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)