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The Evolution Of Boiler Design & Maintenance

      Early manufactured commercial boilers were essentially, “tea kettles.”  A fire was laid under a closed vessel filled with water, and steam was drawn out of the “spout.”  These tea kettle boilers powered the first steam locomotives.  Many other industries, notably the logging industry, used this type of boiler to produce steam.  They were fired by anything that would burn, coal and wood being the primary heat sources.  These boilers were pretty much limited to being located outdoors.

The industrial revolution speeded up the development of boilers into fire tube boilers and this became the standard design for over 50 years.  Boiler manufacturers produced vertical fire tube boiler in all sizes and some variations, but they were all the same basic design as shown in Figure 1.

Figure 1.

Hot water heaters used for non-potable water also were, and still are, called boilers, even though they may not boil the water and generate steam.  The hot water boiler was of similar construction.

As new materials, fuels, control systems, and improved manufacturing processes were developed, basic boiler designs changed.

Three types of boilers now dominate the market:  fire tube boilers, water tube boilers, and cast iron sectional boilers.  (There are others, such as tubeless steel and electric boilers, but they consist a very small share of the boiler market.)

The three dominant types are further broken down in classifications such as “Scotch Marine,” “Horizontal-Return-Tubular,” “Bent Tube,” etc., but the basic design of each type remains the same.  Figure 2 shows these typical designs.  All can be built and trimmed as steam or hot water boilers.

Figure 2.

Many companies derive income from the fact that boilers fail.  Some boilers may still be shoddily made, but most boilers are of good quality and are well made.  Unfortunately, the conditions under which many boilers operate are not ideal.  Failures that sustain the boiler repair and replacement business are, for the most part, due to poor maintenance and unfavorable conditions of operation. The leading causes of boiler failure are:

 

1.  Introduction of raw water

2.  Rapid changes in boiler operating temperature

3.  Too low a flue gas temperature

    (We are not talking about burner or control failures.  Only the boiler itself.)

    A hot water hydronic system is a “closed system,” at least it is supposed to be.  It becomes an “open” system when there are leaks in the system, or the expansion tank becomes waterlogged, resulting in the relief valve opening every time the system temperature rises, expelling the expanded water.  To make up the lost water, the feed valve opens and feeds “raw” water into the system.  Even in small quantities, raw water causes problems.  The nature of these problems depends on the quality of the raw feed water.  If the raw water is “soft” water, it will be of high conductivity, high in oxygen, and have a low pH value.  Soft water will attack the metallic surfaces of the boiler, piping, and radiation.  Corrosion occurs and particles of solid material are carried through the system.  Suspended solids always drop out at the place of lowest velocity, which is usually the boiler.  The boiler will accumulate sludge.  Sludge has an adverse affect on the cast iron and water tube boilers.  Sludge settles to the lowest part of the boiler, which is the most intensely fired surface of the whole boiler.  The accumulation of sludge at these critical points prevents proper heat transfer to the water and results in section cracks and tube failure.  In the case of a fire tube boiler, sludge settles in the drum, which is unfired and no harm is done.

    If the incoming water is “hard” water, the effects are somewhat different.  Scale forming minerals are precipitated out and accumulate on the hottest surfaces.  An accumulation of scale only 1/16th of an inch thick cuts down on heat transfer appreciably.  The results of using “hard water” include cracked sections in the cast iron boiler and burned out tubes in the water and fire tube boilers.

    Steam boilers that have condensate loss, especially process boilers with large condensate loss, some even having no return to the boiler, should have make up water treatment systems that are designed to add chemicals to the make up water to maintain a certain pH level, usually between 7.5 and 8.5 pH, slightly alkaline.  Other devices may be used to “clean up” the make up water before allowing it to be injected into the boiler.

    When make up water exceeds 25% of total supply, a de-aerator is also needed to remove oxygen and carbon dioxide from the water.  They cause corrosion.  When make up water exceeds 50% of supply, preheating the make up water is a necessity.  Preheating can usually be done using the de-aerator.  Typically, exhaust steam is used to preheat the feed water to 200 to 210°F.  Rapidly feeding cold water into a steam or hot water boiler will damage the boiler.  Systems must be designed to prevent the introduction of cold water to prevent boiler “shock.”  The rapid chilling of a boiler is detrimental in many ways.  In the case of cast iron boilers, it creates mechanical stress, due to differential expansion of the wetted sections, causing section cracks.  In the water tube and fire tube boilers, the varying expansion of the tubes results in tube pulling and leaks.  Cold water may also create condensation of flue products on the surfaces exposed to the cold water and flue gases.  This corrodes the metals on the flue side of the boiler.

    Shocking a hot water boiler is more common than many realize.  It is usually due to poor system design, improper installation, or tampering with a control system after installation.  In fact, it is so common that the American Gas Association (AGA), requires the following statement be included in instruction sheets for every gas-fired boiler with the AGA seal of approval.

    “Boilers shall be accompanied by detailed printed instructions which shall state and illustrate that boilers when used in conjunction with refrigeration systems shall be installed so that the chilled medium is piped in parallel with the heating boiler with appropriate valves to prevent chilled medium from entering the heating boiler.” 

    While the above is obviously concerned with heating-cooling systems with a chiller, the statement goes on to address any situation where cold water could be circulated through a boiler.

    Over the years, boilers have been getting smaller and smaller in regards to their ratings —especially hot water boilers, but even steam boilers.  Consequently, they contain less water.  Improved burners, better heat transfer designed surfaces, and fast response controls made it possible to design physically smaller boilers and retain output ratings.  Of course, cost was a driving factor too.  Lowering the water content has advantages.

    Large water content in a boiler makes it respond slowly to system demands.  There may be long warm-up times.  Often, this results in keeping the boiler hot at all times waiting for system demand.  These standby periods could be costly in fuel.  The slow rate of temperature or pressure change was necessary with the controls available at that time.  Controls were slow acting, less responsive than they are today.  Control systems now have quick response to changes, making a low water content boiler operate as smoothly as before.

    As was noted, one of the main reasons sludge deposits form in hot water boilers is due to low velocity in the boiler.  If higher velocity could be maintained in the boiler, sludge forming particles will not drop out in the boiler.  This was well known, but it was impractical to move a lot of water in the boiler for two reasons.  The sheer quantity of water to be moved would require a huge and expensive pump.  The electric energy to run such a pump would add to the expense.  Other system problems would be created by “over pumping” to increase the velocity in the boiler.

    For example, a 1,000,000 BTU/HR input horizontal water tube boiler containing 41 gallons of water usually has a system design velocity of 4 ft/second, a common design figure.  This would result in a velocity in the boiler of only 0.145 ft/second.  In order to increase the velocity in the boiler to 4 ft/second, the pump would need to move about 2350 GPM.  This would require a 30 H.P. pump with 10” inlet and outlet.  Such a pump would cost about $3,500.00!  (It gets worse if the boiler is a cast iron sectional or fire tube boiler.)  A modern 1,000,000 BTU/HR hot water boiler may contain as little as two and one half gallons of water.  Using such a boiler in the same system with no difference in the pump used for the low velocity high water content boiler, results in 7 ft/second velocity in the low water content boiler.

    Low water content hot water boilers are inherently safer boilers.  Boilers do, from time to time, run away and explode.  True, two or more safety devices have to fail at the same time, but it does happen.  When it happens, the destruction that takes place is in direct proportion to the amount of water in the boiler.  Think of what happens when a domestic 30-gallon hot water heater with low heat input explodes.  The danger lies in the energy stored in the water, not in the heat input!

    If our boiler with 41 gallons of water ruptured at 500 psi, it would be the equivalent of 25 pounds of dynamite exploding — an unscheduled boiler relocation to your neighbor’s parking lot!  The explosive force is related to the amount of water that can suddenly turn to steam.

     In the process of transferring heat from hot gases to the water in a boiler, two barriers to heat flow are encountered:  gas film on the fireside and liquid film on the waterside.  Water tends to form a stagnant liquid film that clings to metal surfaces.  This film acts as an insulator.  High velocity helps to scrub away this film, and heat transfer to the water is greatly improved.  Improvement can be as much as 4 to 10 times per square foot of surface.

    Not very long ago, boilermakers made only the boiler; that is, the “wet” part.  Others provided the firebox, burners, trim, controls, etc.  The boiler buyer used the square feet of heating surface per boiler horsepower as a means of comparing what one boilermaker made to another boilermaker.  Buyers felt that the more surface area a boiler had the better it must be.  The buyer bought the most square feet he could get for his money in the size needed.  The burner, controls, trim, etc., were then added to the boiler and it was fired up.  On some occasions, serious problems developed, due to mismatching of components.

    The practice of buying surface area led to the thought that the more surface area a boiler had, the more heat it would extract from the flame and be more efficient.  Less heat would be wasted out the vent system.  This is true, as far as it goes.  All other things being equal, adding more heat transfer surface to a boiler will lower the flue gas temperature, increasing efficiency.  However, lowering the flue gas temperature too much can be dangerous.  A boiler could be too efficient.

    Flue gases must leave a boiler with enough heat to perform two vital functions; keep the column of gas in the vent or chimney hot enough to maintain good draft, and offset the heat loses of the venting systems.  If the flue gases do not have enough heat to do both those tasks, in the coldest weather, combustion products will spill back into the boiler room.  This spillage will lower the oxygen content of the combustion air, resulting in fuel rich combustion, forming excess CO. 

    Warning:  CO (carbon monoxide) is a colorless, odorless gas.  Prolonged exposure to high concentrations will  result in death!

    In addition, flue products contain large amounts of moisture.  The flue gases must be hot enough to keep the vent system and boiler above the dew point, or moisture will condense on the vent system.  This condensate is acid, and will cause corrosion.  Flue gases for a gas-fired boiler should normally be in a range of 305 to 410°F above ambient.  About 80% combustion efficiency is the upper limit for safe practice.

    Note:   Recently there are some condensing boilers on the market that claim efficiencies into the 90% range.  It is doubtful that flue gas temperatures could be less than the water temperature in a boiler, even a condensing boiler.  Therefore, these super high efficiency ratings are suspect, unless the boiler water temperature is very low.

    The modern “packaged” boiler has pretty much eliminated the mismatching of boilers and burners.  Packaged boilers come with burners, trim, controls, etc.  Very large boilers that are constructed on-site are usually very well engineered.  If low flue gas temperatures are encountered nowadays, it’s usually due to field tampering, such as de-rating a burner in an attempt to save fuel, or changes in the vent system.

    A properly sized, installed, and maintained boiler will last over 25 years.  It is not unusual to find boilers still in satisfactory condition after 50 years of service.

    To sum up: 

    •  When sizing a new boiler, whether steam or hot water, match the boiler to the load and add a 10% safety factor.
    •  For hot water boilers in closed systems, follow recommended initial start-up procedures.  Keep the system a closed system.  Fix all leaks and flooded expansion tanks promptly.  Avoid introducing raw water into the system.  Do not drain and re-fill the system unnecessarily!  If it is necessary to drain a system to make a repair, follow the startup procedures after re-filling the system.
    • If a chilled water system, make sure no chilled water can ever enter the boiler. 
    •  Steam boilers that have condensate loss should have make up water treated and preheated if large amounts of condensate are lost.
    • Steam boilers should be periodically shut down, drained, and the wet side cleaned to remove sludge and scale.  Even with treated make up water and pre heating, steam boilers need periodic cleaning.  This will vary according to usage, frequency of blow down, and the quality of the make up water.
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