CONDENSERS

Condensers are devices for reducing a gas or vapour to a liquid.  There is a wide range of technical applications for condensers which are used, for example, in power plants to condense exhaust steam from turbines and in refrigeration plants to condense refrigerant vapours

Historical Background

Samuel Hall was a British engineer and inventor of surface condensers.  Hall attempted to find a means by which steamships could charge their boilers with fresh water at the beginning of their voyage and use it over and over again, so as to avoid the use of corrosive salt water.

In 1838 Hall patented a surface condenser in which steam passed through a number of small condensing tubes cooled on the outside.  Following extensive trials, Halls invention proved to be unsuccessful.  However, the principle of the tubular condenser survives and is now mostly used in cooling devices.

Application

Condensers are used to reduce a gas or a vapour to a liquid.  As refrigeration plant uses condensers to condense refrigerant vapours, petro-chemical industries employ condensers for the condensation of hydrocarbons and other chemical vapours.  In distilling operations, the device in which the vapour is transformed to a liquid state is called a condenser.

All condensers operatte be removing heat from the gas or vapour.  Once sufficient heat is eliminated, liquefaction occurs.  For some applications all that is necessary is to pass the gas through a long tube (usually arranged in a coil or other compact shape) to permit heat to move into the surrounding air.

A heat-conductive metal, such as copper, is commonly used to transport the vapour.  The efficiency of a condenser is often enhanced by attaching fins, which are flat sheets of conductive metal, to the tubing so as to accelerate the removal of heat.  Typically, such condensers employ fans to force air through the fins and carry the heat away.  In many cases, large condensers for industrial applications use water or some other liquid in place of air to achieve heat removal.

Criteria for the selection of condensers

To put the physical process of condensation of a gas into practice, let’s look at the necessary pre-requisites for this.  If one wants to liquify the hot gas of the refrigerant coming from the compressor, a proper contact is needed between the medium to be cooled (gaseous refrigerant) and the cooling medium, e.g. cold air or cold water.  Direct contact is, of course, impossible, so the media must be led past each other in a suitable way so that the desired exchange of heat between the media, which results in the cooling of the refrigerant gas, can take place.

Pipes that allow good heat transfer are suitable for this.  In addition, a correspondingly large surface area is required for heat exchange.  So in practice, for example, metal tubes and metal fins are used for air cooled condensers to increase the surface area.  For water cooled condensers, finned copper tubes are used.

In refrigeration technology, copper is used almost without exception for the tubes, which at 398W/(m.K) has the highest thermal conductivity after silver.  Aluminium is usually used for fins although by comparison to copper it only has a value of 234W/(m.K).  Therefore, if high efficiency is required, copper can also be used as the material for fins.

The finned tube heat exchangers typically used for air cooled condensers are considered to be an efficient standard solution.  Alternatively, aluminium constructions may be used in which the refrigerant flows through microchannels and is condensed there.  Microchannel condensers offer a low cost alternative solution that also requires up to 30% less refrigerant and has a reduced weight.  It should be noted however, that microchannel coil condensers are non-repairable and more vulnerable to environmental damage than the copper/copper or copper/aluminium options.

It is helpful to summarise the primary differences between copper and aluminium as applicable to condensers:

Properties

Heat transfer

Specific heat

Corrosion

Weight

Material required

Durability

Malleability

Maintenance

Repairability

Environmental impact

Cost

Copper
much better
low, heats up and cools down quickly
slow and easy to clean
heavier
more metal needed
strong and more durable
higher tensile strength
easy to maintain
lon-lasting products
durable and energy efficient
more expensive
Aluminium
lower
higher, heats up and cools down more slowly
difficult to clean, more frequent replacement
nearly half the weight of copper
less metal needed
less durable, requires sturdy support
lower strength, easier to manufacture
difficult to maintain
prone to leakage, no on-site repair
easily damaged, releases harmful gases
cheaper

About Heat Transfer in Condensers

 

The energetic processes in a correctly designed condenser can be divided into three different areas:

  • Desuperheating

The superheated vapour refrigerant coming from the discharge port of the compressor is cooled down from the superheat temperature to the condensation temperature (desuperheated)

  • Condensation (liquefaction)

The refrigerant continuously gives off heat to the cooling fluid and condenses at constant pressure and temperature, resulting in a completely liquified refrigerant

  • Subcooling

The refrigerant, which is already fully condensed, continues to give off heat to the cooling fluid.  The liquid refrigerant is cooled to below the condensing temperature

New trends

The efficiency of most industrial plants depends crucially on water vapour condensing on metal plates or condensers, and how easily the condensed water can fall away allowing for more droplets to form.

Typically, on a flat-plate condenser water vapour quickly condenses to form a thin liquid film on the surface.  The formation of this thin liquid film consequently reduces the condenser’s ability to collect more water and ultimately acts as a barrier to heat transfer.  By creating hydrophobic surfaces, either through chemical treatment or through surface patterning, researchers have been able to prevent this problem by prompting water droplets to form and fall away.  This has been achieved by making surfaces that are patterned at multiple scales.  It has been found that the energy released as tiny droplets of water that then merge to form larger ones, is enough to propel the droplets upward from the surface.  The removal of droplets doesn’t depend solely on gravity which causes the droplets to just fall from the surface, but they actually jump away from this.

This process produces surfaces that resemble a bed of tiny pointed-leaves sticking up from the surface.  These nano-scale points minimise contact between the droplets and the surface, making the release easier.  After the leaf-like pattern is created, a hydrophobic coating is applied easing a solution that barns itself to the pattern surface without significantly altering its shape.

This patterning can be made on a film that can be applied to a variety of surfaces including the copper tubes which are widely used for heat exchangers acting as condensers.  This technology can be used for all purposes where heat transfer is important, such as for de-humidifiers and for heating and cooling systems in buildings.