You own or operate an industrial boiler or a thermal fluid heater (oil heater). Maybe you are thinking about adding a boiler or upgrading your existing boiler/heater. As with every investment, you want to make sure that you get the best “bang for your buck.” This article will help you do that.

The first place to start is with some basic knowledge about terms used with relation to the equipment and the way it is sized. Keep in mind that one BTU, or British thermal unit, is the amount of heat required to raise the temperature of one pound of water one degree Fahrenheit.

  • One term used with boilers is boiler horsepower. Not to be confused with electric motor or shaft horsepower, one boiler horsepower equals roughly 33,500 BTU/hr, or 34.5 pounds per hour of steam evaporated from 212°F (100°C) water at zero pressure.
  • Another way boilers are sized is in the amount of steam they will produce in a given timeframe. For instance, the steam boiler is sized in steam output per hour, measured in pounds per hour (pph). One boiler horsepower equals 34.5 pph of steam. Given that one gallon of water weighs 8.33 pounds per gallon, one boiler horsepower will convert (evaporate) 4.14 gallons of 212°F (100°C) water into steam at zero pressure.
  • A hot water boiler is sized by the number of BTUs per hour that the boiler will supply (output). Smaller boilers — those producing under 200,000 BTU/hr — are usually called water heaters rather than boilers.

An alternative to a boiler is a thermal fluid heater. Thermal fluid heaters are sized by the number of BTU/hr that the heater will produce. That heat is transferred from the flame of the burner through the steel pressure vessel and into the thermal fluid (hot oil). While in operation, the thermal fluid is circulated through the heater by a pump, which moves the hot oil through pipelines and delivers the hot oil to the process. The process removes heat from the thermal fluid and the pump returns the oil to the heater for reheating.

See the sidebar, Comparing Costs

Now that basic equipment is understood, it is time to define your process and heat processing needs. Use the following questions to define your needs before investing in a boiler, oil heater or any fuel-burning heat transfer equipment.

  • Exactly how much heat does the process need? What is the minimum required for optimum levels of operation? Is there a possibility for growth? Where do I want to be next year? Five years from now? Can I afford to plan for the future?
  • If the heat can be in the form of steam, you must also ask: How many pounds per hour of steam are required at the process? If it can be in the form of hot water or hot oil, how many BTUs per hour are needed at the process? What are the losses between the boiler/heater and the process?
  • Is there a need for greater heat output during startup? What is the maximum warmup period for the entire plant or process for which the boiler/heater supplies heat? How much extra heat is needed to minimize the warmup time and get into full production faster? How often does the process start up (go from ambient temperature to operating temperature)? If it is once a year, a lengthy warmup period is no big deal. If it is once a week, a lengthy warmup time can waste labor hours.

Once the amount of heat needed at the process is defined, one last question must be addressed before the final decision of sizing the boiler/heater.

Every engine in a diesel truck has a “sweet spot,” meaning the engine rpm and road speed produce the best efficiency, or the best miles per gallon. Likewise, a boiler or heater also will have a sweet spot — the firing rate that produces the best efficiency.

For boilers or heaters, the sweet spot usually is between 60 and 75 percent of maximum firing rate. If the boiler or heater is sized to provide all of the desired heat or steam at a firing rate of 75 percent for normal operation, that leaves an additional 25 percent of capacity that can be used during startup. Also, when the boiler/heater grows old and does not operate at peak (new) efficiency, the plant will still have enough heating capacity to operate at normal production.

Selecting the Heat Source

In many cases, the type of heat source is predetermined by what is available in the plant and the cost of utilities. However, if more than one heat source can be used, there are questions to consider to narrow the choice. Consider the following questions.

  • What heat source (fuel) is available? Typical choices include natural gas, liquefied petroleum (propane or butane) gas, diesel and electricity.
  • What is the cost of each fuel per million BTUs? Electric is the most expensive many parts of the country.

All fuels can be rated in dollars per BTU. For the last several years, in my experience, the best value has been natural gas. Given that, it is worth considering for most applications. If it is not available at your plant, how far away is a main line? What would the gas company charge to run a new line to your facility? In most cases, the utility company can provide a proposal. However, it will be up to you to do the math that will reveal how quickly the new gas line will pay back for the cost of installation.

If a natural gas main line is nowhere near you, liquefied natural gas (LNG) or compressed natural gas (CNG) may be the next best option. It is available in some areas and growing all the time. In most instances, LNG or CNG is around double the cost per BTU over pipeline natural gas. That said, often, it is less costly than the other fuels.

If natural gas is simply not available, the next best choice is usually liquefied petroleum gas (LP), which includes both propane and butane. For the large quantities that are needed to fire boilers and heaters, only propane is available in the United States.

The use of diesel fuel may be limited by local air quality regulations. Be sure and check with the local Department of Environment Quality (DEQ). Also, burning any fuel may have restrictions on the amount of nitrous oxide (NOX) gas that can be produced from any fuel-burning equipment.

If burning diesel is allowed in your area, it usually is less costly per BTU than electric power. If the cost is close (and it usually is), LP gas usually is preferable over diesel (see sidebar). Diesel requires a pump and filter to deliver the fuel to the boiler/heater, and the pump and filter require constant maintenance. The oil burner is more complex and requires more regular maintenance than a gas burner. Any gas — natural or LP — burns more cleanly than any oil, making it easier to keep the burner in tune. If the cost is anywhere close, gas usually is preferable to oil for a heat source in a boiler or heater.

The next step is determining which equipment and options will deliver the best efficiency and produce the most heat for the least investment.

Understanding Boiler Efficiency

Just as automobiles have varying gas mileage depending upon the size of vehicle, every size of boiler or heater will have varying degrees of efficiency. So, just what is “efficiency?” While it is easy to compare the gas mileage of automobiles, in boilers and heaters, efficiency is determined by how much heat is delivered to the process when compared with how much heat was put into the burner (or heat producer) of the boiler or heater.

This is determined using a formula: output over input equals percent of efficiency. For example, a boiler or heater with one million BTU/hr input (heat produced at the burner) with an output of 800,000 BTU/hr (heat delivered the process) has an efficiency of 80 percent. If you can increase the output of the unit without increasing the input, then you have increased the efficiency and saved money.

The efficiency number of 80 percent has been a standard in the boiler industry for years. Efficiency was not of much concern in oil heaters until recent years. Most thermal fluid heaters built before the year 2000 were 60 to 70 percent efficient.

Efficiency is one area that electric-powered boilers and heaters have an advantage. They are 100 percent efficient: for every kilowatt of heat that goes into the boiler or heater, it “all” stays there. By contrast, when using any carbon-based fuel, the burner uses the atmosphere to deliver oxygen to the combustion process. The atmosphere only contains 20.7 percent oxygen. The other 79.3 percent is nitrogen (and small amounts of other inert gases), which does not contribute anything to the heat. The inert gases must be heated in the burner, and whatever heat does not get transferred into the boiler or heater is discharged into the atmosphere.

See the followup to this article: Optimizing Efficiency for Boilers and Thermal Fluid Heaters

In conclusion, following this advice will prevent you from undersizing or oversizing and help you determine exactly what size boiler or heater is the best for the process.