night, and sometimes day to day.
2. Stack Exit Location – A stack exit that is located in an area where it is subject to very high and varying winds can create operational problems that appear to have no logical explanation. These problems are more difficult to find if the high and varying winds are created or amplified by the buildings located at the site.
3. Breeching – When required, the breeching should be as short as possible. They should be constructed of round ducts only. Round ducts are more efficient, and the large flat sides of square or rectangular ducts contribute to noise caused by resonance. The breeching must be installed with a slight upward pitch toward the stack to ensure it is self-venting.
4. Breeching Turns – From turns as small as 30° up to 90°, the design of the turn can have a major effect on the boiler’s operation. Never use an abrupt turn. ―Changes in direction should be accomplished as gradually as possible. Mitered elbows should be avoided. Circular elbows should be of a minimum of four piece construction and have a minimum R/D ratio of 2.0 where R is the centerline radius of the arc and D is the diameter of the duct.
(Source: Packaged Firetube Boiler –Engineering Manual—―, American Boiler Manufacturers Association, First Edition, page 35)
Poorly designed turns in an exhaust system can develop reflective harmonics, or noise, that can upset or destroy stable combustion. These disturbances may not be seen at startup, but develop later as the exhaust system warms up.
5. Multiple Boiler Stack – A common type of installation is two or more boilers using a common exhaust system. With such a system, it is absolutely essential that the two boilers be isolated from each other. The most common method to achieve separation is the use of sequencing draft controls. Even with their installation, a detailed review of the system is required to ensure trouble-free operation. Multiple breeching connections must never enter the stack at the same height. The flow of exhaust gases into each other can create excessive turbulence that results in combustion stability problems of flue gases being vented back into the boiler room.
6. Breeching to Stack Connections – The breeching connection to the stack should always be made with a 45° upward sweeping elbow or mitered turn. A straight in 90° connection, or bullhead type connection, is almost guaranteed to create reflective harmonics or other problems that appear to be combustion problems.
7. Stack Size – Pressure drops through the boiler exhaust system should be calculated and in general should not exceed 0.2″ WC. Undersized stacks create excess back pressure on the boiler resulting in combustion problems, inability to make full capacity, or both. There are several very competent manufacturers of boiler exhaust systems. They should be contacted and asked to review your exhaust system whenever there are any complications beyond a simple, short, straight stack. A little extra time and a few extra dollars will pay off in a smoother and quicker startup followed by years of successful operation.
There are several very competent manufacturers of boiler exhaust systems. They should be contacted and asked to review your exhaust system whenever there are any complications beyond a simple, short, straight stack. A little extra time and few extra dollars to provide a correct complete system will pay off in a smoother and quicker start up followed by years of successful operation.