Tech Talk

Thermal efficiency for refrigerated trucks

Does the wall colour or thickness of a refrigerated truck effect its thermal efficiency? You may be surprised at the significant impact these factors – and others – have on a unit’s efficiency and fridge plant fuel consumption.

To get the hard data, renowned MaxiPARTS engineer Greg Brown conducted a series of experiments by measuring the skin temperature of a white unit and a dark coloured unit placed in the sun, each with 25mm foam walls.

“At its peak temperature, the white unit reached just under 30 degrees Celsius, while the dark unit got up to nearly 70 degrees Celsius,” Brown said.

“With a temperature difference of 38 degrees Celsius, we can expect an extra heat leak of approximately 1200 watts. Even when the temperature difference is lower at 23 degrees Celsius, the calculated extra heat leak is 965 watts.”

Brown also points out that the darkness of a unit is not just about what colour it is painted.

Well-known MaxiPARTS engineer Greg Brown conducted a series of experiments.

“Dirt and grime can impact temperature – the dirtier a truck, the darker it is. This is especially the case for a truck’s roof which is less noticeable when dirty,” Brown said.

“This tells us that whether exposed to the sun or not, there is a big temperature difference between dark, dirty, and light units. But it is not the only factor that contributes to thermal efficiency.”

The thicker the foam in the walls, the better the insulation and heat leak. When we increase the foam from 40mm to 65mm, there is a heat leakage reduction of 460 watts. If we go even further and increase the foam thickness to 90mm, we can see a further reduction of 200 watts.

When talking about refrigerated units, you may have heard the term ‘K-factor’.

“K-factor is the material’s thermal conductivity or ability to conduct heat,” Brown said.

“The unit of measurement for thermal conductivity is W/(m²k), and the lower the number the better.”

When comparing the complete trailer, the ‘K’ is of the complete body and the unit is W/m²K. This is the average energy in watts per square meter of body that will leak per degree Kelvin or degree C temperature difference between the inside and the outside.

When comparing two trailers of the same specs but different lengths (10 metres and 14 metres), Brown noted that the longer trailer had a lower K (0.53) but leaked more (2525 watts), while the shorter trailer leaked less (2006 watts) but had a higher K (0.57).

“This is because as the size of the unit increases there is more surface area so it will leak or transfer more energy. This leaked energy is the energy that the fridge plant needs to remove to maintain the required internal temperature,” Brown said.

The K factor of the complete unit can be misleading as is the larger the unit the lower the K factor which does not equate to lower energy transfer, in fact it is larger. The K factor reduces as the size of the body increases because the panels, walls and roof get proportionally bigger faster than the joins and corners which are a bigger leaks for their size.

The final factor that impacts thermal efficiency and fuel consumption is circulation and multiple temperature zones.

“It’s quite common to have multiple temperature zones in one trailer. Because of customer requirements, they are not often in the logical sequence in the unit,” Brown said.

“Circulation is particularly important for maintaining temperature, and an effective way to manage this is by utilising recessed tracks.

“Recessed tracks maintain the same internal width but increase the insulation thickness for substantial percentage of wall.”

A full overview and visual examples of Brown’s findings can be found on the ATA YouTube channel. 

About the author:

Bob Woodward is the ATA’s chief engineer.

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