Wednesday, August 21, 2013
A heat exchanger is a piece of equipment in
which two fluids are separately circulated
in adjacent spaces so that some of
the heat in the fluid at the higher temperature is transferred into the fluid
at the lower temperature. The fluid having the higher
temperature is therefore
cooled and the fluid having the lower temperature is
heated.
The
heat from a fluid can therefore be transferred to another colder fluid by means
of:
a.
Separately
circulating them in adjacent spaces.
b.
Mixing
them together.
Surface
heat transfer is defined as heat flowing from a “hot” fluid through a tube wall
or other membrane to another cooler fluid.
Contact
heat transfer is defined as heat flowing from a hot fluid to a cooler fluid due
to mixing of one with the other.
A heat exchanger is a device for
transferring thermal energy from one medium to another. It generally consists
of two channels or system of channels, one for each medium, and separated from
each other by partitions through which heat is transferred from the hot medium
to the cold medium.
Most
coolers used on board ship transfer heat from a hot fluid to sea water. For the
main propulsion engine of the motor ship, the engine jacket water, lubricating
oil and charge air must be cooled and generally also water or oil used in
cooling the pistons. In a steamship, apart from the heat yielded to the main
condenser, the turbine and gearbox lubricating oils provide the principal
sources of heat rejected to the circulating cooling water. Auxiliary prime
movers require cooling and air compressors have intercoolers and aftercoolers.
Steam
heated heat exchangers include heavy fuel oil heaters, boiler air pre-heaters,
units to heat sea water for tank washing, evaporators, feed heaters and
calorifiers.
Shipboard
heat exchangers may therefore be classified under the following categories:
COOLER
The
temperature of a hot liquid is lowered by transferring part of its heat to sea
water.
CONDENSER
Heat
from a hot gas or vapour is transferred to seawater to the extent that the gas
or vapour liquefies.
HEATER
The
temperature of a liquid or gas is raised by transferring heat energy from a hot
medium, usually steam.
EVAPORATOR
Heat
energy is transferred to a liquid until it evaporates (changes from liquid to
gas).
FLOW PATH IN A SHELL AND TUBE
HEAT EXCHANGER
The
flow path of the fluids are fixed by the division plates in the heat exchanger
heads for the cooling fluid and the internal baffles or tube support shets
within the body of the heat exchanger for warm fluid:
The
division plates in the heads or boxes fix the position of the inlet and outlet
branches for the fluid passing through the tubes.
The
internal baffles fix the position of the inlet and outlet branches for the
fluid passing through the body of the heat exchanger on the outside of the
tubes.
If
the heat exchanger heads do not contain any division plates, he fluid passing
through the tubes enters at one end of the heat exchanger and leaves at the
other. This arrangement is referred to as a single pass heat exchanger. If the
heat exchanger is a double pass type, a division plate is fitted in one head.
The inlet and outlet connections for the fluid passing through the tubes are
fitted on this head. The division plate prevents the fluid bypassing and causes
it to pass through half the tubes in the heat exchanger which is referred to as
the inlet bank. After the fluid passes through the inlet bank it enters the
other head which is just a bobbin piece and a cover. The direction of fluid
flow is reversed in this head or box and it passes back through the outlet bank
of tubes and leaves at the outlet branch. The fluid has passed through the
tubes in two different paths from which it gets the name two pass, or double
pass.
In
recent designs, the guided flow concept has been introduced, i.e. a secondary
cooling surface in the form of radial fins integral with the tubes between
which flow is guided radially, alternately out and in from section to section.
This gives better heat transfer surface and better heat transfer, lower metal
surface temperature and increase performance.
Baffles.
They have thermal significance because the shell-side fluid is made to flow to
and fro across the bundle from one end of the heat exchanger to the other.
Another importance of baffles is that they are spaced to provide support to the
tubes and prevent sagging of tubes and flow-induced vibration.
MATERIALS
Depending
on the size and duty of the cooler the cylindrical shell may be of fabricated
steel, cast iron or occasionally aluminium bronze. The tube plates are usually
of Naval brass and the tubes of aluminium brass; occasionally 70/30
copper-nickel tubes are used. The usual method of securing the tubes to the
tube plate is to roll-expand them. The cooler headers may be of cast iron. In
such instances the headers act as sacrificial anodes, wasting in preference to
the aluminum-brass tubes. Unless soft iron or mild steel sacrificial anodes or
impressed current cathode protection is used to protect the cooler from
corrosion such headers should not be painted internally.
Shell:
Gun metal (copper, tin, zinc ); or cast iron.
Tubes:
Aluminum brass (copper, zinc, aluminum) or cupro-nickel (copper, nickel)
Tube
plates: Cast naval brass ( copper, zinc, tin ).
Arrangements
for Linear Expansion of Tube Stack.
There
are three arrangements:
Ø
One
tube plate expands, shell and tubes fixed. In heat exchangers with cast bodies,
one tube plate is fastened to the flange on one end of the body. The other tube
plate is made to slide within the end of the body. Sealing is effected by
fitting O-rings in circumferentially cut grooves. The O-rings contact the
circular bored end in the body and the tube plate to make a seal. When there is
a difference in expansion between the tubes and the body the differences are
accommodated by the tube plate which slides relative to the body. With this
arrangement the tubes can only be arranged for single or double pass flow. This
arrangement is more commonly used for engine cooling water and lubricating oil
coolers. It also has the advantage of allowing very easy removal of the tube
stack from the exchanger body for chemical cleaning or repair. This is further
facilitated if the cooler is mounted vertically and the sliding tube plate is
at the lower end.
Ø
Shell
expands, tube plate and tubes fixed. In some types of heat exchangers the body
is made of mild steel plates, and in order to cater for the differential
expansion between the body and the tubes, a bellows ring is welded
circumferentially a round the body of the heat exchanger. Differences in
expansion are then catered for by the bellows ring that deforms slightly to
accommodate the changing length of the tubes. This type of expansion
arrangement can be used for any number of fluid passes through the tubes.The
tube plates are bolted directly onto the flanges of the body and the tubes may
be roller-expanded at both ends.
Ø
Tubes
expand, shell and tube plates fixed. A third arrangement is to fasten the tube
plates to the shell of the exchanger and roller-expand and bell-mouth the inlet
ends of the tubes. The outlet end of the tubes is fitted with a small stuffing
box in the tube late and sealing is made by using cotton cord packing tightened
by a threaded gland ferrule screwed into the threaded end of the stuffing
boxes. Differences in expansion between the tubes and the exchanger body is
accomodated by the tube ends sliding through the packing in the tube plate
gland. This is a common arrangement for exhaust steam condensers.
Shell and tube geometric terminology
|
1
|
Stationary (Front)
Head—Channel
|
20
|
Slip-on Backing Flange
|
|
2
|
Stationary (Front)
Head—Bonnet
|
21
|
Floating Tubesheet
Skirt
|
|
3
|
Stationary (Front) Head
Flange
|
22
|
Floating Tubesheet
Skirt
|
|
4
|
Channel Cover
|
23
|
Packing Box Flange
|
|
5
|
Stationary Head Nozzle
|
24
|
Packing
|
|
6
|
Stationary Tubesheet
|
25
|
Packing Follower Ring
|
|
7
|
Tubes
|
26
|
Lantern Ring
|
|
8
|
Shell
|
27
|
Tie Rods and Spacers
|
|
9
|
Shell Cover
|
28
|
Transverse Baffles or
Support Plates
|
|
10
|
Shell Flange—Stationary
Head End
|
29
|
Impingement Baffle or
Plate
|
|
11
|
Shell Flange—Rear Head
End
|
30
|
Longitudinal Baffle
|
|
12
|
Shell Nozzle
|
31
|
Pass Partition
|
|
13
|
Shell Cover Flange
|
32
|
Vent Connection
|
|
14
|
Expansion Joint
|
33
|
Drain Connection
|
|
15
|
Floating Tubesheet
|
34
|
Instrument Connection
|
|
16
|
Floating Head Cover
|
35
|
Support Saddle
|
|
17
|
Floating Head Flange
|
36
|
Lifting Lug
|
|
18
|
Floating Head Backing
Device
|
37
|
Support Bracket
|
|
19
|
Split Shear Ring
|
SEPARATION OF THE FLUIDS
Arrangements to prevent the two mediums from mixing with each
other consists of:
1.
Tubes
expanded to the plates.
2.
Gasket
between end box(header) and shell flange.
3.
O-
rings between shell and free end of tube plate.
ARRANGEMENT FOR INDICATING
LEAKAGE AT FREE END
Separation of the shell and tube fluids at
the floating head is obtained by means of packing rings installed between the outside
of the floating tube plate and recesses in the rear- head flanges. The shell
and tube side fluids each have their own packing rings, which are separated by
a lantern ring provided with weep holes for leak detection. Leakage at the
packing will not cause mixing of the shell and tube side fluids within the
exchanger itself.
The width of the machined bearing surface
at the flating tube plate periphery must accommodate the two packing rings, the
lantern ring and thermal movements of the bundle. In some cases the pressure of
thickness of floating tube plate is sufficient, but if not, the extra bearing
width is obtained by providing a machined skirt at the tube periphery. The
skirt may be an integral part of the plate or attached by welding.
PLATE TYPE HEAT EXCHANGER
The
basic plate consists of a thin, rectangular, metal sheet into which a
corrugated pattern has been formed by precision-pressing. One side of each
plate has a full peripheral gasket. The complete unit comprises a number of
such plates, mounted on a frame, and clamped together, face to face, by a
bolting system. The pace between adjacent plates forms a flow channel and the
system is arranged so hat the hot and cold fluids flow through alternate flow
channels, parallel to the long side of the plates. Openings(ports) in the four
corners of each plate direct the two fluids into their respective channels.
CONSTRUCTION
The most common metals use in plate type heat exchangers are titanium and
stainless steel Others include titanium-palladium alloy and monel metal Carbon
steel is rarely used because thin plates are required for pressing, which would
have little corrosion resistance. Titanium provides very high resistance to
corrosion by sea water.
PLATE CORRUGATION TYPES
A
wide range of corrugation types are available and the pattern and geometry are
proprietary. The most widely used corrugated types are the inter-mating or
washboard type, and the chevron or herringbone type. The plates are corrugated
for two basic reasons:
1) Corrugations
increase the effective area of the plate and promote turbulence. In he
washboard type, turbulence is promoted by a continuously changing flow
direction and velocity of the fluid. In the herringbone type adjacent plates
are assembled such that the flow channel imparts a swirling motion to the
fluid.
2) To
enable such thin plates to withstand differential pressures up to at least 10
bar, and in special cases up to 25 bar, the corrugations are essential for
strengthening and mutual support of the plates.
GASKETS
The
periphery of each plate is grooved to house a molded gasket, which are usually
cemented in; some are ‘snap-on’ type. The gasket is designed to provide a tight
joint without local distortion of the thin plate. The integrity of the complete
nit depends greatly on gasket performance.
Gasket
materials range from nitrile rubber( maximum temperature of 135°C) to Viton(175°C) and compressed asbestos fiber(260°C). Compressed
asbestos fiber gaskets, however, are not widely used.
FRAME
The
frame consists of a fixed head or fixed frame at one end, and a movable head or
pressure plate at the other. The vertical sides of both the fixed frame and
pressure plates are notched at intervals to engage tie bolts which join the
heads together. The pack of plates are compressed together as the tie bolts are
tightened at the movable head end. A horizontal carrier bar at the top of the
frame, and a horizontal guide bar at the bottom, are attached to the fixed head
plate at one end and an end support column at the other. Every heat transfer
plate is free to slide along both bars. The movable head plate is similarly
notched and free to slide along both bars. The
frame is normally constructed in carbon steel.
PORTS
The
fluids enter the unit via ports located in one or both end plates. If both
inlet and outlet ports for both fluids are located in the fixed head end, then
the unit may be opened up without disturbing the external piping. This applies
to single-pass arrangements, but with multi-pass arrangements the ports must
always be located on both heads. This means that the unit cannot be opened up
without disturbing the external piping at the movable-head end.
The
port entrance area is the weakest part of the plate because, unlike the
peripheral gasket, the gasket in an adjacent plate is not supported by a
corresponding one.
FLOW ARRANGEMENTS
Each
traverse of either fluid from top to bottom of the pack, or vice-versa, is
termed a pass. Usually, all passes have the same number of flow channels in
each pass, but this is not always the case. Single- or multi- pass flow is
possible.
LOOPED FLOW
A
single pass by both fluids provides counter current or co-current and two arrangements,
termed loop flow, are possible. In the preferred arrangement, all four ports
will be on the fixed head-plate, with the advantage that the pack can be opened
without disturbing any external piping.
ADVANTAGES OF PLATE COOLERS OVER TUBE COOLERS
1) Plate
coolers are smaller and lighter than a tube cooler giving the same performance.
2) No
extra space is needed for dismantling (a tube cooler requires enough clearance
at one end to remove the tube nest).
3) Their
higher efficiency is shown by the smaller size.
4) Plates
can be added in pairs to increase capacity and similarly damaged plates are
easily removed, if necessary without replacement.
5) Inspection
and cleaning is simple as is maintenance.
6) Turbulent
flow helps to reduce deposits which would interfere with heat flow.
7) With
titanium plates virtually no corrosion or erosion would occur.
DISADVANTAGES OF PLATE COOLERS
1) In comparison with tube coolers in which leakage
tubes are easily located and plugged, leaks in plates are sometimes difficult
to find because the plates cannot be pressurized and inspected with the same
ease as the tube coolers.
2) Deterioration
of joints are also a problem; they may be difficult to remove and there are
sometimes problems with bonded new joints.
Tube
coolers may be preferred for lubricating oil cooling because of the pressure
differential.
3) Cost
is another drawback; there are a large number of expensive joints on plate
coolers and the plates are expensive.
VARIOUS TYPES OF HEAT EXCHANGERS IN USE ON
SHIPS
1.
Charge
Air Coolers. These are finned-type tubular heat exchangers to provide for
better heat transfer, since air is a poor conductor of heat.These coolers are
normally rectangular in form.
2.
Electric
Machine Coolers. Some large alternators and electric motors are designed for
operation with closed ci5rcuit air cooling. Heat must be extracted from the
circulating air and, for this purpose, finned tube units are employed, inserted
into the air ducting. Sea-water flows through the tubes, while the air passes
over them, flowing between the fins. Spray baffles are usually fitted to these
units so that, in the event of puncture of one of the tubes, sea-water will not
low directly into the alternator or motor.
3.
Oil
Heaters. For burning heavy fuel oil in a boiler furnace, or a compression
ignition engine, it is necessary to pre-heat it. Steam is used as the heating
medium in fuel systems and lubrication systems. The type of heater used is
multi-tubular; the tubes are bent into a U- shape and fitted with expanded ends
into a single tube plate. The liquid head or box is fitted with a division
plate in its middle so that the oil flow takes a two-pass flow. The oil enters
the inlet branch of the liquid head, passes through one leg of the U, round the
bend and leaves by the other leg. The
division plate separates the inlet and outlet legs of the U-shaped heater
tubes. The inlet branch is always on the lower side. Tubes are sometimes fitted
with fins bonded to them(the oil flowing outside the tubes) and tubes fitted
internally to promote turbulence in the oil flowing through them.
AIR PRE-HEATERS
The combustion air required for forced-
draught boiler furnaces may be heated by the exhaust gas in order to improve
the overall efficiency of the whole cycle. This may be carried out in a
conventional heat exchanger, air passing outside the tubes and exhaust gas
inside them.
SEA-WATER HEATERS
These are usually the shell-and-tube type construction, with a fully
floating head. The sea water to be heated flows in several passes through the
tubes, being heated by the condensation of steam in one part of the tube bundle
and sub-cooling the condensate in another.
REFRIGERATION SYSTEM
Shell-and tube type condensers
are used, sea water circulating through
the tubes and refrigerant condensing in the shell. External fins to improve
heat transfer are sometimes used outside of the tubes.
CONDENSERS
A condenser is a vessel in which vapor is deprived of its latent heat of
vaporization and so changed to its liquid state, usually by cooling at constant
pressure. In a steam system, steam enters at an upper level, passes over rubes
in which cold water circulates, falls as water to the bottom and is removed by
a pump or flows to a feed tank. The tube arrangement provides for condensation
with minimum loss of heat: the cooling water normally circulates in two passes,
entering at the bottom.
CONTROL OF TEMPERATURE IN HEAT EXCHANGERS
There
are three basic ways by which the temperature of the hot fluid being cooled in
a heat exchanger may be controlled, when the cooling medium is sea water.
a)
By
by-passing a proportion of the hot fluid flow, the remainder being passed
through the heat exchanger.
b)
By
throttling the sea water flow, or, alternatively, by-passing a proportion of
it.
c)
By
controlling the temperature of the sea water entering the heat exchanger—this
is done in the sea water system as a whole, by spilling part of the heated
discharge back into the pump suction.
MAINTENANCE
OF HEAT EXCHANGERS
The
only attention that heat exchangers should require is to ensure that the heat
transfer surfaces remain substantially clean and the flow passages generally
clear of obstruction. Indication that undue fouling is occurring is given by a
progressive increase in the temperature difference between the two fluids, over
a period of time, usually accompanied by a noticeable rise in pressure loss at
a given flow.
Fouling
on the sea water side is the most usual cause of deterioration in performance.
The method of cleaning the sea water side surfaces depends upon the type of
heat exchanger. With the shell-and-tube type, the removal of the header covers
or, in the case of smaller heat exchangers, the headers themselves, will
provide access to the tubes. Obstructions, dirt, scale etc., can then be
removed, using the tools provided by the manufacturer. Flushing through with
fresh water is recommended before a heat exchanger is returned to service. In
some applications, such as piston oil cooling, progressive fouling may take
place on the outside of the tubes. Most manufacturers recommend a chemical
flushing process to remove this in- situ, without dismantling the heat
exchanger.
Plate
heat exchangers may be cleaned by unclamping the stack of plates and
mechanically cleaning the surface of each plate as recommended by the
manufacturers. The plate seals may require replacement from time to time and
here the manufacturers’ instructions should be closely followed.
Although maintenance is a rapid, simple operation, the heat transfer
plates must be handled with care.
Corrosion
by sea water may occasionally cause perforation of heat transfer surfaces. This
will cause leakage of one fluid into the other but this is not always easy to
detect whilst the leakage is small, although substantial leaks may become
evident through rapid loss of lubricating oil, jacket water etc.
Location
of a perforation is a straight forward matter in the case of a tubular heat
exchanger, whether this is of the shell-and-tube type or of other tubular
construction. Having drained the heat exchanger of sea water and removed the
covers or headers to expose the tube ends, some flow of the liquid on the other
side of the surface will be apparent, in the case of oil and water coolers,
from any tubes which are perforated. To test for leaks in air coolers drain
coolers etc. each tube in turn can be plugged at each end and pressurized with
air; inability to hold pressure indicates a leak.
To
aid the detection of leaks in a large cooler such as a main condenser, in which
it is difficult to get the tubes dry enough to witness any seepage, it is usual
to add a special fluorescent dye to the shell side of the cooler. When a
ultra-violet light is shone on to the tubes and tube plates any seepage is seen
since the dye glows with a vivid green light. In plate heat exchangers, the
only way to locate leaks is by visual inspection of the plate surfaces.
On
docking for any protracted period, such as for repairs, refitting etc. it is
advisable to drain the sea water side of heat exchangers, clean and flush
through with fresh water, after which the heat exchanger should be left
drained, if possible until the ship re-enters service.
