Page 286 - Fluid mechanics, heat transfer, and mass transfer

CONVECTIVE HEAT TRANSFER 267

are all flammable from 180 to 540 C. Most of these . Why is it necessary to deaerate water used for steam
fluids have toxic effects and irritate eyes and skin. generation?
. Summarize fluids used as heat transfer media and their & Dissolved air in water gets released on vaporization
working temperature ranges. of water for steam generation. Thus, steam will be
contaminated by air, which is a noncondensable gas.
& Table 9.5 gives a summary of heat transfer fluids and
the temperature ranges involved in their use. & When steam is used as a heating medium, it con-
denses in heat transfer equipment with accumulation
. What are the capacities and steam pressures involved in
of air on heat transfer surfaces.
generation of steam for use as heating medium in
process plants? & Air has very low heat transfer coefficient, being a bad
conductor, thus retarding heat transfer.
& Medium-pressure boilers for generating steam for
use as a heat transfer fluid generally have capacities & Even 1% air in steam will reduce heat transfer
up to about 10,000 kg/h at an operating pressure of coefficients for steam very appreciable by as much
around 1000 kPa. as over 50–60%.
. What are the quality requirements for the water used for & Due to this reason, deaeration of boiler feed water
steam generation? becomes a necessity for maintaining high heat trans-
fer rates with steam as a heating medium.
& Deaeration of boiler feed water and blowdown
control. & Presence of dissolved gases in process streams will
have similar effect on heat transfer as presence of air
in steam.
TABLE 9.5 Operating Temperature Ranges of Different Heat
Transfer Fluids . What are the other effects of presence of dissolved gases
Operating Temperature in process streams? How are they removed?
Heat Transfer Fluid Range ( C) & Apart from reducing heat transfer rates in heat ex-

Refrigerants changers, dissolved gases on release inside equip-
Ethane and ethylene 60 to 115 ment or pumping systems can create operational
Propane and propylene 5 to 46 problems such as vapor lock, cavitation in pumps,
Butanes 12 to 16 increased entrainment problems inside equipment
Ammonia 32 to 27 and increased losses of liquids as presence of dis-
Fluorocarbon (R-12) 29 to 27 solved gases increases vaporization rates by their
(dichlorodifluoromethane) partial pressure effect on the liquid, interference in
Water þ ethylene glycol 50 to 90
measurements, corrosion problems in downstream
(50%/0%)
equipment, environmental pollution, and safety
Water
problems.
Water (wells, rivers, and lakes) 32–49
& Water containing dissolved oxygen can be highly
Chilled water 1.7–16
Cooling tower water 30 corrosive to boiler system components. Oxygen in
High-temperature water 300–400 water gives rise to localized pitting corrosion that can
Air 65–260 very rapidly lead to catastrophic failure of steel boiler
Steam components.
Low pressure (2.7 bar) 126
& Besides power plants, breweries require deaerated
Low pressure (4.6 bar) 148
Organic oils a 50 to 430 water for acceptable quality of beer.
& If dissolved gases are present in the liquid, the liquid
Silicone oils 23 to 399
Molten salts should be passed through a separation vessel to allow
75–500 for vapor disengagement prior to transferring heat to
25% AlCl 3 , 75% AlBr 3
40% NaNO 2 , 7% NaNO 3 , 204–454 or from the stream or sending for further processing
the stream. Dissolved gases may be present in sig-
53% KNO 3
Liquid metals
nificant amounts in solvents recycled in absorption
56% Na, 44% K or 22% Na, 204–454
and other gas processing units.
78% K
. What is the maximum dissolved oxygen content per-
Mercury (not in current use 316–538
due to toxicity) missible in steam condenser systems in power plants for
Combustion gases >500 efficient heat transfer?
& 7 ppb of dissolved oxygen. 5 ppb is recommended for
a
For example, diphenyl-diphynyl oxide, hydrogenated terphenyl, aliphatic,
and aromatic oils. nuclear power plants.