Don't Overspend When Buying an Economizer

Don’t Overspend When Buying an Economizer

Most of us know that economizers save you money by recycling heat that would otherwise be wasted up the stack.  Typical fuel savings range from 3-8%.  But economizers come in many different shapes and sizes and are manufactured by a wide variety of companies that have their own ideas as to specific design features.  On the E-Tech website, there is a white paper that will guide you through what makes sense (and CENTS) in economizer designs.  Here is a link:

E-Tech White Paper on Choosing an Economizer

But beyond the nuts and bolts discussion detailed in the white paper are other plain sense ideas that apply to all economizer selection criteria, even if an E-Tech design is not your first choice.

1.        Gas side pressure drop (a function of velocity) plays a large role in determining the size and cost of an economizer: the more pressure drop that can be taken through the vessel, the less expensive the cost.  This particular variable has perhaps the highest impact on the size and price of the unit.

2.       In determining the size and shape of an economizer, one must also consider the entire boiler setting.  Buying the least expensive economizer may cost you more in the long run when taking into account support steel and transitions.  Think about the whole package before making a decision.

3.       Fin pitch will have a large bearing on the cost of an economizer so follow good engineering practices to resolve this issue.  Here are a few guidelines to follow:

a.       Serrated finning, generally used with clean-burning fuels, is less expensive than solid fins.

b.      Carbon steel fins are much less expensive than stainless.  Not only that, but the conductivity of stainless fins is less than one-half that of its carbon counterpart.  Unless there is a chance of condensation of moisture in the gas stream, use carbon steel fins.

c.       Fin pitch recommendations:

                                                               i.      Coal/Wood, two fins/inch or less

                                                             ii.      #6 Fuel oil, three fins/inch

                                                            iii.      #2 or #4 Fuel oil up to 4.5 fins/inch.  Serrated is acceptable

                                                           iv.      Natural gas, hydrogen, butane, 6 fins/inch or more, serrated.  Note that with this fuel, sootblowers should not be necessary.

4.       Tube material.  You’ve already spent money on an expensive deaerator.  Why waste additional dollars paying for stainless tubing?  Not only is the cost of stainless tubes significantly higher, it is also not allowed under Section I of the ASME Code.  Your boiler is stamped to Section I.  Make sure the economizer is too.

5.       Along the same lines, unless you have a condensing economizer (or very hot flue gases), there is no good reason not to have carbon steel inner casing.  Spending money on a stainless steel casing offers no benefit for a higher cost.

6.       There is a valid difference of opinion as to whether an economizer needs removable end panels to facilitate tube removal should that become necessary.  Those who say “no”, argue that it’s just as easy to torch off the end casing as it is to remove all the bolts.  This is a matter of personal preference so pick your side and stick to it.

7.       Don’t oversize your economizer!  If you have a 500 HP boiler that never, ever will run more than 300 HP, then buy a smaller unit, one designed for 300 HP.  Remember, though, that once you choose this path, you’re pretty much committed to it.

8.       Finally, on smaller boilers, a cylindrical design is a great cost cutter as there are usually no transitions needed.  Much is made of the circular design not being repairable which is true in many cases.  However the circular design has far fewer tube welds and a much better frequency of repair record than rectangular designs.

To finish, here’s my blog quote of the day.  I’ve always been intrigued by the sayings of Confucius and I found one that’s perfect.

He who will not economize, will have to agonize.  ~ Confucius (551 BC – 479 BC)

The Skinny on Boiler Emissions

Emissions

Because of all the political dialogue, as a population, we tend to have heard about emissions in one form or another.  With my recent blogs on Boiler MACT regulations, I thought it would make a good article to talk about the types of emissions we see in our industry and what can be done to do away with emissions or mitigate their effects.  Although my slant as an employee of E-Tech, Inc. is to offer solutions to emission issues, boiler economizers such as we design and manufacture, can only moderate these pollutants to a certain degree.  Furthermore, my knowledge base is limited to our products to an extent.  Boiler manufacturers have more impact on this and therefore, more smarts.

At E-Tech, my company, we concern ourselves primarily with:

·         Nitrogen compounds (NO_X)

·         Sulfur Oxides (SO_X)

·         Volatile Organic Compounds (VOC’s)

Certainly there are other things to worry about such as particulates but that is not within the scope of this blog.

NITROGEN COUPONDS (NO_X)

NO_X is a generic term for nitric oxide (NO) and nitrogen dioxide (NO2) and is formed from the reaction of nitrogen and oxygen during the combustion process.  NO_X is not to be confused with nitrous oxide (laughing gas, for example).   Although this discussion is related to boilers, NO_X is produced from any combustion process, particularly from automobiles.  NO_X Pollution from cars, trucks, planes, etc., can be particularly high in areas of high motor vehicle activity and around densly populated parts of the country. 

While it is true that NO_X can encourage the formation of acid rain, it is not the main reason we look at reducing them.  It is because of the effects on the production of ozone that we endeavor to lower this pollutant.  To reduce NO_X a variety of methods have been introduced over the years, and as time has gone on, improvements have been made.  Here, in no order, are several.

·         Selective Catalytic Reduction (SCR) – In this method, ammonia is injected in the boiler exhaust gases with a catalyst that allows the ammonia to reduce the NO_X

·         Flue Gas Recirculation (FGR) – This is been around for a while and is an effective means of reducing emissions on smaller boilers (30,000 PPH or less is a good range).  In essence FGR involves recirculating cooler exhaust gases back into the combustion process to lower the temperature.

·         Burner Modifications – Fifteen years ago or more, burners having NO_X levels of 30-50 PPM were considered pretty cutting edge.   Since then, some of the ultra-low NO_X burners offer as low as 5 PPM

·         Efficiency – Finally, this is where E-Tech comes in.  By adding heat recovery equipment to the boiler, the firing rate for any given output is reduced, thereby lowering emissions.

SULFUR COMPOUNDS (SO_X)

I remember years ago when I first heard about acid rain.  It was in New England where I was born and it was having an impact on the lakes and trees of the area.  Although I didn’t understand it at that time, I remember there were certain parts of the country – the rust belt (before it got rusty) – that were blamed for the phenomenon.  While acid rain has been around for centuries the issue became more urgent in the 1970s and 1980s in the US.  Many laws have been promulgated through Congress and the EPA to help turn back the clock to cleaner times.  One of the biggest culprits in acid rain is sulfur.  Even though sulfur compounds can be naturally formed (volcanoes, etc.), much is produced by man through the burning of sulfur-laden fuels such as coal and heavy oil.

As a large portion of our nation’s utilities produce electricity through the burning of coal, the industry has done much to reduce its pollution footprint.  Many coal-burning plants use flue gas desulfurization, a process by which 95% or more of the SO₂ will be removed.  In addition, as natural gas has become more plentiful and less costly, it is being used more and more; its future looks bright not only in the boiler market but also for fleets of cars and trucks.

Although I wish I could plug economizers here, there’s not much an economizer can do except lower the firing rate of the boiler.

VOLATILE ORGANIC COMPOUNDS (VOC)

VOCs are compounds with high vapor pressures and low water solubility.  They are comprised mainly of carbon and hydrogen.  Many VOCs are dangerous to both human health and the environment and may include both manmade as well as naturally occurring elements.  E-Tech has worked with many companies involved in the design and manufacturing of thermal oxidizers, one method of destroying hazardous pollutants.

My blog quote for the day is about autumn, my favorite time of year.

Autumn is a second spring when every leaf is a flower.  ~ Albert Camus

Useful Formulas

For the readers of my last two blogs who slogged through the boring subject matter of government regulations, I hope to move along to something less sterile as well as being more helpful.  In our business, we get so used to calculating various formulas in our day-to-day work that we somehow lose sight of the fact not everyone involved knows the intricacies of our field.  Earlier this week through my LinkedIn connection someone asked about calculating paybacks.  I gave an answer and found that this led to more questions so I hope a few simple equations here will be helpful to all.

First, how do we calculate heat recovery in an economizer?

Q=(m) (Cp) (Tin-Tout)

Where:

Q = heat recovered (Btu/hr)

m = mass flow rate of flue gas (lb/hr)

Cp = Specific heat of the flue gas (Btu/Hr °F)

T = temperature of the flue gas (°F)

Example:

50,000 PPH Boiler with 54,000 PPH flue gas

2,916,000 Btu/hr = (54,000 PPH)(.27)(500-300)

Note that this holds true for both the water-side as well as the gas side.

Another way to calculate Q is:

Q= (U) (A) (LMTD)

Where:

Q= Heat recovered (Btu/hr)

U= Heat Transfer Coefficient (Btu/hr • ft²)

A= Effective Surface Area (sq. ft) 

LMTD = Log Mean Temperature Difference across the heating surface

Note in this formula the most efficient way to lower the amount of surface area is by increasing the heat transfer coefficient. 

Next, how do we calculate fuel savings?

S=(Fuel) (Q) (H) / (E) (1,000,000)

Where:

S = Savings $/yr

Fuel = cost of fuel in $/MMBtu

Q = heat recovered Btu/hr

H = hours of annual operation

E = Boiler efficiency w/o economizer

Example:

$142,155= ($6) (2,916,000 Btu) (6500) / (.8) (1,000,000)

My blog quote for the day is about our mortality, something especially poignant as I grow older.

Eternity is really long, especially near the end. ~ Woody Allen