Selection and Design of Condensers

GBH Enterprises, Ltd.

Process Engineering Guide:
GBHE-PEG-HEA-508

Selection and Design of Condensers

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Process Engineering Guide:

Selection and Design of
Condensers

CONTENTS

SECTION

0

INTRODUCTION/PURPOSE

3

1

SCOPE

3

2

FIELD OF APPLICATION

3

3

DEFINITIONS

3

4

CHOICE OF COOLANT

4

5

LAYOUT CONSIDERATIONS

5

5.1
5.2

Distillation Column Condensers
Other Process Condensers

5
5

6

CONTROL

5

6.1
6.2
6.3

Distillation Columns
Water Cooled Condensers
Refrigerant Condensers

5
5
6

7

GENERAL DESIGN CONSIDERATIONS

6

7.1
7.2
7.3
7.4
7.5

Heat Transfer Resistances
Pressure Drop
Handling of Inerts
Vapor Inlet Design
Drainage of Condensate

6
7
7
8
8

8

SUMMARY OF TYPES AVAILABLE

9


8.1
8.2
8.3
8.4
8.5
8.6
8.7
8.8

Direct Contact Condensers
Shell and Tube Exchangers
Air Cooled Heat Exchangers
Spiral Plate Heat Exchangers
Internal Condensers
Plate Heat Exchangers
Plate-Fin Heat Exchangers
Other Compact Designs

9
11
15
15
16
16
16
16

9

BIBLIOGRAPHY

17

FIGURES

1

DIRECT CONTACT CONDENSER WITH INDIRECT COOLER
FOR RECYCLED CONDENSATE

9

2

SPRAY CONDENSER

10

3

TRAY TYPE CONDENSER

10

4

THREE PASS TUBE SIDE CONDENSER WITH INTERPASS
LUTING FOR CONDENSATE DRAINAGE

12

CROSS FLOW CONDENSER WITH SINGLE PASS COOLANT

13

5

DOCUMENTS REFERRED TO IN THIS PROCESS
ENGINEERING GUIDE

18


0

INTRODUCTION/PURPOSE

This Process Engineering Guide is one of a series on heat transfer prepared for
GBH Enterprises.

1

SCOPE

This Guide is designed as an aid to the selection of condensers for process
duties. It describes the various factors which influence the choice of exchanger,
giving some of the options and detailing their merits and draw-backs. For more
general information on selection of heat exchanger type see GBHE-PEG-HEA506.
The Guide also gives advice on design methods for condensers. It does not
attempt to give detailed design procedures, most of which are in any case
performed by computer programs, but points the reader to sources of
information. It does give advice on many of the additional design features which
are not covered by the programs. General recommendations on computer
programs for exchanger design are given in GBHE-PEG-HEA-502.

2

FIELD OF APPLICATION

This Guide is intended for process engineers and plant operating personnel in
GBH Enterprises world-wide, who may be involved in the specification or design
of condensers.

3

DEFINITIONS

For the purposes of this Guide, the following definitions apply:
HTFS

Heat Transfer and Fluid Flow Service. A co-operative research
organization, in the UK, involved in research into the fundamentals
of heat transfer and two phase flow and the production of design
guides and computer programs for the design of industrial heat
exchange equipment.


HTRI

Heat Transfer Research incorporated. A co-operative research
organization, based in the USA, involved in research into heat
transfer in industrial sized equipment, and the production of design
guides and computer programs for the design of such equipment.

TEMA

The Tubular Exchanger Manufacturers’ Association. An
organization of US manufacturers of shell and tube exchangers.
Their publication ‘Standards of the Tubular Exchanger
Manufacturers Association’ is widely accepted as the basis
specification of heat exchangers

4

CHOICE OF COOLANT

The choice of coolant is obviously Influenced by the condensing temperature of
the process stream. In the case of multi-component systems, which condense
over a temperature range, the key temperature is the final temperature to which
the product is to be cooled.
For temperatures above ambient the choice is usually between air and cooling
tower water. GBHE-PEG-HEA-513 discusses the relative merits of air and water
cooling. This indicates that water is likely to be more economic if the condensing
temperature is less than 20°C above the design air temperature, and air if it is
more than 30°C above. These figures are ‘rules of thumb’, and may be
influenced by layout considerations. See Clause 5.
For condensing temperatures less than 5 - 10°C above the supply temperature of
the cooling water the size of a water cooled exchanger may become excessive,
and it will be economical to use some form of refrigeration. The cold service fluid
used in the process exchangers may be either a single phase liquid or a boiling
liquid.
For single phase liquid coolants, chilled water can be provided at down to 2 5°C; below that, either a brine such as 25% CaCl2, or an organic liquid such as
trichloroethylene or kerosene is used.
With boiling liquid coolants, the working fluid of the refrigeration plant is generally
used directly as the coolant, the process exchanger becoming the evaporator of
the refrigeration cycle, and the vapor being returned to the suction of the
compressor. Typical fluids are halocarbons, such as R22 or KLEA 134A, or
ammonia. On some plants, e.g. olefins, process fluids such as ethylene may be
used as the boiling refrigerant.

For condensers for fluids with a wide condensing range, cooling to low
temperatures, it may prove more economic to perform the condensation in two
stages, taking out most of the heat load in an air or water cooled unit, and
following it with a refrigerated condenser for the final cooling. This will reduce the
load on the refrigeration system, which is an expensive form of cooling.
A combination of air cooling followed by water cooling is sometimes used for
wide temperature ranges above ambient. However, GBHE-PEG-HEA-513
suggests that this is often not economic.
In recent years there has been an increased tendency to heat integration on
plants. If a suitable process stream is available, it can be used as the coolant.
However, it should be remembered that the control requirements of the plant may
impose a fluctuating demand on the coolant, which may influence the operability
of the total exchanger network.
A special case of heat integration in distillation columns is mechanical vapor
re-compression, where the overhead vapor is compressed and condensed at a
higher temperature to provide the heat input to the column reboiler.

5

LAYOUT CONSIDERATIONS

5.1

Distillation Column Condensers

One of the major factors influencing the layout, and sometimes the choice, of the
condenser for a distillation column is whether the reflux liquid is to return to the
column under gravity, or a pumped reflux system is to be used. In the former
case, the condenser has to be mounted above the top plate of a trayed column
or the distributor of a packed column. The use of a gravity reflux return may also
impose a lower design pressure drop for the condenser than is necessary for a
pumped system.
With a pumped reflux system, the designer has more flexibility. However, the
potential savings in mounting the condenser near grade level must be offset
against need for a pump and the increased cost of vapor and liquid pipework to
link the condenser with the top of the column. Moreover, a pumped reflux system
usually requires a reflux drum; for a gravity return system, particularly if the top
product is to be removed as vapor, it may be possible to dispense with this.


With a free standing column, a pumped reflux system has to be used unless
either the column is located close to a structure as high as the column on which
the condenser can be mounted, or the condenser is supported directly from the
column. The latter favors either an internal condenser or a reflux condenser
(dephlegmator), where the condensate flows counter-current to the vapors. See
GBHE-PEG-HEA-516 for a discussion of reflux condensation.
Air cooled exchangers require a relatively large plot area. Because of this, it may
be difficult to find a suitable location for one which would allow gravity
condensate return.

5.2

Other Process Condensers

For most other plant condenser duties, layout does not have a major influence on
the choice of condenser: the structure is usually present to support any design at
a suitable elevation.

6

CONTROL

6.1

Distillation Columns
The operation of a distillation column is usually coupled to the column
pressure control. There are many ways in which this can be done. The
choice of method depends amongst other factors on whether the vapor is
totally condensed, or the top product leaves as a vapor. Some of the
methods can be applied to any type of condenser, but others are specific
to certain designs. Reference [1] and GBHE-PEG-MAS-608 give good
reviews of alternative methods, discussing their advantages and
drawbacks.

6.2

Water Cooled Condensers
It is often necessary to control the heat load on a condenser, for example
if it is a partial condenser. With a single phase coolant the obvious way to
do this is to throttle back the supply of coolant. However, if the coolant
side is prone to fouling which is velocity dependent, this can cause
problems. Cooling water is particularly bad in this respect; low
velocities may lead to both high fouling and corrosion of metals.


It is preferable to control the heat load either by bypassing the hot fluid or
by recycling the cooling water round the exchanger, thus maintaining an
optimum velocity. See GBHE-PEG-HEA-511 for more details.
6.3

Refrigerant Condensers

When using a boiling refrigerant as coolant, it is usually necessary to ensure that
complete evaporation of the liquid refrigerant takes place, to avoid returning
liquid to the compressor suction. This requires control of the refrigerant feed rate
to match the varying plant demands. Partly because of this, most process
refrigerant cooled exchangers are designed as shell and tube exchangers with
shell side boiling, usually as kettle boilers, as this allows easy control of the liquid
level and disengagement of the vapor. Control of heat load can be effected by
varying the pressure, and hence evaporation temperature, of the refrigerant.

7

GENERAL DESIGN CONSIDERATIONS

7.1

Heat Transfer Resistances

The heat transfer resistance on the condensing side of an exchanger is made up
from two parts (apart from fouling): the resistance of the condensate film and the
resistance of the vapor film between the bulk vapor and the vapor-condensate
interface. For a single component, there is no vapor film resistance, but for multicomponent systems, including single condensables with inerts, it can be
substantial.
The value of the condensate film resistance depends on the geometry of the
condensing surface, and on whether the process is dominated by gravity or
vapor shear. In the absence of vapor shear, on vertical surfaces the condensate
film resistance initially rises with increased condensate loading, and then falls
again. On the outside of horizontal tubes, or inside horizontal passages, the
resistance generally rises with increased loading. Vapor shear effects reduce the
resistance, unless the vapor is flowing counter-current to the condensate, when
they may increase it. Further information on the condensate film
resistance can be found in References [2], [3] and [4]


The vapor phase resistance for multi-component condensation arises from the
difference in composition between the bulk vapor and the interface, and is
complicated by mass transfer effects. The full analysis for other than binary
systems is extremely complex; even for binary mixtures the calculations are
tedious. It is therefore usual for engineering calculations to use an approximate
method known as either the Silver or Bell and Ghally method, which is given in
Reference [5] In its simplest form, this relates the local vapor film heat transfer
coefficient α f to the heat transfer coefficient that the gas/vapor phase
would have at that point in the exchanger in the absence of liquid film and
condensation α g by the equation:

Later workers refined this approach, introducing additional correction terms.
Details of the modified method as used in the HTFS computer programs may be
found in Reference [6].

It can be seen from the above that high condensing side coefficients are favored
by high vapor velocities, both to improve the condensate film coefficient and to
give a good gas phase coefficient. Unfortunately, pressure drop also rises with
vapor velocity, limiting the velocity that can be used. During the course of
condensation, the vapor flow falls, causing a reduction in coefficients. This can
be particularly important in the latter stages of a condensation with small
quantities of non-condensables present, where very poor coefficients can result.
These effects can be reduced in some designs of condenser by reducing the flow
area as the condensation proceeds. For example, the number of channels per
pass in a multi-pass exchanger can be reduced for the later passes. For a
shell and tube exchanger with condensing on the shell side, the baffle pitch can
be decreased at the cold end of the exchanger. Where it is not practical to
reduce the flow area, it may be worth considering dividing the duty into two, with
a small vent condenser following the main unit.


Although experience indicates that the Silver/Bell and Ghally method, as used in
most computer programs, generally results in a reasonable estimate of the heat
duty of the exchanger, it cannot give accurate information on non-equilibrium
effects. In practice, the exit condensate is generally at a lower temperature than
the vapor, and the vapor may be super- or sub-saturated with respect to one or
more components. This can be particularly important when the vent from a
condenser is exiting to the environment. Unfortunately, suitable reliable programs
for calculating these cases are not available and hand calculations are difficult. If
this is important, a heat transfer specialist should be consulted.

7.2

Pressure Drop

A high pressure drop is usually undesirable in a condenser; the condensing
temperature falls with reducing pressure, lowering the temperature driving force
for the condensation. Moreover, for a given flowrate the pressure drop rises as
the pressure falls. This can be particularly critical for vacuum condensers. As the
high velocities which tend to give high pressure drops also result in good heat
transfer, some compromise may be necessary, but particular attention should be
given to minimizing the parasitic pressure losses which occur in regions away
from the heat transfer surface, such as the nozzle pressure drops.
The acceleration component of the pressure gradient in a condenser is positive,
due to momentum recovery. The maximum accelerational pressure rise is equal
to two inlet velocity heads. Depending on the magnitude of the frictional terms,
the static pressure in a condenser can actually rise as the condensation
proceeds.

7.3

Handling of lnerts

Many process fluids, even when nominally totally condensable, contain trace
quantities of non-condensable gases, often referred to as ‘inerts’. Unless some
care is taken at the design stage these may build up in the exchanger and, by
lowering the local dew point, result in significant loss of performance. lnerts can
generally be removed from an exchanger by a suitable purge, either continuous
or intermittent, depending on the inlet concentration.


Bell in Reference [7] claims that more than half of all the operational troubles with
condensers are caused by poor venting. He states the four principles of
condenser venting as:
(a)

All vapors to be condensed contain non-condensable gases

(b)

Condensers must be constructed positively to guide the non-condensable
gases to a convenient. identifiable point or region in the condenser.

(c)

The vent must be located in that region.

(d)

A vent condenser may be required.

7.4

Vapor Inlet Design

Because the volume flow rate decreases as the condensation proceeds, it is
economical to employ a high vapor velocity at the inlet. However, this may
present problems:
(a)

High velocities may result in excessive pressure drop.

(b)

There may be erosion damage due to impingement of liquid droplets in the
incoming vapor. An impingement plate is ALWAYS required for the inlet
nozzle of a shell and tube condenser with shell side condensation unless it
can be guaranteed that the vapor is always significantly superheated, say
by 50°C or more. Even then, a plate will be necessary if the product of the
fluid density and the square of the nozzle velocity exceeds the TEMA
recommended maximum of 2200 kg/m.s2.

(c)

Vibration damage may occur, particularly with shell and tube exchangers.
For shell and tube exchangers, a vibration analysis should always be
performed using either the methods within the computer programs or a
suitable hand method such as Reference [8].

(d)

Mal-distribution may occur. This problem is particularly important for
condensers with a large number of parallel flow channels, such as shell
and tube exchangers with tube side condensation. It arises if the jet of
vapor issuing from the inlet nozzle has a momentum that is high relative to
the pressure drop in the tubes. If the nozzle is parallel to the tubes, the
flowrate in the tubes immediately opposite the nozzle can be well above
the mean flowrate, giving severe mal-distribution.


This can result in accumulation of non-condensable gases in the tubes
subjected to lower flow rates, resulting in severe downgrading of the
condenser performance. To avoid severe mal-distribution, the total
pressure drop across the tubes should be at least five times the inlet
nozzle pressure drop. Mal-distribution can be reduced by fitting restriction
devices into each tube inlet, by the use of a perforated deflector plate in
the inlet header, by increasing the size of the inlet nozzle or by using a
radial nozzle.

7.5

Drainage of Condensate

If condensate and uncondensed vapors and gases are to be withdrawn from the
condenser through a single nozzle, this must be adequately sized for the flow,
remembering that two phase pressure drops are considerably greater than single
phase ones. It is more usual to arrange for vapor-liquid disengagement within the
exchanger, with separate outlet nozzles for the two phases. If liquid carry-over
with the vapors is undesirable, a demister before the vapor outlet, or a separate
separator may be necessary.
The liquid outlet should be designed to avoid carry under of the vapors, using the
recommendations in GBHE-PEG-FLO-301 for self-venting flow. A vortex breaker
is recommended; for shell side condensation in a shell and tube exchanger this is
not necessary provided that the bottom tubes are close to the outlet, but is
required if tubes have been removed in this region to aid condensate drainage.

8

SUMMARY OF TYPES AVAILABLE

8.1

Direct Contact Condensers

Direct contact condensers are the simplest and cheapest form of condenser.
They are also less prone than other types to problems associated with dirty or
corrosive fluids. However, they are only suitable where there is no objection to
mixing the process fluid and the coolant.
They are mainly used in vacuum or low pressure applications, particularly for
condensing steam; a typical example is for the final condenser in a multi-effect
evaporation train in the production of salt. For vacuum duties, it is usual to mount
the condenser at a sufficient elevation to enable the coolant and condensate to
flow by gravity through a ‘barometric leg’, thus obviating the need for a pump.

Another area of use for direct contact condensers is as a quench condenser for a
hot corrosive gas. Although principally used for condensing steam, organic
vapors can also be condensed. If water is used as the coolant and the organic
material is immiscible with water, a phase separator will be necessary. Often,
however, the product itself is used as the coolant, the mixture of freshly
condensed material and recycled cold product being passed through a separate
single phase cooler and a portion returned to the condenser. See Figure 1. This
may enable a large surface condenser to be replaced by a simple direct
condenser and a small cooler.
FIGURE 1

DIRECT CONTACT CONDENSER WITH INDIRECT
COOLER FOR

The most common form of direct contact condenser is the spray chamber shown
in Figure 2. The key to this design is to produce fine sprays which are positioned
so that the liquid is well distributed. This produces a large surface area for heat
and mass transfer. The liquid pressure must be considerably above that of the
vapor to ensure a good spray. However, too fine a spray results in excessive
liquid carryover. Design methods for spray condensers are given in References
[9] and [10].


FIGURE 2

SPRAY CONDENSER

The spray condenser has the disadvantage that the nozzles can block if using a
dirty coolant such as river water. The tray type of direct condenser shown in
Figure 3 does not have this limitation. Unfortunately, it does not produce a very
high interfacial area. References [9] and [10] can be used for design of this type
also.


FIGURE 3

TRAY TYPE CONDENSER

Reference [11] describes a design of direct contact condenser in which the
interfacial area is created by feeding the liquid over a set of V-notch weirs
mounted at the top of the chamber. This design probably produces an interfacial
area between that of the tray and spray designs.
A packed column gives the highest thermal efficiency of all types of direct contact
condenser, with evenly distributed counter-current flow of vapor and liquid, a high
interfacial area and a good heat transfer coefficient. However, it is more
expensive than the other designs, and gives a larger pressure drop.
The simplest method of contacting a high pressure vapor with a liquid is to
bubble it through the liquid in some sort of sparge device. The major drawbacks
of such devices are the noise and possible cavitation damage associated with
collapse of the vapor bubbles. In can be particularly difficult to design equipment
to operate reasonably over a wide turndown range. Reference [12] gives some
useful advice on design of sparging systems for condensable vapors or very
soluble gases.

8.2

Shell and Tube Exchangers

With shell and tube condensers, there is a choice between shell side or tube side
condensation. Tube side condensation is favored when the condensing fluid is at
high pressure, is corrosive or prone to cause fouling. Shell side condensation is
generally used for low pressures, and almost always for vacuum duties. There
are two basic types of shell side condenser, cross-flow condensers and baffled
exchangers where the overall flow direction is along the shell.
Shell and tube condensers can be rated using commercially available computer
programs.
8.2.1 Tube Side Condensation
Vertical tube side condensers are almost always designed as single pass
units, usually with co-current downwards flow of condensate and vapor. It
is feasible to design a two pass condenser with vertical up-flow in the first
pass and down-flow in the second, provided that it can be guaranteed that
the vapor velocity at the top of the first pass is sufficient to carry the
condensate upwards, but this is not recommended. Some condensers are
built in which substantially all the condensation takes place in a down-flow
pass with condensate removal from the base. The non-condensables then
flow back up a second pass having far fewer tubes to effect a final cooling.
This second pass operates with a counter-current flow of condensate and
vapor. The design of such units cannot be performed directly with
computer programs. See GBHE-PEG-HEA-516 for a discussion of reflux
condensers (dephlegmators) in which the condensate and vapors flow
counter-current.
For single pass down flow, the cheapest designs are generally obtained
by using the maximum allowable tube length, subject to pressure drop
constraints. Use of shorter tubes may result in a substantial increase in
required heat transfer surface, particularly for condensation with inerts.
Horizontal tube side condensers give the designer more flexibility in
principle, as multi-pass units are possible. However, except for two pass
U-tube designs, problems can arise in distribution of the two phase
mixture after the first pass. Computer programs for the rating of
condensers generally assume a uniform distribution of liquid and vapor,
but in practice, most of the liquid will flow in the lower tubes of the second
and subsequent passes. For a multi-component mixture, this means that
the vapor in the upper tubes will contain less of the heavier components,
and so be more difficult to condense.

There are also problems with assessing the pressure drop, as different
tubes will have different flow rates and vapor fractions.
Reference [13] recommends the removal of condensate from the end of each
pass in a multi-pass exchanger, with suitable luting where the individual
condensate streams are combined to prevent vapor bypassing. (See Figure 4).
Although this does get round the problem of phase distribution, because the
composition and flowrate will vary from pass to pass, each pass will have to be
modeled separately; conventional computer programs are not suitable directly for
this task, although they can assist, given some skill on the part of the designer. If
this approach is used, there is a possibility that the mixed stream from the
different passes will flash when the mixing takes place. A check on the vaporliquid equilibrium of the mixture should be made.
FIGURE 4

THREE PASS TUBE SIDE CONDENSER WITH INTERPASS
LUTING FOR CONDENSATE DRAINAGE

A multi-pass design allows the number of tubes per pass to be reduced in later
passes, to maintain the vapor velocity, and hence heat transfer coefficient.
However, none of the computer programs available is able directly to model
exchangers with differing numbers of tubes per pass, so some ingenuity is
necessary on the part of the designer.


8.2.2 Cross-flow Shell Side Condensers (TEMA X Shell Type)
This type of condenser is used for condensing large volume flowrate vapors
containing only small quantities of incondensable gases where only a low
pressure drop can be tolerated, particularly in vacuum duties. The most common
application is steam turbine exhaust condensers in the power industry.
The main problems in the design are:
(a)

Avoiding a very high cross flow velocity in the inlet region, which could
lead to tube damage from erosion or vibration. This is usually done by
providing multiple inlet nozzles to spread the vapor along the shell.
Suitable impingement devices should be provided for each nozzle. There
should be an adequate number of support plates along the shell to prevent
serious tube vibration. These must be shaped to support all the tubes, but
cut clear of the upper and lower spaces where there are no tubes, to allow
longitudinal flow of vapor and condensate.

(b)

Minimizing the pressure drop, to avoid loss of temperature difference.

(c)

Avoiding stagnant regions where inerts can accumulate. There should be
a positive flow of vapor towards the vent region, which should be
maintained as cold as possible to reduce the condensable components as
far as possible. In some designs, particularly with a single pass coolant,
inclined longitudinal baffles running the full length of the shell are used in
the region near the vents, as shown in Figure 5. Because of the high
concentration of incondensable gases in the region of the vent, the heat
flux to these tubes will be low, so the coolant in these tubes will not heat
up much, thus reducing the temperature of the vent stream.


FIGURE 5

CROSS FLOW CONDENSER WITH SINGLE PASS COOLANT

8.2.3 Baffled Shell Side Condensers
The most common type of condenser in the process industries is the horizontal
shell side condenser. Vertical shell side condensation is normally limited to the
heating of vertical boilers etc., usually with condensing steam. For process
duties, a nominally horizontal design is more usual, although the exchanger is
generally inclined at a small angle, say 5°, to assist drainage.
The baffles should have a vertical cut, and have a triangular notch in the bottom
to assist in the drainage of condensate. This should have a minimum size of 20
mm for clean fluids and 40 mm for dirty fluids. At the cold end of the shell, the
baffle spacing can be reduced to maintain an adequate vapor velocity.
Note:
Commercially available programs can presently model variable baffle pitch.
An impingement plate in the shell opposite the inlet nozzle just above the first
row of tubes is mandatory unless using a vapor belt inlet. With small nozzles, this
should be slightly bigger than the nozzle; with large nozzles it should extend to
the shell to avoid a high velocity stream impacting the outer tubes.


Tubes are removed from the top of the shell (assuming a top vapor inlet) to avoid
excessive pressure drop of the incoming fluid and to reduce the velocities off the
impingement plate. Tubes may be removed from the bottom of the shell to assist
condensate drainage. The spaces left by the removal of tubes and the pass
partition lanes for multi-pass designs represent by-pass routes for the vapor.
Sealing devices or longitudinal baffles may be necessary to reduce by-passing.
A vent should be provided on the top of the shell at the cold end, opposite the
liquid nozzle. The liquid nozzle should be sized for self venting flow as in GBHEPEG-FLO-301, with a vortex breaker unless the tubes extend to close to the
liquid nozzle. The exchanger should be mounted to give a slight downwards
slope to the cold end, to assist in drainage.
Most process shell and tube exchangers are of TEMA E-type, with a single side
pass. For large volumes of vapors or low pressure drops, it may be preferable to
use a split flow arrangement (TEMA J-shell). See GBHE-PEG-HEA-506 for more
information on the TEMA designations.

8.2.4 Extended Surfaces
If the heat transfer resistance on the condensing side is dominant, it may be
worth considering the use of extended surfaces. The most common form of this
is the use of low-finned tubing. This is produced by rolling from a thick walled
base tube to give fin heights typically around 1 mm and frequencies from 400 1500 fins per meter. The ends of the tubes are left plain for joining to the tubesheets, and sometimes a plain section is left at each baffle position. The tubes
come in standard fin sizes and frequencies; consult manufacturers’ brochures for
exact dimensions. The finned tubing typically has an outside area from 1.5 to 4
times that of the plain tube.
Finned tubing is normally used in horizontal exchangers, as this gives good
condensate drainage. The fins provide additional surface, and also may give
some direct increase In local coefficient. This is evidenced by the fact that the
enhancement in the condensing heat transfer coefficient, based on the plain tube
area, may be even greater than the increase in area, but generally it is less, due
to flooding of the lower portion of each tube by condensate. This flooding is the
result of hold-up of condensate by capillary forces. There is a theoretical
optimum fin spacing for a given fluid which will result in the smallest
exchanger. The optimum depends on the surface tension of the fluid. Finned
tubes are likely to be of most benefit for low surface tension fluids which have
less tendency to flood.

Low-fin tubing is more expensive per meter than plain tubing. However, its use
will result in a smaller exchanger. Whether there is an overall saving can only be
determined by performing designs for both plain and enhanced tubes.

8.2.5 Subcooling
It is often desired to subcool the condensate from a condenser. The condensate
from a vertical tube side exchanger is usually subcooled to some extent, and
designs can be performed to give reasonable levels of subcooling.
Note:
Caution: some commercially available programs severely underestimates the
performance in the subcooling zone, as their model is based on full bore pipe
flow rather than the film flow which usually occurs.
In horizontal tube side condensers, subcooling can be achieved by arranging for
the last pass to run full of condensate. It may be advantageous to reduce the
number of tubes in this pass to give a reasonable velocity.
Note:
The computer programs cannot directly model exchangers with different numbers
of tubes per pass.
Subcooling can be done in shell side condensation by arranging for the
condensate to maintain a level in the shell above the bottom few rows of tubes.
However, it must be remembered that this is not a very efficient way of
performing the subcooling. The liquid velocities in the bottom of the shell are low,
so that cooling is performed mainly by natural convection. It is not possible to
predict with any certainty what level of subcooling will be achieved, and the
calculations of subcooling in the thermal rating programs are based on a
physical model of the system which does not correspond to reality. If shell side
subcooling is used, it will usually be necessary to arrange for level control of the
condensate in the shell. Any level control device should preferably be adjustable.
Designs have been produced in which the lower’ part of the shell is separated
from the condensing region by a horizontal baffle, open to the upper part in the
last baffle space from the vapor inlet end. Condensate is then directed back
along the shell below this baffle. Extra cross baffles are used to give a good flow
pattern and hence heat transfer to subcool the condensate. This type of design is
frequently used for the design of boiler feed water heaters, and has been used
for process condensers.

There are no computer programs that we are aware of suitable for modeling this
design except for the special case of steam condensation. If it is essential to
guarantee a given level of subcooling, it is preferable to perform this part of the
heat duty in a separate exchanger designed for the purpose.
Vertical shell side condensers, such as steam heated vaporizers, are sometimes
designed to have a variable level of condensate as a means of controlling the
heat duty (see GBHE-PEG-HEA-515). This will lead to some subcooling of the
condensate. Again, it is not possible to predict this with great accuracy.

8.3 Air Cooled Heat Exchangers
Consult GBHE-PEG-HEA-513 for full information on air cooled heat exchangers,
including their use as condensers.
Note:
Commercially available programs: air cooled exchanger rating subroutines
allows for different numbers of tubes per pass. It is normal design practice to
have fewer tubes in the subcooling pass(es) than in the condensing pass(es).

8.4 Spiral Plate Heat Exchangers
There are several forms of exchanger based on the spiral plate concept. All of
these are suitable for condensation duties, but have different merits.
The first of these, referred to as Type I by Alfa Laval, one of the principal
manufacturers of spiral plate exchangers, has spiral flow on both sides. This
results in almost pure countercurrent flow, and is thus suitable for duties which
require a long flow path, such as wide condensing range mixtures.
The Alfa Lava1 Type II has spiral flow on the coolant side, but the condensate
flows in cross flow, with a relatively short flow path. It is normally used where
there is a very large volume of vapor to be handled with low pressure drop. It will
be better suited to single component systems than wide condensing range
mixtures. It can also be mounted directly onto a distillation column to act as a
dephlegmator. See GBHE-PEG-HEA-516 for more details.
The Alfa Lava1 Type III offers on the process side a combination of cross flow for
the initial bulk condensation followed by counter-current spiral flow to subcool the
condensate and non-condensables.

The Alfa Lava1 Type G is designed specifically as a condenser for mounting
directly onto distillation columns or reactors. Unlike the Type II, it is not a
dephlegmator. Instead, the incoming vapor is directed up a central tube and can
then be allowed to flow back down the spiral, in cross flow to the coolant and cocurrent with the condensate, outward in spiral flow counter-current to the coolant
as in Type I or a combination as in Type III.
As explained in GBHE-PEG-HEA-506, spiral plate exchangers are considered
proprietary items, and are designed by the manufacturers.

8.5

Internal Condensers

Rather than provide a separate heat exchanger, it is possible to mount a cooling
bundle inside the top of a distillation column. In large diameter columns this can
be in the form of a horizontal tube bundle. For smaller columns a vertical bundle
is used, generally in the form of a set of U tubes. The latter is a more common
arrangement. This design acts as a dephlegmator. The design of such a unit is
discussed in GBHE-PEG-HEA-516.
In some cases, to avoid the problems with dephlegmators such as flooding,
internal baffling is used to direct the vapor past the bundle and back in
downwards flow.

8.6

Plate Heat Exchangers

Experience of the use of plate heat exchangers for condensing duties, other than
using steam for heating, is limited. Brazed plate exchangers are used as
condensers in some small refrigeration units. The relatively high pressure drop
usually associated with plate exchangers will reduce their applicability. However,
some of the plate manufacturers are pursuing a policy of extending the use of
plate exchangers into new areas, developing modified forms of plate where
necessary, so this situation could change.


8.7

Plate-Fin Heat Exchangers

Plate-fin exchangers are well established as condensers in the cryogenics
industry, but the restriction until recently to aluminium as a material of
construction has limited their applications elsewhere. With the development of
the large brazed stainless steel units from various manufacturers, recently, and
the continuing work by others to produce an all stainless exchanger, other outlets
should be considered.

8.8

Other Compact Designs

Experience in the use of other compact exchangers in condensing duties is at
present very limited. As condensing duties are usually non-fouling, the small
passage size of many compact designs should not be a limitation, although
fouling from the coolant side, especially cooling water could be a problem.


DOCUMENTS REFERRED TO IN THIS PROCESS ENGINEERING GUIDE
This Process Engineering Guide makes reference to the following documents:
PROCESS ENGINEERING GUIDES
GBHE-PEG-FLO-301

Overflows and Gravity Drainage Systems
(referred to in 7.5 and 8.2.3)

GBHE-PEG-HEA-502

Computer Programs for the Thermal Design of Heat
Exchangers (referred to in Clause 1)

GBHE-PEG-HEA-506

Selection of Heat Exchanger Type
(referred to in Clause 1, 8.2.3 and 8.4)

GBHE-PEG-HEA-511

Shell and Tube Heat Exchangers Using Cooling
Water (referred to in 6.2)

GBHE-PEG-HEA-513

Air Cooled Heat Exchanger Design
(referred to in Clause 4 and 8.3)

GBHE-PEG-HEA-515

The Design and Layout of Vertical Thermosyphon
Reboilers (referred to in 8.2.5)

GBHE-PEG-HEA-516

Refluxing Condensation Systems (Dephlegmators)
(referred to in 5.1, 8.2.1, 8.4 and 8.5)

GBHE-PEG-MAS-608

Control of Continuous Distillation Columns (referred to
in 6.1)


Selection and Design of Condensers

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