The simplest air-cooled condenser consists of a plain tube containing the refrigerant, placed in still air and relying on natural air circulation. An example is the condenser of the domestic refrigerator, which may also have some secondary surface in the form of supporting and spacer wires.
Above this size, the flow of air over the condenser surface will be by forced convection, i. e. fans. The high thermal resistance of the boundary layer on the air side of the heat exchanger leads to the use, in all but the very smallest condensers, of an extended surface. This takes the form of plate fins mechanically bonded onto the refrigerant tubes in most commercial patterns. The ratio of outside to inside surface will be between 5 : 1 and 10 : 1.
Flow of the liquefied refrigerant will be assisted by gravity, so the inlet will be at the top of the condenser and the outlet at the bottom. Rising pipes should be avoided in the design, and care is needed in installation to get the pipes level.
The flow of air may be vertically upwards or horizontal, and the configuration of the condenser will follow from this (see Figure 6.2). Small cylindrical matrices are also used, the air flowing radially inwards and out through a fan at the top.
Forced convection of the large volumes of air at low resistance leads to the general use of propeller or single-stage axial flow fans. Where a single fan would be too big, multiple smaller fans give the advantages of lower tip speed
And noise, and flexibility of operation in winter (see Section 6.12). In residential areas slower-speed fans may be specified to reduce noise levels. A smaller air flow will derate the condenser, and manufacturers may give ratings for ‘standard’ and ‘quiet’ products.
The low specific heat capacity and high specific volume of air implies a large volume to remove the condenser heat. If the mass flow is reduced, the temperature rise must increase, raising the condensing temperature and pressure to give lower plant efficiency. In practice, the temperature rise of the air is kept between 9 and 12 K. The mass flow, assuming a rise of 10.5 K, is then
1 0.093 kg/(s kW)
10.5 X 1.02
Where 1.02 is the specific heat capacity of ambient air.
As an example of these large air flows required, the condenser for an air — conditioning plant for an office block, having a cooling capacity of 350 kW and rejecting 430 kW, would need 40.85 kg/s or about 36m3/s of air. This cooling air should be as cold as possible, so the condenser needs to be mounted where such a flow of fresh ambient air is available without recirculation.
The large air flows needed, the power to move them, and the resulting noise levels are the factors limiting the use of air-cooled condensers.
As the condenser load increases the temperature difference between the air inlet (ambient) temperature and the condensing temperature will increase in order to reject heat at a faster rate with the same surface. This is with a constant air flow.
A condenser rating, kW/K, where the condenser load is in kW and the K is the temperature difference, can be considered to be constant, as a first approximation.
A condenser is sized to reject 12 kW heat at a condensing temperature of 50°C when the maximum outdoor temperature is 35°C, what is the rating and what will be the approximate condensing temperature when the outdoor temperature is at 15°C and the load is reduced to 8kW?
Condenser Rating =———— Load————- = — = 0.8kW/K
Temperature Difference 15
Temperature Difference at 15°C = — = 10
Condensing temperature at 15°C = 15 + 10 = 25°C
The condenser must be sized to meet the design load at the maximum ambient condition, but during typical running conditions with the air temperature at 15°C, the load will fall because the cooling load will tend to be less, and the compressor power will certainly be less. A condenser, which may appear to be small and require a high condensing temperature at the design condition, balances out to give an acceptable condensing temperature most of the time.
Materials of construction are aluminium fins on stainless steel tube for ammonia, or aluminium or copper fins on aluminium or copper tube for the halocarbons. Aluminium tube is not yet common, but its use is expected to increase.
In view of the high material cost for air-cooled condensers, a higher temperature difference than for water cooled is usually accepted, and condensing temperatures may be 5-8 K higher for a given cooling medium temperature. Air-cooled condensers are very widely used in sizes ranging from a few kW to several hundred kW. They can be seen as wall mounted fan-coil units on air conditioners and on large roof mounted systems, Figure 6.3. They must, of course, be used on land transport systems. They will also be used in desert areas where the supply of cooling water is unreliable.
It is frequently necessary to vary the air flow, for example to ensure that condensing pressure does not fall too low for proper control of the low side system, to reduce sound levels at night time, or to reduce the fan power required under low ambient conditions. The control parameter is usually condensing pressure, and an intelligent controller will reduce fan power when this reduces the total power consumption, including that of the compressors. The air flow reduction can be achieved by switching off fans on multiple fan units or by varying fan speed. Single phase fan motors can often be speed controlled with a simple pressure-sensing controller that varies the voltage to the motor. A hot coil will
Figure 6.3 Multiple section air cooled condenser (Searle)
Induce an air flow even with the fan idle. Large condensers may be arranged in two or more sections to overcome over-capacity situations. The effective size of the condenser is reduced by shutting off the appropriate section.
Arranging the coil in sections allows the condenser to serve more than one refrigeration system. They can have different operating conditions or refrigerants. Most manufacturers offer units with two rows of fans: a two section coil can be incorporated for this purpose.
Posted in Refrigeration and Air Conditioning