When Is A Door Not A Door

The very nature of public buildings is that they are public.  The high footfall and pedestrian traffic levels through museums, galleries, hospitals, civil offices and the like, demands considerable thought to entrance design.  Here, Andrew Saxon of Biddle Air Curtains provides a guide to the use of air curtain entry solutions and explores the technology behind them.

It’s probably true to say that outside of the domains of consulting engineers, architects and H&V contractors, most people’s understanding of air curtains is reasonably inaccurate and out of date, yet they are now commonplace in commercial office, retail and manufacturing environments.

Air curtains and the technology that they employ have developed dramatically since the days when they were nothing more than over door heaters.  Certainly, there are still units available today which are still fundamentally unchanged from their heater origins, but a ‘true’ air curtain, whilst still capable of providing heat as part of its design, does much more.

To define the applications and benefits of air curtains in civil and public buildings, it is necessary to first clarify their function, operation and technology.

In essence, air curtains are designed to separate external and internal climates without the use of physical barriers, such as doors or shutters, for example.  Beyond this, they also serve to condition the incoming air by controlling the velocity, volume, air direction, flow and temperature.

Where entrances and access points are in regular use and no climate separation equipment is deployed, the temperature within a building can gradually fall or rise to within just a few degrees of the external ambient temperature.

This is caused by a difference in air densities and a process called ‘natural convection’.  In a warm building the heat will escape at the top of the doorway and cooler air will replace it by entering the building lower down the opening.

However, the loss of heat through open doorways or the loss of cooled air, where air conditioning is deployed, produces the same effects.  It reduces heating or cooling efficiency, increases energy costs and impacts on the level of comfort for the building’s occupants.

Ideal illustrations of where ‘open door’ environments demand consideration be given to all of these points are retail stores, leisure centres, libraries and local authority buildings.

To overcome these effects, air curtains must create a clean separation between internal and external climates and yet be efficient and cost effective to operate.

Whilst the five key air curtain design and performance factors of air velocity, volume direction, flow and temperature are well known, it wasn’t until Biddle initiated research into the inter-relationship of these variables that a true understanding of their importance was identified.  The results of this research provided the foundation for a new generation of air curtains with exceptional performance.

The research, conducted jointly by Biddle Holland; Novem, (the Dutch Energy Conservation Institute); TNO (the Dutch Research Institute); The University of Groningen and BSIRA used computer simulation, wind tunnel testing and analysis of real buildings to determine a true picture of the air behaviour, air curtain performance and the temperature ‘profiles’ created when mapped by thermal imaging techniques.

Whilst this explanation considerably undersells the scale and depth of the research programme, the most important aspects are the results.  The intelligence gained enabled Biddle to completely re-evaluate the design of air curtains in the knowledge that the development made in the improvement of performance, efficiency and operation could be measured accurately.

Equally important was the fact that the information gained enabled Biddle to develop a computer modelling software process, in conjunction with Warwick University, called CFD.  CFD accurately models heat and mass transfer in ‘real’ buildings to enable ‘energy cost savings’ to be predicted along with a realistic guide to the pay-back period.

From a product perspective, the culmination of the research was the creation of a new range called ‘Invisidor’ which incorporates all of the design developments suggested by the study including a new Biddle patented air rectifier that ensures the flow from the unit is laminar rather than turbulent to give a highly effective air screen.  Six operating speeds are also available to maintain the air screen integrity in situations where the outside wind velocities may threaten to penetrate the curtain.

The development process also included the ability for Invisidor air curtains to be fully integrated into sophisticated building management and climate control systems or operate as a stand alone unit controlled by and intelligent touchpad controller.  When set to automatic, the Invisidor’s fan speed, air discharge temperature and velocity are controlled by the unit itself.

With regard to access design, public buildings present the same challenges to architects, facilities managers and engineers as commercial or retail operations – that of presenting an ‘open and welcoming’ environment which does not compromise the comfort of employees or add to energy bills.

Air curtains are designed to meet these specific needs as they not only provide an invisible climate barrier but can actually help reduce energy costs by minimising the heat losses through constantly opening doors.

So, when is a door not a door? When its an air curtain.