Page 284 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER 265

are the highest in the heat transfer equipment oper- & Acid number and viscosity measurements of the fluid
ating at high temperatures. in use give an indication of the degradation of the
. What are the performance problems involved in the use fluid and replacement requirements.
of high-temperature heat transfer fluids? . What are the consequences of contamination of heat
& Fluid Degradation: Fluid degradation starts in the transfer fluid?
fluid film, as the fluid is hottest. Fluid degradation & Reduced heat transfer rates.
occurs due to oxidation of the heat transfer oils giving & Decreased fuel efficiency.
risetoformationofsolidsandhigh-viscosityproducts. & Flow blockages in small diameter or low-velocity
& For typical heat transfer oils, the fluid decomposition
areas.
rate doubles for roughly every 10 C. & Extended start-up times at low temperatures.

➢ Increase in degradation rates in the film can exceed
& Fouling of heat transfer surfaces.
those in the bulk fluid by a factor of 20 or more.
& Overheating and damage, or even complete failure of
& Fouling: Overheating of the fluid film may result in
heater tubes.
fouling of the heat transfer surface.
. Steam is the most commonly used heat transfer fluid.
& Coke Formation: Formation of abrasive coke parti-
Would you recommend high-pressure steam or low-
cles due to fluid cracking reduces the life of pump
pressure steam for this purpose? Give reasons for your
seals, bearing surfaces, and valve seats.
choice.
& Contamination: Accumulation of dirt, contamina-
& High-pressure steam is normally not a good choice
tion from the process systems, and corrosion pro-
for heating, though it gives larger DT, for the fol-
ducts from piping and equipment.
lowing reasons:
& Low Boiling Fluid Formation: Formation of low ➢ It is more valuable for power generation, the
boiling fluid or gas formation due to decomposition exhaust from the turbines being normally used for
at high temperatures requires vapor venting that heating purposes.
contributes to fluid loss apart from lowering heat
➢ It has lower l v , heat of condensation, thus releas-
transfer coefficients.
ing less energy per unit quantity compared to low-
. What are the reasons for heat transfer fluid degradation? pressure steam.
& When a heat transfer fluid reacts with oxygen, ➢ Equipment and distribution lines must be designed
organic acids are formed and oxidation occurs. and fabricated to withstand higher pressures, an
& Coke deposits, sludge, and increased viscosity are the expensive proposition.
symptoms of oxidation and are the most common ➢ Where higher DT is required, some other heat
reasons for severe fluid degradation. transfer fluid such as Dowtherm with high l v or
& Unless water is present, these acids are not corrosive. other high boiling fluids should be selected.
& The problem is because the acids, which have poor . To get high DT, high-pressure steam is preferred as a
thermal stability, break down inside the heat transfer heating medium than low-pressure steam. Explain the
equipment. pros and cons in supporting such a proposition.
& One of the consequences of this acid deterioration is & While high-pressure steam increases DT for heat
the extremely fine carbon particles (soot) that can transfer, it is not preferred as a heating medium for
agglomerate to form sludge deposits. reasons given under the previous question.
➢ These particles will remain suspended in the fluid
. Use of superheated steam as a heating medium in-
while it is flowing, but can stick together and form
creases DT for heat transfer. Is it advisable to use
blockages where excessive turbulence will pack
superheated steam for such an application?
the particles together.
& Superheated steam behaves as a gas while it is giving
➢ These carbon particles will also settle out of the
up its superheat. Heat transfer coefficients are very
suspension and form sediment when the fluid is
low for gases compared to heat transfer coefficients
stagnant and cool.
for condensation of vapors. Condensing film coeffi-
& Another way that acids cause problems in heat cients are over 800–1000 times higher than gas film
transfer systems is that they increase the viscosity coefficients.
of the heat transfer fluid when they polymerize.If & Area requirements for heat transfer will be very
localized hot surfaces exist inside the tubes or on
high compared to the benefits obtainable due to
electrical elements, high acid levels will cause hard
getting higher DT. Thus, equipment costs will
deposits (coke) to form on the hot spot.
increase.