Scaled Water Tube Boiler Efficiency

[BronYrAur]

Are there any rules of thumb on how much efficiency drops from scaled tubes? For example, let's say that you have a uniform 1/8" thick scaling on all tubes. What can we say about the efficiency?

Also, just a general question on efficiency of B&W boilers. I have an old unit operating at 90 psig. Burner efficiency has 80% rating, but how can I estimate the overall efficiency (gas to steam)? There is no economizer.

 

[BronYrAur]

I guess I should have done some looking before asking the question. I found this:

http://www.energysolutionscenter.org/boilerburner/Eff_Improve/Efficiency/Scale_Deposits.asp