TRU Educational Series: A closer look at the Primary Design Purpose of the TRU

The TRU is designed to maintain the product setpoint during the transport journey. This may seem silly to repeat here again, but we must understand something about that statement, and it has to do with the word "maintain."

This word maintain assumes that the product has already been pre-cooled to a safe set-point for transport. The TRU then is simply keeping that set point as it goes from point A to Point B.

So in effect, it is just maintaining. Again, all of this sounds obvious, right? Let's keep moving, nothing to see here. Well, wait a second, because there's actually something very important we need to see here.

Just like with anything else, there's "the way" something is designed to work, and then there's the way that people use it. Something as simple as a pocket knife is used to cut things, but we use it for more than just cutting. We use it to cut, pry, screw, and scrap.

People do the same thing with the TRU. They don't just use it to keep products at pre-cooled set points, they use it as the primary method for cooling products too. So is this okay, or is it frowned upon? Well that depends on who you ask.

For our purposes, we're going to focus on the issues that this practice can create. When this practice is used, what can happen is that the unit may perform without issue, but there is going to be an increased risk to the product.

In some cases, it's perfectly okay, especially if the TRU is being used in a cold storage application. The risk begins to show up when the product is also being pre-cooled by the trailer during the transport phase. This is especially true of things like produce.

For example, fruits and vegetables produce heat and they usually contain high amounts of moisture. This is especially true when they are just harvested. Cooling these products requires the slow removal of heat and moisture so there's an even temperature distribution that doesn't affect product quality.

The TRU is not designed to do this, instead, it's designed to keep the produce at its current temperature by removing the heat trying to get in the trailer and mitigating the small amount of heat that is produced by the produce.

What happens in these scenarios is that the TRU will inevitably struggle during the pull-down process because of the product's ability to make its heat while also giving off lots of moisture. The unit just can't keep up.

Remember, the components of the TRU are designed to remove moisture, but not like this. They will become overwhelmed, while also failing to keep the desired set-point. Again, this is especially true if only the driver is the one keeping watch over the load.

What's going on inside the box during this process is that moisture begins to freeze over the evaporator and all the surrounding metal surfaces. This results in diminished cooling capacity and the potential for cooling alarm shut-downs.

Another common problem comes from the intensity of the airflow inside the compartment. With fast-moving colder air circulating the top of the product keeping it cold, the product on the inside of the stack is going to remain warmer. This kind of uneven cooling throughout the load can to hot spots which may increase the likelihood of the load being rejected upon delivery. There are other problems but this gives you some idea of what can happen if you use the TRU this way without some idea of what you should do to combat the problems.

I've experienced these kinds of scenarios as a mobile technician with Thermo King. I'd be dispatched to a unit that was shut down due to a cooling capacity alarm code. Usually, the driver would say to me that the load was put in the trailer at room temperature. It was now tightly stacked in cardboard boxes inside the trailer.

Initially, the TRU did what it was supposed to do, but after 3 or 4 hours, it just started not being able to keep up. Eventually, it set a few alarm codes and shut down, which led to an emergency call to dispatch and a visit from me. I knew right away what the problem was: a frozen evaporator.

The room temperature fruit inside the trailer was producing enough heat to make the trailer struggle to get it out; the problem was made worse by the fact that the shipper was attempting to bring the 75º load down to 45º before arriving at the distribution warehouse the following afternoon. The product, which was some kind of fruit, was giving off tons of moisture as it cooled too. This moisture overwhelmed the evaporator drain system and started collecting on the evaporator cores itself completely restricting airflow.

Eventually, we got the TRU cooling again, but for the remainder of the trip, the driver would have to stop every hour and force the unit into a defrost simply to keep the moisture inside the trailer from overwhelming the TRU by restricting the evaporator. What a headache for him I'm sure.

There is a method to removing heat from produce and it involves time and space, something the TRU does not provide. So just because it possesses the capacity to remove a bunch of heat, doesn't mean it's a good idea to use it this way.

Listen the speed-o-meter on my Honda Accord says 160 mph at the far right side of the display, but that doesn't mean it's a good idea to go try and do 160 miles an hour, even if I'm legally doing it on a race track.

The thing to remember here is what the design limitations can be with the TRU, how it's designed to be utilized in the transport process; as well as what different products can do to the performance of the TRU.

Now I know that maybe some in the industry will disagree and that's okay. They've done this kind of thing and had no issues. That's cool. My goal isn't to say what is right or wrong here, my goal is to inform and educate you about the design purposes of the TRU, and the different kinds of issues that you might face in supporting them.

The bottom line here is that it's important to understand the design strategy behind the TRU so when you see a customer using the TRU to try and get his product down to set point before leaving the dock, you might consider educating them on the risk involved in this kind of practice. The fact is, many people don't understand these issues. They just see a big refrigerator hanging off the front of the unit and assume that it will make things cold no matter what.

Helping them understand things like this goes a long way in mitigating future problems and the risk associated with transporting refrigerated products.