Wednesday, June 20, 2012

High Volume Air Curtains

Air curtains cut cold air loss and reduce humidity, thus reducing the load on refrigeration or air-conditioning plant and thereby saving energy by reducing compressor running time, maintenance, gas and recharging intervals. These units are an alternative to fast-acting roller doors and PVC slat curtains, and can be situated above or to the side of industrial doorways to create a powerful seal across the opening, keeping cold air in while providing complete visibility and access. This means that forklift drivers don't have to get out of their cabs to open and close doors so that moving from one area to another is a lot easier and productive. Unlike plastic slat curtains, which quickly become opaque with use, an air curtain provides 100% visibility at all times. Units have been installed in loading bays, factories, hangars and production facilities, such as food processing and pharmaceuticals, where differential temperatures are encountered overseas.

Insect an pest control a benefit
An air curtain supplying a high-velocity sheet of air across a door opening will prevent flies and other flying insects from entering a building. This is particularly important in restaurants and bars, and any premises where food is manufactured or served - where strict environmental health regulations apply. It is an extremely beneficial side effect as an air curtain is primarily employed for climate control yet units are also installed overseas for insect control alone.

In retail environment, research has shown that an open door increases trade by up to 40% as it is an invitation to come in and shop yet retail stores and supermarket air-conditioning systems are 'energy-hungry' so it would therefore seem logical to save as much of the cold air as possible, considering the cost to make it cold in the first place. Public buildings, like airports and convention centres, which have high demand air-conditioning systems need to have open doors to allow access and, while some facilities have motion detector-activated sliding doors, usually of glass, this solution is costly, thermally not very efficient and generally just a way of reducing draughts.

Air curtains are widely accepted across North America and Europe as a standard fitment in public buildings, retail stores and industrial applications such as coolers, freezers and cold storage facilities. Air curtains are generally associated with retaining heat energy in cold climates, and the energy savings are substantial.

The idea of having an invisible barrier across the doorway to enable unimpeded access yet effectively prevent temperature flow, dust, odours and insects to cross, is appealing in itself but add to this the cost-saving in reducing energy use and the idea becomes more compelling, especially as air curtains are relatively inexpensive items.

Appropriate for industrial applications
Keeping warm air out of insulated cold stores or refrigerated facilities is the Number 1 objective yet a mockery is made of this premise when freezer of chiller doors are left wide open and, in some cases, chocked open to facilitate the movement or storage of perishables.

The most effective air curtain design for insect control has a nozzle that can angle the air stream away from the area to be protected. An angle of about 20 from the vertical is usually optimal. The unit should meet or exceed UK Department of Agriculture and US Federal Drug Administration standards, which call for an air stream of 50 mm to 120 mm wide at the nozzle, capable of producing a minimum velocity of 8 m/s of air 1 m above the floor and across the entire door opening to ensure there are no gaps for the insects to enter.

Apart from flying insects, air-curtain manufacturers say rodents are also discouraged from entering a building. They claim that rats, mice and other furry intruders do not like the sensations of an air curtain on their fur and will avoid it!

Design and installation
The construction an design of the equipment that generates an air curtain is quite simple - a cross flow or axial fan, driven by a constant or variable speed electric motor contained in a simple enclosure with an air inlet and outlet, sized to produce a rate of flow to suit the opening and the velocity required to prevent heat transfer.

Off-the-shelf models are available from a number of suppliers and units are also custom made but, where wide entrances are involved that exceed the capacity of one fan unit, tow or more are stacked side by side (or on top of one another in the case of side mounting of very tall openings) and operated simultaneously, meaning that there is probably no limit to the size of the opening that can be accommodated.

By directing a jet of air either from top to bottom or horizontally across a doorway, this invisible 'barrier' reduces the rate of heat and moisture flow through the opening and, according to independent research conducted by the US-based Refrigerated Research Foundation and the University of Illinois, this can be as much as 60% to 80% in refrigerated facilities such as cold storage buildings, cold rooms and freezers.

These simple devices can be retrofitted or specified as part of the initial refrigeration or climate control design, reducing compressor running time or using a smaller capacity unit to achieve the same performance.

A variety of installation options are available to cater for building and architectural features such as suspended ceilings but the most common is simply bolting the unit to the wall above the doorway and connecting a single phase supply and switch to the unit.

Power consumption is negligible for a normal sized doorway opening - about 300 W. Some units can even be specified with a heating element to warm an enclosed area.

Air curtains block the flow
An air curtain simply creates a block in the air flow through an opening. The air velocity of the curtain must then be great enough to direct the resulting velocity downward and ensure that a small part of the air stream goes out while the main air flow comes back into the room.

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