The higher heat capacity and density of water make it an ideal medium for condenser cooling and, by comparison with the 350 kW plant cited above, the flow is only 9.8 litre/s. Small water-cooled condensers may comprise two concentric pipes (‘double pipe’), the refrigerant being in either the inner tube or the annu — lus. Configurations may be straight, with return bends or headers, or coiled. The double-pipe condenser is circuited in counterflow (media flowing in opposite directions) to get the most subcooling, since the coldest water will meet the outgoing liquid refrigerant.
Larger sizes of water-cooled condenser require closer packing of the tubes to minimize the overall size, and the general form is shell-and-tube, having the water in the tubes (Figure 6.4/ . This construction is a very adaptable mechanical design and is found in all sizes from 100 mm to 1.5m diameter and in lengths from 600mm to 6m, the latter being the length of commercially available tubing. Materials can be selected for the application and refrigerant, but all mild steel is common for fresh water, with cupronickel or aluminium brass tubes for salt water.
Some economy in size can be effected by extended surfaces on the refrigerant side, usually in the form of low integral fins formed on the tubes. On the water side, swirl strips can be fitted to promote turbulence, but these interfere with
Figure 6.4 Shell-and-tube condenser (Titan)
Maintenance cleaning and are not much in favour. Water velocity within the tubes is of the order of 1 m/s, depending on the bore size. To maintain this velocity, baffles are arranged within the end covers to direct the water flow to a number of tubes in each ‘pass’. Some condensers have two separate water circuits (double bundle, Figure 6.5), using the warmed water from one circuit as reclaimed heat in another part of the system. The main bundle rejects the unwanted heat. Where
^ Hot gas
Figure 6.5 Double-bundle shell-and-tube condenser
The mass flow of water is unlimited (sea, lake, river or cooling tower), the temperature rise through the condenser may be kept as low as 5 K, since this will reduce the head pressure at the cost only of larger water pumps and pipes.
If, however, water is used once through only, and is then rejected to a drain, the range can be much higher, possibly 10-12 K.
A condenser uses water from a river with a temperature rise of 5.2 K. Total duty at the condenser is 930 kW. How much water flow is required?
5.2 x 4.187
Brazed plate heat exchangers are used as condensers and can be a lower cost alternative to shell and tube. The construction and characteristics are discussed in Chapter 7, and illustrated in Figure 7.9. Because the refrigerant volume is small, they can work with a lower charge. To be fully effective a BPHX needs to be kept fully drained into a liquid receiver.
The supply of water is usually limited and requires the use of a cooling tower. Other possibilities are worth investigation; for example, in the food industries, large quantities of water are used for processing the product, and this could be passed first through the condensers if precautions are taken to avoid contamination. Also, where ground water is present, it could be taken from a borehole and afterwards returned to the ground at some distance from the suction. In both these cases, water would be available at a steady temperature and some 8-10 K colder than summer water from a cooling tower.
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