Deciding which steam boiler to purchase is commonly determined by equipment cost. And while cost is an important consideration, this single criterion does not take into consideration a product’s operational inefficiencies, which can lead to excessive lifetime costs. Rather, equipment selection should be based on the overall performance of the equipment relative to system demand requirements and lifetime operational costs. To that end, the evaluation process should examine the following important attributes of the steam boiler and related equipment:
Today, the term “efficiency” can be interpreted in a variety of ways, such as combustion efficiency, thermal efficiency, fuel-to-steam efficiency, etc. By definition, “efficiency” is “the measure of a machine's energy effectiveness” or “the amount of energy used by a machine to the amount of work done by it” as shown in Figure 1. For example, the amount of heat [energy] produced per unit of fuel when all of a fuel has been burned is a measure of a steam boiler’s efficiency.
When evaluating a steam boiler, one should understand the variable factors that influence a boiler’s operating or “in-service efficiency” under all operating conditions of the steam plant. No “fuel” fired boiler operates at 100% efficiency.
Depicted in Figure 1 are variable factors that influence the efficiency of a steam boiler. [The nominal steady state efficiency of a gas fuel fired boiler without an economizer, operating at 125 psig is 82%.]
The energy lost [unused] comes in several forms. The greatest relative source is the exhaust gases from the process of combustion, which can decrease efficiency by 18% or more. The next area of loss pertains to the heat that is radiated from the boiler, whether operating or in standby mode, which can result in losses up to 4%. Too frequent blowdown cycles can cause as much as 3% or more in heat loss.
As one can see, reducing [or recovering] the losses of radiation, blowdown and flue gases will increase the operating efficiency of the equipment. The final boiler selection should be evaluated on what the vendor can provide to reduce these losses and increase the operating efficiency of the boiler. Overall system design should also consider the piping losses, proper insulation of the steam system, wasted or returned condensate, system operation, etc., as these factors also combine to determine the cost of each pound of steam produced.
Aside from the energy losses, one also needs to consider the location of the steam boiler. Is there sufficient space for service and maintenance of the equipment on the fireside and waterside? Sufficient head room for overhead piping? If space is limited, will the reduced floor space interfere with equipment access to perform yearly maintenance and inspection? If space is limited and the equipment is not designed for easy access, service, and tube replacement, properly maintaining the equipment will be difficult, which can lead to increased operating costs and reduced life expectancy of the boiler.
Decidedly, boilers do not operate at a static condition or at one specific firing rate for a typical steam demand requirement. Changes in steam demand, inlet feedwater temperature, blowdown cycles, chemical treatment, radiation losses from the equipment during operation standby, excess air settings, combustion air temperature, ambient room temperature, burner load tracking ability, fireside cleanliness, and waterside cleanliness all influence the “in-service” efficiency of the boiler and ultimate production of steam at a reasonable cost. Each of these “influencers” determines the efficiency of the boiler to produce each pound of steam to the system. Yet, other factors such as electrical consumption, steam piping, condensate return, and maintenance costs will also factor into the overall cost of efficient steam production.
The boiler room footprint should be a consideration during product selection. Installing a product that is difficult to service, inspect, or repair due to limited access space can lead to higher overall maintenance costs. The generous heating surface of a Firetube boiler is the most efficient boiler without an economizer; however limited space may preclude its selection. In the case of a small boiler room, a Flex-Watertube (or Flextube) design should be considered.
The Flextube boiler does not require additional space for tube removal, keeping tube surface maintenance area to a minimum on each side. The Flextube boiler is on line within minutes from a cold start and with a full modulation burner [up to 10:1 turndown on Gas Fuel] and proportional level controls, it tracks the steam load efficiently with steam quality of 99.5%. Enhancements to the basic Flextube package can include: an economizer to increase operating efficiency and decrease fuel consumption; linkageless controls for reduction in air/fuel ratio maintenance, superior load tracking and a reduction is excess air levels; and blowdown heat recovery and feedwater treatment packages from the simple feedwater tank to a more sophisticated deaerator package. These enhancements can increase in-service package efficiency to more than 85%.
The Firetube boiler requires boiler room length extension for tube removal; however, maintenance area on the boiler sides is minimal. Newer Firetube designs have reduced water volume allowing for a 25% reduction in response time compared to “traditional” Firetube boilers. Turndown of 10:1 on gas fuel enables the boiler to follow load requirements without boiler cycling. Cycling reduces boiler efficiency. Steam quality of 99.5% is easily maintained without the requirement of a steam separator. Firetube boilers without heat recovery have the highest efficiency, and adding an economizer will enhance operating efficiency and decrease fuel consumption and emissions. Linkageless controls will increase the operating efficiency of the Firetube boiler. Add to this blowdown heat recovery and a sophisticated deaerator package, and in-service package efficiency increases to more than 85%.
In summary, consider the total cost of ownership. Choose the boiler that is highly efficient, has a load tracking capability, and is easy to operate and maintain.