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UVC light - reducing chiller energy in the summer heat

Now that the summer months are upon us, buildings and facilities managers face the challenge of keeping building occupants cool and comfortable while minimising energy consumption.

In this blog we’ll examine the importance of cooling coil performance within the air handling unit (AHU) of a heating, ventilation and air-conditioning (HVAC) system.

Cooling coils are heat exchangers whose performance is governed by the U value or coefficient of heat transfer. The larger the U value, the easier it is to transfer heat energy from the air to the chilled water.

The damp environment of a cooling coil supports the growth of bacteria, fungi and viruses on the fins of the coil. This mixture of living organisms combines with dust particles to form a very effective insulating layer on the coil fins known as biofilm.

In many cases, the build-up of biofilm reduces the coil’s U value to half its ‘as new’ value, severely reducing its ability to remove heat from the air. As a consequence, the amount of chilled water needed to keep the building temperature under control significantly increases.

Bathing the cooling coil in ultraviolet-C (UVC) light kills the organisms on the coil fins, improving the hygiene of the coil and, crucially, increasing the U value of the coil back to its ‘as new’ value. The graph below shows U value improvement for a coil in a large London office block following the installation of UVC, which is typical of many buildings where such systems have been fitted.

The chillers have a maximum chilled water flow rate, so, for example, if the addition of UVC to all the coils on a building reduces the chilled water flow by 50%, only half the number of chillers will be required compared with contaminated coils.

It follows that with contaminated coils that require twice as much chilled water and twice the number of chillers, each chiller is only 50% loaded compared to the clean coil situation.

For example, if the cooling load on a building on a particular day was 600 kW (600 kW of heat being pumped out of the building) and with contaminated coils the number of chillers needed was six, it follows that each chiller would be pumping out 100 kW.

With clean coils requiring only half the amount of chilled water, the number of chillers required would be three, each pumping out 200 kW. The efficiency of the chillers is generally considerably less at partial load, meaning that with contaminated coils, there would be six chillers running with poor efficiency compared to three chillers running with higher efficiency for the same heat load, resulting in considerable savings.

Where buildings have on-floor recirculating AHUs, the fan speed is often linked to the cooling valve demand signal. Fitting UVC to these systems improves the effectiveness of the cooling coil, reducing the cooling coil demand and fan speed. The power absorbed by the fan goes down with the cube of the speed, so if the fan speed is reduced by 20%, power absorbed goes down by 50%.

At Gibbons Ultraviolet Solutions, we install our innovative UVC disinfection systems within air handling units (AHUs) across the industrial and commercial sectors. The flexible, chemical-free and low-maintenance units can be installed in just a few hours by our engineers.

For more information on Gibbons Ultraviolet Solutions and our UVC systems, call Colin Hardman on 07940 544206 or email

Thanks to Steve Rix, Director of FED Ltd, the independent third-party testing partners for Gibbons Ultraviolet Solutions.

© Gibbons Ultraviolet Solutions 2018. All rights reserved. No part of this document may be reproduced or transmitted in any form or by any means, or stored in any retrieval system of any nature without prior written permission.

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