Corrosion And Limescale Build-Up; Flow Velocity; Materials - Bitzer K033N Instructions De Service

It is advisable to clean the coolant pipes regularly. The
contamination depends directly on the quality of the
coolant used. The pipe bundles can be cleaned on the
coolant side without intervention in the refrigerating cir-
cuit.
• Dissolved or solid components of the coolant such
as lime, sand, algae or silt may settle in the pipes.
• Organic substances such as algae can form local
elements. In the worst case, this can lead to pitting.
• If the pipes are cooled with seawater, shells may
grow on the inside of the pipes.
3.2.1

Corrosion and limescale build-up

The influences on the lifetime of coolant pipes are com-
plex. Dissolved oxygen and the gases CO
the coolant contribute significantly to corrosion. Sus-
pended solids may deposit in the pipe profile. In the vi-
cinity of dust, sand or deposits and decomposition
products of organic components, pitting may occur
within a short time. For this reason, the proportion of
dissolved gases and solids must be kept as low as pos-
sible. The growth of shells in the pipe profiles must be
prevented in any case.
To assess the risk of corrosion and limescale build-up
for gas- and solids-free coolants, the salinity "S", the al-
kalinity "Alc", the CaCO
value of the water must be known:
Langelier saturation index
The respective negatives logarithms of these values
are used to calculate this index:
▶ LSI = pH - pS - pAlc - pCa
➙ LSI < 0: The coolant can cause corrosion.
➙ LSI = 0: No corrosion or limescale build-up is to be
expected.
➙ LSI > 0: The coolant can cause limescale build-up.
Ryznar stability index
This measure also takes into account the influence of
temperature. The calculation is more complex; the tem-
perature is taken into account as an absolute temperat-
ure T , which corresponds to the temperature in °C
abs
plus 273 K.
▶ RSI = 2 x (44.25 + lg((S - 1) / 10) - (13.12 x lgT
lgAlc - lgCa) - pH
➙ RSI < 5.5: Coolant is highly prone to limescale build-
up.
➙ 5.5 < RSI < 6.2: Coolant is prone to limescale build-
up.
DB-200-6
and H
2
concentration "Ca" and the pH
3
➙ 6.2 < RSI < 6.8: Very little limescale build-up is to be
expected.
➙ 6.8 < RSI < 8.5: The coolant is corrosive.
➙ 8.5 < RSI: The coolant is very corrosive.
3.2.2

Flow velocity

Flow velocity
Minimum
Recommended
Maximum
These values apply to clean and gas-free water directly
at the inlet. When using water containing a low amount
of solids or gases, the shell and tube condenser can be
S in
2
operated with a flow velocity of up to approx. 1.5 m/s.
However, positive experience must first have been
gained from comparable applications.
The minimum flow velocity of the coolant provides suffi-
cient heat transport at a low thermal load. A high flow
rate may lead to vibrations in the pipes and, depending
on the coolant quality, to abrasion of the tube profile or
to cavitation.
NOTICE
!
!
Too high flow velocity will damage the coolant
pipes.
Never exceed the maximum flow velocity.
A low flow rate through the coolant pipes is required
even if the system is at standstill. This will avoid depos-
its and reduce the risk of limescale build-up and corro-
sion.
In case of parallel operation:
▶ The flow velocity must be monitored at each shell
and tube condenser in each operating condition.
▶ It is preferable to install a coolant pump for each
shell and tube condenser.

3.3 Materials

• Heat exchanger tubes
– standard designs: copper (ISO Code Cu-DHP;
UNS Code C12200)
– seawater resistant design: copper-nickel 90/10
) -
abs
(ISO Code CuNi10Fe1Mn; UNS Code C70600)
• Shells: carbon steel P265GH
• Tube sheets: carbon steel P265GH, plastic-coated
• Coolant reversing covers:
Cu-DHP CuNi10Fe1Mn
1.0 m/s 1.0 m/s
1.5 .. 2.5 m/s 1.5 .. 1.8 m/s
3.0 m/s 2.0 m/s
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