MTS Water Quality Guide - Manual (Page 11)

Heat Exchanger Care and Water Quality Guide Heat Exchanger Care 11
Contaminated water
supply
The most likely cause of corrosion failure in heat exchangers is their use with
cooling towers. These towers use fan propelled ambient air to evaporate a portion
of the cooling water and thus cool the remaining water. This action transfers
whatever air pollution exists into the cooling water. Also, since the towers are
open to the environment, they are prone to collecting animal and vegetable
matter which is damaging to heat exchangers.
How can I prevent heat exchanger failure?
There are four types of heat exchanger failures that can occur: mechanical,
chemically induced corrosion, a combination of mechanical and chemically
induced corrosion, and fouling due to the accumulation of scale, solids, and
algae.
Mechanical failure
Mechanical failures may take one of the following forms: metal erosion, water
hammer, vibration fatigue, thermal fatigue, or freeze-up.
• Metal Erosion: Excessive fluid velocity through the heat exchanger can
cause damaging erosion as the metal elements wear away. Any existing
corrosion is accelerated as erosion removes protective films from the metal,
exposing fresh metal to further attack.
Most metal erosion problems occur in the chambers and at the entrances to
the heat exchange. The entrance areas experience severe metal loss when
high-velocity fluid from a nozzle is divided into much smaller streams upon
entering the heat exchanger. This stream dividing results in extreme
turbulence with very high localized velocities.
The maximum recommended velocity in the chambers and at the entrance
nozzle is dependent on the chamber material, fluid handled, and
temperature. Materials such as stainless steel and copper-nickel can
withstand velocities of 10-11 ft/s (3.0-3.4 m/s). Copper is normally limited
to 7.5 feet per second (2.3 m/s). The water flow velocity in copper should be
kept less than 7.5 ft/s when it contains suspended solids or is softened.
• Water Hammer: Damaging pressure surges or shock waves can result from
an interruption in the flow of cooling water. To reduce your risk of water
hammer, the cooling water flow should always be started before heat is
applied to the exchanger.
Fluid flow control valves that open or close suddenly also produce water
hammer. Modulating control valves are preferable to on-off types.
• Vibration fatigue: Excessive vibration from equipment or transients in the
fluid flow (pulsating) can cause failures in the form of a fatigue stress crack
or erosion at the welded joints. Heat exchangers should be isolated from
excessive vibration.
• Thermal Fatigue: Fatigue resulting from accumulated stresses associated
with repeated thermal cycling can also cause failures. In thermal fatigue,
the temperature differences cause flexing, which produces a stress that acts
additively until the tensile strength of the material is exceeded and it cracks.
The crack usually runs radially and many times results in a total break.
A thermostatic or spring loaded by-pass relief valve installed ahead of the
heat exchanger will hasten warm-up and relieve the system of excessive
pressures, as well as control the hydraulic fluid temperature in certain
installations.