Air curtains:faster is not always better!

Comfort air curtains separate the indoor from the outdoor climate in retail shops and public buildings. Separation is achieved through minimising natural convection and heating the incoming cold ventilation air. The heating capacity of conventional air curtains is often increased in winter by adjusting the discharge velocity.This article demonstrates that it would be better adjusting the discharge width rather than adjusting the discharge velocity when more heat is required. The benefits of this CA- (Constant Air velocity) technology with regard to energy consumption and comfort will also be outlined. BASICS OF COMFORT AIR CURTAINS
by Ir. B.E. Cremers, Biddle bv Kootstertille (The Netherland Comfort air curtains are used in retail shops for creating a comfortable indoor climate while the shop home door stays open. For a better understanding of how this works, a brief explanation about air curtains is given.In the situation of an open door, the temperature difference (indoor versus outdoor) will trigger an air mass exchange where warm air escapes and cold air enters the building. In addition, there is a ventilation flow that results in cold air entering the building. Such ventilation flow is caused by extraction inside the building (mechanical ventilation) and by pressure differences in and around the building (infiltration).Air curtain application is serving two purposes, briefly explained in the following paragraphs (see also fig.1). These matters are explained at length in an earlier TVVL publication [1].
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Reducing the outflow of heat

The first purpose of an air curtain is to minimise the outflow of heat to the outside. The air curtain draws in warm air that is about to escape through the top part of the door opening, creates a vertical air stream and the air returns backinto the shop at floor level. The reduction of heat loss, expressed as a percentage, is called the efficiency of the air curtain. If heat loss is presented as
Fig. 1 Technological principles of comfort air curtains out , the formula for the efficiency is: