Vol 16 – Direct Expansion Water to Refrigerant Evaporators

Author Mike Creamer, Business Edge Ltd

 

Volume 16        

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 & tubeevaporators 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 fairly straightforward,

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 of vaporisation. 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 removed.

A typical arrangement of a dry type or direct expansion shell and tube chiller is shown in Figure 1

FIGURE 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 Figure 2.

 

 FIGURE 2

These are normally used with positive displacement compressors such as reciprocating, rotary or screw machines.

 

Flooded Evaporator

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 hoodedarrangement or floodedtype.  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 hoodedor floodedtype known as the semi-flooded type where only the bottom row of tubes are immersed in the liquid refrigerant and 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 shall above the tubes and liquid below results in a low velocity flow and allows the liquid mist and droplets to me 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 if of insufficient volume.

 

Recirculating Medium

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 requires both the entering and leaving water temperature to be sub-zero.  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 minus 50 Deg C are attainable.  These additives will be covered later in the series.

 

Baffle Plates

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 platesis 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 the entire cooling power of the evaporator is dedicated to only part of the water flowing through the exchanger thus leading to a greater temperature reduction, possibly below freezing.  This is especially likely to occur in any flow dead spot regions.

However, if the water were forced to change direction over a series of semi-circular baffle plates, alternately arranged as shown in Figure 5, 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.

FIGURE 3

 

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 0.6 m/s (2 ft/s) to continually clean the tubes and less than 3 m/s (10 ft/s to avoid tube erosion.

 

Baffle Plate Spacing

The number of baffles and baffle pate spacing is varied to produce different ca1pacities form 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.

 

Refrigerant Distribution

The distribution of the liquid refrigerant in direct expansion, dry shell and tube evaporators must be absolute 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 & 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 tube bundle.

In a 2 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 180 degrees.  Whilst tube surface enhancement can therefore be avoided in this design, an ideal arrangement would be the combination of a 2 pass evaporator and tube surface enhancement.

Multiple pass evaporators can be constructed to provide up to 6 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.

NEXT MONTH:  Vol 17 – Water and Brine Evaporators continued

DISCLAIMER:  Whilst every effort is made to ensure absolute accuracy, Business Edge Ltd/EvoMart Ltd will not accept any responsibility or liability for direct or indirect losses arising from the use of the data contained in this series of articles.