Oversized shoes are rarely a good fit

The problem with overcapacity

– by Peter Bos, 12 December 2016

As a sales manager, I’m often asked to quote for air conditioning systems which take into account extreme ambient conditions. I even regularly receive tender requests for ambient temperatures of 50°C with a relative humidity of 100%.

Big is not always better

Worried about having a system that is too ‘light’, clients often provide extreme values so that they cover all the bases. Without realising it, this can create serious problems, especially for systems where the cooling is realised via direct expansion and one compressor. These popular systems are very efficient, favourably priced and can be made very compact. The refrigerant (generally known as Freon) is vaporised directly in the cooling battery and the air flowing through the cooling battery is cooled directly. This also means there are no temperature losses, which can be the case in chilled water systems which also serve as a transfer medium.

An over-specced system

While a system that is too light is an issue, the opposite can also lead to problems. Just as shoes which are too small are uncomfortable, oversized shoes are also rarely a good fit. When the AC system is specified based on exaggerated conditions it may result in an over-dimensioned system. This can work while the vessel stays in the Middle East or tropical regions, but as soon as it enters areas with lighter loads – further north, for instance – the problems start.

The problem with extra cooling capacity

Partly due to the fact that seawater is colder in the northern regions, cooling equipment may produce more cooling. This means that the cooling capacity expands – and this extra capacity is not a good thing. From a technical viewpoint, the capacity of a compressor can only be reduced to a certain extent and will stop once the desired inside temperature is reached. This will result in a mix of recirculated air and untreated outside air flowing into the vessel. In addition to fluctuations in the sensory temperature this often also leads to moisture on cold surfaces on board.

The compressor will switch on and off repeatedly which – in the case of a large compressor – can be compared to continually pushing the throttle in and out when driving. It’s an uncomfortable sensation and we all prefer a smooth ride within certain limits. A smaller, properly dimensioned compressor operates much more smoothly than one that is over-dimensioned. It may also be better to install two smaller compressors instead of a single larger one and thus considerably expand the capacity adjustment range.

Download our study into extreme ambient conditions

To specify realistic ambient conditions, Heinen & Hopman made a study of the maximum ambient conditions a vessel may encounter. The document can be downloaded here and I recommend using the results of this study to determine realistic ambient conditions for your specifications. Which ambient conditions actually apply onboard? ‘The larger the better’ often doesn’t apply and, once again, we see why HVAC systems are a truly custom product.

Peter Bos | Sales Manager

Peter Bos has been working at Heinen & Hopman for almost 30 years. He devotes most of his attention to selling HVAC&R systems to clients like Damen Shipyards and Feadship. Additionally, Peter offers sales support to our subsidiaries abroad as well. He highly values reliability and a no-nonsense approach and stands behind what he sells.