AIR CONDITIONING
work, which in practice results in the shutting down of compressors. For building owners, it means a smaller energy bill at the end of the month.
To accomplish this, rooftops are fitted with an additional component known as the ‘economiser’ – a simple two-damper component installed inside the unit that manages fresh air and exhaust air streams. Economisers nowadays are fitted with Modbus-driven actuators which communicate directly with the unit controller to regulate the damper opening based on the building requirements. When combining variable airflow and free cooling, the energy benefit is astounding. A typical payback will range between six months and two years, depending on the equipment usage and local price of electricity.
power consumptions and avoid cold drafts.
Let’s recall fan laws, which tells us that when
the fan speed decreases, the power consumption reduces by a factor of three. In other words, lowering the fan speed by 15% reduces the fan power consumption by 53%. Impressive, isn’t it? Variable fan speed control is not something
new and could also be done with AC fan motors. However, the solution was usually complex, bulky and therefore costly. Now, with the incorporation of EC technologies, components have become smaller and lighter, and fan speed control has been drastically simplified. Speed modulation is managed by applying a simple 0-10V signal to the control input. The input can come from multiple sources, depending on the application and type of load. For example, cinema rooms can automatically manage ventilation and air-conditioning based on a predefined schedule, supermarkets can regulate based on carbon dioxide concentration levels, etc. Now, thanks to improved technology
and more advanced software algorithms, rooftop units are managed in a smarter way, guaranteeing better comfort and performance. Energy savings can increase with a proper rooftop system controller, or even a simplified building management system (BMS).
Free cooling
One advantage packaged rooftops are known for is their ability to manage fresh air. This is especially important for applications involving human comfort, in order to respect local building codes that require a minimum amount of air renewal. For retail applications, for example, the common practice is to regulate
with 30% of fresh air in relation to the nominal supply airflow.
Rooftops are typically provided with a mixing chamber, in which the mixing between fresh air and return air streams occur. This improves the indoor air quality inside the building, which is also an important indication of comfort. Building managers will often have a CO2 sensor installed in the space to monitor the concentration of carbon dioxide, and regulate the amount of fresh air using the sensor as an input.
However, what few building managers realise is that fresh air can also be a quick and easy way to reduce energy costs. During spring and autumn, in which outdoor temperatures in Europe are mild and there is still a cooling requirement, it is possible to simply regulate a damper (much like opening a window) and cool down the building space using fresh air. This reduces the need for mechanical
Design
Comfort, efficiency, investment cost. No matter who you talk to in the HVAC industry, these three criteria will always come into play when selecting the right HVAC equipment for your commercial building. Independent of new regulations proposed by the European Commission, there continues to be an increase in demand for higher efficiency products with a lower carbon footprint This demand continuously pushes HVAC manufacturers to innovate, seeking better performance at the same or lower cost. Rooftop units are a formidable example of
an HVAC technology which has benefitted from this innovation, providing building designers and owners with a wider range of options suited to their budget and energy requirements – and giving traditional air handling systems some real competition.
www.acr-news.com
February 2020 31
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