The water-cooled condenser
The rejection of heat from condensers was discussed in chapter 11. The treatment here is confined to water-cooled condensers. There are four types of condenser in use: shell-and — tube, shell-and-coil, tube-in-tube, and plate heat exchanger.
Hot, superheated gas usually enters at the top of the shell and is desuperheated and condensed to a liquid at high pressure and temperature by coming in contact with horizontal tubes which convey the cooling water. Some sub-cooling of the liquid below the saturation temperature corresponding to the pressure prevailing inside the shell also occurs and is desirable. The condensed liquid is usually only enough to fill about the lower fifth of the shell, but the shell must be large enough to contain the full charge of refrigerant when repair or maintenance work is carried out on other components of the system. There is thus no need for a separate liquid receiver.
The capacity of a condenser to reject heat depends on the difference of temperature between the condensing refrigerant and the cooling water. Thus, its capacity may be increased by raising the condensing temperature or by increasing the rate of water-flow. Raising the condensing temperature has the side effect of lowering the capacity of the compressor, as has been observed earlier.
Some choice is open to the designer of a refrigeration system: a small condenser may be chosen, giving a high condensing temperature, or vice-versa. The first choice will result in a motor of larger horsepower to drive the compressor, with consequent increased running costs. The alternative will give lower running costs, but the size and capital cost of the condenser will be greater.
A typical choice of water flow rate is about 0.06 litres s’1 for each kW of refrigeration, rising in temperature from 27°C to 32°C, as it flows through the condenser.
Pressure settings for safety and other purposes are expressed in BS 4434 and are related to good practice. The values chosen are particularly important if the plant is to be used as a heat pump, the heat normally rejected at the condenser being put to good use. Recommended settings are:
High-pressure cut-out setting: design working pressure + 10%
Maximum working pressure: high-pressure cut-out setting/0.9
Test pressure: maximum working pressure
+ 30% (if a steel shell) or
+ 50% (if a cast iron shell).
Condensers are generally used with dirty water from a cooling tower or other source and a proper allowance for the fouling factor must be made. A typical value is 0.000 175 6 m2 K W-1 (0.001 ft2 h°F/Btu) but, if tubes with an enhanced internal surface area (such as longitudinal or spiral grooves) are used, the allowance for fouling must be greatly increased.
Starner (1976) claims that making an allowance for a fouling factor does more than merely increase the thermal resistance. To achieve the same overall heat transfer it becomes necessary to increase the surface area of the evaporator and the presence of additional tubes lowers the water velocity therein, giving a further fall in the overall heat transfer coefficient. Suitor et al. (1976) imply that the build-up of fouling as time passes does not
Increase the thermal resistance in a linear fashion: it is claimed that the fouling resistance
Increases asymptotically with time.
Only about 1 degree of sub-cooling is normally achieved but greater sub-cooling can be obtained if it is arranged to submerge the tubes in the refrigerant condensate.
It is important, when considering plant layout, that at least the full length of the condenser shell is allowed for tube withdrawal or mechanical cleaning.
If the design working pressure for a refrigeration plant is 15 bar determine the setting of the high-pressure cut-out switch, the maximum working pressure and the test pressure.
HP cut-out setting: Max working pressure: Test pressure:
15 x 1.1 = 16.5 bar 16.5/0.9 = 18.3 bar
18.3 x 1.3 = 23.8 bar if steel
Or 18.3 x 1.5 = 27.4 bar if cast-iron
Cooling water flows through one or more coils within a shell and refrigerant condenses on the outer surface of the coil(s). Condensers of this sort are used for small, self-contained packaged plant in the range 2 to 50 kW of refrigeration. Mechanical cleaning is not possible for the inside of the tubes but sometimes chemical methods are adopted. Water pressure drops may be greater than for shell-and-tube condensers.
These are used in packaged plant over the range 1 to 180 kW of refrigeration. They comprise a coil of tubes, one within the other. The water may flow through the central tube with the refrigerant condensing in the annular tube, or vice-versa. Mechanical cleaning of the water tube is not possible but chemical methods may be used. Sometimes the tubes are straight and then mechanical cleaning on the water side is possible, provided that the waterflow is through the central tube.
An array of parallel, stainless steel plates is used, hot gas condensing on one side of alternate plates with cooling water flowing on the other side. Gaskets of appropriate materials are used to separate the plates. Heat transfer rates are usually higher than with shell-in-tube condensers but mechanical cleaning is not possible. Heat exchangers of this type are also sometimes used as water chillers (see section 12.4).
Posted in Engineering Fifth Edition