Page 285 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER BASICS
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& Use of superheated steam is not advisable as an ➢ Consequently, piping and vessel costs will also
option for obtaining higher DT as the amount of heat rise rapidly. Thus, other high-temperature heat
obtainable from the superheat is verysmall compared transfer fluids must be considered.
to latent heat obtainable on condensation of the ➢ A low vapor pressure at a high temperature is the
saturated steam. major reason for choosing an organic fluid over
& Heat exchangers operating at higher temperature steam.
levels involve increased heat losses or require more ➢ Pressurized water could be used from 300 to
insulation to offset the increased heat losses. 400 C, but high pressures are required to maintain

& However, sometimes, superheated steam is used as a the water in the liquid state.
heating medium in desuperheaters for energy recov- ➢ Above 500 C, combustion gases and liquid metals

ery or while heating highly viscous liquids where are possibilities.
high DT is desirable.
➢ Liquid metals are used for cooling nuclear
. What are the practices that involve wasteful energy reactors.
losses in steam heating systems? ➢ Temperatures from 50 to 1000 C can also be

& Many refineries and petrochemical plants use expen-
achieved by electrical heating.
sive medium-pressure (MP) steam for low-tempera- & Environmental Effects: Since accidental chemical
ture heating duties when they could instead be using
spills occur occasionally, the effect of the heat trans-
cheaper and more readily available low-pressure
fer fluid on the environment and health must be
(LP) steam. A common reason to use MP steam is
considered.
that if there is significant back pressure in the con-
➢ Since the use of chemicals may be governed by
densate return system, the pressure of the LP steam is
laws, the process engineer must comply. For ex-
too low to drive condensate through a steam trap and
ample, EPA has banned use of polychlorinated
into the condensate main, resulting in condensate
biphenyls (PCBs) because of the concern over
back up in the equipment impeding heat transfer.
environmental contamination.
& This MP steam demand for heating purposes is met
& Viscosity: High viscosity reduces heat transfer rates,
often by letting down high-pressure steam, which is a
defeating the very purpose of heat transfer.
wasteful operation.
& Thermal Stability: Organic heat transfer fluids can
& LP steam is a versatile heat source and is frequently
degrade somewhat, either by oxidation or thermal
vented to atmosphere, or condensate from heat trans-
cracking.
fer equipment is sent to drain, both involving wastage
➢ The primary cause is thermal degradation. In
of energy.
thermal degradation, chemical bonds are broken
& The above practices add to ever increasing energy
forming new smaller compounds that lower the
costs.
flash point of the fluid.
. How can the energy losses by the above practices be
➢ At the flash point, flammable fluids will momen-
reduced?
tarily ignite on application of a flame or spark.
& Combined steam trap and pump systems are avail-
➢ Organic fluids will also degrade to form active
able that avoid the pressure drop problems by return-
compounds.Thesecompoundswillthenpolymerize
ing LP condensate to the boiler house even against
to form large molecules thereby increasing the fluid
significant back pressure, thus avoiding use of MP
viscosity, which reduces heat transfer.
steam.
& Corrosivity: Generally, a heat transfer fluid should
. If steam used as a heat transfer fluid contains some air,
be noncorrosive to carbon steel because of its low
what is its effect on heat transfer rates?
cost.
& Presence of air reduces heat transfer.
➢ Carbon steel may be used with all the organic
. What are the factors that are to be considered in

fluids and with molten salts up to 450 C. With the
evaluating heat transfer fluids?
sodium–potassium alloys, carbon and low-alloy
& Working Temperature Range: This is the primary steels can be used up to 540 C, but above 540 C

consideration to match process temperature require- stainless steels should be used. Cryogenic fluids
ments with the selected fluid. require special steels. For example, liquid methane
➢ Steam is normally considered first for high tem- requires steels containing 9% nickel.
peratures, but above 180 C steam pressure & Toxicity and Flammability: Organic heat transfer

increases rapidly with increasing temperature.
fluids require stringent leakage control because they