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The Science of Condensation

Published: February 1, 2021

Marty Dunningham, FPG Refrigeration Design Engineer

CONDENSATION

It can lead to fogging on the glass of a refrigerated display cabinet. In the worst cases, it can lead to pooling of water. Why does it occur, and what can be done to prevent it? Recently eNews caught up with FPG senior refrigeration engineer Marty Dunningham, to talk about the issue of condensation.

Marty, what causes condensation on the outside of refrigerated cabinets?

It is important to understand that condensation is a naturally occurring phenomenon and can be explained scientifically. It is the result of the temperature of the outer surface of the refrigerated cabinet being at, or less than, the Dew Point temperature. ‘Dew Point’ is the temperature at which moisture in the air will condense on a surface. The Dew Point temperature is dependent on:

  • the air temperature and
  • the air Relative Humidity.

‘Relative Humidity’ is the ratio of the amount of water vapour in the air at a specific temperature to the maximum amount that the air could hold at that temperature, expressed as a percentage (%RH). Whether condensation will form on the cabinet is therefore determined by the:

  • The temperature of a refrigerated cabinet’s outer surface
  • Air temperature
  • Relative Humidity

So, a change in either or both of the air temperature and Relative Humidity will impact the Dew Point temperature.

Correct. Here are some examples:

Scenario One

Cabinet outer surface temp16°C
Air temperature21°C
Relative Humidity45%
Dew point8.6°C

Condensation?

No. The temperature of the cabinet outer surface 16°C is well above the Dew Point temperature of 8.6°C.

Scenario Two

Cabinet outer surface temp16°C
Air temperature21°C
Relative Humidity65%
Dew point14.2°C

Condensation?

No. The temperature of the cabinet outer surface 16°C is still above the Dew Point temperature of 14.2°C

Scenario Three – Climate Class 3

Cabinet outer surface temp17°C
Air temperature25°C
Relative Humidity60%
Dew point16.7°C

Condensation?

No. The temperature of the cabinet outer surface 17°C is above the Dew Point temperature of 16.7°C

Scenario Four

Cabinet outer surface temp17°C
Air temperature25°C
Relative Humidity85%
Dew point22.3°C

Condensation?

Yes. Condensation forms on the outer surface of the cabinet. The temperature of the cabinet outer surface 17°C is well below the Dew Point temperature of 22.3°C

To illustrate the point, the cabinets in the following photos are the same model and were installed on different sites in Queensland, Australia.

Photo 1: The air conditioning was turned off over night while the refrigerated cabinets were left stocked and running. Unsurprisingly, in the morning, condensation and pooling of water was observed.

Photo 2: The environment at this site is controlled via a fit-for-purpose air conditioning system. The site has no issues with condensation on their cabinets.

What can be done to stop condensation on the outside of refrigerated cabinets?

There are several steps that can be taken to mitigate condensation:

  1. Reduce the store humidity. Store humidity levels are usually controlled by the store air conditioning system. Ensure that the store air conditioning system is working properly and maintaining a low humidity. This will lower the Dew Point temperature which in turn will ease or stop the condensation.
  1. Don’t allow the store temperature to get too warm. The warmer the store air temperature is, the higher the Dew Point temperature will become. Ensure that the store air conditioning system is working properly and maintaining the correct store temperature.
  1. Is the refrigerated cabinet running too cold? The colder the cabinet internal temperature is, then the colder the cabinet outer surface will be. A cold outer surface temperature is more likely to be below the Dew Point and form condensation. Raising the cabinet internal operating temperature will have the effect of raising the cabinet outer surface temperature helping to keep it above the Dew Point.
  1. Keep the outer surface of the cabinet warm. It is important that there is unrestricted air movement over the outer surfaces of the cabinet. If any of the refrigerated cabinet outer surfaces are too close to other cabinets, joinery, walls or any other objects that inhibit natural air movement, then condensation can occur. The reason for this is that without the warming effect of natural or forced air movement, the refrigerated cabinet outer surface can become cold enough to reach the Dew Point temperature when condensation forms.

Are there other options to reduce the likelihood of condensation?

Prevention is the best course of action. As noted above, if you understand the science you can take steps to mitigate it before it becomes an issue.

In some cases, however, the cabinet specification needs to be considered. The industry standard for the design and testing of refrigerated cabinets is Climate Class 3. If your environmental conditions are outside of Climate Class 3 (25°C/60%RH) then be sure to mention this to the supplier of your refrigerated display cabinet before purchase.

There are options such as engineering with heated glass to increase the temperature of the cabinet outer surface. At FPG we have an environmental test chamber so we can test cabinet performance to the different environmental settings. While adding heated glass comes at an additional cost, the upside is a clear display to help you sell more.

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