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Category Archives: Technical Library

 

While many boiler rooms and power plants have been swapping out old coal and oil-fired burners/boilers for natural gas and bio-fuels, there still will always be the need for oil burners/boilers in the world. With this need, come the multiple options of how to burn the oil. While there are many different variations, the two main methods of oil atomization for the burner are pressure and air atomization.

Pressure atomization depends on the oil pressure inside the nozzle tip to spray a fine mist of oil, very similar to a Windex spray bottle. The micronized oil droplets are flung into burner head, where they are thoroughly mixed with the combustion air and ignited. As mentioned above, the pressure at the oil nozzle is the key factor in the atomization process, therefore your oil pump and pressure regulator are the key components in this system. The pump needs to be able to meet the gallons per hour (gph) requirement for the burner/boiler to meet their load capacity. The pressure regulator is set in accordance to the firing rate which is normally between 100-300 (psi). The turndown ratio for a pressure atomizing burner is normally only 3:1 or 4:1.

Air atomization adds another variable to the equation. Like pressure atomization, the oil is pumped through the system and into the oil nozzle. There, the oil is sheared by a intersecting stream of air. These two elements are mixed rapidly and forced out the nozzle tip into the burner head where they mix with the combustion air and ignited. As mentioned above, not only is the oil pressure of importance, as seen in the pressure atomization, but the injection air is as well. The oil and atomizing air are both varied based on the firing rate of the burner. While the oil pressure will remain around 100 (psi), the atomizing air pressures can range from 5-75 (psi) based on the design of the burner and firing rate. The turndown ratio for an air atomized burner is normally 6:1 or 8:1.

Both main forms of oil atomization are acceptable and widely used throughout the industry. Depending on the application and resources available one may be better suited than the other. Pressure atomization requires a more powerful pump and motor assembly to create the amount of pressure at the oil tip, while air atomization does not. Air atomization requires an air compressor to be able to create the atomizing air needed to shear the oil in the tip while pressure atomization does not. The whole piping rack for the oil is not as complicated and requires fewer parts for the pressure atomization while air atomization requires piping for air, solenoid valves, air pressure regulators, etc. Air atomization allows for a better turndown ration, which will allow a burner to run at a lower rate while keeping up with the load, while the pressure atomized burner will have to run at a higher or lower rate and either vent the excess steam or cycle on and off to keep the base load. As mentioned before, the application and resources available will determine whether pressure or air atomization will be the better choice for an oil burner application.

 

Robert Bohn, Mechanical Engineer

 

Are there any fuel oil handling systems that don’t need return pumps? Are there any advantages to not having return pumps?  While it is true that I have worked on system designs and executions that have not included return pumps, my general answer is “no,” there are not any advantages.  And there are many different functional reasons why, especially within Mission Critical and First Response handling systems.  The purpose of this white paper is to list several different uses for return pumps under various circumstances, and to demonstrate how every single installation can benefit from them.

 

We all know that return pumps are great for keeping things under control, even if humans seem determined to find a way to spill fuel—running supply pumps in “hand,” or opening valves that should remain closed, and preventing a day tank overflow.  This is the most basic function of a return pump, and by far the most popular reason for their inclusion in both older and more modern designs.

 

FUEL RETURN:  Sending fuel from a day tank, to a main tank

Fuel return is a return pump’s most basic function.  During typical, normal operation, the return pump will be a lonely piece of equipment, but will also be one of the important safety features within the entire fuel oil handling system (FOHS), potentially saving very significant environmental and financial damages.

 

On many basic systems, the return pump will only activate when a “high” level float switch is activated, or similar signal, from a Controller.  An improvement is to have the return pump interlocked with a “high” level switch, which will start the return pump even in the event the Controller is offline or disabled.

 

Be sure to size the return pump appropriately to the application.  Calculate the highest possible rate of fuel entering the day tank.  On multiple day tank systems, this may mean the highest flow rate possible from all supply pumps running simultaneously, while filling that one particular day tank, and no others.  The return pump should be sized at a significantly higher rate than that potential high-flow rate.  How much higher will depend on pump type, available flow rates, etc., but should be 150-200% of the highest flow rate possible from the supply pumps.

 

 

INSPECTIONS AND REPAIRS:  Emptying a day tank for repair or replacement

What if you find fuel in your day tank’s secondary containment?  After looking for the usual leaking suspects, such as a threaded connection, weld, valve, etc., you may come to the conclusion that your primary tank is leaking.  Whether you intend on inspecting the tank, repairing or replacing it, you will need to drain it first.  But, if you have only a top/side-mounted overflow, how do you get the fuel out?

 

A return pump makes this process much faster and less messy, sending the good, clean fuel exactly to where you want it—back into the main tank.  You are also eliminating potential problems with big, messy barrels, which will contaminate the fuel and cause you to dispose of hazardous materials, increasing the hassle—not to mention the expense—even more.

 

 

FUEL FILTRATION:  Cleaning ALL of the fuel, not just the fuel in the main tank

One of the most common reasons for emergency generators to fail to run is bad fuel.  The best way to prevent bad fuel is to filter it.  ALL of it.  This means polishing the fuel that is trapped in supply lines, return lines, and day tanks.  Generators get their fuel directly from their day tanks, but most fuel is only polished in the main tank!

 

Return pumps are great for circulating fuel.  Modern control systems can be programmed to run polishing sequences, including activating return pumps (and supply pumps) that will circulate the fuel through the day tanks and allow it to be cleaned at the main tank(s).  Running a day tank “turnover” sequence, in combination with a filtration/polishing sequence, ensures much cleaner fuel throughout your entire fuel system.  The best generators on Earth won’t run on bad fuel!*

 

*ALL Mission Critical facilities should be on a strict fuel filtration and polishing regimen (Please look out for other white papers about Filtration from Preferred Utilities).  In fact, any facility that requires a backup generator of any type has a responsibility to ensure, to the best of their ability, that the generator runs in an emergency.  That’s what they’re there for!

 

 

THERMAL MANAGEMENT:  Decreasing fuel supply temperatures for generators

Generators use diesel fuel oil not only for internal combustion, but also for cooling the engine’s injectors.  Less than half of the fuel that the generator draws from the day tank is actually used for combustion; the remainder of the fuel is returned to the day tank by the engine, and at a higher temperature.  As continuous running creates a continuous fuel temperature increase, this can adversely affect the performance of the generator, up to, and including, generator shutdown.

 

There are two main contributors to overheating a day tank’s fuel:

  1. Ambient temperature.  Perhaps the day tank is outside, or on a rooftop, in a warm climate, or all of the above.  If the fuel in the day tank is already at 95 degrees F, for example, it’s going to rise fairly rapidly when the generator engine starts.  Unfortunately, in places like California, many power outages occur during the hottest days of the year, due to excessive demand on the grid.
  2. Day tank size versus generator engine size.  A large generator engine paired with a small day tank will increase the day tank fuel temperature quickly, regardless of any other environmental conditions.  Local, state, or national code may inhibit the installation of larger day tanks.

 

Return pumps can assist in decreasing fuel temperatures for generators under both scenarios.  By simply circulating the hot fuel out of the day tanks, and replacing it with cooler fuel from the main tank(s), the generators will continue to run, and run more efficiently.

 

This fuel circulation can be automated.  The day tanks can be equipped with Resistance Temperature Detector (RTD) probes, which will monitor the day tank temperature.  When the day tank temperature reaches a pre-determined threshold, the RTD will signal the master control system (“Controller”), which will start a day tank cooling sequence.  We sometimes refer to a day tank cooling sequence as, “level bouncing.”  A level bouncing sequence would look something like this hypothetical example:

  1. RTD reports temperature threshold met on Day Tank 1 to Controller.
  2. Controller activates Day Tank 1 Return Pump.  Day Tank 1 begins to pump out.
  3. Day Tank 1 reaches “Supply-Pump-On” lower level, which creates a Call For Fuel.
  4. Controller turns Return Pump off.
  5. Controller activates Supply Pump, and opens Day Tank 1’s inlet valve.
  6. Supply Pump fills Day Tank 1.
  7. Day Tank 1 reaches “Supply-Pump-Off” high level.
  8. Controller deactivates Supply Pump.
  9. RTD monitors temperature, and…
    1. RTD reports temperature threshold met on Day Tank 1 to Controller, and sequence repeats… or…
    2. RTD reports temperature acceptable.  No action occurs.

Return pumps are useful for far more than just pushing fuel back to a main tank.  They are an integral part of any system and do not only save us from a messy cleanup and a lot of explaining, but also enable us to truly and completely clean a system, cool a day tank, or just do a more thorough inspection.

 

For more information, or if you have any questions, please contact:

 

Lee Carnahan

District Sales Manager, West

PREFERRED UTILITIES MFG. CORP.

209.890.9993 cell

LCarnahan@preferred-mfg.com

 

combustion-theory-efficiency

Understanding Combustion Efficiency

The efficiency of a burner-boiler combination is simply the amount of useful energy leaving the system expressed as a percentage of the chemical energy in the fuel entering the system.

Why should I care about efficiency?

Accounting for the loss of useful energy is an important step in evaluating overall cost.

For instance, a change in efficiency by as little as 5% can have a major impact on the operational expense of a facility. The larger the facility’s consumption of fuel and electricity, the more drastic the cost.

General rules of conservation of energy are:

  • Fuel energy “in” equals heat energy “out.”
  • Energy leaves in steam or in losses.
  • Efficiency + 100% minus all losses.

The typical efficiency of a boiler is 80% to 85%. The remaining 15% to 20% is lost. These losses usually fall in the following percentages:

  • 15% typical “stack loss.”
  • 3% radiation loss.
  • 1% to 2% miscellaneous loss.

The table below shows you how to calculate your cost with your efficiency as the variable.

table-operating-costs

Operator / Install Tips

Since a burner must be set up to operate cleanly under worst-case conditions, you must provide enough excess air in order to burn any additional fuel that the metering device at the burner may introduce.

You should also ensure that there is sufficient excess air available on a hot, humid summer day. Remember: there is no way to prevent heat and humidity, but with the proper control systems, you should be able to control fuel flow precisely and efficiently.

Other posts in this series:

  • Understanding Local Law 87 – and laws like it
  • Combustion Theory: The Basics
  • Combustion Theory: Variables – Account for variations in oxygen and fuel
  • Combustion Theory: Efficiency – Calculate efficiency and losses
  • Combustion Theory: FGR – See how flue gas recirculation reduces NOx
  • Combustion Theory: Combustion Controls – Learn how cutting-edge tech can cut your emissions
  • Combustion Systems: Design – Basic principles to follow when designing your combustion system
  • Combustion Systems: Troubleshooting: Burner problems and their causes
  • Combustion Control: Strategies – Linkage vs. Linkageless, and why you should care
 

combustion-theory-variables-header

 Accounting for variations in oxygen and fuel

For any burner-boiler combination, there is an ideal “minimum excess air” level for each firing rate over the turn-down range. Usually, burners require much higher levels of excess air when operating near their minimum firing rates than they do at “high fire.”

More serious factors than dirty fan wheels and dampers inhibit air flow. Varying oxygen content in the air changes the ambient air conditions and effects the input of oxygen into the combustion process.

Hot and Cold Days

On a “standard” day of 60°F, 30 inches barometric pressure, and 45% relative humidity, seasonal temperature and pressure changes, you must take into account that a burner has to deal with additional variables.

When it seems harder for us to breathe on a hot, humid summer day, burners have a problem too. On a hot, humid day, the oxygen flow drops by almost 20% and burners that can’t adapt for this “oxygen lean” air will smoke, soot, and produce noxious emissions.

On a cold, dry winter day, the air flow would increase by 10%, and the burner must adapt.

Viscosity

Variations in pressure across the metering valve and fluid viscosity have the greatest effect on fuel flow.  Viscosity can vary from delivery to delivery and can be affected further by temperature changes.

Having thick oil in burner supply line can reduce the pressure at the metering valve while having thick oil in the return line can increase the pressure at the valve.

Since the burner must be set up to operate cleanly under worst case conditions, enough excess air must be provided to burn any additional fuel that the metering device at the burner may introduce as well as to ensure that the metering device at the burner may introduce as well as to ensure that there will be sufficient excess air available on a hot, humid summer day.

There is no way to prevent heat and humidity, but fuel flow can be closely controlled with the appropriate controls.

Controls are essential to the boiler-burner combination because they will reduce the amount of excess air wasted during weather and fuel changes.

Other posts in this series:

  • Understanding Local Law 87 – and laws like it
  • Combustion Theory: The Basics
  • Combustion Theory: Variables – Account for variations in oxygen and fuel
  • Combustion Theory: Efficiency – Calculate efficiency and losses
  • Combustion Theory: FGR – See how flue gas recirculation reduces NOx
  • Combustion Theory: Combustion Controls – Learn how cutting-edge tech can cut your emissions
  • Combustion Systems: Design – Basic principles to follow when designing your combustion system
  • Combustion Systems: Troubleshooting: Burner problems and their causes
  • Combustion Control: Strategies – Linkage vs. Linkageless, and why you should care
 

Combustion Theory

Introduction

Welcome to the Combustion blog series by Preferred Utilities Manufacturing Corporation. To read the introductory post, click here.

This series was inspired by Local Law 87, an environmental regulation passed by New York City legislators. LL87 seeks to reduce the city’s emissions by 50% while increasing the overall efficiency of large residential buildings (over 50,000 gross sq. ft.).

With additional state and local governments instituting similar environmental regulations across the United States, combustion system design and theory is more important now than ever.

Whether you’re a building owner, plant operator, building designer, or system engineer, this blog series will help you make informed decisions on your projects, especially as they pertain to LL87 and laws like it.

Why listen to us?

Because we’ve been doing combustion since 1920. Our rotary-style burners, invented in the 1960s, are still in operation all across New York City–almost half-a-century later.

But we’ve learned a lot since then.

We’re not like a lot of other burner companies. We don’t cut corners. Our products aren’t flimsy and they don’t come cheap. They last. And they perform.

Ultra low emissions. High efficiency. High turn down. Rugged durability. We reached for these marks because we believe in what we do. We love combustion. We love doing it right.

If this sounds like you, then read on.

Basics

The most common industrial fuels are hydrocarbons. This means that they are predominantly composed of carbon and hydrogen. Table 1 lists some common fuels and gives typical values for the hydrogen and carbon contents as percentages by weight. Note that there are some other components besides hydrogen and carbon. Some of these, such as sulfur, are combustible and will contribute to the heat released by the fuel. Other components are not combustible and contribute no positive energy to the combustion process.

Combustion Theory - the basics [table 2]

Table 1

The Chemistry

Table 2 reviews the basic chemical equations, which represent the most common combustion reactions. Note that nitrogen (N2) is shown on both sides of the equations. Except for the formation of NOx (in the parts per million range) nitrogen does not react in the combustion process. The nitrogen must be considered in fan sizing and stoichiometry calculations. Each atom of carbon in the fuel will combine with two atoms of oxygen (or one molecule of O2) from the atmosphere to form one molecule of CO2. On a weight basis, each pound of carbon requires 2.66 pounds of oxygen for complete combustion resulting in the production of 3.66 lb of carbon dioxide.

Combustion Theory - the basics [table 1]

Table 2

Each pair of hydrogen atoms (or each molecule of H2) will combine with one atom of oxygen (or one half molecule of O2) to form one molecule of H2O, or water. On a weight basis, each pound of hydrogen requires 7.94 pounds of oxygen for complete combustion, resulting in the production of 8.94 pounds of water.

By the Numbers

The air we breathe is only about 21% oxygen by volume. For all practical purposes, the remaining 79% is nitrogen. Since oxygen is a little heavier than nitrogen, the percentages by weight are somewhat different. The percentage of oxygen by weight is 23%, and the remaining 77% is nitrogen. Thus, it requires about 4.35 pound of air to deliver one pound of oxygen. Table 3 shows the composition of air.

Combustion Theory - the basics [table 3]

Table 3

A typical gallon of No. 6 fuel oil weighs 8 pounds and is 87% carbon and 12 % hydrogen (the missing percent is sulfur, ash, water and sediment). This gallon contains 6.95 pounds of carbon and 0.96 pound of hydrogen. From the data presented earlier, we can compute that 18.49 pounds of oxygen are needed to burn the carbon and 7.62 pounds of oxygen must be provided to burn the hydrogen in this gallon of fuel oil. This represents a total requirement of 26.11 pounds of oxygen. Since air is only 23% oxygen by weight, it will take 113.5 pounds of air (26.1 ÷ 0.23) for the complete and perfect (0% excess air) combustion of this gallon of fuel. Assuming there are 13 cubic feet of air to the pound, 1476 cubic feet of air are required to burn each gallon of fuel. A 50 gallon per hour burner (about 200 boiler HP) would need nearly 74,000 cubic feet of air per hour (or 1230 scfm) to fire without any allowance for excess air.

The Real World

In the real world, however, there must always be more air supplied to the combustion process than the theoretical or stoichiometric air requirement. This is because no burner made is this “perfect”. This “extra” air is referred to as “excess air.” If 20% more than the theoretical air requirement is supplied, we say that the burner is operating at 20% excess air. Another way of stating the same thing is to say that the burner is operating with 120% “total air.”

Complete combustion of our one gallon of No. 6 fuel oil with 20% excess air would require 136 pounds of air. The 50 gallon per hour burner would actually require about 90,000 cubic feet of air per hour.

For any particular burner-boiler combination, there is an ideal “minimum excess air” level for each firing rate over the turn-down range. Greater air flows would waste fuel because of the increased mass flow of hot gases leaving the stack. Lesser amounts of air would cause fuel waste because the fuel would not be burned completely. Typically, burners require much higher levels of excess air when operating near their minimum firing rates than they do at “high fire.” Table 4 shows a typical relationship between percent firing rate and the excess air required to insure complete combustion of the fuel. In many cases, even though stack temperature might decrease at low fire, efficiency suffers because so much of the fuel energy is lost to heat this excess air.

Combustion Theory - the basics [table 4]

Table 4

 

Other posts in this series:

  • Understanding Local Law 87 – and laws like it
  • Combustion Theory: The Basics
  • Combustion Theory: Variables – Account for variations in oxygen and fuel
  • Combustion Theory: Efficiency – Calculate efficiency and losses
  • Combustion Theory: FGR – See how flue gas recirculation reduces NOx
  • Combustion Theory: Combustion Controls – Learn how cutting-edge tech can cut your emissions
  • Combustion Systems: Design – Basic principles to follow when designing your combustion system
  • Combustion Systems: Troubleshooting: Burner problems and their causes
  • Combustion Control: Strategies – Linkage vs. Linkageless, and why you should care