INCORPORATING COLD CHAIN
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iii.
iv.
Control of condensing temperature is simple. Cooling water can be distributed to more than one condenser.
The disadvantages of the water-cooled condenser system are: i. Higher initial cost. ii. Higher maintenance cost. iii. Water treatment is required.
Note: The cost and maintenance requirements of the water circulating pump, as well as the cooling tower fan, must be taken into consideration when evaluating the running cost difference between an air-cooled system and water-cooled system.
The temperature of the water entering the condenser depends on the outside air wet bulb temperature. Theoretically the water can be cooled to this temperature. This is not done for economic considerations. In most cases the water is cooled to approximately 9K above the wet bulb temperature of the entering air.
This temperature difference between the water leaving the tower and the wet bulb temperature of the air entering the tower is called‘ the approach’.
Thus, if the design wet bulb temperature in a region is 20 ° C the design condenser water temperature is approximately 29 ° C.
The condenser water temperature difference across the cooling tower is called‘ the range’. This‘ range’ is the same as the
water temperature difference across the condenser plus the temperature difference across the pump due to the power input into it. Water temperature difference across the condenser can range from 3-10K. Standard ratings are often for a difference of 5.5K.
Types and construction The three most common types of watercooled condensers used are: i. The tube-in-tube condenser ii. The shell-and-coil condenser iii. The shell-and-tube condenser.
In the tube-in-tube condenser, shown in figure below, water flows through the inner pipe and the refrigerant flows in the opposite direction( counter-flow) in the space between the inner and outer tube. In this arrangement, the refrigerant is cooled by the water in the inside tube and also by the air on the outside of the tube. Its advantages are its low cost, high efficiency and its flexibility as to size, adaptability and arrangement.
Its disadvantage is that it cannot be mechanically cleaned and must be cleaned by circulating special chemicals through the water tubes. Tube-in-tube condensers are extensively used in small packaged equipment.
The tube-in-tube condenser can be made with headers for bigger applications which make it possible to clean the water tubes. The big advantage of low cost is, however, lost by doing this.
The shell-and-coil condenser consists of a welded shell containing a coil of bare or finned tubing. The coil is continuous and without joints. The condenser water is circulated through the coil and the refrigerant is contained in the shell. Refrigerant gas enters at the top, condenses on the cold coils and the liquid drains off into the bottom of the shell.
Shell-and-coil condensers are used in medium range packaged equipment.
Cleaning of the water circuit can only be done by chemical means.
Shell-and-coil condenser.
As you can see there are many ways to achieve heat removal from the system to the ambient air but in essence all condensers perform the same basic function.
The shell-and-tube condenser consists of a cylindrical steel shell in which a number of straight tubes are arranged in parallel.
These tubes are held in place by tube sheets in which holes have been drilled. The tubes are expanded into the holes to seal them off. End-plates are provided on both ends through which the water enters and leaves the condenser. These endplates are removable and constructed in such a way that a number of passes can be provided for the water. Increasing the number of passes increases the water velocity in the tubes to provide better heat transfer.
Note: The number of passes is the number of times the water passes through the condenser. As you can see there are many ways to achieve heat removal from the system to the ambient air but in essence all condensers perform the same basic function. This concludes this module. Should you feel competent you may request an assessment to receive credits against this module.
Should you have any queries require assistance, please contact Anria Pieterse( SDF) at ACSA on 0027( 11) 609 1118.
REFERENCES S. A. I. R. A. C. | Modern refrigeration and air conditioning. | merSETA Training ACRA training material. CLA
Tube-in-tube condenser.
Shell-and-tube condenser.
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