Striving to keep the land as clean as we found it, while improving life for those who occupy it......

Selecting not only the correct size boiler, but the right boiler design for an application can mean the difference of tens or hundreds of thousands of dollars in capital and operating cost. Firetube boilers can be built to 2500 HP at a fraction of the cost of a watertube. With consideration of operating pressure and expected equipment life, a firetube may be an economical consideration. 

The ability to understand the equipment, the operation, and importantly, the specific application is a key element to a successful analysis.  MavTech Energy collects the appropriate data, conducts interviews with Plant Management and Operations Personel, Build represenative demand and load profile models, and ultimately help the owner select the correct boiler.  

This section attempts to support that decision, and provide an analysis of the two (2) major boiler types, Firetube and Watertube, which are being considered for the new boiler plant. 

The majority of operating steam plants are not equipped with instrumentation to accurately measure overall boiler efficiency. If you consider the efficiency equation, 82 to 84% of the efficiency is in the fuel conversion, 3 to 5 % is in the radiant losses of the boiler to the atmosphere, and the balance leaves the cycle via the stack. Most plants run without any accurate steam metering, so many real-world boiler efficiency comparisons are hard to come by. It is generally accepted in the industry, that new watertubes and firetubes, produce stream at efficiencies in the range of 80 to 84% at design conditions. While selecting and operating a boiler at or near its peak design efficiency is important, there are many other parameters which should be considered. 

At first glance, there are some obvious and quantifiable differences. Watertube boilers tend to be much larger, than firetubes. This is a result of method of construction and size of the boiler drums. Watertubes have large drums, usually one for water and the other for steam. Connecting these large drums are boiler tubes which are bent around the furnace. This method of construction produces less thermal and mechanical stress when the boiler cycles up and down through low and high firing rates. However, this construction method results in more material, steel, resulting in much higher cost.  

The typical design of a firetube, is a 3 pass wet back. This design has a large Morison tube or furnace area, and then 2 boiler tube passes. These boiler tubes are connected by a tube sheet. It is this connection that is the wink link in the firetube boiler, compared to the bent water tubes. It is important to select a boiler with as large a tube sheet as possible. Generally, high quality manufacturers use  ¾” sheets with ¾” thick ligets separating the firetube holes. Additionally, tube diameters are important. High quality manufacturers select tube thickness over 0.1 inches thick.  This connection of the boiler tube to the tube sheet needs to be made with beefy material and the method of attachment performed by a skilled boiler craftsman. Large firetubes are typically designed for 5 sqft per horsepower. ( Some European manufacturer’s designs have as little as 3.5 sqft.)   

The cleaning of a firetube boiler may also appear easier than that of the watertube. It can be done without getting inside the unit, which is not the case of the watertube. Special tools, some automated, may be required for watertube cleaning, which can add to the cost. Generally, units built in the US are built to ASME standards and with ample safety margin.  

If the unit is properly maintained, water treatment is done right, and it is used within the parameters imposed by the vendor, then usually the pressure vessel should fail as a result of water treatment. The water treatment costs will be similar for both the firetube and watertube, however, an advantage of fire tubes is that with the water all around the tubes, it's less likely to plug a firetube. Poor water treatment, can result in water tubes plugging, and ultimately bursting. This applies to scaling and fouling, however, fire tubes will corrode just as likely as water tubes. With the proper pretreatment and chemical programs available, this should not be a major decision point.. A large percentage of the water treatment cost is associated with the amount of condensate actually returned to the boiler. In applications where all, or almost all of the steam that leaves comes back to the boiler as condensate, then the chemical costs will be very minimal, regardless of what's going on with the boiler itself.   

An advantage of firetube boilers is that they have almost a zero tramp air factor which may not be, or probably is not the case with the older watertubes. This tramp air produces inefficiencies in older watertubes, so there will be additional savings regardless of boiler selection. And finally, since this a new project you want to make sure that new, state of the art, controls are used to keep the boiler operating at maximum efficiency. That alone, compared to the old (often manual) controls and adjustments, will be your biggest savings. You need to make sure a burner management system with O2 control is definitely included. It is also recommended to consider Variable Speed Drive (VSD) with soft start features for the combustion fan motors to reduce overall electric use and demand. 

Another advantage of a firetube is the shorter steaming time, due partly to less mass or material to heat up. However, when left operating at warm standby, the watertube may prove more efficient at low firing rates.   

Independent of boiler selection, is operating pressure. While there is no simple way to determine the energy and fuel savings due to reducing your steam system operating pressure. Savings can come from at least seven sources. Each source's contribution depends heavily upon how your steam system is currently designed and operated. In addition, some operational disadvantages and steam quality concerns may arise from operating at low pressure.

As important as the boiler selection, is understanding specifically where losses occur in a boiler, and attempting to minimize these losses. These methods are well documented, however, for completeness, listed below;:

·         Reduced thermal losses through steam distribution line insulation (due to the lower steam temperature). The amount of the reduction depends upon the distribution line size, length, and insulation type and thickness.

·         Reduced losses through uninsulated steam distribution system valves and fittings. The amount of the reduction depends upon the number, size, and type of valves and fittings, and whether removable insulated jackets are used at the facility.

·         Reduced boiler blowdown losses. This energy savings benefit occurs because the liquid being blown down is at a lower temperature. The savings depend upon the blowdown rate (which, in turn, is dependent upon whether an automatic blowdown control system is in use), and whether a blowdown heat recovery system is in place.

·         Reduced energy losses from steam leaks and failed steam traps. At lower pressures, the leak rate or mass passing through an orifice is reduced resulting in energy savings. In addition, each pound of steam lost has a lower enthalpy and contains less energy. Savings also depend upon the magnitude of current steam losses.

·         Reduced makeup-water heating requirements. The boiler makeup water must be heated to 320°F, rather than 350°F. The quantity of makeup water required depends upon water treatment considerations (which dictate the boiler blowdown rate), plus steam leaks, deaerator steam venting, condensate return, and other steam losses. Makeup water heating requirements also depend upon whether the makeup water is preheated by a blowdown heat exchanger and/or a stack gas economizer.

·         Reduced stack gas temperature. The boiler flue gas exit temperature must be greater than—on the order of 100°F—the boiler water temperature in order for efficient heat transfer to occur. A reduction in steam temperature can allow for a corresponding decrease in flue gas temperature. A rule of thumb is that a 40°F reduction in stack gas temperature leads to a 1% improvement in boiler efficiency. The amount of energy saved depends upon whether or not an economizer is used to recover stack waste heat. Flue gas minimum temperature limits must also be maintained to minimize the formation of acidic condensate that can cause corrosion in flue gas passages.

·         Reduced boiler skin losses. Since the temperature of the liquid in the boiler is reduced, there is a reduction in surface heat transfer losses. Energy savings are dependent upon boiler type (firetube versus watertube), surface area, and current degree of insulation.

A number of operational issues must be considered when reducing pressure. By running a boiler at a lower pressure, the boiling action in the boiler becomes much more violent, causing water to be carried over into the steam system. Producing low-quality steam can lead to boiler shutdowns due to low water level trips; damaged steam pipes and valves due to water hammer, vibration, corrosion and erosion; reduced capacity of steam heaters; and, overloaded steam traps.

It is recommended to conduct a detailed investigation to determine operational effects before reducing steam pressure on an individual boiler. As the steam density and enthalpy are reduced, the steam velocity in the distribution system must be increased to supply the same thermal energy to various loads. Increased velocity increases friction losses and leads to pressure drops within the system. Insufficient steam supply could result. Pressure reducing stations (if present) must be reset, and mud blowdown must be timed at a point when the boiler is operating at partial load to avoid upsetting circulation. A change in pressure relief valve sizes may also be necessary.