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Open doorways cause of rising energy costs

Article: African Heating & Cooling - September 2006

South Africa's energy wake-up has focused the spotlight on energy-saving devices (and costs) by preventing the escape of cold or hot air from a climate-controlled space. By Robin Hayes
An open supermarket door is welcoming but at what cost? Retailers could incur massive energy (and cost) loss due to refrigerated air escaping through unprotected doors.
Solution for energy savings
Retail outlets, as well as many public and industrial buildings could achieve significant energy savings by installing air curtains in strategic positions.
Energy losses
The extent of energy loss through an unprotected oorway can be significant.
Significant savings
The graph shows savings to be achieved with an air curtain
It is odd that a product so widely accepted across Europe and North America as a standard fitment in public buildings, retail stores and industrial applications such as coolers, freezers and cold storage facilities, is rarely seen in South Africa. Perhaps it is because air curtains are generally associated with retaining heat energy in cold climates rather than cold in hot climates but, in both applications, the energy savings are substantial.

In the 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.

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.

Properly installed 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.

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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Why not in South Africa?
All consultants, clients and contractors should consider 'Why not?' Perhaps the real reasons are that energy costs have never been an issue or supply has been consistent and abundant, certainly in the major metropolitan areas. However persistent outages and warnings from Eskom that tariffs will have to increase means that the halcyon days of 'cheap', plentiful power are gone for ever and we'd better use what we have more sparingly and wisely!
Size of air flow
This varies according to temperature difference between internal and external air and air exchange is therefore caused by thermal temperature differences. Using known values for the indoor and outdoor temperature difference, the densities, pressure differences and ultimately the air flow through the opening can be calculated.
Design considerations
Self-generated pressure evolves from a non-circulatory type of air curtain as the jet induces ambient air into the stream. When the air curtain is installed on the outside of a cold room, the air spills back into the room and continues to do so until the pressure builds up to such a point that the air stream deflects to the outside.

The pressure-form-stack effect evolves when the cold air in the room is denser than the warmer outside air. The higher the ceiling, the higher the pressure becomes inside the cold room. The velocity of the air curtain should therefore be high enough to compensate for the outward pressure at the lower part of the cold room.

Wind pressures occur because of the conversion of the velocity pressure in the wind into static pressure. Of particular significance is the stability of the air curtain, which is directed by its outlet momentum. The curtain of air must be able to withstand pressure variations between the inside and the outside of the cold room. The outlet momentum should be sufficiently sustained and strong enough to stabilise the air at the floor. It is therefore essential to incorporate controls in the design of the air curtain to increase the strength of the velocity and ensure a stiff curtain of air and to decrease the velocity to prevent unnecessary heat and moisture transfer at the door.

In refrigeration applications, a high volume of turbulent air hitting the floor tens to form ice on the floor of freezers - this is not desirable or necessary so air flow needs to be adjusted so that it barely hits the floor. This can be accomplished by adjusting air louvers or fan speed to obtain the desired velocity. Drafts are eliminated by ensuring that air flow is vertical. The air curtain may be mounted on either the warm or cold side of the cooler with equal effectiveness.
How much energy is lost through an open doorway?
A standard doorway (900 mm wide x 2 m high) could cause energy loss of close to 12,5 MW a year - this could be prevented by installing a relatively inexpensive air curtain.
Hannes Steyn, general manager of Grasso, and Derick Truscott of AMS in Cape Town maintain that a normal doorway (900mm wide by 2 m high) will have a minimum loss of 12 kW/h with a 10° Kelvin temperature difference, 5° Kelbin will give you 6 kW/h. So realistically you would need a 5,4 kW capacity air conditioner just to take care of the loss through an open door on a normal summer day.

Truscott added: "If we work on an average 5 Kelvin difference and an eight-hour day over 52 weeks, one can safely say that you will lose 12,5 MW a year. An air curtain for the average doorway can be installed for about R3 000 and reduce this loss by 80% to 90%. The energy saving alone makes it common sense to install an air curtain much more than say energy-saving light bulbs and geyser lagging as being promoted by Eskom".
Air curtain pays for itself in UK example

Zwetloots - a major UK flower retailer - has successfully maintained a chilled air atmosphere with the main door left open by installing an industrial air curtain. Zwetsloots needs to keep its blooms at a temperature of 4° to 5°C to maintain freshness and allow easy access in and out of the cold store while not chilling the adjoining area.

Tests showed that, with the air curtain turned on above the open door, the floor temperature directly outside the cold store remained above 12°C - exactly the same as the ambient temperature outside the store.

The 2 200 m2 cold store is linked to a staff-occupied packing area by a fast-acting roller shutter door. The problem had been the movement of warm air, at a high level, into the cold store when the shutter door was open, effectively pushing the heavy cold air out at low level into the staff area. For the staff, this phenomenon provided an uncomfortable and draughty work environment and Zwetsloots was faced with unhappy staff and increasing refrigeration costs in order to maintain the cold store's temperature.

David Zwetsloot, technical director, said: "We monitored the air curtain for two weeks as we were concerned that the down draught would damage the flowers and that it wouldn't create a seal across the door. However the down draught had no effect on the flowers and it sealed up the cool and ambient working area very well - a lot better than I expected. Tests showed that there weren't any cold spots near the entrance when compared to the ambient temperature for the rest of the building. Staff who work near the chill store used to complain that it was cold. However, since the air curtain was installed, there've been no complaints."

He said it would be difficult to quantify a financial benefit from any energy savings but, based on maintenance costs for the existing fast-acting roller door alone, the payback period would be between eight and 17 months.

The air curtain, creating a continuous wall of ambient air over the doorway, is installed outside the cold room and switches on when the roller door opens.

An air curtain across a roller shutter door opening will pay for itself simply in reduced door maintenance costs.
Article: African Heating & Cooling - September 2006