Fans in Refrigeration - Axial fans for Unit Coolers
Whether the 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, unit coolers using axial fan technology provides the solution.
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 most 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 Unit cooler?
Larger cooling requirements for retail, commercial or industrial scale processes separate the various components of the system. Cold rooms or areas that need to be kept cold are usually highly insulated or have high thermal mass. This ensures that the refrigeration energy stays where it is needed. The Unit Cooler is located inside the cold area and comprises of a heat exchanger - referred to as the evaporator coil - and the thermal expansion valve.
Air inside the cold room is drawn through a finned heat exchanger most commonly by using an axial fan. The spacing between the refrigerant tubes and fins of the heat exchanger offer a low resistance to flow which is the ideal operating area for an axial fan. In addition, the surface area of an axial fan provides a large area of coil coverage and ensures that cooling occurs across most of the evaporator coil.
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 heat energy within the air passing over the fins is absorbed by the refrigerant changing its phase from a liquid to a gas. The cool gas is then drawn out of the heat exchanger by the compressor which is part of the heat rejection process.
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, the axial fans used in a unit cooler draw air through the coil exhausting chilled air out through its mounting guard. With ambient temperatures down to -40oC, high humidity with the risk of condensation and ice, robust protection is needed to avoid electrical and mechanical failures. For this type of environment high humidity class protection is recommended, (H1 or H2).
Examples of this type of fan can be found on the following RS Components Web pages…
230V EC - (701-3583)
115V EC - (825-7942)
115V EC - (825-7951)
230V AC - (701-3592)
115V EC - (825-7967)
115V EC - (825-7960)
230V AC - (825-8171)
How is a Unit Cooler Used?
When placed inside a sealed insulated room or in a cold process area with a high thermal mass a unit cooler is installed at a high level, drawing air from underneath through the coil and out at high level via the fan. The air circulating in the room picks up heat from the products or process contained within, returning the air to the unit cooler where the heat energy is taken away and rejected to the ambient environment.
An important consideration for the cold room is to avoid any areas where still airs can develop warm spots within the cooled area. Good housekeeping, maintenance and regular checking by the room operators can resolve issues that may occur during the use of the cold room.
Fan selection and design are also critical to ensure the air is delivered throughout the whole temperature-controlled area. Factors such as the velocity and the airflow pattern of the air as it leaves the fan will determine if the cold air reaches the full length of the temperature-controlled area.
The pattern of the air leaving an axial fan will be affected by the position of the fan within the mounting orifice and the operating point.
The further the fan is placed through the orifice plate, the more diffuse the exhaust air pattern will be.
The operating point on the fan curve also influences the shape of the exhaust air pattern.
Another effect on the air as it passes through the unit cooler is that the axial fan induces a swirl on the outlet. This swirling energy also causes diffusion of the air pattern reducing the capacity of the unit cooler to reach distant points of the cold room.
To ensure mixing of cold air coming from the unit cooler throughout the cold room the air should be thrown as far as possible away from the fan. To achieve this, the air must be straightened as it leaves the fan, creating a tubular air pattern that maintains its form for as long as possible. The device which makes this possible is called a streamer and it comes in several forms…
For shallow and basket guard mounted axial fans, streamers can clip to the outlet guard
Using a basket guard axial fan in a plain orifice as described so far has been a common method of installing the fan on to the unit cooler. This style of mounting 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.
A compromise solution would be to add a short radius around the orifice in which the fan is mounted 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.
AxiCool fans for evaporators and cooling units – 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 Unit cooler that include:
- Optimised radiused mounting ring to eliminate turbulence loss and minimise noise
- Smooth surfaces to prevent dirt accumulation
- Integrated condensate drainage channels for accumulated moisture during the defrost cycle
- Optional built-in variable speed control with EC motors
- Optional streamer flow straightener to maximise air throw
- Optional hinge mounting for coil cleaning and unit maintenance
- Optional heater band to prevent ice blockage on the fan during coil defrost cycle
If this product is of interest, further information can be requested via the product plus area on the RS Components web pages - https://uk.rs-online.com/web/generalDisplay.html?id=our-services/procurement-solutions/product-plus
For the low-pressure high-volume operating point of a unit cooler application, 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 the effect it has on the airflow pattern as the cooled air circulates around the cold room:
- Using a full bell-mouth mounting around the fan will ensure maximum air is delivered through the unit cooler
- Straightening the air as it leaves the fan ensures that the air will circulate to all areas of the cold room
- Additional mounting features ensure reliable operation, ease of maintenance ensure that a high standard of hygiene can be maintained
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/refrigeration.html