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Condenser for Heat Transfer
In systems that involve heat transfer (such as distillation processes), a condenser device is installed, to transform a vapour to its liquid equivalent, by cooling. What happens in a condenser system is that, the latent heat in the gas is given up and transferred, to the surroundings.
There are many examples of this in the industry, and notable examples include, the condensation of refrigerant vapours, such as ammonia and chlorofluorocarbons (CFCs), in refrigeration plants. Exhaust steam, released from operating turbines is also necessarily condensed, as are chemical vapours and hydrocarbons, in petroleum and chemical industries.
Put another way, in one cooling cycle of a refrigeration system, the vapour refrigerant in the evaporator absorbs heat. Then the compressor compresses the refrigerant. The high temperature and high pressure of the vapour refrigerant, is now converted to liquid at the condenser, built to effectively condense the compressed refrigerant vapour.
How condensers work
Without exception, condensers operate by extracting heat from the gas or vapour. This results in inevitable liquefaction, because sufficient heat has been removed from the system. In a few applications, the core apparatus consists of a long tube that is often arranged in a coil, or other shape that is compact. The tube allows heat to escape from the surrounding air. The vapour is commonly transported via a heat-conductive metal, such as copper.
Types of condensing units
- Surface condensers physically separate the condensing medium and vapours. They are used when direct contact is undesirable. It is a heat exchanger of the shell and tube type, installed at the outlet of steam turbines, in thermal power stations. In common implementations, cooling water flows through the tube, and steam is channelled through the shell. Condensation happens on the external surface of the heat transfer tubes. The condensate is collected at the bottom, in a hot well (built-in pan) as it collects. The shell side, functions at a partial vacuum or vacuum. The vacuum is the result of a differential in specific volume, between the steam and the condensate.
However, vapour is conveyed through the tubes, with the coolant water or air flowing around the outside.
- A condenser in a chemistry investigation cools hot vapours, condensing them into a liquid. Laboratory condensers are expectedly different from industrial condensers. The Liebig, Graham, and Allihn condensers are key examples. It is key not to mix this with condensation reactions, where two fragments are linked into one molecule, via an addition reaction and an elimination reaction.
Laboratory distillation, reflux, and rotary evaporators involve several condenser types, the Liebig being the simplest. It is a single straight tube, within a cooling jacket.
A spiral tube within a water jacket is what the Graham condenser is essentially, while the Allihn boosts several large and small constrictions, on the inside tube, effectively boosting the surface area, upon which the vapour constituents are able to condense. It is more complex to manufacture, and thus more expensive.
All three condenser types are glassware available in 100, 200, and 400 mm standard lengths
- Large industrial condensers are used in industrial-scale distillation, to produce liquid distillates, from cool distilled vapour.
- Direct-contact condenser, in which hot vapour and cooled liquid, are mixed directly in a vessel. There is no barrier. As the vapour gives up its latent heat, condensing to a liquid in the process, while the liquid absorbs this latent heat, and experiences a temperature rise. The vapour and liquid involved, contain a single condensable substance, such as a water spray, to cool the air and adjust its humidity.
- Condenser units as used in central air conditioning units, have a heat exchanger component, to cool and condense refrigerant vapour, into a liquid. There is also a compressor, to raise the refrigerant pressure and move it along, and a fan to blow external air, through the heat exchanger area, to cool the refrigerant.
Regular configurations have the heat exchange section, wrapping around the sides of the unit, with the compressor within. In the exchange section, the gaseous refrigerant is passed through several tubes, surrounded by heat transfer fins that convey cooling air to the inside of the unit. A fan running on a motor is present in the top part of the condenser unit. This is covered by some grating, to prevent objects from falling into the fan. The fan pulls in cooling air from the outside, through the heat exchanger section at the sides, and blows it out through the top grating.
Condenser units are placed outside the building they are cooling. There is tubing provided, linking the unit and the building. One tubing is for the entering vapour refrigerant. The other is for the liquid refrigerant, exiting the unit. Electrical power is needed to power the compressor and the fan in the unit.
Applying condenser systems in the real world
Condenser units are of three basic variants, depending on the type of condensing medium used in the removal of heat. The condensing media are air, water, or a combination of both.
Often used in homes and small offices (domestic refrigerators, upright freezers, and home air conditioning installations). Usually small-capacity systems, below 20 tons. Such systems have a design that offers the advantage of not needing to install any water piping, or water disposal system. This saves water costs and avoids scaling problems, due to the mineral content of the water. It is also easier to install, and comes with an initial lower cost.
Dirt can be lethal to condenser performance in these systems, and while maintenance costs are minimal in air-cooled systems, they require much high power per ton of refrigeration. There is also a shorter compressor life, often providing much less cooling, on days when most cooling is required.
Positioning an air-cooled condenser device outside the unit is often the easiest implementation, ejecting heat to the external environment.
The circulation of the air-cooled type can be via natural convection or forced convection. Natural convection is used in smaller systems, like freezers and refrigerators, whereas forced convection uses circulated air through a fan, or blower that draws atmospheric air via its finned coils. Internally, the refrigerant circulates through the coil, with air flowing across the outside of the tubes
Three common types of this variant are available: shell and tube, shell and coil, and double tube. The shell and tube type is most-widely adopted, being usually adopted from 2 tons to several hundred tons.
They are more expensive to install than the air-cooled variants. It is the most power-efficient design, per ton of refrigeration, with compressors lasting longer than in air-cooled types. For higher-capacity applications, a water cooling tower is installed.
A combination of water- and air-cooled condensation systems. It is the least known of the three. They are of great utility when water supply is inadequate, to use a water-cooled condenser or the condensation temperature is lower than is possible, with air-cooled condensers.
They can be used within a building or outside it, operating at a low condensation temperature, under normal conditions. In terms of efficiency, they probably lag behind, compared to the other two, yet they are widely employed in large commercial air conditioning units.
Comparing air-cooled and water-cooled systems
Air-cooled systems require a superior head pressure, or condensing pressure, effectively reducing the compressor capacity, and increasing power intake. A 2 hp water-cooled unit, will need similar refrigeration as a 3 hp air-cooled system.
It costs three to four times more, to maintain water-cooled systems, than the air-cooled types. Air-cooled types only need frequent lubrication of fan, and motor bearings. Water-cooled types necessitate constant cleaning, because of proliferating algae and bacteria. Constant supply of make-up water, along with water treatment, also helps control the growth of these organisms. Tube scales are taken off using an acid compound and proper water treatment is critical, to the operation of the condenser.
Improving condenser efficiency
To enhance condenser efficiency, fins – flat laminae (sheets) of conductive metal, are attached to the tubing, to cause heat removal to speed up. Such condensers use fans to force air through the fins, and convey the heat away. Mostly, industrial condensers which are expectedly large, use water or another liquid rather than air, to make heat removal possible.