Page 324 - Fluid mechanics, heat transfer, and mass transfer

HEAT EXCHANGERS 305

& These scales should be scraped off by ensuring should be avoided as residual solvents can create
wetting such surfaces by water. severe corrosion problems. Also disposal of haloge-
. What is rubber ball cleaning of heat exchanger tubes? nated compounds is more expensive.
What are its advantages?
& Baking Soda: Aqueous solution is forced at high
& Sponge rubber balls are used to clean inside of tubes pressure through nozzles as an alternative to high-
while the unit is in operation. pressure water jets.
& Balls are slightly larger in diameter than the tube and ➢ Cleans oil, grease, polymers, dirt, carbon soot, and
are compressed as they travel the length of the tube. so on.
This constant rubbing action keeps tube walls clean ➢ Foulant does not redeposit.
and virtually free from all types of deposits.
. How does EDTA work as a cleaning fluid?
& Suspended solids are kept moving and not allowed to
& Many chemical cleanings are performed with EDTA.
settle, while bacterial fouling is wiped quickly.
& EDTA belongs to a class of compounds known as
& Balls are forced to move due to DP between inlet and
chelants.
outlet.
& EDTA is particularly effective in complexing diva-
& Abrasive-coated balls are available to remove heavy
lent and trivalent cations.
fouling.
& The most popular procedure uses tetraammonium
& Balls are circulated in a closed loop.
EDTA, in which the two hydrogen atoms at the end of
& This technique is used in natural gas pipelines to flush
each molecule (four total) are replaced with ammo-
out condensates, keeping pipeline capacities for gas þ
nium (NH 4 ) ions.
flow high.
& Ammonia is alkaline and a typical EDTA cleaning is
. When dirty liquids like waste streams are involved on
performed at a pH of 9–9.5.
the tube side of a heat exchanger, what are the ways to
& Tetraammonium EDTA is not as aggressive toward
maintain clean tube surfaces?
deposits as HCl, so common practice calls for the
& Use of recirculated sponge balls or reversing brushes
boiler to be filled with a 5% solution and then fired
without shutting down the unit, that is, online
until liquid temperatures reach about 135 C. This

cleaning.
increases reactivity of the chemical.
. What are the various chemicals employed in chemical
& In natural circulation units, following the initial
cleaning?
firing, the boiler must be alternately cooled to about
& Acids:

135 C or so and then refired to 135 C to circulate the
➢ Strong acids damage equipment.
chemical.
➢ HCl with 0.25–1% ammonium bifluoride and & The iron removal stage of an EDTA cleaning may
copper complexing agents as additives. Effective take from 12 to 36 h.
for siliceous deposits. Not generally recom- ➢ Free EDTA concentrations should not be allowed
mended for austenitic alloys.
to fall below 0.4%.
➢ H 2 SO 4 for austenitic alloys.
➢ Once iron removal is complete, the system is
➢ Weak acids such as citric, formic, sulfamic, and so allowed to cool to about 65 C and then an oxidant

on are less effective and more expensive but less (such as air with sodium nitrite, oxygen, or hy-
corrosive compared to HCl. drogen peroxide) is injected into the solution to
- Citric acid is sometimes preferred for preoper- effect copper removal.
ational cleaning, where iron oxides constitute ➢ This may take 4–8 h.
the bulk of deposits.
& The oxidant converts copper to the þ 2 oxidation
- Hydroxyacetic acid and formic acid mixtures
state, whereupon it is complexed by the EDTA.
(typically 2% hydroxyacetic acid þ 1% formic
. What are the advantages and disadvantages of tetra-
acid).
ammonium EDTA?
➢ Primarily used to clean once-through boilers.
& Advantages:
- EDTA (ethylene diamine tetra acetic acid) for-
➢ EDTA is much less corrosive than HCl.
mulations (ammoniacal solutions).
➢ The process is performed at an alkaline pH, so if a
- Ammonium citrate.
bit of residual remains in the boiler after the
& Chlorine: Being discouraged/eliminated by govern-
cleaning and rinses, it will not attack tubes unlike
ment regulations. Use of chlorine-containing solvents
HCl.