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Fans in Refrigeration - Condensing Units

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Fans in Refrigeration - Axial fans for Refrigeration Condensing Units

When cooling is required for bulk food storage, maintaining a low temperature for food preparation, controlling the ambient temperature in a processing plant or drinks cellar cooling; evaporators are used to absorb heat from the controlled environment. But where does the absorbed heat go?

In a refrigeration application there are four key stages of what is called the vapour compression cycle where a refrigerant changes state from liquid to gas (evaporation), and from gas to liquid (condensing) in a continuous closed loop process. During evaporation (creation of cold) and condensing (release of heat), axial fans are commonly selected for their efficiency and surface area coverage when forcing air over heat exchange coils.

How Does a Refrigeration system work?

A refrigeration system relies on a cyclical process that removes heat from an area required to be cold and rejects it to the ambient surroundings away from the cold area. This process is achieved using a refrigerant which during its journey changes state as the heat moves through the system. An example that we will be mostly familiar with is our household refrigerator.

When we fill our fridges at home with cold or ambient produce bought from the supermarket, its temperature will usually be higher than the temperature inside the fridge. This extra warmth in the food has the effect of increasing the temperature inside the fridge by convection, conduction and radiation.

This heat is gathered in by the refrigerant circuit - the evaporator - inside the fridge and is drawn to the outside via the compressor. The compressor sends the heat gathered inside the fridge, outside to a heat rejection circuit – the condenser – which returns refrigerant back to the cold part of the fridge via a thermal expansion valve.

This closed-circuit refrigeration cycle ensures that the temperature inside the fridge is maintained to a level that ensures our food stays fresher for longer.

What is a Condenser?

Larger cooling requirements for retail, commercial or industrial scale processes separate the various components of the refrigeration system.  Condensers are usually situated outdoors in the open air, exposed to the elements and there are two common configurations: ‘Flatbed’ & ‘V-Block’.

Both configurations consist of a finned heat exchange coil, a supporting structure that supports and encases the heat exchange coil and a deck on which the fans are mounted to force air through the condenser. The air is commonly drawn through the heat exchange coil using an axial fan.

The heat exchanger comprises a number of metal tubes attached to strips of thin sheet metal called ‘fins’. The fins are bonded to the tubes to increase the surface area over which the heat exchange can take place thereby increasing the heat exchange efficiency. The cool ambient air passing over the fins is absorbed by the refrigerant changing its phase from a hot gas to a warm liquid. The warm liquid refrigerant is delivered to the thermal expansion valve which feeds the evaporator providing the cold energy where it absorbs heat and boils off, changing state from liquid to gas.

The spacing between the refrigerant tubes and fins of the heat exchanger offer a low resistance to flow. In addition, the surface area of an axial fan provides a large area of coil coverage ensuring that condensing occurs across most of the evaporator coil. This combination of operating point (High flow versus low pressure), and large surface area coverage make an axial fan the choice for delivering effective system efficiency.

In order to ensure full coverage of the coil a minimum distance between the fan and the heat exchange coil is required. The recommendation is that there should be at least 1/3 of the fan diameter distance as a minimum.

Typically, axial fans used in a condenser draw ambient air through the heat exchange coil exhausting air out through its mounting guard. This type of condenser uses a “V-Flow” direction fan. As the air passes through the heat exchange coil it picks up heat from the hot refrigerant gas via the heat exchanger enabling the condensing process. The temperature of the air as it passes over the fan can be much warmer than local ambient conditions dependent on the refrigerant used and the demand from the refrigeration system. For this reason, care must be exercised when choosing the fan to ensure that it will be able to withstand the temperature of the air passing over the motor at the fan operating point.

Alternatively, a fan with the opposite airflow direction can be used drawing ambient air in and forcing it through the heat exchange coil. This type of condenser uses an “A-Flow” direction fan. The motor in this case will not see an elevated temperature however care needs to be taken to ensure that there is sufficient clearance between fan and heat exchanger to maximise heat exchanger efficiency.

Examples of both A & V-Flow fans can be found on the following RS Components Web pages…

A-Flow

Diameter (mm)

Link

250

2-Pole : (826-1054)
4-Pole : (218-3113)

300

4-Pole : (165-1373)

400

4-Pole : (825-8193)

 

V-Flow

Diameter (mm)

Link

250

2-Pole : (826-1058)
4-Pole : (825-8171)

300

4-Pole : (826-1076)

400

4-Pole : (825-8193)

Minimum Efficiency Performance Standards and the Energy Related Product Directive

The mounting method of the fan has a significant influence on the performance it will deliver. The performance stated by the fan manufacturer is measured under laboratory conditions using what is called a full bell-mouth or full radius mounting. This type of casing for the fan allows the air to enter onto the blades smoothly with a minimum amount of disturbance. Other types of mounting design can be used which include a plane orifice (a hole in a plate), or an orifice with a short radius. 

Using an axial fan in a plain orifice creates turbulence at the tips of the impeller blade which can cause up to 40% fan performance loss dependent on the fan operating point and mounting clearances.

The compromise orifice with a short radius provides better inlet conditions for the fan impeller however this still creates turbulence and loses up to 15% of the fan performance. To ensure that the fan achieves its full performance capability, using a full radiused mounting ring eliminates most of the turbulence that can happen and minimise noise generation.

In 2013 the European Union introduced regulations that set a minimum efficiency performance standard for fans. The minimum efficiency which fans supplied into Europe must achieve is a calculated based on the power consumption at its best efficiency operating point. In 2015, the minimum efficiency level increased to a higher level and is set to increase again some time in 2020 / 2021. To achieve the required operating efficiencies and to get the most from the fan selection, mounting the fan in a full radius mounting plate is essential.

AxiBlade fans for Condensers – The solution to meet current energy efficiency performance targets

ebm-papst have developed a complete range of products focussed toward the operating requirements of a Condenser that include:

  • Optimised radiused mounting ring to eliminate turbulence loss and minimise noise
  • High aerodynamic efficiency impeller with winglets to maximise the effective diameter
  • Airflow straightener to direct the air away from the condenser and avoid recirculation of warm air
  • Optimised contact protection guard; set at a distance with maximised free area to minimise flow resistance
  • High efficiency, Permanent Magnet, Electronically Commutated motor with on-board intelligence

Further details can be obtained from the ebm-papst UK Microsite : https://ref.ebmpapst.com/de/en/ref/products/commercial-refrigeration/axiblade.html

 

Summary

For the low-pressure high-volume operating point of a refrigeration condenser, the selection of an axial fan with its wide area of coil coverage ensures that maximum heat exchange efficiency can be achieved.

In addition, the fan mounting style and mounting position have a significant effect on the airflow pattern as ambient air is drawn through the hot condenser coil, absorbing and removing heat energy to facilitate condensing and propelling it away. The enhancements the AxiBlade fan provide include:

  • A full bell-mouth mounting around the fan, ensuring maximum air is delivered through the condenser
  • Straightening the air as it leaves the fan ensuring that the air carrying the heat from the condenser is propelled away, eliminating recirculation
  • A high efficiency motor with on-board intelligence delivering powerful performance on demand.

For more information on products designed for the refrigeration industry that are available from ebm-papst, please visit our web page https://ref.ebmpapst.com/de/en/ref/products/commercial-refrigeration/axiblade.html

 

My background is in Mechanical & Production Engineering however working for ebm-papst that has expanded into electro-mechanical, some electronic and acoustic engineering. When it comes to acquiring and passing on knowledge, I try to keep it as simple and as least painful as possible. I am happy to receive feedback and if there are any questions that arise from anything that gets published. If I don't know the answer to your question immediately, I am sure that I know someone that can help.

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