Another way to determine whether a backup boiler is a wise decision is to compute the cost of downtime to the owner or the user, as shown in the following three examples:
A chemical company manufactures dry cell battery compound in a batch process. The process temperature must be maintained within 2 degrees. The boiler shuts down on a flame failure. They have 20 minutes to recover steam or the batch is scrap. The value of the product is $250,000.
A Midwestern insurance company building has comfort heat supplied by one boiler. There are over 2000 workers in the building. The boiler shuts down due to a failed gas valve. Outside, it's 11 °F. Inside, the temperature continues to drop and, at 1:30 in the afternoon, all 2,000
workers are sent home.
A meat processing company makes its entire packaged ham line in a Southern plant. It operates 24 hours a day, every day. A single boiler provides heat for curing, sterilizing, and cleaning. The boiler goes down due to a lack of feedwater. Each hour of steam loss results in four hours of lost production.
Boiler turndown is the ratio between full boiler output and the boiler output when operating at low fire. Typical boiler turndown is 4:1. For example, a 400 horsepower boiler, with a 4:1 turndown burner, will modulate down to 100 horsepower before cycling off. The same boiler
with a 10:1 turndown burner will modulate down to 40 horsepower.
The ability of the burner to turn down reduces frequent on and off cycling. Fully modulating burners are typically designed to operate down to 25% of rated capacity. At a load that is 20% of the rated capacity, the boiler will turn off and cycle frequently.
A boiler operating at low load conditions can cycle as frequently as 12 times per hour, or 288 times per day. With each cycle, pre- and postpurge air flow removes heat from the boiler and sends it out the stack. The energy loss can be eliminated by keeping the boiler on at low firing rates. Every time the boiler cycles off, it must go through a specific start-up sequence for safety assurance. It requires about one to two minutes to place the boiler back on line. And, if there's a sudden load demand, the start-up sequence cannot be accelerated. Keeping the boiler on line assures the quickest response to load changes. Frequent cycling also accelerates wear of boiler components. Maintenance increases and, more importantly, the chance of component failure increases.
As discussed earlier, boiler(s) capacity requirement determined by many different types of load variations in the system. Boiler over-sizing occurs when future expansion and safety factors are added to assure that the boiler is large enough for the application. If the boiler is oversized, the ability of the boiler to handle minimum loads without cycling is reduced. Therefore, capacity and turndown should be considered together for proper boiler selection to meet overall system load requirements.
Three important considerations pertain to fuels, emissions, and efficiency. All three have important impact on boiler performance, and can affect long-term boiler operating costs.
Fuels Remember, from an operating perspective, fuel costs typically account for approximately 10% of a facility's total operating budget. Therefore, fuel is an important consideration. Normally, the fuels of choice are natural gas, propane, or light oil. Increasingly stringent emission standards have greatly reduced the use of heavy oil and solid fuels such as coal and wood. Of the fossil fuels, natural gas burns cleanest and leaves fewer residues; therefore less maintenance is required.
It can be advantageous to supply a boiler with a combination burner that can burn two fuels independently - for example, oil or natural gas. A combination burner allows the customer to take advantage of "peak time" rates, which substantially reduces the costs of a therm of gas when operating "off peak" by merely switching to the back up fuel. Dual fuel capability also is beneficial if the primary fuel supply must be shut down for safety or maintenance reasons.
Some waste streams can be used as fuel in the boiler. In addition to reducing fuel costs, firing an alternate fuel in a boiler can greatly reduce disposal costs. Waste streams are typically used in combination with standard fuels to ensure safe operation and to provide additional
Emission standards for boilers have become very stringent in many areas, because of the new clean air regulations. The ability of the boiler to meet emission regulations depends on the type of boiler and burner options.
Efficiency is used in the measure of economic performance of any piece of equipment. In the boiler industry, there are four common definitions of efficiency, but only one true measurement. Following are the definitions and how to measure efficiency. Combustion Efficiency Combustion efficiency is the effectiveness of the burner only and relates
to its ability to completely burn the fuel. The boiler has little bearing on combustion efficiency. A well- designed burner will operate with as little as 15 to 20% excess air, while converting all combustibles in the fuel to thermal energy.
Thermal efficiency is the effectiveness of the heat transfer in a boiler. It does not take into account boiler radiation and convection losses - for example, from the boiler shell, water column piping, etc.
The term "boiler efficiency" is often substituted for combustion or thermal efficiency. True boiler efficiency is the measure of fuel-to-steam efficiency.
Cleaver-Brooks guaranteed boiler efficiencies are fuel-to- steam efficiencies.
Fuel-to-steam efficiency is the correct definition to use when determining boiler efficiency. Fuel-to-steam efficiency is calculated using either of two methods, as prescribed by the ASME Power Test Code, PTC 4.1. The first method is input-output, which is the ratio of
Btu output divided by Btu input x 100.
The second method is heat balance which considers stack temperature and losses, excess air levels, and radiation and convection losses. Therefore, the heat balance calculation for fuel-to-steam efficiency is 100 minus the total percent stack loss and minus the percent radiation and convection losses.
Stack Temperature and Losses
Stack temperature is the temperature of the combustion gases (dry and water vapor) leaving the boiler. A well-designed boiler removes as much heat as possible from the combustion gases. Thus, lower stack temperature represents more effective heat transfer and lower heat loss up the stack. The stack temperature reflects the energy that did not transfer from the fuel to steam or hot water. Stack temperature is a visible indicator of boiler efficiency. Any time efficiency is guaranteed, predicted stack temperatures should be verified.
Stack loss is a measure of the amount of heat carried away by dry flue gases (unused heat) and the moisture loss (product of combustion), based on the fuel analysis of the specific fuel being used, moisture in the combustion air, etc.
Excess air provides safe operation above stoichiometric conditions. A burner is typically set up with 15 to 20% excess air. Higher excess air levels result in fuel being used to heat the air instead of transferring it to usable energy, increasing stack losses.
Radiation and Convection Losses
Radiation and convection losses will vary with boiler type, size, and operating pressure. The losses are typically considered constant in Btu/hr, but become a larger percentage loss as the firing rate decreases. Boiler design factors that also impact efficiencies of the boiler are heating surface, flue gas passes, and design of the boiler and burner package.
Heating surface is one criterion used when comparing boilers. Boilers with higher heating surface per boiler horsepower tend to be more efficient and operate with less thermal stress. Many packaged boilers are offered with five square feet of heating surface per boiler horsepower as an optimum design for peak efficiency.
Flue Gas Passes
The number of passes that the flue gas travels before exiting the boiler is also a good criterion when comparing boilers. As the flue gas travels through the boiler it cools and, therefore, changes volume. Multiple pass boilers increase efficiency because the passes are designed to maximize flue gas velocities as the flue gas cools.
Integral Boiler/Burner Package
Ultimately, the performance of the boiler is based on the ability of the burner, the boiler, and the controls to work together. When specifying performance, efficiency, emissions, turndown, capacity, and excess air all must be evaluated together. The efficiency of the boiler is based, in part, on the burner being capable of operating at optimum excess air levels. Burners not properly designed will produce CO or soot at these excess air levels, foul the boiler, and substantially reduce efficiency. In addition to the boiler and burner, the controls included on the boiler (flame safeguard, oxygen trim, etc.) can enhance efficiency and reduce
overall operating costs for the customer. A true packaged boiler design includes the burner, boiler, and controls as a single, engineered unit.
Replacement Boilers, If the boiler is to be placed in an existing facility, there are a number of considerations:
Floor space required.
Total space requirements.
Access space for maintenance.
Size and characteristics of the boiler to be replaced, including location of existing piping, the boiler stack and utilities.
Boiler weight limitations.
With little or no access to the boiler room, the cast iron boiler and some bent-tube type boilers can be carried into the boiler room in sections or pieces and easily assembled, with no welding required.
Electric boilers should also be considered, especially since they do not require a stack.
Vertical firetube boilers have a small floor space requirement
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