Now, this is an official answer. While not backing off of what I said above, because in the "spirt" of the matter, it is still true, it is not the "letter" of the matter, and someone else had recently asked me the same question, so I wanted to give the HEI version, and then e mail the thread to the other person.
The U value determined by the conventional Q = U x A x dT method shown by TEMA uses the fouling factor in the "U" value calculation formula to artificially increase the surface area required as described, and as you already know.
HEI, however, does it differently. HEI is an association of equipment manufacturers who deal specifically with Feedwater heaters, Surface Condensers, Deaerators, and air removal (from condensers) equipment (to mention most of them) which establishes standards that their industry (and the users of their equipment,) in general, adheres to, whether or not they are members. Visit them at www.heatexchange.org
to see who they are and what all they cover.
The term "cleanliness factor" pertains principally to turbine exhaust surface condensers. In a condenser calculation, the ideal "U" value is determined by the formula Ui = C X sq. root of V(velocity). Or, Ui = C X V^1/2.
C is an empirical number determined by testing by HEI, ranging from 247 for 1-7/8 and 2 inch tubes to 267 for 5/8 and 3/4 inch tubes.
This yields, remember, the "IDEAL U" value, Ui.
The design "U" value, then is determined by Ud = Ui X Cm X Ct, where Cm is a tube metal correction factor, material and gage specific, and Ct is a circ water temperature correction factor. They are both picked from tables in HEI, but based on a value of 1.0 for 18 ga admiralty, arsenic copper, or aluminum tubes in the case of Cm, and 1.0 for 70F for Ct.
Cm ranges from .55 for 304/316 SS, and Ti tubing at 12 ga. to 1.06 for 24 ga admiralty brass tubing. (Who would ever use such a thin brass tube???)
Ct ranges from .55 for 30F CW to 1.143 for 120F CW.
All these factors may have changed in the latest edition of the HEI standards. The numbers given here are from a previous edition.
Therefore, the Cleanliness factor, expressed as a percentage, is expressed calculated by; Cf = Ua/Ud X 100%, where Ua is the actual U value, either picked as a beginning point by the designer, or determined by testing in an operating condenser, and Ud is the design "U" value calculated above.
So, Ua is picked initially by the designer, based on some criteria for different metals. Brass and copper based alloys, for example are usually designed at 85% cleanliness factor, while harder tubing materials, like SS, duplex, and Ti tubing is picked at 90% cleanliness factor.
Condensers are rarely designed with a cleanliness factor higher than 90%, hence, it becomes an "artificial" parameter to increase the condenser overall surface area, as does the "fouling factor" in the conventional formula.
A brand new condenser, on day one, before the CW fouled it would theorhetically, then, would show a cleanliness factor of 100%. The designer then hopes, that once it reaches its initial fouled condition that it does not exceed his design cleanliness factor of 90%. That is because he put enough surface in it to perform the duty at that cleanliness factor, and less than that means loss of performance.
So, now you have the official answer. I hope this answers your question. Thanks for the opportunity to set the record straight, because I now understand it exactly, as according to HEI, and not "intuitively" as before. And, yes, it is still 'sixes' in my mind.