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post by David Parrin, Senior Advisor Cordstrap Knowledge Center

 

In a previous post I answered a question we regularly receive about the use of dunnage air bags in airplanes.


Another question we are often asked at Cordstrap Knowledge Center is if inflatable dunnage bags can be used to secure cargos inside reefer containers. Luckily the answer is YES. Due to the lack of lashing points in a reefer container, dunnage bags are one of the few securing methods available. However, care has to be taken that they are used properly to allow unhindered air circulation and prevent loss of pressure in the bags.

 


 

Temperatures inside reefer containers can go down to -23°C (-9.4°F). I am sure you could imagine that filling the dunnage bag during the summer at an outside temperature of 25°-30°C (77°-86°F) and then cooling to -23°C (-9.4°F) could lead to the bags hanging loose in the gaps between the cargo.
 

The bags will of course lose some pressure but, in the example above and at a maximum filling pressure of 0.2 Bar (20 kPa), Boyles’s Law tells us that the pressure change will be only around 16% as the temperature drops down to -23°C (-9.4°F).
 

Filling each bag up to the maximum filling pressure (taking care as always not to damage the packaging), waiting a couple of minutes to allow the pressure in the bag to settle and then adding a little more pressure will compensate for the expected small pressure drop.


To allow an unhindered flow of conditioned air around the cargo there are a number of important points to be taken care of during loading:


1. Never load the reefer beyond the red “No cargo above this line” indicators in the reefer.


2. Always leave sufficient room between the dunnage bag and the floor and/or roof of the reefer container to allow the air to circulate fully around the packaging. Failure to do this could end up with blocking the air flow resulting in possible damage to the product being transported.


3. When dunnage bags are used down the center of the reefer, allow sufficient space between the bags to allow the air to circulate freely around the cargo.


4. Do not use dunnage bags between the cargo and the doors as this will prevent the necessary air-flow and could lead to doors bursting open at the destination.


Hopefully you found this article helpful. I love feedback, so feel free to share your experience, questions, critique or wisdom in the comment box below.


Thanks!


David Parrin

 

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Comments

Dirk Pieter
Dirk Pieter
Tuesday, July 31, 2012 10:25 AM
Dear David,

Thanks for the interesting article!

I was only wondering if we can apply the same technique here in Saudi Arabia where the temperatures go over 50 degrees celcius in summer? Is the change as well 16% or is this actual percentage higher with these temperatures?

Br,
Dirk Pieter
David Parrin
David Parrin
Tuesday, July 31, 2012 3:39 PM
Hi Dirk-Pieter,

Many thanks for your question! You can apply the same technique in Saudi Arabia but from +50 degrees C down to for example -23 degrees C will mean a pressure drop of +/- 23%. Mostly the temperature of the air coming out of the air line is not as hot as 50 degrees which helps but I would certainly recommend giving the bags a little more pressure after initial filling to compensate for the pressure drop. If your inflator doesn't have a pressure gauge it's a question of feeling how tight the bag based on you experience.

Let me know how it goes on - I'll be pleased to hear!

Best regards,

David
paul
paul
Thursday, September 27, 2012 5:50 PM
Hello David

thanks for your wisdom, however I have always wondered about the following:
using the dunnage bags down the middle means you're forcing the pallets against the wall. That also means there is no longer an optimal airflow around the boxes. Are we then partially losing out on the benefits of the reefer as only the space left between the dunnage bags and between top of load and ceiling will be left for air circulation?
(in this case we have boxes that are fully closed on all sides)

thanks
David Parrin
David Parrin
Tuesday, October 02, 2012 9:07 PM
Hi Paul,

Many thanks for your question. It's always good to get a reaction. I am sorry my response took a little longer, but I had a few days off...

I think we have 2 damage issues here:

1. Preventing damage by securing the cargo against movement - depending on the degree of "tightness" between the boxes or pallets in the reefer we have to ensure that the cargo may not move around and become damaged during the jouney. Too much space will lead to damage. The best way to do this is to fill gaps and use the container walls to support the cargo.
2. Preventing damage due to temperature/moisture changes - this requires a good air circulation and the reefer has been designed to distribute the air mainly under the cargo via the floor grating and over the top of the cargo. To my mind this is sufficient and going around the walls should not be necessary.

I think in this way we solve both problems.

Best regards,

David
Peter Wedell
Peter Wedell
Wednesday, November 28, 2012 10:51 AM
Hi David,

Good article with many professional observations, but I feel it fair to stress, that You should also include, that wrong use of dunnage bags in reefer containers, can create considerable damage, not neccessarily to the cargo, but to the reefer container itself. We have seen several issues where dunnage bags placed against sidewalls in reefer containers have damaged the entire side, with severe repair costs as a consequence, if not a total loss.
Just a comment, that focus is not always on the cargo itself.

Brgds
peter
Peter Wedell
Peter Wedell
Wednesday, November 28, 2012 2:32 PM
Hi David,

Sorry but in my previous answer to Your blog, I forgot to mention that all reefer containers have securing points in the T-Bar flooring, minimum 6 each side, capable of withstanding 1 ton pull. Brgds Peter
paul
paul
Wednesday, November 28, 2012 4:42 PM
Peter

from your experience, how do you see the effective use of the securing points to keep pallets with loosly loaded boxes on top from shifting against the wall where they would be blocking the airflow on these sides of the load and I have understood that the airflow must be well distributed and passing along all sides of the entire load

thanks for your insight
Peter Wedell
Peter Wedell
Friday, November 30, 2012 1:57 PM
Paul, the entire secret about safe transport of perishables or other fragile commodities, is using the correct pallet. A standard 1,00 x 1,20m will ensure a tight and correct stowage where pallets themselves and side walls are sufficient to secure the cargo. The air flow in a container is, as You correctly state, a vertical movement, where the air must flow upwards throughout the cargo, thereby removing respiration heat and/or gasses such as CO2 or ethylene. In this case You can freely stuff against sidewalls, as the airflow here is not important. (same procedure as on reefer vessel tweendecks). When carrying frozen goods we recommend a block stowage, where stuffing against walls is also allowed. A good air distribution in reefer containers is solely dependent on respecting the red loadlines at top and door end. You can find more on this topic at "Ask Captain Peter" www.maersklinereefer.com.
David Parrin
David Parrin
Friday, December 07, 2012 4:52 PM
To Peter and Paul... Many thanks for your contribution and making it very clear to us what can and cannot be done in a reefer. We rely on the expert experience and knowledge of those directly concerned with such topics and are glad that you share this with us.

Peter you are of course right! We advise to use dunnage bags down the middle of the container between the pallets as much as possible. If (and only if) they are used against the side walls much care should be taken to ensure that the reefer walls are not damaged! Also thanks for the remark regarding the securing points in the flooring. I know they exist but did not realise that they are officially securing points so this is welcome news to me and may help to secure reefer cargos better.

Best regards,
David
Anonymous
Anonymous
Monday, December 24, 2012 7:11 AM
2012, Christmas and a New Year

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