Page 178 - CW E-Magazine (7-11-2023)

Special Report

tives used for fouling and corrosion remove sludge or similar soft is to stop operation and clean the heat
mitigation were hazardous to the envi- deposits. Soft salt deposits may be exchanger. Off-line cleaning can be
ronment. But these days chemicals washed out by circulating hot fresh classified into off-line chemical clean-
benign to the environment are preferred. water. ing or by mechanical means. A cleaning
Chlorination can be used for com- method without the need to dismantle
Besides fouling detection, cleaning bating bio-fouling(11). the heat exchangers is preferred, but
and routine maintenance activities are A variety of polymeric and non- usually it is necessary to have access to

crucial to maintain a heat exchangers’ polymeric scale inhibitors such as the inside surfaces. It would be prudent
performance. polyacrylates (molecular weight to consider the installation of a standby
900–250,000), acrylate-based co- heat exchanger, thereby providing the
Cleaning of heat exchangers polymers, polyphosphates, etc. are opportunity to clean the fouled and
In order to maintain or restore the used(12). scaled heat exchanger while at the same
efficiency of a heat exchanger, it is often When hard water is used, it is im- time maintaining production.

necessary to clean it. Methods of clean- portant to arrange for magnetic
ing may broadly be classified into two water treatment for scale prevention Off-line mechanical cleaning
groups: ‘On-line’ and ‘Off-line’ clean- as soon as possible. If one waits Water jetting is one of the saf-

ing. In some applications, the cleaning too long, the treatment may be less est cleaning methods in the rained
can be done on-line to maintain accept- effective at breaking up the scale and hands. It lowers the maintenance
able performance without interruption removing the bacteria. As its name cost by negating the need for hazar-
of operations. In other cases, off-line implies, it works by water flowing dous chemicals disposal services.
cleaning must be used. through a chamber and being ex- It also enables testing of heat ex-
posed to a series of reversing pola- changers by ‘Eddy Current’ or
On-line cleaning rity, alternating permanent magnetic ‘Ultrasonic Internal Rotating Inspec-
On-line cleaning generally utilizes fields. As water passes through a hot tion System (IRIS)’ (Figure-4 & 5).
a mechanical method designed for only water heater, it reacts to the magne-
tube side and requires no disassembly. tic field by changing the fundamen-
The advantage of on-line cleaning is tal structure of the hard water. It
the continuity of service of the heat does not remove any minerals from
exchanger with the hope that no cleaning- the water. Rather, it makes it more
mandated downtime will occur. How- difficult for minerals to stick to the
ever, there is no assurance that all the wetted surfaces of the pipes and
tubes will be cleaned sufficiently. equipment it is traveling through
and form scale build-up(3, 4).
The various on-line cleaning methods High and low frequency sound

include: emitters are also used to relieve
Circulation of sponge rubber balls – fouling problems. The use of sound
the technique is capable of prevent- is much less effective in sticky and
ing the accumulation of particulate tenacious deposits generally associa- Fig. 4: Internal Cleaning of Heat Exchanger
matter, bio-film formation, and ted with slag. Tubes by Water-jetting
scale and corrosion product deposi- Commercially available chemical
tion(10). compounds may be injected into
Two phases of the ferrous sulphate the process streams for removing
treatment – the first phase involves sludge, scale or coke, in case hot
the initial laying down of the pro- water or wash oil, is ineffective.
tective film. The second phase Radiation sterilization of microbial-

involves the maintenance of the laden water, ultraviolet light, and
film, which would be otherwise Gamma rays have also been utilized
destroyed by the shear effects of flow. for cleaning purposes.
Circulating hot wash oil or light
distillate through the tubes or shell Off-line cleaning Fig. 5: External Cleaning of Tube Bundle by
at high velocity may effectively An alternative to on-line cleaning Water-jetting

178 Chemical Weekly November 7, 2023

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