In the last article we completed our
review of BPHE’s. This month we move on to
direct expansion water to refrigerant
evaporators. These are often referred to as shell & tube evaporators or chillers.
The primary purpose of shell and tube chillers is
that of heat exchange, in vessel form, to cool a
closed circuit, recirculating fluid flow, using
refrigerant as the cooling medium. The
thermodynamic process was shown in Part 3 of
this series in the P-E diagram.
Shell and tube chillers are extremely efficient.
Moreover, they are very compact, requiring only a
small footprint and overall height. Maintenance,
an important consideration in costs terms, is also
The concept of the shell and tube chiller is
based on a large number of tubes formed into
what is known as a tube bundle. Refrigerant
flowing from the expansion device is passed into
the tubes and progressively evaporates thereby
producing a cooling effect through the latent heat
The tube bundle is mounted within a steel shell
and end caps are fitted to both ends of the shell.
Water is passed over the tubes and gives up heat
energy to the surface of the tubes at lower
temperature. The water therefore leaves the shell
and tube chiller several degrees lower than the
entering water temperature. This is known as a
dry type or direct expansion evaporator. The
water enters the side of the shell at one end and
leaves the side of the shell at the other. Shell and
tube chillers are supplied with screwed or flanged
water connections. A drain connection is normally
incorporated to allow the water in the shell to be
A typical arrangement of a dry type or direct
expansion shell and tube chiller is shown in Fig 1.
It is a more common arrangement for the
refrigerant inlet and outlet to be at the same end
as shown in the cutaway illustration in Fig 2.
These are normally used with positive
displacement compressors such as reciprocating,
rotary or screw machines.
Figure 1: A typical arrangement of a dry type or direct expansion shell and tube chiller
There is also a type of shell and tube chiller where
the water runs through the tubes and the
refrigerant flows over the outside of the tubes
within a closed shell. This is known as a hooded
arrangement or flooded type. However, this
arrangement is not as common as the dry type of
construction. Approximately 50% to 75% of the
tubes are immersed in liquid refrigerant and the
space above provides an allowance for the vapour
generated through evaporation of the liquid below.
This type is more often used with screw or
centrifugal type compressors.
There is a variant of the hooded or flooded type known as the semi-flooded type where only the
bottom row of tubes are immersed in the liquid
refrigerant. A trough is often employed to ensure
good distribution of liquid refrigerant along the full
length of the evaporator as correct refrigerant
distribution within the shell is important to ensure
the tube bundle above is adequately wetted.
The refrigerant liquid and vapour mixture is
normally supplied to the bottom of the shell via a
distributor that supplies the refrigerant evenly
under the tubes.
The warmer liquid (water or brine) flowing
through the tubes causes evaporation of the liquid
portion of the refrigerant. The resulting vapour
bubbles rise through the tube bundle and the
liquid surrounding the tubes becomes frothy and
also foams of oil is present in reasonable quantity.
The vapour leaving the surface of the liquid will
contain liquid droplets in the form of a mist.
This liquid mist must not be allowed to leave
the evaporator shell or a loss of performance will
result, coupled with possible accumulation of
droplets into a sufficiently large quantity leading to
partial compressor damage.
The provision of a large free volume in the shell
above the tubes and liquid below results in a low
velocity flow and allows the liquid mist and
droplets to be retained whilst the remaining
vapour is drawn out through the suction outlet.
Mist eliminators or a coalescing filter can be used
to separate the liquid droplets form the vapour if
the upper free chamber is of insufficient volume.
Figure 2:A dry type or direct shell and tube chiller with inlet and outlet at the same end
Water is normally used as the recirculating
medium for transferring heat energy from the
building to the shell and tube chiller. There are
many applications however that require both the
entering and leaving water temperature to be subzero.
It may also be necessary to have the entering
water temperature above zero and the leaving
water below zero. In either case, the water must
contain an additive that will prevent freezing and
severe damage to the shell and tube chiller. These
additives include the following:
• calcium chloride
• sodium chloride
• propylene glycol
• ethanol brine
• ethylene glycol
By varying the concentration of the additive in
water, the freezing point can be varied to suit the
application. A safety margin is also provided.
Freeze point temperatures exceeding -50°C are
attainable. These additives will be covered later in
The tube bundle may be several metres in length
and the tubes require supporting to prevent
sagging, which would otherwise impair correct
distribution of water over the outer surface of the
tubes. A series of baffle plates is therefore fitted
along the length of the evaporator to support the
tubes but these also serve another important role.
Were the water to be simply pumped in at one
end of the tube bundle and allowed to run along
the tube bundle without any disturbance or
turbulence, a large proportion of the water might
pass through the heat exchanger without making
direct contact with the colder tubes. This would
limit the heat exchange and thermal performance.
Partial freezing of water might also occur under
certain circumstances if the entire cooling power
of the evaporator is dedicated to only part of the
water flowing through the exchanger. This is
especially likely to occur in any flow dead spot
However, if the water were forced to change
direction over a series of semi-circular baffle
plates, the resulting turbulence will ensure
thorough mixing of the water throughout the
length of the heat exchanger thus leading to
maximum performance and freeze protection.
The baffles also increase the velocity of the
water throughout the exchanger, thereby
increasing heat transfer coefficient. The velocity of
the water flowing perpendicular to the tubes
should be at least 0.6m/s (2ft/s) to continually
clean the tubes and less than 3m/s (10ft/s to
avoid tube erosion.
Baffle plate spacing
The number of baffles and baffle pate spacing is
varied to produce different capacities from the
same tube bundle assembly. An increase in the
number of baffles increase the performance and
capacity of the evaporator. The pressure drop
experienced at the water pumps and subsequent
pump input energy level increases however.
The distribution of the liquid refrigerant in direct
expansion, dry shell and tube evaporators must be
absolutely uniform to all tubes in order to realise
the required performance, efficiency and capacity.
If some tubes receive more refrigerant than others,
they will not be able to fully evaporate the liquid
thereby leading to compressor flooding and
possible failure. Superheat within these tubes will
therefore be zero. Since the overall refrigerant flow
is controlled by maintaining a pre-set superheat
value at the expansion device, the remaining tubes
will be forced to work at a higher than normal
superheat level to evaporate the liquid passing
through the other tubes thus forcing these tubes to
operate at low efficiency and heat transfer.
Even distribution of liquid refrigerant is
achieved with a distributor or by ensuring that the
volume of the inlet chamber is kept to a minimum
to achieve constant flooding of the chamber and
even flow through the entire tube bundle. Either
method must maintain an even distribution of
liquid refrigerant and flash gas vapour to all tubes.
Number of passes
The number of passes directly affects the
performance of a direct expansion shell and tube
evaporator. In a single pass evaporator, the
refrigerant flows in at one end of the bundle and
out at the other. This arrangement requires total
evaporation of all liquid refrigerant and some
superheat by the time the refrigerant leaves the
tubes. This can require either a long tube bundle or
some form of tube surface enhancement to
increase the heat transfer and thereby a shorter
In a two-pass evaporator, the refrigerant
normally travels in one direction through 50% of
the tubes and returns to the same end of the
bundle through the remaining 50% of tubes. This
halves the length of the overall bundle that would
otherwise be required but after the first pass, a
large proportion (approximately 50%) of the
refrigerant has been evaporated and even
distribution of the remaining 50% is more difficult
to attain, especially as this also has to turn
through 180°. Whilst tube surface enhancement
can therefore be avoided in this design, an ideal
arrangement would be the combination of a
two-pass evaporator and tube surface
Multiple pass evaporators can be constructed
to provide up to six passes in order to capitalise
on the advantages described above. This is
achieved by fitting different heads at either end of
the shell in order to create the desired flow
pattern back and forth through a standard tube
bundle to suit the required capacity for the
application. A similar technique is applied to water
cooled condensers described earlier in the series.
Figure 3: An increase in the number of baffles increase the performance and capacity of the evaporator
Tube surface enhancement
The performance of a two-pass evaporator, where
the refrigerant travels in one direction through
50% of the tubes and returns to the same end of
the bundle through the remaining 50% of tubes,
suffers where, after the first pass, a large
proportion (approximately 50%) of the refrigerant
has been evaporated and even distribution of the
remaining 50% is more difficult to attain,
especially as the balance of refrigerant also has to
turn 180° to return through the remaining tubes.
Whilst this halves the length of the overall
bundle that would otherwise be required, thereby
leading to a more practical design for integration
within a packaged water chiller, the partial loss of
performance must be offset by increasing the
length of the bundle accordingly.
Tube surface enhancement, for improved heat
transfer, would offset the aforementioned
disadvantages and the two-pass shell and tube
evaporator incorporating tube enhancement is
therefore an ideal arrangement.
The use of a tube-within-a-tube, and a spirally
wound copper crimped fin insert promoted
annular flow, and led to a great improvement in
heat transfer unmatched by others at the time of
its introduction. Since then, heat transfer
development has continued throughout the air
conditioning industry and a number of other
enhancement techniques have been used. The
most recent of these, the use of rifled microbore
DX evaporator tubing, has allowed many
manufacturers to offer compact shell and tube
heat exchangers that can provide similar
capacities to inner fin evaporators.
The need to protect the environment against
one of the causes of ozone depletion, ie CFCs, and
to a much lesser extent HCFCs, has led to the
devdevelopment of HFC refrigerants such as
R407C and R134a. These refrigerants, although
they have an ozone depletion potential (ODP) of
zero, required package chiller manufacturers to
replace the mineral oils normally used with CFCs
and HCFCs with polyolester lubricants, in order to
maintain satisfactory oil transport within their
It has been established, through extended
factory testing, that the use of ester oils in
evaporators having small bore rifled tubing can
lead to a very significant reduction in capacity, as
the grooved micro-rifling can become clogged
with oil, partially transforming rifled tubes into
smooth bore tubes. Evaporators that rely on
performance enhancement via grooved tube may
therefore be unable to translate the calculated
performance specification into reality, resulting in
a high evaporator approach, reduced efficiency
and possible shutdown due to occasional tripping
of safety devices.
The inner fin shell and tube evaporator does
not suffer from loss of capacity when operating
with high viscosity ester based lubricants. The
comparatively coarse nature of the inner finning
prevents oil logging, and a low approach is
maintained. For this reason, many manufacturers
have for some years used at least one viscosity
grade higher than the mineral oil it replaces when
selecting ester oil for a particular application. This
provides adequate protection against thinning of
the oil by the HFC refrigerant used, preventing
accelerated compressor wear rates. The ability of
'Inner Fin' evaporators to maintain their
performance when operated with HFC refrigerants
and thicker oils offers an assurance that the
specified performance will be attained.
The capacity range of inner fin’ direct expansion
evaporators available can extend to 1200kW
cooling capacity with the ability to specify up to
four refrigerant circuits.
All evaporators feature single refrigerant and
water passes in counter-flow configuration for
optimal heat transfer and the greatest protection
against freeze up. The smaller CH and DCH type
evaporators feature copper tubes brazed into
brass tube sheets.
The tube bundle is brazed into a steel shell, and
steel end caps are then welded to the shell.
Finally, the threaded or flanged water spigots are
welded to the shell before pressure testing.
Vessels may be supplied with the chilled water
connection spigots arranged for vertical or
Where potable water is to be chilled, shell and
tube evaporators can be supplied with a standard
copper tube bundle but with a stainless steel
Where more aggressive fluids are to be chilled,
the entire tube bundle, shell, baffle plates, tube
plates and connections can be constructed from
316 grade stainless steel.
The use of HFC refrigerants and ester oils
places the manufacturer under a number of
obligations. To ensure long term reliability of a
chiller, it is vitally important that a proper pressure
test and evacuation of the product is carried out to
eliminate leaks. Typically, a DX evaporator would
be pressure tested in accordance with BSEN 378.
Thorough evacuation and dehydration between
200 and 250 microns provides additional
reassurance that the vessel is totally leak free.
The use of a helium leak detection system may be
employed if a vacuum shows signs of
deteriorating over the 15 minute hold period.
Finally, each vessel is charged with dry nitrogen at
0.5bar to prevent corrosion or moisture ingress
during storage or transit.
As HFCs do have a reasonable global warming
potential, and ester oils have a great affinity to
absorb moisture, the importance placed on
achieving a leak free and dry vessel cannot be
overstated or this could lead to cross leaks, tube
leaks and gasket problems.
Shell and tube evaporators continued